full_paragraphs: 1.1a: - Atmosphere Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. The period from 1983 to 2012 was very likely the warmest 30-year period of the last 800 years in the Northern Hemisphere, where such assessment is possible and likely the warmest 30-year period of the last 1400 years. The globally averaged combined land and ocean surface temperature data as calculated by a linear trend show a warming of 0.85 [0.65 to 1.06] °C20 over the period 1880 to 2012, for which multiple independently produced datasets exist. The total increase between the average of the 1850–1900 period and the 2003–2012 period is 0.78 [0.72 to 0.85] °C, based on the single longest dataset available. For the longest period when calculation of regional trends is sufficiently complete, almost the entire globe has experienced surface warming. In addition to robust multi-decadal warming, the globally averaged surface temperature exhibits substantial decadal and interannual variability. Due to this natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over the past 15 years , which begins with a strong El Niño, is smaller than the rate calculated since 1951 . Based on multiple independent analyses of measurements, it is virtually certain that globally the troposphere has warmed and the lower stratosphere has cooled since the mid-20th century. There is medium confidence in the rate of change and its vertical structure in the Northern Hemisphere extratropical troposphere. Confidence in precipitation change averaged over global land areas since 1901 is low prior to 1951 and medium afterwards. Averaged over the mid-latitude land areas of the Northern Hemisphere, precipitation has likely increased since 1901 . For other latitudes area-averaged long-term positive or negative trends have low confidence. 1.1b: - Ocean Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010 with only about 1% stored in the atmosphere. On a global scale, the ocean warming is largest near the surface, and the upper 75 m warmed by 0.11 [0.09 to 0.13] °C per decade over the period 1971 to 2010. It is virtually certain that the upper ocean warmed from 1971 to 2010, and it likely warmed between the 1870s and 1971. It is likely that the ocean warmed from 700 to 2000 m from 1957 to 2009 and from 3000 m to the bottom for the period 1992 to 2005. It is very likely that regions of high surface salinity, where evaporation dominates, have become more saline, while regions of low salinity, where precipitation dominates, have become fresher since the 1950s. These regional trends in ocean salinity provide indirect evidence for changes in evaporation and precipitation over the oceans and thus for changes in the global water cycle. There is no observational evidence of a long-term trend in the Atlantic Meridional Overturning Circulation. Since the beginning of the industrial era, oceanic uptake of CO2 has resulted in acidification of the ocean; the pH of ocean surface water has decreased by 0.1, corresponding to a 26% increase in acidity, measured as hydrogen ion concentration. There is medium confidence that, in parallel to warming, oxygen concentrations have decreased in coastal waters and in the open ocean thermocline in many ocean regions since the 1960s, with a likely expansion of tropical oxygen minimum zones in recent decades. 1.1c: - Atmosphere Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. The period from 1983 to 2012 was very likely the warmest 30-year period of the last 800 years in the Northern Hemisphere, where such assessment is possible and likely the warmest 30-year period of the last 1400 years. The globally averaged combined land and ocean surface temperature data as calculated by a linear trend show a warming of 0.85 [0.65 to 1.06] °C20 over the period 1880 to 2012, for which multiple independently produced datasets exist. The total increase between the average of the 1850–1900 period and the 2003–2012 period is 0.78 [0.72 to 0.85] °C, based on the single longest dataset available. For the longest period when calculation of regional trends is sufficiently complete, almost the entire globe has experienced surface warming. In addition to robust multi-decadal warming, the globally averaged surface temperature exhibits substantial decadal and interannual variability. Due to this natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over the past 15 years , which begins with a strong El Niño, is smaller than the rate calculated since 1951 . Based on multiple independent analyses of measurements, it is virtually certain that globally the troposphere has warmed and the lower stratosphere has cooled since the mid-20th century. There is medium confidence in the rate of change and its vertical structure in the Northern Hemisphere extratropical troposphere. Confidence in precipitation change averaged over global land areas since 1901 is low prior to 1951 and medium afterwards. Averaged over the mid-latitude land areas of the Northern Hemisphere, precipitation has likely increased since 1901 . For other latitudes area-averaged long-term positive or negative trends have low confidence. - Ocean Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010 with only about 1% stored in the atmosphere. On a global scale, the ocean warming is largest near the surface, and the upper 75 m warmed by 0.11 [0.09 to 0.13] °C per decade over the period 1971 to 2010. It is virtually certain that the upper ocean warmed from 1971 to 2010, and it likely warmed between the 1870s and 1971. It is likely that the ocean warmed from 700 to 2000 m from 1957 to 2009 and from 3000 m to the bottom for the period 1992 to 2005. It is very likely that regions of high surface salinity, where evaporation dominates, have become more saline, while regions of low salinity, where precipitation dominates, have become fresher since the 1950s. These regional trends in ocean salinity provide indirect evidence for changes in evaporation and precipitation over the oceans and thus for changes in the global water cycle. There is no observational evidence of a long-term trend in the Atlantic Meridional Overturning Circulation. Since the beginning of the industrial era, oceanic uptake of CO2 has resulted in acidification of the ocean; the pH of ocean surface water has decreased by 0.1, corresponding to a 26% increase in acidity, measured as hydrogen ion concentration. There is medium confidence that, in parallel to warming, oxygen concentrations have decreased in coastal waters and in the open ocean thermocline in many ocean regions since the 1960s, with a likely expansion of tropical oxygen minimum zones in recent decades. 1.1d: - Ocean Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010 with only about 1% stored in the atmosphere. On a global scale, the ocean warming is largest near the surface, and the upper 75 m warmed by 0.11 [0.09 to 0.13] °C per decade over the period 1971 to 2010. It is virtually certain that the upper ocean warmed from 1971 to 2010, and it likely warmed between the 1870s and 1971. It is likely that the ocean warmed from 700 to 2000 m from 1957 to 2009 and from 3000 m to the bottom for the period 1992 to 2005. It is very likely that regions of high surface salinity, where evaporation dominates, have become more saline, while regions of low salinity, where precipitation dominates, have become fresher since the 1950s. These regional trends in ocean salinity provide indirect evidence for changes in evaporation and precipitation over the oceans and thus for changes in the global water cycle. There is no observational evidence of a long-term trend in the Atlantic Meridional Overturning Circulation. Since the beginning of the industrial era, oceanic uptake of CO2 has resulted in acidification of the ocean; the pH of ocean surface water has decreased by 0.1, corresponding to a 26% increase in acidity, measured as hydrogen ion concentration. There is medium confidence that, in parallel to warming, oxygen concentrations have decreased in coastal waters and in the open ocean thermocline in many ocean regions since the 1960s, with a likely expansion of tropical oxygen minimum zones in recent decades. 1.1e: - Cryosphere Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass. Glaciers have continued to shrink almost worldwide. Northern Hemisphere spring snow cover has continued to decrease in extent. There is high confidence that there are strong regional differences in the trend in Antarctic sea ice extent, with a very likely increase in total extent. Glaciers have lost mass and contributed to sea level rise throughout the 20th century. The rate of ice mass loss from the Greenland ice sheet has very likely substantially increased over the period 1992 to 2011, resulting in a larger mass loss over 2002 to 2011 than over 1992 to 2011. The rate of ice mass loss from the Antarctic ice sheet, mainly from the northern Antarctic Peninsula and the Amundsen Sea sector of West Antarctica, is also likely larger over 2002 to 2011. The annual mean Arctic sea ice extent decreased over the period 1979 to 2012. The rate of decrease was very likely in the range 3.5 to 4.1% per decade. Arctic sea ice extent has decreased in every season and in every successive decade since 1979, with the most rapid decrease in decadal mean extent in summer . For the summer sea ice minimum, the decrease was very likely in the range of 9.4 to 13.6% per decade . It is very likely that the annual mean Antarctic sea ice extent increased in the range of 1.2 to 1.8% per decade between 1979 and 2012. However, there is high confidence that there are strong regional differences in Antarctica, with extent increasing in some regions and decreasing in others. There is very high confidence that the extent of Northern Hemisphere snow cover has decreased since the mid-20th century by 1.6 [0.8 to 2.4] % per decade for March and April, and 11.7% per decade for June, over the 1967 to 2012 period. There is high confidence that permafrost temperatures have increased in most regions of the Northern Hemisphere since the early 1980s, with reductions in thickness and areal extent in some regions. The increase in permafrost temperatures has occurred in response to increased surface temperature and changing snow cover. 1.1g: - Sea level Over the period 1901–2010, global mean sea level rose by 0.19 [0.17 to 0.21] m. The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia. It is very likely that the mean rate of global averaged sea level rise was 1.7 [1.5 to 1.9] mm/yr between 1901 and 2010 and 3.2 [2.8 to 3.6] mm/yr between 1993 and 2010. Tide gauge and satellite altimeter data are consistent regarding the higher rate during the latter period. It is likely that similarly high rates occurred between 1920 and 1950. Since the early 1970s, glacier mass loss and ocean thermal expansion from warming together explain about 75% of the observed global mean sea level rise. Over the period 1993–2010, global mean sea level rise is, with high confidence, consistent with the sum of the observed contributions from ocean thermal expansion, due to warming, from changes in glaciers, the Greenland ice sheet, the Antarctic ice sheet and land water storage. Rates of sea level rise over broad regions can be several times larger or smaller than the global mean sea level rise for periods of several decades, due to fluctuations in ocean circulation. Since 1993, the regional rates for the Western Pacific are up to three times larger than the global mean, while those for much of the Eastern Pacific are near zero or negative. There is very high confidence that maximum global mean sea level during the last interglacial period was, for several thousand years, at least 5 m higher than present and high confidence that it did not exceed 10 m above present. During the last interglacial period, the Greenland ice sheet very likely contributed between 1.4 and 4.3 m to the higher global mean sea level, implying with medium confidence an additional contribution from the Antarctic ice sheet. This change in sea level occurred in the context of different orbital forcing and with high-latitude surface temperature, averaged over several thousand years, at least 2°C warmer than present . 1.2a: - 'Natural and anthropogenic radiative forcings Atmospheric concentrations of GHGs are at levels that are unprecedented in at least 800, years. Concentrations of carbon dioxide, methane and nitrous oxide have all shown large increases since 1750 . CO2 concentrations are increasing at the fastest observed decadal rate of change for 2002– 2011. After almost one decade of stable CH4 concentrations since the late 1990s, atmospheric measurements have shown renewed increases since 2007. N2O concentrations have steadily increased at a rate of 0.73 ± 0.03 ppb/yr over the last three decades. The total anthropogenic radiative forcing over 1750–2011 is calculated to be a warming effect of 2.3 [1.1 to 3.3] W/m , and it has increased more rapidly since 1970 than during prior decades. Carbon dioxide is the largest single contributor to radiative forcing over 1750–2011 and its trend since 1970. The total anthropogenic radiative forcing estimate for 2011 is substantially higher than the estimate reported in the IPCC Fourth Assessment Report for the year 2005. This is caused by a combination of continued growth in most GHG concentrations and an improved estimate of radiative forcing from aerosols. The radiative forcing from aerosols, which includes cloud adjustments, is better understood and indicates a weaker cooling effect than in AR4. The aerosol radiative forcing over 1750–2011 is estimated as –0.9 [–1.9 to −0.1] W/m . Radiative forcing from aerosols has two competing components: a dominant cooling effect from most aerosols and their cloud adjustments and a partially offsetting warming contribution from black carbon absorption of solar radiation. There is high confidence that the global mean total aerosol radiative forcing has counteracted a substantial portion of radiative forcing from wellmixed GHGs. Aerosols continue to contribute the largest uncertainty to the total radiative forcing estimate. Changes in solar irradiance and volcanic aerosols cause natural radiative forcing. The radiative forcing from stratospheric volcanic aerosols can have a large cooling effect on the climate system for some years after major volcanic eruptions. Changes in total solar irradiance are calculated to have contributed only around 2% of the total radiative forcing in 2011, relative to 1750.' - Human activities affecting emission drivers About half of the cumulative anthropogenic CO2 emissions between 1750 and 2011 have occurred in the last 40 years . Cumulative anthropogenic CO2 emissions of 2040 ± 310 GtCO2 were added to the atmosphere between 1750 and 2011. Since 1970, cumulative CO2 emissions from fossil fuel combustion, cement production and flaring have tripled, and cumulative CO2 emissions from forestry and other land use22 have increased by about 40% 23. In 2011, annual CO2 emissions from fossil fuel combustion, cement production and flaring were 34.8 ± 2.9 GtCO2/yr. For 2002–2011, average annual emissions from FOLU were 3.3 ± 2.9 GtCO2/yr. About 40% of these anthropogenic CO2 emissions have remained in the atmosphere since 1750. The rest was removed from the atmosphere by sinks, and stored in natural carbon cycle reservoirs. Sinks from ocean uptake and vegetation with soils account, in roughly equal measures, for the remainder of the cumulative CO2 emissions. The ocean has absorbed about 30% of the emitted anthropogenic CO2, causing ocean acidification. Total annual anthropogenic GHG emissions have continued to increase over 1970 to 2010 with larger absolute increases between 2000 and 2010. Despite a growing number of climate change mitigation policies, annual GHG emissions grew on average by 1.0 GtCO2-eq per year, from 2000 to 2010, compared to 0.4 GtCO2-eq per year, from 1970 to 2000 24. Total anthropogenic GHG emissions from 2000 to 2010 were the highest in human history and reached 49 GtCO2-eq/yr in 2010. The global economic crisis of 2007/2008 reduced emissions only temporarily. CO2 emissions from fossil fuel combustion and industrial processes contributed about 78% to the total GHG emission increase between 1970 and 2010, with a contribution of similar percentage over the 2000–2010 period. Fossil-fuel-related CO2 emissions reached 32 GtCO2/yr, in 2010, and grew further by about 3% between 2010 and 2011, and by about 1 to 2% between 2011 and 2012. CO2 remains the major anthropogenic GHG, accounting for 76% of total anthropogenic GHG emissions in 2010. Of the total, 16% comes from CH4, 6.2% from N2O, and 2.0% from fluorinated gases 25. Annually, since 1970, about 25% of anthropogenic GHG emissions have been in the form of non-CO2 gases. Total annual anthropogenic GHG emissions have increased by about 10 GtCO2-eq between 2000 and 2010. This increase directly came from the energy, industry, transport and building sectors. Accounting for indirect emissions raises the contributions by the building and industry sectors. Since 2000, GHG emissions have been growing in all sectors, except in agriculture, forestry and other land use 22. In 2010, 35% of GHG emissions were released by the energy sector, 24% from AFOLU, 21% by industry, 14% by transport and 6.4% by the building sector. When emissions from electricity and heat production are attributed to the sectors that use the final energy, the shares of the industry and building sectors in global GHG emissions are increased to 31% and 19%, respectively. See also Box 3.2 for contributions from various sectors, based on metrics other than 100-year Global Warming Potential. Globally, economic and population growth continue to be the most important drivers of increases in CO2 emissions from fossil fuel combustion. The contribution of population growth between 2000 and 2010 remained roughly identical to that of the previous three decades, while the contribution of economic growth has risen sharply. Between 2000 and 2010, both drivers outpaced emission reductions from improvements in energy intensity of gross domestic product. Increased use of coal relative to other energy sources has reversed the long-standing trend in gradual decarbonization of the world’s energy supply. 1.2c: - Attribution of climate changes and impacts The evidence for human influence on the climate system has grown since AR4. Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, and in global mean sea level rise; and it is extremely likely to have been the dominant cause of the observed warming since the mid20th century. In recent decades, changes in climate have caused impacts on natural and human systems on all continents and across the oceans. Impacts are due to observed climate change, irrespective of its cause, indicating the sensitivity of natural and human systems to changing climate. The causes of observed changes in the climate system, as well as in any natural or human system impacted by climate, are established following a consistent set of methods. Detection addresses the question of whether climate or a natural or human system affected by climate has actually changed in a statistical sense, while attribution evaluates the relative contributions of multiple causal factors to an observed change or event with an assignment of statistical confidence. Attribution of climate change to causes quantifies the links between observed climate change and human activity, as well as other, natural, climate drivers. In contrast, attribution of observed impacts to climate change considers the links between observed changes in natural or human systems and observed climate change, regardless of its cause. Results from studies attributing climate change to causes provide estimates of the magnitude of warming in response to changes in radiative forcing and hence support projections of future climate change. Results from studies attributing impacts to climate change provide strong indications for the sensitivity of natural or human systems to future climate change. 1.3a: - Observed impacts attributed to climate change In recent decades, changes in climate have caused impacts on natural and human systems on all continents and across the oceans. Impacts are due to observed climate change, irrespective of its cause, indicating the sensitivity of natural and human systems to changing climate. Evidence of observed climate change impacts is strongest and most comprehensive for natural systems. Some impacts on human systems have also been attributed to climate change, with a major or minor contribution of climate change distinguishable from other influences. Impacts on human systems are often geographically heterogeneous because they depend not only on changes in climate variables but also on social and economic factors. Hence, the changes are more easily observed at local levels, while attribution can remain difficult. In many regions, changing precipitation or melting snow and ice are altering hydrological systems, affecting water resources in terms of quantity and quality. Glaciers continue to shrink almost worldwide due to climate change , affecting runoff and water resources downstream . Climate change is causing permafrost warming and thawing in high-latitude regions and in high-elevation regions . Many terrestrial, freshwater and marine species have shifted their geographic ranges, seasonal activities, migration patterns, abundances and species interactions in response to ongoing climate change. While only a few recent species extinctions have been attributed as yet to climate change , natural global climate change at rates slower than current anthropogenic climate change caused significant ecosystem shifts and species extinctions during the past millions of years. Increased tree mortality, observed in many places worldwide, has been attributed to climate change in some regions. Increases in the frequency or intensity of ecosystem disturbances such as droughts, windstorms, fires and pest outbreaks have been detected in many parts of the world and in some cases are attributed to climate change . Numerous observations over the last decades in all ocean basins show changes in abundance, distribution shifts poleward and/ or to deeper, cooler waters for marine fishes, invertebrates and phytoplankton, and altered ecosystem composition , tracking climate trends. Some warm-water corals and their reefs have responded to warming with species replacement, bleaching, and decreased coral cover causing habitat loss . Some impacts of ocean acidification on marine organisms have been attributed to human influence, from the thinning of pteropod and foraminiferan shells to the declining growth rates of corals. Oxygen minimum zones are progressively expanding in the tropical Pacific, Atlantic and Indian Oceans, due to reduced ventilation and O2 solubility in warmer, more stratified oceans, and are constraining fish habitat. Assessment of many studies covering a wide range of regions and crops shows that negative impacts of climate change on crop yields have been more common than positive impacts . The smaller number of studies showing positive impacts relate mainly to high-latitude regions, though it is not yet clear whether the balance of impacts has been negative or positive in these regions. Climate change has negatively affected wheat and maize yields for many regions and in the global aggregate. Effects on rice and soybean yield have been smaller in major production regions and globally, with a median change of zero across all available data which are fewer for soy compared to the other crops. Observed impacts relate mainly to production aspects of food security rather than access or other components of food security. Since AR4, several periods of rapid food and cereal price increases following climate extremes in key producing regions indicate a sensitivity of current markets to climate extremes among other factors . At present the worldwide burden of human ill-health from climate change is relatively small compared with effects of other stressors and is not well quantified. However, there has been increased heat-related mortality and decreased cold-related mortality in some regions as a result of warming. Local changes in temperature and rainfall have altered the distribution of some water-borne illnesses and disease vectors. ‘Cascading’ impacts of climate change can now be attributed along chains of evidence from physical climate through to intermediate systems and then to people. The changes in climate feeding into the cascade, in some cases, are linked to human drivers , while, in other cases, assessments of the causes of observed climate change leading into the cascade are not available. In all cases, confidence in detection and attribution to observed climate change decreases for effects further down each impact chain. 1.4a: - Extreme events Changes in many extreme weather and climate events have been observed since about 1950. Some of these changes have been linked to human influences, including a decrease in cold temperature extremes, an increase in warm temperature extremes, an increase in extreme high sea levels and an increase in the number of heavy precipitation events in a number of regions. It is very likely that the number of cold days and nights has decreased and the number of warm days and nights has increased on the global scale. It is likely that the frequency of heat waves has increased in large parts of Europe, Asia and Australia. It is very likely that human influence has contributed to the observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-20th century. It is likely that human influence has more than doubled the probability of occurrence of heat waves in some locations. There is medium confidence that the observed warming has increased heat-related human mortality and decreased coldrelated human mortality in some regions. Extreme heat events currently result in increases in mortality and morbidity in North America , and in Europe with impacts that vary according to people’s age, location and socio-economic factors. There are likely more land regions where the number of heavy precipitation events has increased than where it has decreased. The frequency and intensity of heavy precipitation events has likely increased in North America and Europe. In other continents, confidence in trends is at most medium. It is very likely that global near-surface and tropospheric air specific humidity has increased since the 1970s. In land regions where observational coverage is sufficient for assessment, there is medium confidence that anthropogenic forcing has contributed to a global-scale intensification of heavy precipitation over the second half of the 20th century. There is low confidence that anthropogenic climate change has affected the frequency and magnitude of fluvial floods on a global scale. The strength of the evidence is limited mainly by a lack of long-term records from unmanaged catchments. Moreover, floods are strongly influenced by many human activities impacting catchments, making the attribution of detected changes to climate change difficult. However, recent detection of increasing trends in extreme precipitation and discharges in some catchments implies greater risks of flooding on a regional scale. Costs related to flood damage, worldwide, have been increasing since the 1970s, although this is partly due to the increasing exposure of people and assets. There is low confidence in observed global-scale trends in droughts, due to lack of direct observations, dependencies of inferred trends on the choice of the definition for drought, and due to geographical inconsistencies in drought trends. There is also low confidence in the attribution of changes in drought over global land areas since the mid-20th century, due to the same observational uncertainties and difficulties in distinguishing decadal scale variability in drought from long-term trends. There is low confidence that long-term changes in tropical cyclone activity are robust, and there is low confidence in the attribution of global changes to any particular cause. However, it is virtually certain that intense tropical cyclone activity has increased in the North Atlantic since 1970. It is likely that extreme sea levels have increased since 1970, being mainly the result of mean sea level rise. Due to a shortage of studies and the difficulty of distinguishing any such impacts from other modifications to coastal systems, limited evidence is available on the impacts of sea level rise. Impacts from recent climate-related extremes, such as heat waves, droughts, floods, cyclones and wildfires, reveal significant vulnerability and exposure of some ecosystems and many human systems to current climate variability . Impacts of such climate-related extremes include alteration of ecosystems, disruption of food production and water supply, damage to infrastructure and settlements, human morbidity and mortality and consequences for mental health and human well-being. For countries at all levels of development, these impacts are consistent with a significant lack of preparedness for current climate variability in some sectors. Direct and insured losses from weather-related disasters have increased substantially in recent decades, both globally and regionally. Increasing exposure of people and economic assets has been the major cause of long-term increases in economic losses from weather- and climate-related disasters. 2.1b: - Key drivers of future climate and the basis on which projections are made Cumulative emissions of CO 2 largely determine global mean surface warming by the late 21st century and beyond. Projections of greenhouse gas emissions vary over a wide range, depending on both socio-economic development and climate policy. Climate models are mathematical representations of processes important in the Earth’s climate system. Results from a hierarchy of climate models are considered in this report; ranging from simple idealized models, to models of intermediate complexity, to comprehensive General Circulation Models, including Earth System Models that also simulate the carbon cycle. The GCMs simulate many climate aspects, including the temperature of the atmosphere and the oceans, precipitation, winds, clouds, ocean currents and sea-ice extent. The models are extensively tested against historical observations. In order to obtain climate change projections, the climate models use information described in scenarios of GHG and air pollutant emissions and land use patterns. Scenarios are generated by a range of approaches, from simple idealised experiments to Integrated Assessment Models. Key factors driving changes in anthropogenic GHG emissions are economic and population growth, lifestyle and behavioural changes, associated changes in energy use and land use, technology and climate policy, which are fundamentally uncertain. The standard set of scenarios used in the AR5 is called Representative Concentration Pathways. The methods used to estimate future impacts and risks resulting from climate change are described in Box 2.3. Modelled future impacts assessed in this report are generally based on climate-model projections using the RCPs, and in some cases, the older Special Report on Emissions Scenarios. Risk of climate-related impacts results from the interaction between climate-related hazards and the vulnerability and exposure of human and natural systems. Alternative development paths influence risk by changing the likelihood of climatic events and trends, through their effects on GHGs, pollutants and land use, and by altering vulnerability and exposure. Experiments, observations and models used to estimate future impacts and risks have improved since the AR4, with increasing understanding across sectors and regions. For example, an improved knowledge base has enabled expanded assessment of risks for human security and livelihoods and for the oceans. For some aspects of climate change and climate change impacts, uncertainty about future outcomes has narrowed. For others, uncertainty will persist. Some of the persistent uncertainties are grounded in the mechanisms that control the magnitude and pace of climate change. Others emerge from potentially complex interactions between the changing climate and the underlying vulnerability and exposure of people, societies and ecosystems. The combination of persistent uncertainty in key mechanisms plus the prospect of complex interactions motivates a focus on risk in this report. Because risk involves both probability and consequence, it is important to consider the full range of possible outcomes, including low-probability, high-consequence impacts that are difficult to simulate. - Response options for mitigation Mitigation options are available in every major sector. Mitigation can be more cost-effective if using an integrated approach that combines measures to reduce energy use and the greenhouse gas intensity of end-use sectors, decarbonize energy supply, reduce net emissions and enhance carbon sinks in land-based sectors. A broad range of sectoral mitigation options is available that can reduce GHG emission intensity, improve energy intensity through enhancements of technology, behaviour, production and resource efficiency and enable structural changes or changes in activity. In addition, direct options in agriculture, forestry and other land use involve reducing CO2 emissions by reducing deforestation, forest degradation and forest fires; storing carbon in terrestrial systems; and providing bioenergy feedstocks. Options to reduce non-CO2 emissions exist across all sectors but most notably in agriculture, energy supply and industry. An overview of sectoral mitigation options and potentials is provided in Table 4.4. Well-designed systemic and cross-sectoral mitigation strategies are more cost-effective in cutting emissions than a focus on individual technologies and sectors with efforts in one sector affecting the need for mitigation in others . In baseline scenarios without new mitigation policies, GHG emissions are projected to grow in all sectors, except for net CO2 emissions in the AFOLU sector. Mitigation scenarios reaching around 450 ppm CO2-eq concentration by 21004328 show largescale global changes in the energy supply sector . While rapid decarbonization of energy supply generally entails more flexibility for end-use and AFOLU sectors, stronger demand reductions lessen the mitigation challenge for the supply side of the energy system. There are thus strong interdependencies across sectors and the resulting distribution of the mitigation effort is strongly influenced by the availability and performance of future technologies, particularly BECCS and large scale afforestation . The next two decades present a window of opportunity for mitigation in urban areas, as a large portion of the world’s urban areas will be developed during this period. Decarbonizing electricity generation is a key component of cost-effective mitigation strategies in achieving low stabilization levels . In most integrated modelling scenarios, decarbonization happens more rapidly in electricity generation than in the industry, buildings and transport sectors. In scenarios reaching 450 ppm CO2-eq concentrations by 2100, global CO2 emissions from the energy supply sector are projected to decline over the next decade and are characterized by reductions of 90% or more below 2010 levels between 2040 and 2070. Efficiency enhancements and behavioural changes, in order to reduce energy demand compared to baseline scenarios without compromising development, are a key mitigation strategy in scenarios reaching atmospheric CO2-eq concentrations of about 450 to about 500 ppm by 2100 . Near-term reductions in energy demand are an important element of cost-effective mitigation strategies, provide more flexibility for reducing carbon intensity in the energy supply sector, hedge against related supply-side risks, avoid lock-in to carbon-intensive infrastructures and are associated with important co-benefits . Emissions can be substantially lowered through changes in consumption patterns and dietary change and reduction in food wastes. A number of options including monetary and non-monetary incentives as well as information measures may facilitate behavioural changes. Decarbonization of the energy supply sector requires upscaling of low- and zero-carbon electricity generation technologies. In the majority of low-concentration stabilization scenarios , the share of low-carbon electricity supply, nuclear and CCS, including BECCS) increases from the current share of approximately 30% to more than 80% by 2050 and 90% by 2100, and fossil fuel power generation without CCS is phased out almost entirely by 2100. Among these low-carbon technologies, a growing number of RE technologies have achieved a level of maturity to enable deployment at significant scale since AR4 and nuclear energy is a mature low-GHG emission source of baseload power, but its share of global electricity generation has been declining. GHG emissions from energy supply can be reduced significantly by replacing current world average coal-fired power plants with modern, highly efficient natural gas combined-cycle power plants or combined heat and power plants, provided that natural gas is available and the fugitive emissions associated with extraction and supply are low or mitigated. Behaviour, lifestyle and culture have a considerable influence on energy use and associated emissions, with high mitigation potential in some sectors, in particular when complementing technological and structural change . In the transport sector, technical and behavioural mitigation measures for all modes, plus new infrastructure and urban redevelopment investments, could reduce final energy demand significantly below baseline levels. While opportunities for switching to low-carbon fuels exist, the rate of decarbonization in the transport sector might be constrained by challenges associated with energy storage and the relatively low energy density of low-carbon transport fuels. In the building sector, recent advances in technologies, know-how and policies provide opportunities to stabilize or reduce global energy use to about current levels by mid-century. In addition, recent improvements in performance and costs make very low energy construction and retrofits of buildings economically attractive, sometimes even at net negative costs. In the industry sector, improvements in GHG emission efficiency and in the efficiency of material use, recycling and reuse of materials and products, and overall reductions in product demand and service demand could, in addition to energy efficiency, help reduce GHG emissions below the baseline level. Prevalent approaches for promoting energy efficiency in industry include information programmes followed by economic instruments, regulatory approaches and voluntary actions. Important options for mitigation in waste management are waste reduction, followed by re-use, recycling and energy recovery. The most cost-effective mitigation options in forestry are afforestation, sustainable forest management and reducing deforestation, with large differences in their relative importance across regions. In agriculture, the most cost-effective mitigation options are cropland management, grazing land management and restoration of organic soils . About a third of mitigation potential in forestry can be achieved at a cost <20 USD/tCO2-eq emission. Demand-side measures, such as changes in diet and reductions of losses in the food supply chain, have a significant, but uncertain, potential to reduce GHG emissions from food production . Bioenergy can play a critical role for mitigation, but there are issues to consider, such as the sustainability of practices and the efficiency of bioenergy systems . Evidence suggests that bioenergy options with low lifecycle emissions, some already available, can reduce GHG emissions; outcomes are site-specific and rely on efficient integrated ‘biomassto-bioenergy systems’, and sustainable land use management and governance. Barriers to large-scale deployment of bioenergy include concerns about GHG emissions from land, food security, water resources, biodiversity conservation and livelihoods. Mitigation measures intersect with other societal goals, creating the possibility of co-benefits or adverse side-effects. These intersections, if well-managed, can strengthen the basis for undertaking climate mitigation actions . Mitigation can positively or negatively influence the achievement of other societal goals, such as those related to human health, food security, biodiversity, local environmental quality, energy access, livelihoods and equitable sustainable development . On the other hand, policies towards other societal goals can influence the achievement of mitigation and adaptation objectives. These influences can be substantial, although sometimes difficult to quantify, especially in welfare terms. This multi-objective perspective is important in part because it helps to identify areas where support for policies that advance multiple goals will be robust. Potential co-benefits and adverse side effects of the main sectoral mitigation measures are summarized in Table 4.5. Overall, the potential for co-benefits for energy end-use measures outweigh the potential for adverse side effects, whereas the evidence suggests this may not be the case for all energy supply and AFOLU measures. 2.1c: - Climate system responses Climate system properties that determine the response to external forcing have been estimated both from climate models and from analysis of past and recent climate change. The equilibrium climate sensitivity 3325is likely in the range 1.5°C to 4.5°C, extremely unlikely less than 1°C, and very unlikely greater than 6°C. Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond. Multiple lines of evidence indicate a strong and consistent near-linear relationship across all scenarios considered between net cumulative CO2 emissions and projected global temperature change to the year 2100. Past emissions and observed warming support this relationship within uncertainties. Any given level of warming is associated with a range of cumulative CO2 emissions , and therefore, for example, higher emissions in earlier decades imply lower emissions later. The global mean peak surface temperature change per trillion tonnes of carbon emitted as CO2 is likely in the range of 0.8°C to 2.5°C. This quantity, called the transient climate response to cumulative carbon emissions, is supported by both modelling and observational evidence and applies to cumulative emissions up to about 2000 GtC. Warming caused by CO2 emissions is effectively irreversible over multi-century timescales unless measures are taken to remove CO2 from the atmosphere. Ensuring CO2-induced warming remains likely less than 2°C requires cumulative CO2 emissions from all anthropogenic sources to remain below about 3650 GtCO2, over half of which were already emitted by 2011. Multi-model results show that limiting total human-induced warming to less than 2°C relative to the period 1861–1880 with a probability of >66% would require total CO2 emissions from all anthropogenic sources since 1870 to be limited to about 2900 GtCO2 when accounting for non-CO2 forcing as in the RCP2.6 scenario, with a range of 2550 to 3150 GtCO2 arising from variations in non-CO2 climate drivers across the scenarios considered by WGIII. About 1900 [1650 to 2150] GtCO2 were emitted by 2011, leaving about 1000 GtCO2 to be consistent with this temperature goal. Estimated total fossil carbon reserves exceed this remaining amount by a factor of 4 to 7, with resources much larger still. 2.1d: - Climate system responses Climate system properties that determine the response to external forcing have been estimated both from climate models and from analysis of past and recent climate change. The equilibrium climate sensitivity 3325is likely in the range 1.5°C to 4.5°C, extremely unlikely less than 1°C, and very unlikely greater than 6°C. Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond. Multiple lines of evidence indicate a strong and consistent near-linear relationship across all scenarios considered between net cumulative CO2 emissions and projected global temperature change to the year 2100. Past emissions and observed warming support this relationship within uncertainties. Any given level of warming is associated with a range of cumulative CO2 emissions , and therefore, for example, higher emissions in earlier decades imply lower emissions later. The global mean peak surface temperature change per trillion tonnes of carbon emitted as CO2 is likely in the range of 0.8°C to 2.5°C. This quantity, called the transient climate response to cumulative carbon emissions, is supported by both modelling and observational evidence and applies to cumulative emissions up to about 2000 GtC. Warming caused by CO2 emissions is effectively irreversible over multi-century timescales unless measures are taken to remove CO2 from the atmosphere. Ensuring CO2-induced warming remains likely less than 2°C requires cumulative CO2 emissions from all anthropogenic sources to remain below about 3650 GtCO2, over half of which were already emitted by 2011. Multi-model results show that limiting total human-induced warming to less than 2°C relative to the period 1861–1880 with a probability of >66% would require total CO2 emissions from all anthropogenic sources since 1870 to be limited to about 2900 GtCO2 when accounting for non-CO2 forcing as in the RCP2.6 scenario, with a range of 2550 to 3150 GtCO2 arising from variations in non-CO2 climate drivers across the scenarios considered by WGIII. About 1900 [1650 to 2150] GtCO2 were emitted by 2011, leaving about 1000 GtCO2 to be consistent with this temperature goal. Estimated total fossil carbon reserves exceed this remaining amount by a factor of 4 to 7, with resources much larger still. 2.2e: - Water cycle Changes in precipitation in a warming world will not be uniform. The high latitudes and the equatorial Pacific are likely to experience an increase in annual mean precipitation by the end of this century under the RCP8.5 scenario. In many mid-latitude and subtropical dry regions, mean precipitation will likely decrease, while in many mid-latitude wet regions, mean precipitation will likely increase under the RCP8.5 scenario. Extreme precipitation events over most mid-latitude land masses and over wet tropical regions will very likely become more intense and more frequent as global mean surface temperature increases. Globally, in all RCPs, it is likely that the area encompassed by monsoon systems will increase and monsoon precipitation is likely to intensify and El Niño-Southern Oscillation related precipitation variability on regional scales will likely intensify. 2.2f: - Ocean, cryosphere and sea level The global ocean will continue to warm during the 21st century. The strongest ocean warming is projected for the surface in tropical and Northern Hemisphere subtropical regions. At greater depth the warming will be most pronounced in the Southern Ocean . It is very likely that the Atlantic Meridional Overturning Circulation will weaken over the 21st century, with best estimates and model ranges for the reduction of 11% for the RCP2.6 scenario, 34% for the RCP8.5. Nevertheless, it is very unlikely that the AMOC will undergo an abrupt transition or collapse in the 21st century. Year-round reductions in Arctic sea ice are projected for all RCP scenarios. The subset of models that most closely reproduce the observationsproject that a nearly ice-free Arctic Oceanin September is likely for RCP8.5 before mid-century. In the Antarctic, a decrease in sea ice extent and volume is projected with low confidence. The area of Northern Hemisphere spring snow cover is likely to decrease by 7% for RCP2.6 and by 25% in RCP8.5 by the end of the 21st century for the multi-model average. It is virtually certain that near-surface permafrost extent at high northern latitudes will be reduced as global mean surface temperature increases. The area of permafrost near the surface is likely to decrease by 37% to 81% for the multi-model average. The global glacier volume, excluding glaciers on the periphery of Antarctica, is projected to decrease by 15 to 55% for RCP2.6 and by 35 to 85% for RCP8.5. Global mean sea level will continue to rise during the 21st century. There has been significant improvement in understanding and projection of sea level change since the AR4. Under all RCP scenarios, the rate of sea level rise will very likely exceed the observed rate of 2.0 [1.7–2.3] mm/yr during 1971–2010, with the rate of rise for RCP8.5 during 2081–2100 of 8 to 16 mm/yr . Sea level rise will not be uniform across regions. By the end of the 21st century, it is very likely that sea level will rise in more than about 95% of the ocean area. Sea level rise depends on the pathway of CO2 emissions, not only on the cumulative total; reducing emissions earlier rather than later, for the same cumulative total, leads to a larger mitigation of sea level rise. About 70% of the coastlines worldwide are projected to experience sea level change within ±20% of the global mean. It is very likely that there will be a significant increase in the occurrence of future sea level extremes in some regions by 2100. 2.2g: - Carbon cycle and biogeochemistry Ocean uptake of anthropogenic CO2 will continue under all four RCPs through to 2100, with higher uptake for higher concentration pathways. The future evolution of the land carbon uptake is less certain. A majority of models projects a continued land carbon uptake under all RCPs, but some models simulate a land carbon loss due to the combined effect of climate change and land use change. Based on Earth System Models, there is high confidence that the feedback between climate change and the carbon cycle will amplify global warming. Climate change will partially offset increases in land and ocean carbon sinks caused by rising atmospheric CO2. As a result more of the emitted anthropogenic CO2 will remain in the atmosphere, reinforcing the warming. Earth System Models project a global increase in ocean acidification for all RCP scenarios by the end of the 21st century, with a slow recovery after mid-century under RCP2.6. The decrease in surface ocean pH is in the range of 0.06 to 0.07 for RCP2.6, 0.14 to 0.15 for RCP4.5, 0.20 to 0.21 for RCP6.0, and 0.30 to 0.32 for RCP8.5 . It is very likely that the dissolved oxygen content of the ocean will decrease by a few percent during the 21st century in response to surface warming, predominantly in the subsurface mid-latitude oceans. There is no consensus on the future volume of low oxygen waters in the open ocean because of large uncertainties in potential biogeochemical effects and in the evolution of tropical ocean dynamics. 2.2h: - Ocean, cryosphere and sea level The global ocean will continue to warm during the 21st century. The strongest ocean warming is projected for the surface in tropical and Northern Hemisphere subtropical regions. At greater depth the warming will be most pronounced in the Southern Ocean . It is very likely that the Atlantic Meridional Overturning Circulation will weaken over the 21st century, with best estimates and model ranges for the reduction of 11% for the RCP2.6 scenario, 34% for the RCP8.5. Nevertheless, it is very unlikely that the AMOC will undergo an abrupt transition or collapse in the 21st century. Year-round reductions in Arctic sea ice are projected for all RCP scenarios. The subset of models that most closely reproduce the observationsproject that a nearly ice-free Arctic Oceanin September is likely for RCP8.5 before mid-century. In the Antarctic, a decrease in sea ice extent and volume is projected with low confidence. The area of Northern Hemisphere spring snow cover is likely to decrease by 7% for RCP2.6 and by 25% in RCP8.5 by the end of the 21st century for the multi-model average. It is virtually certain that near-surface permafrost extent at high northern latitudes will be reduced as global mean surface temperature increases. The area of permafrost near the surface is likely to decrease by 37% to 81% for the multi-model average. The global glacier volume, excluding glaciers on the periphery of Antarctica, is projected to decrease by 15 to 55% for RCP2.6 and by 35 to 85% for RCP8.5. Global mean sea level will continue to rise during the 21st century. There has been significant improvement in understanding and projection of sea level change since the AR4. Under all RCP scenarios, the rate of sea level rise will very likely exceed the observed rate of 2.0 [1.7–2.3] mm/yr during 1971–2010, with the rate of rise for RCP8.5 during 2081–2100 of 8 to 16 mm/yr . Sea level rise will not be uniform across regions. By the end of the 21st century, it is very likely that sea level will rise in more than about 95% of the ocean area. Sea level rise depends on the pathway of CO2 emissions, not only on the cumulative total; reducing emissions earlier rather than later, for the same cumulative total, leads to a larger mitigation of sea level rise. About 70% of the coastlines worldwide are projected to experience sea level change within ±20% of the global mean. It is very likely that there will be a significant increase in the occurrence of future sea level extremes in some regions by 2100. 2.2m: - Ocean, cryosphere and sea level The global ocean will continue to warm during the 21st century. The strongest ocean warming is projected for the surface in tropical and Northern Hemisphere subtropical regions. At greater depth the warming will be most pronounced in the Southern Ocean . It is very likely that the Atlantic Meridional Overturning Circulation will weaken over the 21st century, with best estimates and model ranges for the reduction of 11% for the RCP2.6 scenario, 34% for the RCP8.5. Nevertheless, it is very unlikely that the AMOC will undergo an abrupt transition or collapse in the 21st century. Year-round reductions in Arctic sea ice are projected for all RCP scenarios. The subset of models that most closely reproduce the observationsproject that a nearly ice-free Arctic Oceanin September is likely for RCP8.5 before mid-century. In the Antarctic, a decrease in sea ice extent and volume is projected with low confidence. The area of Northern Hemisphere spring snow cover is likely to decrease by 7% for RCP2.6 and by 25% in RCP8.5 by the end of the 21st century for the multi-model average. It is virtually certain that near-surface permafrost extent at high northern latitudes will be reduced as global mean surface temperature increases. The area of permafrost near the surface is likely to decrease by 37% to 81% for the multi-model average. The global glacier volume, excluding glaciers on the periphery of Antarctica, is projected to decrease by 15 to 55% for RCP2.6 and by 35 to 85% for RCP8.5. Global mean sea level will continue to rise during the 21st century. There has been significant improvement in understanding and projection of sea level change since the AR4. Under all RCP scenarios, the rate of sea level rise will very likely exceed the observed rate of 2.0 [1.7–2.3] mm/yr during 1971–2010, with the rate of rise for RCP8.5 during 2081–2100 of 8 to 16 mm/yr . Sea level rise will not be uniform across regions. By the end of the 21st century, it is very likely that sea level will rise in more than about 95% of the ocean area. Sea level rise depends on the pathway of CO2 emissions, not only on the cumulative total; reducing emissions earlier rather than later, for the same cumulative total, leads to a larger mitigation of sea level rise. About 70% of the coastlines worldwide are projected to experience sea level change within ±20% of the global mean. It is very likely that there will be a significant increase in the occurrence of future sea level extremes in some regions by 2100. 2.3a: - Exposure and vulnerability The character and severity of impacts from climate change and extreme events emerge from risk that depends not only on climate-related hazards but also on exposure and vulnerability of human and natural systems. Exposure and vulnerability are influenced by a wide range of social, economic and cultural factors and processes that have been incompletely considered to date and that make quantitative assessments of their future trends difficult . These factors include wealth and its distribution across society, demographics, migration, access to technology and information, employment patterns, the quality of adaptive responses, societal values, governance structures and institutions to resolve conflict. Differences in vulnerability and exposure arise from non-climatic factors and from multidimensional inequalities often produced by uneven development processes. These differences shape differential risks from climate change. People who are socially, economically, culturally, politically, institutionally or otherwise marginalized are especially vulnerable to climate change and also to some adaptation and mitigation responses . This heightened vulnerability is rarely due to a single cause. Rather, it is the product of intersecting social processes that result in inequalities in socio-economic status and income, as well as in exposure. Such social processes include, for example, discrimination on the basis of gender, class, ethnicity, age and ability. Climate-related hazards exacerbate other stressors, often with negative outcomes for livelihoods, especially for people living in poverty. Climate-related hazards affect poor people’s lives directly through impacts on livelihoods, reductions in crop yields or the destruction of homes, and indirectly through, for example, increased food prices and food insecurity. Observed positive effects for poor and marginalized people, which are limited and often indirect, include examples such as diversification of social networks and of agricultural practices. Violent conflict increases vulnerability to climate change . Large-scale violent conflict harms assets that facilitate adaptation, including infrastructure, institutions, natural resources, social capital and livelihood opportunities. - 'Future risks and impacts caused by a changing climate Climate change will amplify existing risks and create new risks for natural and human systems. Risks are unevenly distributed and are generally greater for disadvantaged people and communities in countries at all levels of development. Increasing magnitudes of warming increase the likelihood of severe, pervasive and irreversible impacts for people, species and ecosystems. Continued high emissions would lead to mostly negative impacts for biodiversity, ecosystem services and economic development and amplify risks for livelihoods and for food and human security. Risk of climate-related impacts results from the interaction of climate-related hazards with the vulnerability and exposure of human and natural systems, including their ability to adapt. Rising rates and magnitudes of warming and other changes in the climate system, accompanied by ocean acidification, increase the risk of severe, pervasive, and in some cases, irreversible detrimental impacts. Future climate change will amplify existing climate-related risks and create new risks. Key risks are potentially severe impacts relevant to understanding dangerous anthropogenic interference with the climate system. Risks are considered key due to high hazard or high vulnerability of societies and systems exposed, or both. Their identification is based on large magnitude or high probability of impacts; irreversibility or timing of impacts; persistent vulnerability or exposure; or limited potential to reduce risks. Some risks are particularly relevant for individual regions, while others are global. For risk assessment it is important to evaluate the widest possible range of impacts, including low-probability outcomes with large consequences. Risk levels often increase with temperature and are sometimes more directly linked to other dimensions of climate change, such as the rate of warming, as well as the magnitudes and rates of ocean acidification and sea level rise . Key risks that span sectors and regions include the following : 1. Risk of severe ill-health and disrupted livelihoods resulting from storm surges, sea level rise and coastal flooding; inland flooding in some urban regions; and periods of extreme heat. 2. Systemic risks due to extreme weather events leading to breakdown of infrastructure networks and critical services. 3. Risk of food and water insecurity and loss of rural livelihoods and income, particularly for poorer populations. 4. Risk of loss of ecosystems, biodiversity and ecosystem goods, functions and services. The overall risks of future climate change impacts can be reduced by limiting the rate and magnitude of climate change, including ocean acidification. Some risks are considerable even at 1°C global mean temperature increase above pre-industrial levels. Many global risks are high to very high for global temperature increases of 4°C or more. These risks include severe and widespread impacts on unique and threatened systems, the extinction of many species, large risks to food security and compromised normal human activities, including growing food or working outdoors in some areas for parts of the year, due to the combination of high temperature and humidity. The precise levels of climate change sufficient to trigger abrupt and irreversible change remain uncertain, but the risk associated with crossing such thresholds in the earth system or in interlinked human and natural systems increases with rising temperature. Adaptation can substantially reduce the risks of climate change impacts, but greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits . The potential for adaptation, as well as constraints and limits to adaptation, varies among sectors, regions, communities and ecosystems. The scope for adaptation changes over time and is closely linked to socio-economic development pathways and circumstances. See Figure 2.4 and Table 2.3, along with Topics 3 and 4.' - Climate change beyond 2100, irreversibility and abrupt changes Many aspects of climate change and its associated impacts will continue for centuries, even if anthropogenic emissions of greenhouse gases are stopped. The risks of abrupt or irreversible changes increase as the magnitude of the warming increases. Warming will continue beyond 2100 under all RCP scenarios except RCP2.6. Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions . A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial timescale, except in the case of a large net removal of CO2 from the atmosphere over a sustained period. Stabilization of global average surface temperature does not imply stabilization for all aspects of the climate system. Shifting biomes, re-equilibrating soil carbon, ice sheets, ocean temperatures and associated sea level rise all have their own intrinsic long timescales that will result in ongoing changes for hundreds to thousands of years after global surface temperature has been stabilized. Ocean acidification will continue for centuries if CO2 emissions continue, it will strongly affect marine ecosystems , and the impact will be exacerbated by rising temperature extremes. Global mean sea level rise will continue for many centuries beyond 2100. The few available analyses that go beyond 2100 indicate sea level rise to be less than 1 m above the pre-industrial level by 2300 for GHG concentrations that peak and decline and remain below 500 ppm CO2-eq, as in scenario RCP2.6. For a radiative forcing that corresponds to a CO2-eq concentration in 2100 that is above 700 ppm but below 1500 ppm, as in scenario RCP8.5, the projected rise is 1 m to more than 3 m by 2300 . There is low confidence in the available models’ ability to project solid ice discharge from the Antarctic ice sheet. Hence, these models likely underestimate the Antarctica ice sheet contribution, resulting in an underestimation of projected sea level rise beyond 2100. There is little evidence in global climate models of a tipping point or critical threshold in the transition from a perennially ice-covered to a seasonally ice-free Arctic Ocean, beyond which further sea-ice loss is unstoppable and irreversible. There is low confidence in assessing the evolution of the Atlantic Meridional Overturning Circulation beyond the 21st century because of the limited number of analyses and equivocal results. However, a collapse beyond the 21st century for large sustained warming cannot be excluded. Sustained mass loss by ice sheets would cause larger sea level rise, and part of the mass loss might be irreversible. There is high confidence that sustained global mean warming greater than a threshold would lead to the near-complete loss of the Greenland ice sheet over a millennium or more, causing a sea level rise of up to 7 m. Current estimates indicate that the threshold is greater than about 1°C but less than about 4°C of global warming with respect to pre-industrial temperatures. Abrupt and irreversible ice loss from a potential instability of marine-based sectors of the Antarctic ice sheet in response to climate forcing is possible, but current evidence and understanding is insufficient to make a quantitative assessment. Within the 21st century, magnitudes and rates of climate change associated with medium to high emission scenarios pose a high risk of abrupt and irreversible regional-scale change in the composition, structure and function of marine, terrestrial and freshwater ecosystems, including wetlands, as well as warm water coral reefs . Examples that could substantially amplify climate change are the boreal-tundra Arctic system and the Amazon forest. A reduction in permafrost extent is virtually certain with continued rise in global temperatures. Current permafrost areas are projected to become a net emitter of carbon with a loss of 180 to 920 GtCO2 under RCP8.5 over the 21st century . - 'Characteristics of adaptation pathways Adaptation can reduce the risks of climate change impacts, but there are limits to its effectiveness, especially with greater magnitudes and rates of climate change. Taking a longer-term perspective, in the context of sustainable development, increases the likelihood that more immediate adaptation actions will also enhance future options and preparedness. Adaptation can contribute to the well-being of current and future populations, the security of assets and the maintenance of ecosystem goods, functions and services now and in the future. Adaptation is place- and context-specific, with no single approach for reducing risks appropriate across all settings . Effective risk reduction and adaptation strategies consider vulnerability and exposure and their linkages with socio-economic processes, sustainable development, and climate change. Adaptation research since the IPCC Fourth Assessment Report has evolved from a dominant consideration of engineering and technological adaptation pathways to include more ecosystem-based, institutional and social measures. A previous focus on cost–benefit analysis, optimization and efficiency approaches has broadened with the development of multi-metric evaluations that include risk and uncertainty dimensions integrated within wider policy and ethical frameworks to assess tradeoffs and constraints. The range of specific adaptation measures has also expanded, as have the links to sustainable development. There are many studies on local and sectoral adaptation costs and benefits, but few global analyses and very low confidence in their results. Adaptation planning and implementation at all levels of governance are contingent on societal values, objectives and risk perceptions. Recognition of diverse interests, circumstances, social-cultural contexts and expectations can benefit decision-making processes. Indigenous, local and traditional knowledge systems and practices, including indigenous peoples’ holistic view of community and environment, are a major resource for adapting to climate change, but these have not been used consistently in existing adaptation efforts. Integrating such forms of knowledge into practices increases the effectiveness of adaptation as do effective decision support, engagement and policy processes. Adaptation planning and implementation can be enhanced through complementary actions across levels, from individuals to governments. National governments can coordinate adaptation efforts of local and sub-national governments, for example by protecting vulnerable groups, by supporting economic diversification and by providing information, policy and legal frameworks and financial support. Local government and the private sector are increasingly recognized as critical to progress in adaptation, given their roles in scaling up adaptation of communities, households and civil society and in managing risk information and financing. A first step towards adaptation to future climate change is reducing vulnerability and exposure to present climate variability, but some near-term responses to climate change may also limit future choices. Integration of adaptation into planning, including policy design, and decision-making can promote synergies with development and disaster risk reduction. However, poor planning or implementation, overemphasizing short-term outcomes or failing to sufficiently anticipate consequences can result in maladaptation, increasing the vulnerability or exposure of the target group in the future or the vulnerability of other people, places or sectors. For example, enhanced protection of exposed assets can lock in dependence on further protection measures. Appropriate adaptation options can be better assessed by including co-benefits and mitigation implications. Numerous interacting constraints can impede adaptation planning and implementation. Common constraints on implementation arise from the following: limited financial and human resources; limited integration or coordination of governance; uncertainties about projected impacts; different perceptions of risks; competing values; absence of key adaptation leaders and advocates; and limited tools to monitor adaptation effectiveness. Other constraints include insufficient research, monitoring and observation and the financial and other resources to maintain them. Underestimating the complexity of adaptation as a social process can create unrealistic expectations about achieving intended adaptation outcomes . Greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits. Limits to adaptation occur when adaptive actions to avoid intolerable risks for an actor’s objectives or for the needs of a system are not possible or are not currently available. Value-based judgments of what constitutes an intolerable risk may differ. Limits to adaptation emerge from the interaction among climate change and biophysical and/or socio-economic constraints. Opportunities to take advantage of positive synergies between adaptation and mitigation may decrease with time, particularly if limits to adaptation are exceeded. In some parts of the world, insufficient responses to emerging impacts are already eroding the basis for sustainable development. For most regions and sectors, empirical evidence is not sufficient to quantify magnitudes of climate change that would constitute a future adaptation limit. Furthermore, economic development, technology and cultural norms and values can change over time to enhance or reduce the capacity of systems to avoid limits. As a consequence, some limits are ‘soft’ in that they may be alleviated over time. Other limits are ‘hard’ in that there are no reasonable prospects for avoiding intolerable risks. Transformations in economic, social, technological and political decisions and actions can enhance adaptation and promote sustainable development. Restricting adaptation responses to incremental changes to existing systems and structures without considering transformational change may increase costs and losses and miss opportunities. For example, enhancing infrastructure to protect other built assets can be expensive and ultimately not defray increasing costs and risks, whereas options such as relocation or using ecosystem services to adapt may provide a range of benefits now and in the future. Transformational adaptation can include introduction of new technologies or practices, formation of new financial structures or systems of governance, adaptation at greater scales or magnitudes and shifts in the location of activities. Planning and implementation of transformational adaptation could reflect strengthened, altered or aligned paradigms and consequently may place new and increased demands on governance structures to reconcile different goals and visions for the future and to address possible equity and ethical implications: transformational adaptation pathways are enhanced by iterative learning, deliberative processes, and innovation. At the national level, transformation is considered most effective when it reflects a country’s own visions and approaches to achieving sustainable development in accordance with its national circumstances and priorities. Building adaptive capacity is crucial for effective selection and implementation of adaptation options . Successful adaptation requires not only identifying adaptation options and assessing their costs and benefits, but also increasing the adaptive capacity of human and natural systems . This can involve complex governance challenges and new institutions and institutional arrangements. Significant co-benefits, synergies and trade-offs exist between mitigation and adaptation and among different adaptation responses; interactions occur both within and across regions . Increasing efforts to mitigate and adapt to climate change imply an increasing complexity of interactions, particularly at the intersections among water, energy, land use and biodiversity, but tools to understand and manage these interactions remain limited. Examples of actions with co-benefits include improved energy efficiency and cleaner energy sources, leading to reduced emissions of health-damaging, climate-altering air pollutants; reduced energy and water consumption in urban areas through greening cities and recycling water; sustainable agriculture and forestry; and protection of ecosystems for carbon storage and other ecosystem services.' 2.3b: - 'Future risks and impacts caused by a changing climate Climate change will amplify existing risks and create new risks for natural and human systems. Risks are unevenly distributed and are generally greater for disadvantaged people and communities in countries at all levels of development. Increasing magnitudes of warming increase the likelihood of severe, pervasive and irreversible impacts for people, species and ecosystems. Continued high emissions would lead to mostly negative impacts for biodiversity, ecosystem services and economic development and amplify risks for livelihoods and for food and human security. Risk of climate-related impacts results from the interaction of climate-related hazards with the vulnerability and exposure of human and natural systems, including their ability to adapt. Rising rates and magnitudes of warming and other changes in the climate system, accompanied by ocean acidification, increase the risk of severe, pervasive, and in some cases, irreversible detrimental impacts. Future climate change will amplify existing climate-related risks and create new risks. Key risks are potentially severe impacts relevant to understanding dangerous anthropogenic interference with the climate system. Risks are considered key due to high hazard or high vulnerability of societies and systems exposed, or both. Their identification is based on large magnitude or high probability of impacts; irreversibility or timing of impacts; persistent vulnerability or exposure; or limited potential to reduce risks. Some risks are particularly relevant for individual regions, while others are global. For risk assessment it is important to evaluate the widest possible range of impacts, including low-probability outcomes with large consequences. Risk levels often increase with temperature and are sometimes more directly linked to other dimensions of climate change, such as the rate of warming, as well as the magnitudes and rates of ocean acidification and sea level rise . Key risks that span sectors and regions include the following : 1. Risk of severe ill-health and disrupted livelihoods resulting from storm surges, sea level rise and coastal flooding; inland flooding in some urban regions; and periods of extreme heat. 2. Systemic risks due to extreme weather events leading to breakdown of infrastructure networks and critical services. 3. Risk of food and water insecurity and loss of rural livelihoods and income, particularly for poorer populations. 4. Risk of loss of ecosystems, biodiversity and ecosystem goods, functions and services. The overall risks of future climate change impacts can be reduced by limiting the rate and magnitude of climate change, including ocean acidification. Some risks are considerable even at 1°C global mean temperature increase above pre-industrial levels. Many global risks are high to very high for global temperature increases of 4°C or more. These risks include severe and widespread impacts on unique and threatened systems, the extinction of many species, large risks to food security and compromised normal human activities, including growing food or working outdoors in some areas for parts of the year, due to the combination of high temperature and humidity. The precise levels of climate change sufficient to trigger abrupt and irreversible change remain uncertain, but the risk associated with crossing such thresholds in the earth system or in interlinked human and natural systems increases with rising temperature. Adaptation can substantially reduce the risks of climate change impacts, but greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits . The potential for adaptation, as well as constraints and limits to adaptation, varies among sectors, regions, communities and ecosystems. The scope for adaptation changes over time and is closely linked to socio-economic development pathways and circumstances. See Figure 2.4 and Table 2.3, along with Topics 3 and 4.' - Climate change beyond 2100, irreversibility and abrupt changes Many aspects of climate change and its associated impacts will continue for centuries, even if anthropogenic emissions of greenhouse gases are stopped. The risks of abrupt or irreversible changes increase as the magnitude of the warming increases. Warming will continue beyond 2100 under all RCP scenarios except RCP2.6. Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions . A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial timescale, except in the case of a large net removal of CO2 from the atmosphere over a sustained period. Stabilization of global average surface temperature does not imply stabilization for all aspects of the climate system. Shifting biomes, re-equilibrating soil carbon, ice sheets, ocean temperatures and associated sea level rise all have their own intrinsic long timescales that will result in ongoing changes for hundreds to thousands of years after global surface temperature has been stabilized. Ocean acidification will continue for centuries if CO2 emissions continue, it will strongly affect marine ecosystems , and the impact will be exacerbated by rising temperature extremes. Global mean sea level rise will continue for many centuries beyond 2100. The few available analyses that go beyond 2100 indicate sea level rise to be less than 1 m above the pre-industrial level by 2300 for GHG concentrations that peak and decline and remain below 500 ppm CO2-eq, as in scenario RCP2.6. For a radiative forcing that corresponds to a CO2-eq concentration in 2100 that is above 700 ppm but below 1500 ppm, as in scenario RCP8.5, the projected rise is 1 m to more than 3 m by 2300 . There is low confidence in the available models’ ability to project solid ice discharge from the Antarctic ice sheet. Hence, these models likely underestimate the Antarctica ice sheet contribution, resulting in an underestimation of projected sea level rise beyond 2100. There is little evidence in global climate models of a tipping point or critical threshold in the transition from a perennially ice-covered to a seasonally ice-free Arctic Ocean, beyond which further sea-ice loss is unstoppable and irreversible. There is low confidence in assessing the evolution of the Atlantic Meridional Overturning Circulation beyond the 21st century because of the limited number of analyses and equivocal results. However, a collapse beyond the 21st century for large sustained warming cannot be excluded. Sustained mass loss by ice sheets would cause larger sea level rise, and part of the mass loss might be irreversible. There is high confidence that sustained global mean warming greater than a threshold would lead to the near-complete loss of the Greenland ice sheet over a millennium or more, causing a sea level rise of up to 7 m. Current estimates indicate that the threshold is greater than about 1°C but less than about 4°C of global warming with respect to pre-industrial temperatures. Abrupt and irreversible ice loss from a potential instability of marine-based sectors of the Antarctic ice sheet in response to climate forcing is possible, but current evidence and understanding is insufficient to make a quantitative assessment. Within the 21st century, magnitudes and rates of climate change associated with medium to high emission scenarios pose a high risk of abrupt and irreversible regional-scale change in the composition, structure and function of marine, terrestrial and freshwater ecosystems, including wetlands, as well as warm water coral reefs . Examples that could substantially amplify climate change are the boreal-tundra Arctic system and the Amazon forest. A reduction in permafrost extent is virtually certain with continued rise in global temperatures. Current permafrost areas are projected to become a net emitter of carbon with a loss of 180 to 920 GtCO2 under RCP8.5 over the 21st century . 2.3c: - Ecosystems and their services in the oceans, along coasts, on land and in freshwater Risks of harmful impacts on ecosystems and human systems increase with the rates and magnitudes of warming, ocean acidification, sea level rise and other dimensions of climate change. Future risk is indicated to be high by the observation that natural global climate change at rates lower than current anthropogenic climate change caused significant ecosystem shifts and species extinctions during the past millions of years on land and in the oceans. Many plant and animal species will be unable to adapt locally or move fast enough during the 21st century to track suitable climates under mid- and high range rates of climate change . Coral reefs and polar ecosystems are highly vulnerable. A large fraction of terrestrial, freshwater and marine species faces increased extinction risk due to climate change during and beyond the 21st century, especially as climate change interacts with other stressors. Extinction risk is increased relative to pre-industrial and present periods, under all RCP scenarios, as a result of both the magnitude and rate of climate change . Extinctions will be driven by several climate-associated drivers and the interactions among these drivers and their interaction with simultaneous habitat modification, over-exploitation of stocks, pollution, eutrophication and invasive species. Global marine species redistribution and marine biodiversity reduction in sensitive regions, under climate change, will challenge the sustained provision of fisheries productivity and other ecosystem services, especially at low latitudes . By the mid-21st century, under 2°C global warming relative to pre-industrial temperatures, shifts in the geographical range of marine species will cause species richness and fisheries catch potential to increase, on average, at mid and high latitudes and to decrease at tropical latitudes and in semi-enclosed seas. The progressive expansion of Oxygen Minimum Zones and anoxic ‘dead zones’ in the oceans will further constrain fish habitats. Open-ocean net primary production is projected to redistribute and to decrease globally, by 2100, under all RCP scenarios. Climate change adds to the threats of over-fishing and other non-climatic stressors . Marine ecosystems, especially coral reefs and polar ecosystems, are at risk from ocean acidification. Ocean acidification has impacts on the physiology, behaviour and population dynamics of organisms. The impacts on individual species and the number of species affected in species groups increase from RCP4.5 to RCP8.5. Highly calcified molluscs, echinoderms and reef-building corals are more sensitive than crustaceans and fishes. Ocean acidification acts together with other global changes and with local changes , leading to interactive, complex and amplified impacts for species and ecosystems. Carbon stored in the terrestrial biosphere is susceptible to loss to the atmosphere as a result of climate change, deforestation and ecosystem degradation. The aspects of climate change with direct effects on stored terrestrial carbon include high temperatures, drought and windstorms; indirect effects include increased risk of fires, pest and disease outbreaks. Increased tree mortality and associated forest dieback is projected to occur in many regions over the 21st century, posing risks for carbon storage, biodiversity, wood production, water quality, amenity and economic activity. There is a high risk of substantial carbon and methane emissions as a result of permafrost thawing. Coastal systems and low-lying areas will increasingly experience submergence, flooding and erosion throughout the 21st century and beyond, due to sea level rise. The population and assets projected to be exposed to coastal risks as well as human pressures on coastal ecosystems will increase significantly in the coming decades due to population growth, economic development and urbanization. Climatic and non-climatic drivers affecting coral reefs will erode habitats, increase coastline exposure to waves and storms and degrade environmental features important to fisheries and tourism. Some low-lying developing countries and small island states are expected to face very high impacts that could have associated damage and adaptation costs of several percentage points of gross domestic product . - Water, food and urban systems, human health, security and livelihoods The fractions of the global population that will experience water scarcity and be affected by major river floods are projected to increase with the level of warming in the 21st century . Climate change over the 21st century is projected to reduce renewable surface water and groundwater resources in most dry subtropical regions, intensifying competition for water among sectors . In presently dry regions, the frequency of droughts will likely increase by the end of the 21st century under RCP8.5. In contrast, water resources are projected to increase at high latitudes. The interaction of increased temperature; increased sediment, nutrient and pollutant loadings from heavy rainfall; increased concentrations of pollutants during droughts; and disruption of treatment facilities during floods will reduce raw water quality and pose risks to drinking water quality. All aspects of food security are potentially affected by climate change, including food production, access, use and price stability. For wheat, rice and maize in tropical and temperate regions, climate change without adaptation is projected to negatively impact production at local temperature increases of 2°C or more above late 20th century levels, although individual locations may benefit. Projected impacts vary across crops and regions and adaptation scenarios, with about 10% of projections for the 2030–2049 period showing yield gains of more than 10%, and about 10% of projections showing yield losses of more than 25%, compared with the late 20th century. Global temperature increases of ~4°C or more above late 20th century levels, combined with increasing food demand, would pose large risks to food security, both globally and regionally. The relationship between global and regional warming is explained in 2.2.1. Until mid-century, projected climate change will impact human health mainly by exacerbating health problems that already exist. Throughout the 21st century, climate change is expected to lead to increases in ill-health in many regions and especially in developing countries with low income, as compared to a baseline without climate change . Health impacts include greater likelihood of injury and death due to more intense heat waves and fires, increased risks from foodborne and waterborne diseases and loss of work capacity and reduced labour productivity in vulnerable populations . Risks of undernutrition in poor regions will increase . Risks from vector-borne diseases are projected to generally increase with warming, due to the extension of the infection area and season, despite reductions in some areas that become too hot for disease vectors. Globally, the magnitude and severity of negative impacts will increasingly outweigh positive impacts . By 2100 for RCP8.5, the combination of high temperature and humidity in some areas for parts of the year is expected to compromise common human activities, including growing food and working outdoors. In urban areas, climate change is projected to increase risks for people, assets, economies and ecosystems, including risks from heat stress, storms and extreme precipitation, inland and coastal flooding, landslides, air pollution, drought, water scarcity, sea level rise and storm surges. These risks will be amplified for those lacking essential infrastructure and services or living in exposed areas. Rural areas are expected to experience major impacts on water availability and supply, food security, infrastructure and agricultural incomes, including shifts in the production areas of food and non-food crops around the world . These impacts will disproportionately affect the welfare of the poor in rural areas, such as female-headed households and those with limited access to land, modern agricultural inputs, infrastructure and education. Aggregate economic losses accelerate with increasing temperature, but global economic impacts from climate change are currently difficult to estimate. With recognized limitations, the existing incomplete estimates of global annual economic losses for warming of ~2.5°C above pre-industrial levels are 0.2 to 2.0% of income . Changes in population, age structure, income, technology, relative prices, lifestyle, regulation and governance are projected to have relatively larger impacts than climate change, for most economic sectors. More severe and/or frequent weather hazards are projected to increase disaster-related losses and loss variability, posing challenges for affordable insurance, particularly in developing countries. International dimensions such as trade and relations among states are also important for understanding the risks of climate change at regional scales. From a poverty perspective, climate change impacts are projected to slow down economic growth, make poverty reduction more difficult, further erode food security and prolong existing poverty traps and create new ones, the latter particularly in urban areas and emerging hotspots of hunger . Climate change impacts are expected to exacerbate poverty in most developing countries and create new poverty pockets in countries with increasing inequality, in both developed and developing countries . Climate change is projected to increase displacement of people . Displacement risk increases when populations that lack the resources for planned migration experience higher exposure to extreme weather events, such as floods and droughts. Expanding opportunities for mobility can reduce vulnerability for such populations. Changes in migration patterns can be responses to both extreme weather events and longer term climate variability and change, and migration can also be an effective adaptation strategy. Climate change can indirectly increase risks of violent conflict by amplifying well-documented drivers of these conflicts, such as poverty and economic shocks. Multiple lines of evidence relate climate variability to some forms of conflict. 2.3e: - Water, food and urban systems, human health, security and livelihoods The fractions of the global population that will experience water scarcity and be affected by major river floods are projected to increase with the level of warming in the 21st century . Climate change over the 21st century is projected to reduce renewable surface water and groundwater resources in most dry subtropical regions, intensifying competition for water among sectors . In presently dry regions, the frequency of droughts will likely increase by the end of the 21st century under RCP8.5. In contrast, water resources are projected to increase at high latitudes. The interaction of increased temperature; increased sediment, nutrient and pollutant loadings from heavy rainfall; increased concentrations of pollutants during droughts; and disruption of treatment facilities during floods will reduce raw water quality and pose risks to drinking water quality. All aspects of food security are potentially affected by climate change, including food production, access, use and price stability. For wheat, rice and maize in tropical and temperate regions, climate change without adaptation is projected to negatively impact production at local temperature increases of 2°C or more above late 20th century levels, although individual locations may benefit. Projected impacts vary across crops and regions and adaptation scenarios, with about 10% of projections for the 2030–2049 period showing yield gains of more than 10%, and about 10% of projections showing yield losses of more than 25%, compared with the late 20th century. Global temperature increases of ~4°C or more above late 20th century levels, combined with increasing food demand, would pose large risks to food security, both globally and regionally. The relationship between global and regional warming is explained in 2.2.1. Until mid-century, projected climate change will impact human health mainly by exacerbating health problems that already exist. Throughout the 21st century, climate change is expected to lead to increases in ill-health in many regions and especially in developing countries with low income, as compared to a baseline without climate change . Health impacts include greater likelihood of injury and death due to more intense heat waves and fires, increased risks from foodborne and waterborne diseases and loss of work capacity and reduced labour productivity in vulnerable populations . Risks of undernutrition in poor regions will increase . Risks from vector-borne diseases are projected to generally increase with warming, due to the extension of the infection area and season, despite reductions in some areas that become too hot for disease vectors. Globally, the magnitude and severity of negative impacts will increasingly outweigh positive impacts . By 2100 for RCP8.5, the combination of high temperature and humidity in some areas for parts of the year is expected to compromise common human activities, including growing food and working outdoors. In urban areas, climate change is projected to increase risks for people, assets, economies and ecosystems, including risks from heat stress, storms and extreme precipitation, inland and coastal flooding, landslides, air pollution, drought, water scarcity, sea level rise and storm surges. These risks will be amplified for those lacking essential infrastructure and services or living in exposed areas. Rural areas are expected to experience major impacts on water availability and supply, food security, infrastructure and agricultural incomes, including shifts in the production areas of food and non-food crops around the world . These impacts will disproportionately affect the welfare of the poor in rural areas, such as female-headed households and those with limited access to land, modern agricultural inputs, infrastructure and education. Aggregate economic losses accelerate with increasing temperature, but global economic impacts from climate change are currently difficult to estimate. With recognized limitations, the existing incomplete estimates of global annual economic losses for warming of ~2.5°C above pre-industrial levels are 0.2 to 2.0% of income . Changes in population, age structure, income, technology, relative prices, lifestyle, regulation and governance are projected to have relatively larger impacts than climate change, for most economic sectors. More severe and/or frequent weather hazards are projected to increase disaster-related losses and loss variability, posing challenges for affordable insurance, particularly in developing countries. International dimensions such as trade and relations among states are also important for understanding the risks of climate change at regional scales. From a poverty perspective, climate change impacts are projected to slow down economic growth, make poverty reduction more difficult, further erode food security and prolong existing poverty traps and create new ones, the latter particularly in urban areas and emerging hotspots of hunger . Climate change impacts are expected to exacerbate poverty in most developing countries and create new poverty pockets in countries with increasing inequality, in both developed and developing countries . Climate change is projected to increase displacement of people . Displacement risk increases when populations that lack the resources for planned migration experience higher exposure to extreme weather events, such as floods and droughts. Expanding opportunities for mobility can reduce vulnerability for such populations. Changes in migration patterns can be responses to both extreme weather events and longer term climate variability and change, and migration can also be an effective adaptation strategy. Climate change can indirectly increase risks of violent conflict by amplifying well-documented drivers of these conflicts, such as poverty and economic shocks. Multiple lines of evidence relate climate variability to some forms of conflict. 2.3f: - Water, food and urban systems, human health, security and livelihoods The fractions of the global population that will experience water scarcity and be affected by major river floods are projected to increase with the level of warming in the 21st century . Climate change over the 21st century is projected to reduce renewable surface water and groundwater resources in most dry subtropical regions, intensifying competition for water among sectors . In presently dry regions, the frequency of droughts will likely increase by the end of the 21st century under RCP8.5. In contrast, water resources are projected to increase at high latitudes. The interaction of increased temperature; increased sediment, nutrient and pollutant loadings from heavy rainfall; increased concentrations of pollutants during droughts; and disruption of treatment facilities during floods will reduce raw water quality and pose risks to drinking water quality. All aspects of food security are potentially affected by climate change, including food production, access, use and price stability. For wheat, rice and maize in tropical and temperate regions, climate change without adaptation is projected to negatively impact production at local temperature increases of 2°C or more above late 20th century levels, although individual locations may benefit. Projected impacts vary across crops and regions and adaptation scenarios, with about 10% of projections for the 2030–2049 period showing yield gains of more than 10%, and about 10% of projections showing yield losses of more than 25%, compared with the late 20th century. Global temperature increases of ~4°C or more above late 20th century levels, combined with increasing food demand, would pose large risks to food security, both globally and regionally. The relationship between global and regional warming is explained in 2.2.1. Until mid-century, projected climate change will impact human health mainly by exacerbating health problems that already exist. Throughout the 21st century, climate change is expected to lead to increases in ill-health in many regions and especially in developing countries with low income, as compared to a baseline without climate change . Health impacts include greater likelihood of injury and death due to more intense heat waves and fires, increased risks from foodborne and waterborne diseases and loss of work capacity and reduced labour productivity in vulnerable populations . Risks of undernutrition in poor regions will increase . Risks from vector-borne diseases are projected to generally increase with warming, due to the extension of the infection area and season, despite reductions in some areas that become too hot for disease vectors. Globally, the magnitude and severity of negative impacts will increasingly outweigh positive impacts . By 2100 for RCP8.5, the combination of high temperature and humidity in some areas for parts of the year is expected to compromise common human activities, including growing food and working outdoors. In urban areas, climate change is projected to increase risks for people, assets, economies and ecosystems, including risks from heat stress, storms and extreme precipitation, inland and coastal flooding, landslides, air pollution, drought, water scarcity, sea level rise and storm surges. These risks will be amplified for those lacking essential infrastructure and services or living in exposed areas. Rural areas are expected to experience major impacts on water availability and supply, food security, infrastructure and agricultural incomes, including shifts in the production areas of food and non-food crops around the world . These impacts will disproportionately affect the welfare of the poor in rural areas, such as female-headed households and those with limited access to land, modern agricultural inputs, infrastructure and education. Aggregate economic losses accelerate with increasing temperature, but global economic impacts from climate change are currently difficult to estimate. With recognized limitations, the existing incomplete estimates of global annual economic losses for warming of ~2.5°C above pre-industrial levels are 0.2 to 2.0% of income . Changes in population, age structure, income, technology, relative prices, lifestyle, regulation and governance are projected to have relatively larger impacts than climate change, for most economic sectors. More severe and/or frequent weather hazards are projected to increase disaster-related losses and loss variability, posing challenges for affordable insurance, particularly in developing countries. International dimensions such as trade and relations among states are also important for understanding the risks of climate change at regional scales. From a poverty perspective, climate change impacts are projected to slow down economic growth, make poverty reduction more difficult, further erode food security and prolong existing poverty traps and create new ones, the latter particularly in urban areas and emerging hotspots of hunger . Climate change impacts are expected to exacerbate poverty in most developing countries and create new poverty pockets in countries with increasing inequality, in both developed and developing countries . Climate change is projected to increase displacement of people . Displacement risk increases when populations that lack the resources for planned migration experience higher exposure to extreme weather events, such as floods and droughts. Expanding opportunities for mobility can reduce vulnerability for such populations. Changes in migration patterns can be responses to both extreme weather events and longer term climate variability and change, and migration can also be an effective adaptation strategy. Climate change can indirectly increase risks of violent conflict by amplifying well-documented drivers of these conflicts, such as poverty and economic shocks. Multiple lines of evidence relate climate variability to some forms of conflict. 2.4a: - Climate change beyond 2100, irreversibility and abrupt changes Many aspects of climate change and its associated impacts will continue for centuries, even if anthropogenic emissions of greenhouse gases are stopped. The risks of abrupt or irreversible changes increase as the magnitude of the warming increases. Warming will continue beyond 2100 under all RCP scenarios except RCP2.6. Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions . A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial timescale, except in the case of a large net removal of CO2 from the atmosphere over a sustained period. Stabilization of global average surface temperature does not imply stabilization for all aspects of the climate system. Shifting biomes, re-equilibrating soil carbon, ice sheets, ocean temperatures and associated sea level rise all have their own intrinsic long timescales that will result in ongoing changes for hundreds to thousands of years after global surface temperature has been stabilized. Ocean acidification will continue for centuries if CO2 emissions continue, it will strongly affect marine ecosystems , and the impact will be exacerbated by rising temperature extremes. Global mean sea level rise will continue for many centuries beyond 2100. The few available analyses that go beyond 2100 indicate sea level rise to be less than 1 m above the pre-industrial level by 2300 for GHG concentrations that peak and decline and remain below 500 ppm CO2-eq, as in scenario RCP2.6. For a radiative forcing that corresponds to a CO2-eq concentration in 2100 that is above 700 ppm but below 1500 ppm, as in scenario RCP8.5, the projected rise is 1 m to more than 3 m by 2300 . There is low confidence in the available models’ ability to project solid ice discharge from the Antarctic ice sheet. Hence, these models likely underestimate the Antarctica ice sheet contribution, resulting in an underestimation of projected sea level rise beyond 2100. There is little evidence in global climate models of a tipping point or critical threshold in the transition from a perennially ice-covered to a seasonally ice-free Arctic Ocean, beyond which further sea-ice loss is unstoppable and irreversible. There is low confidence in assessing the evolution of the Atlantic Meridional Overturning Circulation beyond the 21st century because of the limited number of analyses and equivocal results. However, a collapse beyond the 21st century for large sustained warming cannot be excluded. Sustained mass loss by ice sheets would cause larger sea level rise, and part of the mass loss might be irreversible. There is high confidence that sustained global mean warming greater than a threshold would lead to the near-complete loss of the Greenland ice sheet over a millennium or more, causing a sea level rise of up to 7 m. Current estimates indicate that the threshold is greater than about 1°C but less than about 4°C of global warming with respect to pre-industrial temperatures. Abrupt and irreversible ice loss from a potential instability of marine-based sectors of the Antarctic ice sheet in response to climate forcing is possible, but current evidence and understanding is insufficient to make a quantitative assessment. Within the 21st century, magnitudes and rates of climate change associated with medium to high emission scenarios pose a high risk of abrupt and irreversible regional-scale change in the composition, structure and function of marine, terrestrial and freshwater ecosystems, including wetlands, as well as warm water coral reefs . Examples that could substantially amplify climate change are the boreal-tundra Arctic system and the Amazon forest. A reduction in permafrost extent is virtually certain with continued rise in global temperatures. Current permafrost areas are projected to become a net emitter of carbon with a loss of 180 to 920 GtCO2 under RCP8.5 over the 21st century . 2.4b: - Climate change beyond 2100, irreversibility and abrupt changes Many aspects of climate change and its associated impacts will continue for centuries, even if anthropogenic emissions of greenhouse gases are stopped. The risks of abrupt or irreversible changes increase as the magnitude of the warming increases. Warming will continue beyond 2100 under all RCP scenarios except RCP2.6. Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions . A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial timescale, except in the case of a large net removal of CO2 from the atmosphere over a sustained period. Stabilization of global average surface temperature does not imply stabilization for all aspects of the climate system. Shifting biomes, re-equilibrating soil carbon, ice sheets, ocean temperatures and associated sea level rise all have their own intrinsic long timescales that will result in ongoing changes for hundreds to thousands of years after global surface temperature has been stabilized. Ocean acidification will continue for centuries if CO2 emissions continue, it will strongly affect marine ecosystems , and the impact will be exacerbated by rising temperature extremes. Global mean sea level rise will continue for many centuries beyond 2100. The few available analyses that go beyond 2100 indicate sea level rise to be less than 1 m above the pre-industrial level by 2300 for GHG concentrations that peak and decline and remain below 500 ppm CO2-eq, as in scenario RCP2.6. For a radiative forcing that corresponds to a CO2-eq concentration in 2100 that is above 700 ppm but below 1500 ppm, as in scenario RCP8.5, the projected rise is 1 m to more than 3 m by 2300 . There is low confidence in the available models’ ability to project solid ice discharge from the Antarctic ice sheet. Hence, these models likely underestimate the Antarctica ice sheet contribution, resulting in an underestimation of projected sea level rise beyond 2100. There is little evidence in global climate models of a tipping point or critical threshold in the transition from a perennially ice-covered to a seasonally ice-free Arctic Ocean, beyond which further sea-ice loss is unstoppable and irreversible. There is low confidence in assessing the evolution of the Atlantic Meridional Overturning Circulation beyond the 21st century because of the limited number of analyses and equivocal results. However, a collapse beyond the 21st century for large sustained warming cannot be excluded. Sustained mass loss by ice sheets would cause larger sea level rise, and part of the mass loss might be irreversible. There is high confidence that sustained global mean warming greater than a threshold would lead to the near-complete loss of the Greenland ice sheet over a millennium or more, causing a sea level rise of up to 7 m. Current estimates indicate that the threshold is greater than about 1°C but less than about 4°C of global warming with respect to pre-industrial temperatures. Abrupt and irreversible ice loss from a potential instability of marine-based sectors of the Antarctic ice sheet in response to climate forcing is possible, but current evidence and understanding is insufficient to make a quantitative assessment. Within the 21st century, magnitudes and rates of climate change associated with medium to high emission scenarios pose a high risk of abrupt and irreversible regional-scale change in the composition, structure and function of marine, terrestrial and freshwater ecosystems, including wetlands, as well as warm water coral reefs . Examples that could substantially amplify climate change are the boreal-tundra Arctic system and the Amazon forest. A reduction in permafrost extent is virtually certain with continued rise in global temperatures. Current permafrost areas are projected to become a net emitter of carbon with a loss of 180 to 920 GtCO2 under RCP8.5 over the 21st century . 2.4c: - Climate change beyond 2100, irreversibility and abrupt changes Many aspects of climate change and its associated impacts will continue for centuries, even if anthropogenic emissions of greenhouse gases are stopped. The risks of abrupt or irreversible changes increase as the magnitude of the warming increases. Warming will continue beyond 2100 under all RCP scenarios except RCP2.6. Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions . A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial timescale, except in the case of a large net removal of CO2 from the atmosphere over a sustained period. Stabilization of global average surface temperature does not imply stabilization for all aspects of the climate system. Shifting biomes, re-equilibrating soil carbon, ice sheets, ocean temperatures and associated sea level rise all have their own intrinsic long timescales that will result in ongoing changes for hundreds to thousands of years after global surface temperature has been stabilized. Ocean acidification will continue for centuries if CO2 emissions continue, it will strongly affect marine ecosystems , and the impact will be exacerbated by rising temperature extremes. Global mean sea level rise will continue for many centuries beyond 2100. The few available analyses that go beyond 2100 indicate sea level rise to be less than 1 m above the pre-industrial level by 2300 for GHG concentrations that peak and decline and remain below 500 ppm CO2-eq, as in scenario RCP2.6. For a radiative forcing that corresponds to a CO2-eq concentration in 2100 that is above 700 ppm but below 1500 ppm, as in scenario RCP8.5, the projected rise is 1 m to more than 3 m by 2300 . There is low confidence in the available models’ ability to project solid ice discharge from the Antarctic ice sheet. Hence, these models likely underestimate the Antarctica ice sheet contribution, resulting in an underestimation of projected sea level rise beyond 2100. There is little evidence in global climate models of a tipping point or critical threshold in the transition from a perennially ice-covered to a seasonally ice-free Arctic Ocean, beyond which further sea-ice loss is unstoppable and irreversible. There is low confidence in assessing the evolution of the Atlantic Meridional Overturning Circulation beyond the 21st century because of the limited number of analyses and equivocal results. However, a collapse beyond the 21st century for large sustained warming cannot be excluded. Sustained mass loss by ice sheets would cause larger sea level rise, and part of the mass loss might be irreversible. There is high confidence that sustained global mean warming greater than a threshold would lead to the near-complete loss of the Greenland ice sheet over a millennium or more, causing a sea level rise of up to 7 m. Current estimates indicate that the threshold is greater than about 1°C but less than about 4°C of global warming with respect to pre-industrial temperatures. Abrupt and irreversible ice loss from a potential instability of marine-based sectors of the Antarctic ice sheet in response to climate forcing is possible, but current evidence and understanding is insufficient to make a quantitative assessment. Within the 21st century, magnitudes and rates of climate change associated with medium to high emission scenarios pose a high risk of abrupt and irreversible regional-scale change in the composition, structure and function of marine, terrestrial and freshwater ecosystems, including wetlands, as well as warm water coral reefs . Examples that could substantially amplify climate change are the boreal-tundra Arctic system and the Amazon forest. A reduction in permafrost extent is virtually certain with continued rise in global temperatures. Current permafrost areas are projected to become a net emitter of carbon with a loss of 180 to 920 GtCO2 under RCP8.5 over the 21st century . 2.4d: - Climate change beyond 2100, irreversibility and abrupt changes Many aspects of climate change and its associated impacts will continue for centuries, even if anthropogenic emissions of greenhouse gases are stopped. The risks of abrupt or irreversible changes increase as the magnitude of the warming increases. Warming will continue beyond 2100 under all RCP scenarios except RCP2.6. Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions . A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial timescale, except in the case of a large net removal of CO2 from the atmosphere over a sustained period. Stabilization of global average surface temperature does not imply stabilization for all aspects of the climate system. Shifting biomes, re-equilibrating soil carbon, ice sheets, ocean temperatures and associated sea level rise all have their own intrinsic long timescales that will result in ongoing changes for hundreds to thousands of years after global surface temperature has been stabilized. Ocean acidification will continue for centuries if CO2 emissions continue, it will strongly affect marine ecosystems , and the impact will be exacerbated by rising temperature extremes. Global mean sea level rise will continue for many centuries beyond 2100. The few available analyses that go beyond 2100 indicate sea level rise to be less than 1 m above the pre-industrial level by 2300 for GHG concentrations that peak and decline and remain below 500 ppm CO2-eq, as in scenario RCP2.6. For a radiative forcing that corresponds to a CO2-eq concentration in 2100 that is above 700 ppm but below 1500 ppm, as in scenario RCP8.5, the projected rise is 1 m to more than 3 m by 2300 . There is low confidence in the available models’ ability to project solid ice discharge from the Antarctic ice sheet. Hence, these models likely underestimate the Antarctica ice sheet contribution, resulting in an underestimation of projected sea level rise beyond 2100. There is little evidence in global climate models of a tipping point or critical threshold in the transition from a perennially ice-covered to a seasonally ice-free Arctic Ocean, beyond which further sea-ice loss is unstoppable and irreversible. There is low confidence in assessing the evolution of the Atlantic Meridional Overturning Circulation beyond the 21st century because of the limited number of analyses and equivocal results. However, a collapse beyond the 21st century for large sustained warming cannot be excluded. Sustained mass loss by ice sheets would cause larger sea level rise, and part of the mass loss might be irreversible. There is high confidence that sustained global mean warming greater than a threshold would lead to the near-complete loss of the Greenland ice sheet over a millennium or more, causing a sea level rise of up to 7 m. Current estimates indicate that the threshold is greater than about 1°C but less than about 4°C of global warming with respect to pre-industrial temperatures. Abrupt and irreversible ice loss from a potential instability of marine-based sectors of the Antarctic ice sheet in response to climate forcing is possible, but current evidence and understanding is insufficient to make a quantitative assessment. Within the 21st century, magnitudes and rates of climate change associated with medium to high emission scenarios pose a high risk of abrupt and irreversible regional-scale change in the composition, structure and function of marine, terrestrial and freshwater ecosystems, including wetlands, as well as warm water coral reefs . Examples that could substantially amplify climate change are the boreal-tundra Arctic system and the Amazon forest. A reduction in permafrost extent is virtually certain with continued rise in global temperatures. Current permafrost areas are projected to become a net emitter of carbon with a loss of 180 to 920 GtCO2 under RCP8.5 over the 21st century . 3.1a: - 'Foundations of decision-making about climate change Effective decision-making to limit climate change and its effects can be informed by a wide range of analytical approaches for evaluating expected risks and benefits, recognizing the importance of governance, ethical dimensions, equity, value judgments, economic assessments and diverse perceptions and responses to risk and uncertainty. Sustainable development and equity provide a basis for assessing climate policies. Limiting the effects of climate change is necessary to achieve sustainable development and equity, including poverty eradication. Countries’ past and future contributions to the accumulation of GHGs in the atmosphere are different, and countries also face varying challenges and circumstances and have different capacities to address mitigation and adaptation. Mitigation and adaptation raise issues of equity, justice and fairness and are necessary to achieve sustainable development and poverty eradication. Many of those most vulnerable to climate change have contributed and contribute little to GHG emissions. Delaying mitigation shifts burdens from the present to the future, and insufficient adaptation responses to emerging impacts are already eroding the basis for sustainable development. Both adaptation and mitigation can have distributional effects locally, nationally and internationally, depending on who pays and who benefits. The process of decision-making about climate change, and the degree to which it respects the rights and views of all those affected, is also a concern of justice. Effective mitigation will not be achieved if individual agents advance their own interests independently. Climate change has the characteristics of a collective action problem at the global scale, because most GHGs accumulate over time and mix globally, and emissions by any agent affect other agents. Cooperative responses, including international cooperation, are therefore required to effectively mitigate GHG emissions and address other climate change issues. The effectiveness of adaptation can be enhanced through complementary actions across levels, including international cooperation. The evidence suggests that outcomes seen as equitable can lead to more effective cooperation. Decision-making about climate change involves valuation and mediation among diverse values and may be aided by the analytic methods of several normative disciplines. Ethics analyses the different values involved and the relations between them. Recent political philosophy has investigated the question of responsibility for the effects of emissions. Economics and decision analysis provide quantitative methods of valuation which can be used for estimating the social cost of carbon, in cost–benefit and costeffectiveness analyses, for optimization in integrated models and elsewhere. Economic methods can reflect ethical principles, and take account of non-marketed goods, equity, behavioural biases, ancillary benefits and costs and the differing values of money to different people. They are, however, subject to well-documented limitations. Analytical methods of valuation cannot identify a single best balance between mitigation, adaptation and residual climate impacts. Important reasons for this are that climate change involves extremely complex natural and social processes, there is extensive disagreement about the values concerned, and climate change impacts and mitigation approaches have important distributional effects. Nevertheless, information on the consequences of emissions pathways to alternative climate goals and risk levels can be a useful input into decision-making processes. Evaluating responses to climate change involves assessment of the widest possible range of impacts, including low-probability outcomes with large consequences. Effective decision-making and risk management in the complex environment of climate change may be iterative: strategies can often be adjusted as new information and understanding develops during implementation. However, adaptation and mitigation choices in the near term will affect the risks of climate change throughout the 21st century and beyond, and prospects for climate-resilient pathways for sustainable development depend on what is achieved through mitigation. Opportunities to take advantage of positive synergies between adaptation and mitigation may decrease with time, particularly if mitigation is delayed too long. Decision-making about climate change is influenced by how individuals and organizations perceive risks and uncertainties and take them into account. They sometimes use simplified decision rules, overestimate or underestimate risks and are biased towards the status quo. They differ in their degree of risk aversion and the relative importance placed on near-term versus long-term ramifications of specific actions. Formalized analytical methods for decision-making under uncertainty can account accurately for risk, and focus attention on both short- and long-term consequences.' 3.1b: - 'Foundations of decision-making about climate change Effective decision-making to limit climate change and its effects can be informed by a wide range of analytical approaches for evaluating expected risks and benefits, recognizing the importance of governance, ethical dimensions, equity, value judgments, economic assessments and diverse perceptions and responses to risk and uncertainty. Sustainable development and equity provide a basis for assessing climate policies. Limiting the effects of climate change is necessary to achieve sustainable development and equity, including poverty eradication. Countries’ past and future contributions to the accumulation of GHGs in the atmosphere are different, and countries also face varying challenges and circumstances and have different capacities to address mitigation and adaptation. Mitigation and adaptation raise issues of equity, justice and fairness and are necessary to achieve sustainable development and poverty eradication. Many of those most vulnerable to climate change have contributed and contribute little to GHG emissions. Delaying mitigation shifts burdens from the present to the future, and insufficient adaptation responses to emerging impacts are already eroding the basis for sustainable development. Both adaptation and mitigation can have distributional effects locally, nationally and internationally, depending on who pays and who benefits. The process of decision-making about climate change, and the degree to which it respects the rights and views of all those affected, is also a concern of justice. Effective mitigation will not be achieved if individual agents advance their own interests independently. Climate change has the characteristics of a collective action problem at the global scale, because most GHGs accumulate over time and mix globally, and emissions by any agent affect other agents. Cooperative responses, including international cooperation, are therefore required to effectively mitigate GHG emissions and address other climate change issues. The effectiveness of adaptation can be enhanced through complementary actions across levels, including international cooperation. The evidence suggests that outcomes seen as equitable can lead to more effective cooperation. Decision-making about climate change involves valuation and mediation among diverse values and may be aided by the analytic methods of several normative disciplines. Ethics analyses the different values involved and the relations between them. Recent political philosophy has investigated the question of responsibility for the effects of emissions. Economics and decision analysis provide quantitative methods of valuation which can be used for estimating the social cost of carbon, in cost–benefit and costeffectiveness analyses, for optimization in integrated models and elsewhere. Economic methods can reflect ethical principles, and take account of non-marketed goods, equity, behavioural biases, ancillary benefits and costs and the differing values of money to different people. They are, however, subject to well-documented limitations. Analytical methods of valuation cannot identify a single best balance between mitigation, adaptation and residual climate impacts. Important reasons for this are that climate change involves extremely complex natural and social processes, there is extensive disagreement about the values concerned, and climate change impacts and mitigation approaches have important distributional effects. Nevertheless, information on the consequences of emissions pathways to alternative climate goals and risk levels can be a useful input into decision-making processes. Evaluating responses to climate change involves assessment of the widest possible range of impacts, including low-probability outcomes with large consequences. Effective decision-making and risk management in the complex environment of climate change may be iterative: strategies can often be adjusted as new information and understanding develops during implementation. However, adaptation and mitigation choices in the near term will affect the risks of climate change throughout the 21st century and beyond, and prospects for climate-resilient pathways for sustainable development depend on what is achieved through mitigation. Opportunities to take advantage of positive synergies between adaptation and mitigation may decrease with time, particularly if mitigation is delayed too long. Decision-making about climate change is influenced by how individuals and organizations perceive risks and uncertainties and take them into account. They sometimes use simplified decision rules, overestimate or underestimate risks and are biased towards the status quo. They differ in their degree of risk aversion and the relative importance placed on near-term versus long-term ramifications of specific actions. Formalized analytical methods for decision-making under uncertainty can account accurately for risk, and focus attention on both short- and long-term consequences.' - Interaction among mitigation, adaptation and sustainable development Climate change is a threat to equitable and sustainable development. Adaptation, mitigation and sustainable development are closely related, with potential for synergies and trade-offs. Climate change poses an increasing threat to equitable and sustainable development. Some climate-related impacts on development are already being observed. Climate change is a threat multiplier. It exacerbates other threats to social and natural systems, placing additional burdens particularly on the poor and constraining possible development paths for all. Development along current global pathways can contribute to climate risk and vulnerability, further eroding the basis for sustainable development. Aligning climate policy with sustainable development requires attention to both adaptation and mitigation. Interaction among adaptation, mitigation and sustainable development occurs both within and across regions and scales, often in the context of multiple stressors. Some options for responding to climate change could impose risks of other environmental and social costs, have adverse distributional effects and draw resources away from other development priorities, including poverty eradication. Both adaptation and mitigation can bring substantial co-benefits . Examples of actions with co-benefits include improved air quality; enhanced energy security, reduced energy and water consumption in urban areas through greening cities and recycling water; sustainable agriculture and forestry; and protection of ecosystems for carbon storage and other ecosystem services. Strategies and actions can be pursued now that will move towards climate-resilient pathways for sustainable development, while at the same time helping to improve livelihoods, social and economic well-being and effective environmental management. Prospects for climate-resilient pathways are related fundamentally to what the world accomplishes with climate change mitigation. Since mitigation reduces the rate as well as the magnitude of warming, it also increases the time available for adaptation to a particular level of climate change, potentially by several decades. Delaying mitigation actions may reduce options for climate-resilient pathways in the future. 3.1c: - 'Foundations of decision-making about climate change Effective decision-making to limit climate change and its effects can be informed by a wide range of analytical approaches for evaluating expected risks and benefits, recognizing the importance of governance, ethical dimensions, equity, value judgments, economic assessments and diverse perceptions and responses to risk and uncertainty. Sustainable development and equity provide a basis for assessing climate policies. Limiting the effects of climate change is necessary to achieve sustainable development and equity, including poverty eradication. Countries’ past and future contributions to the accumulation of GHGs in the atmosphere are different, and countries also face varying challenges and circumstances and have different capacities to address mitigation and adaptation. Mitigation and adaptation raise issues of equity, justice and fairness and are necessary to achieve sustainable development and poverty eradication. Many of those most vulnerable to climate change have contributed and contribute little to GHG emissions. Delaying mitigation shifts burdens from the present to the future, and insufficient adaptation responses to emerging impacts are already eroding the basis for sustainable development. Both adaptation and mitigation can have distributional effects locally, nationally and internationally, depending on who pays and who benefits. The process of decision-making about climate change, and the degree to which it respects the rights and views of all those affected, is also a concern of justice. Effective mitigation will not be achieved if individual agents advance their own interests independently. Climate change has the characteristics of a collective action problem at the global scale, because most GHGs accumulate over time and mix globally, and emissions by any agent affect other agents. Cooperative responses, including international cooperation, are therefore required to effectively mitigate GHG emissions and address other climate change issues. The effectiveness of adaptation can be enhanced through complementary actions across levels, including international cooperation. The evidence suggests that outcomes seen as equitable can lead to more effective cooperation. Decision-making about climate change involves valuation and mediation among diverse values and may be aided by the analytic methods of several normative disciplines. Ethics analyses the different values involved and the relations between them. Recent political philosophy has investigated the question of responsibility for the effects of emissions. Economics and decision analysis provide quantitative methods of valuation which can be used for estimating the social cost of carbon, in cost–benefit and costeffectiveness analyses, for optimization in integrated models and elsewhere. Economic methods can reflect ethical principles, and take account of non-marketed goods, equity, behavioural biases, ancillary benefits and costs and the differing values of money to different people. They are, however, subject to well-documented limitations. Analytical methods of valuation cannot identify a single best balance between mitigation, adaptation and residual climate impacts. Important reasons for this are that climate change involves extremely complex natural and social processes, there is extensive disagreement about the values concerned, and climate change impacts and mitigation approaches have important distributional effects. Nevertheless, information on the consequences of emissions pathways to alternative climate goals and risk levels can be a useful input into decision-making processes. Evaluating responses to climate change involves assessment of the widest possible range of impacts, including low-probability outcomes with large consequences. Effective decision-making and risk management in the complex environment of climate change may be iterative: strategies can often be adjusted as new information and understanding develops during implementation. However, adaptation and mitigation choices in the near term will affect the risks of climate change throughout the 21st century and beyond, and prospects for climate-resilient pathways for sustainable development depend on what is achieved through mitigation. Opportunities to take advantage of positive synergies between adaptation and mitigation may decrease with time, particularly if mitigation is delayed too long. Decision-making about climate change is influenced by how individuals and organizations perceive risks and uncertainties and take them into account. They sometimes use simplified decision rules, overestimate or underestimate risks and are biased towards the status quo. They differ in their degree of risk aversion and the relative importance placed on near-term versus long-term ramifications of specific actions. Formalized analytical methods for decision-making under uncertainty can account accurately for risk, and focus attention on both short- and long-term consequences.' 3.1d: - 'Foundations of decision-making about climate change Effective decision-making to limit climate change and its effects can be informed by a wide range of analytical approaches for evaluating expected risks and benefits, recognizing the importance of governance, ethical dimensions, equity, value judgments, economic assessments and diverse perceptions and responses to risk and uncertainty. Sustainable development and equity provide a basis for assessing climate policies. Limiting the effects of climate change is necessary to achieve sustainable development and equity, including poverty eradication. Countries’ past and future contributions to the accumulation of GHGs in the atmosphere are different, and countries also face varying challenges and circumstances and have different capacities to address mitigation and adaptation. Mitigation and adaptation raise issues of equity, justice and fairness and are necessary to achieve sustainable development and poverty eradication. Many of those most vulnerable to climate change have contributed and contribute little to GHG emissions. Delaying mitigation shifts burdens from the present to the future, and insufficient adaptation responses to emerging impacts are already eroding the basis for sustainable development. Both adaptation and mitigation can have distributional effects locally, nationally and internationally, depending on who pays and who benefits. The process of decision-making about climate change, and the degree to which it respects the rights and views of all those affected, is also a concern of justice. Effective mitigation will not be achieved if individual agents advance their own interests independently. Climate change has the characteristics of a collective action problem at the global scale, because most GHGs accumulate over time and mix globally, and emissions by any agent affect other agents. Cooperative responses, including international cooperation, are therefore required to effectively mitigate GHG emissions and address other climate change issues. The effectiveness of adaptation can be enhanced through complementary actions across levels, including international cooperation. The evidence suggests that outcomes seen as equitable can lead to more effective cooperation. Decision-making about climate change involves valuation and mediation among diverse values and may be aided by the analytic methods of several normative disciplines. Ethics analyses the different values involved and the relations between them. Recent political philosophy has investigated the question of responsibility for the effects of emissions. Economics and decision analysis provide quantitative methods of valuation which can be used for estimating the social cost of carbon, in cost–benefit and costeffectiveness analyses, for optimization in integrated models and elsewhere. Economic methods can reflect ethical principles, and take account of non-marketed goods, equity, behavioural biases, ancillary benefits and costs and the differing values of money to different people. They are, however, subject to well-documented limitations. Analytical methods of valuation cannot identify a single best balance between mitigation, adaptation and residual climate impacts. Important reasons for this are that climate change involves extremely complex natural and social processes, there is extensive disagreement about the values concerned, and climate change impacts and mitigation approaches have important distributional effects. Nevertheless, information on the consequences of emissions pathways to alternative climate goals and risk levels can be a useful input into decision-making processes. Evaluating responses to climate change involves assessment of the widest possible range of impacts, including low-probability outcomes with large consequences. Effective decision-making and risk management in the complex environment of climate change may be iterative: strategies can often be adjusted as new information and understanding develops during implementation. However, adaptation and mitigation choices in the near term will affect the risks of climate change throughout the 21st century and beyond, and prospects for climate-resilient pathways for sustainable development depend on what is achieved through mitigation. Opportunities to take advantage of positive synergies between adaptation and mitigation may decrease with time, particularly if mitigation is delayed too long. Decision-making about climate change is influenced by how individuals and organizations perceive risks and uncertainties and take them into account. They sometimes use simplified decision rules, overestimate or underestimate risks and are biased towards the status quo. They differ in their degree of risk aversion and the relative importance placed on near-term versus long-term ramifications of specific actions. Formalized analytical methods for decision-making under uncertainty can account accurately for risk, and focus attention on both short- and long-term consequences.' 3.2b: - Climate change risks reduced by adaptation and mitigation Without additional mitigation efforts beyond those in place today, and even with adaptation, warming by the end of the 21st century will lead to high to very high risk of severe, widespread and irreversible impacts globally. Mitigation involves some level of co-benefits and of risks due to adverse side effects, but these risks do not involve the same possibility of severe, widespread and irreversible impacts as risks from climate change, increasing the benefits from near-term mitigation efforts. The risks of climate change, adaptation and mitigation differ in nature, timescale, magnitude and persistence. Risks from adaptation include maladaptation and negative ancillary impacts. Risks from mitigation include possible adverse side effects of large-scale deployment of low-carbon technology options and economic costs. Climate change risks may persist for millennia and can involve very high risk of severe impacts and the presence of significant irreversibilities combined with limited adaptive capacity. In contrast, the stringency of climate policies can be adjusted much more quickly in response to observed consequences and costs and create lower risks of irreversible consequences. Mitigation and adaptation are complementary approaches for reducing risks of climate change impacts. They interact with one another and reduce risks over different timescales . Benefits from adaptation can already be realized in addressing current risks and can be realized in the future for addressing emerging risks. Adaptation has the potential to reduce climate change impacts over the next few decades, while mitigation has relatively little influence on climate outcomes over this timescale. Near-term and longerterm mitigation and adaptation, as well as development pathways, will determine the risks of climate change beyond mid-century. The potential for adaptation differs across sectors and will be limited by institutional and capacity constraints, increasing the long-term benefits of mitigation. The level of mitigation will influence the rate and magnitude of climate change, and greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits. Without additional mitigation efforts beyond those in place today, and even with adaptation, warming by the end of the 21st century will lead to high to very high risk of severe, widespread and irreversible impacts globally . Estimates of warming in 2100 without additional climate mitigation efforts are from 3.7°C to 4.8°C compared with pre-industrial levels; the range is 2.5°C to 7.8°C when using the 5th to 95th percentile range of the median climate response. The risks associated with temperatures at or above 4°C include severe and widespread impacts on unique and threatened systems, substantial species extinction, large risks to global and regional food security, consequential constraints on common human activities, increased likelihood of triggering tipping points and limited potential for adaptation in some cases . Some risks of climate change, such as risks to unique and threatened systems and risks associated with extreme weather events, are moderate to high at temperatures 1°C to 2°C above pre-industrial levels. Substantial cuts in GHG emissions over the next few decades can substantially reduce risks of climate change by limiting warming in the second half of the 21st century and beyond . Global mean surface warming is largely determined by cumulative emissions, which are, in turn, linked to emissions over different timescales. Limiting risks across Reasons For Concern would imply a limit for cumulative emissions of CO2. Such a limit would require that global net emissions of CO2 eventually decrease to zero. Reducing risks of climate change through mitigation would involve substantial cuts in GHG emissions over the next few decades. But some risks from residual damages are unavoidable, even with mitigation and adaptation. A subset of relevant climate change risks has been estimated using aggregate economic indicators. Such economic estimates have important limitations and are therefore a useful but insufficient basis for decision-making on long-term mitigation targets. Mitigation involves some level of co-benefits and risks, but these risks do not involve the same possibility of severe, widespread and irreversible impacts as risks from climate change . Scenarios that are likely to limit warming to below 2°C or even 3°C compared with pre-industrial temperatures involve large-scale changes in energy systems and potentially land use over the coming decades. Associated risks include those linked to large-scale deployment of technology options for producing low-carbon energy, the potential for high aggregate economic costs of mitigation and impacts on vulnerable countries and industries. Other risks and co-benefits are associated with human health, food security, energy security, poverty reduction, biodiversity conservation, water availability, income distribution, efficiency of taxation systems, labour supply and employment, urban sprawl, fossil fuel export revenues and the economic growth of developing countries. Inertia in the economic and climate systems and the possibility of irreversible impacts from climate change increase the benefits of near-term mitigation efforts. The actions taken today affect the options available in the future to reduce emissions, limit temperature change and adapt to climate change. Near-term choices can create, amplify or limit significant elements of lock-in that are important for decision-making. Lock-ins and irreversibilities occur in the climate system due to large inertia in some of its components such as heat transfer from the ocean surface to depth leading to continued ocean warming for centuries regardless of emission scenario and the irreversibility of a large fraction of anthropogenic climate change resulting from CO2 emissions on a multi-century to millennial timescale unless CO2 were to be removed from the atmosphere through large-scale human interventions over a sustained period . Irreversibilities in socio-economic and biological systems also result from infrastructure development and long-lived products and from climate change impacts, such as species extinction. The larger potential for irreversibility and pervasive impacts from climate change risks than from mitigation risks increases the benefit of shortterm mitigation efforts. Delays in additional mitigation or constraints on technological options limit the mitigation options and increase the long-term mitigation costs as well as other risks that would be incurred in the medium to long term to hold climate change impacts at a given level. - Trade-offs, synergies and integrated responses There are many opportunities to link mitigation, adaptation and the pursuit of other societal objectives through integrated responses. Successful implementation relies on relevant tools, suitable governance structures and enhanced capacity to respond. A growing evidence base indicates close links between adaptation and mitigation, their co-benefits and adverse side effects, and recognizes sustainable development as the overarching context for climate policy . Developing tools to address these linkages is critical to the success of climate policy in the context of sustainable development. This section presents examples of integrated responses in specific policy arenas, as well as some of the factors that promote or impede policies aimed at multiple objectives. Increasing efforts to mitigate and adapt to climate change imply an increasing complexity of interactions, encompassing connections among human health, water, energy, land use and biodiversity. Mitigation can support the achievement of other societal goals, such as those related to human health, food security, environmental quality, energy access, livelihoods and sustainable development, although there can also be negative effects. Adaptation measures also have the potential to deliver mitigation co-benefits, and vice versa, and support other societal goals, though trade-offs can also arise. Integration of adaptation and mitigation into planning and decision-making can create synergies with sustainable development. Synergies and trade-offs among mitigation and adaptation policies and policies advancing other societal goals can be substantial, although sometimes difficult to quantify especially in welfare terms. A multi-objective approach to policy-making can help manage these synergies and trade-offs. Policies advancing multiple goals may also attract greater support. Effective integrated responses depend on suitable tools and governance structures, as well as adequate capacity . Managing trade-offs and synergies is challenging and requires tools to help understand interactions and support decision-making at local and regional scales. Integrated responses also depend on governance that enables coordination across scales and sectors, supported by appropriate institutions. Developing and implementing suitable tools and governance structures often requires upgrading the human and institutional capacity to design and deploy integrated responses. An integrated approach to energy planning and implementation that explicitly assesses the potential for co-benefits and the presence of adverse side effects can capture complementarities across multiple climate, social and environmental objectives . There are strong interactive effects across various energy policy objectives, such as energy security, air quality, health and energy access and between a range of social and environmental objectives and climate mitigation objectives . An integrated approach can be assisted by tools such as cost-benefit analysis, cost-effectiveness analysis, multi-criteria analysis and expected utility theory. It also requires appropriate coordinating institutions. Explicit consideration of interactions among water, food, energy and biological carbon sequestration plays an important role in supporting effective decisions for climate resilient pathways . Both biofuel-based power generation and large-scale afforestation designed to mitigate climate change can reduce catchment run-off, which may conflict with alternative water uses for food production, human consumption or the maintenance of ecosystem function and services. Conversely, irrigation can increase the climate resilience of food and fibre production but reduces water availability for other uses. An integrated response to urbanization provides substantial opportunities for enhanced resilience, reduced emissions and more sustainable development. Urban areas account for more than half of global primary energy use and energy-related CO2 emissions and contain a high proportion of the population and economic activities at risk from climate change. In rapidly growing and urbanizing regions, mitigation strategies based on spatial planning and efficient infrastructure supply can avoid the lock-in of high emission patterns. Mixed-use zoning, transport-oriented development, increased density and co-located jobs and homes can reduce direct and indirect energy use across sectors. Compact development of urban spaces and intelligent densification can preserve land carbon stocks and land for agriculture and bioenergy. Reduced energy and water consumption in urban areas through greening cities and recycling water are examples of mitigation actions with adaptation benefits. Building resilient infrastructure systems can reduce vulnerability of urban settlements and cities to coastal flooding, sea level rise and other climate-induced stresses. 3.2d: - 'Future risks and impacts caused by a changing climate Climate change will amplify existing risks and create new risks for natural and human systems. Risks are unevenly distributed and are generally greater for disadvantaged people and communities in countries at all levels of development. Increasing magnitudes of warming increase the likelihood of severe, pervasive and irreversible impacts for people, species and ecosystems. Continued high emissions would lead to mostly negative impacts for biodiversity, ecosystem services and economic development and amplify risks for livelihoods and for food and human security. Risk of climate-related impacts results from the interaction of climate-related hazards with the vulnerability and exposure of human and natural systems, including their ability to adapt. Rising rates and magnitudes of warming and other changes in the climate system, accompanied by ocean acidification, increase the risk of severe, pervasive, and in some cases, irreversible detrimental impacts. Future climate change will amplify existing climate-related risks and create new risks. Key risks are potentially severe impacts relevant to understanding dangerous anthropogenic interference with the climate system. Risks are considered key due to high hazard or high vulnerability of societies and systems exposed, or both. Their identification is based on large magnitude or high probability of impacts; irreversibility or timing of impacts; persistent vulnerability or exposure; or limited potential to reduce risks. Some risks are particularly relevant for individual regions, while others are global. For risk assessment it is important to evaluate the widest possible range of impacts, including low-probability outcomes with large consequences. Risk levels often increase with temperature and are sometimes more directly linked to other dimensions of climate change, such as the rate of warming, as well as the magnitudes and rates of ocean acidification and sea level rise . Key risks that span sectors and regions include the following : 1. Risk of severe ill-health and disrupted livelihoods resulting from storm surges, sea level rise and coastal flooding; inland flooding in some urban regions; and periods of extreme heat. 2. Systemic risks due to extreme weather events leading to breakdown of infrastructure networks and critical services. 3. Risk of food and water insecurity and loss of rural livelihoods and income, particularly for poorer populations. 4. Risk of loss of ecosystems, biodiversity and ecosystem goods, functions and services. The overall risks of future climate change impacts can be reduced by limiting the rate and magnitude of climate change, including ocean acidification. Some risks are considerable even at 1°C global mean temperature increase above pre-industrial levels. Many global risks are high to very high for global temperature increases of 4°C or more. These risks include severe and widespread impacts on unique and threatened systems, the extinction of many species, large risks to food security and compromised normal human activities, including growing food or working outdoors in some areas for parts of the year, due to the combination of high temperature and humidity. The precise levels of climate change sufficient to trigger abrupt and irreversible change remain uncertain, but the risk associated with crossing such thresholds in the earth system or in interlinked human and natural systems increases with rising temperature. Adaptation can substantially reduce the risks of climate change impacts, but greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits . The potential for adaptation, as well as constraints and limits to adaptation, varies among sectors, regions, communities and ecosystems. The scope for adaptation changes over time and is closely linked to socio-economic development pathways and circumstances. See Figure 2.4 and Table 2.3, along with Topics 3 and 4.' - Climate change risks reduced by adaptation and mitigation Without additional mitigation efforts beyond those in place today, and even with adaptation, warming by the end of the 21st century will lead to high to very high risk of severe, widespread and irreversible impacts globally. Mitigation involves some level of co-benefits and of risks due to adverse side effects, but these risks do not involve the same possibility of severe, widespread and irreversible impacts as risks from climate change, increasing the benefits from near-term mitigation efforts. The risks of climate change, adaptation and mitigation differ in nature, timescale, magnitude and persistence. Risks from adaptation include maladaptation and negative ancillary impacts. Risks from mitigation include possible adverse side effects of large-scale deployment of low-carbon technology options and economic costs. Climate change risks may persist for millennia and can involve very high risk of severe impacts and the presence of significant irreversibilities combined with limited adaptive capacity. In contrast, the stringency of climate policies can be adjusted much more quickly in response to observed consequences and costs and create lower risks of irreversible consequences. Mitigation and adaptation are complementary approaches for reducing risks of climate change impacts. They interact with one another and reduce risks over different timescales . Benefits from adaptation can already be realized in addressing current risks and can be realized in the future for addressing emerging risks. Adaptation has the potential to reduce climate change impacts over the next few decades, while mitigation has relatively little influence on climate outcomes over this timescale. Near-term and longerterm mitigation and adaptation, as well as development pathways, will determine the risks of climate change beyond mid-century. The potential for adaptation differs across sectors and will be limited by institutional and capacity constraints, increasing the long-term benefits of mitigation. The level of mitigation will influence the rate and magnitude of climate change, and greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits. Without additional mitigation efforts beyond those in place today, and even with adaptation, warming by the end of the 21st century will lead to high to very high risk of severe, widespread and irreversible impacts globally . Estimates of warming in 2100 without additional climate mitigation efforts are from 3.7°C to 4.8°C compared with pre-industrial levels; the range is 2.5°C to 7.8°C when using the 5th to 95th percentile range of the median climate response. The risks associated with temperatures at or above 4°C include severe and widespread impacts on unique and threatened systems, substantial species extinction, large risks to global and regional food security, consequential constraints on common human activities, increased likelihood of triggering tipping points and limited potential for adaptation in some cases . Some risks of climate change, such as risks to unique and threatened systems and risks associated with extreme weather events, are moderate to high at temperatures 1°C to 2°C above pre-industrial levels. Substantial cuts in GHG emissions over the next few decades can substantially reduce risks of climate change by limiting warming in the second half of the 21st century and beyond . Global mean surface warming is largely determined by cumulative emissions, which are, in turn, linked to emissions over different timescales. Limiting risks across Reasons For Concern would imply a limit for cumulative emissions of CO2. Such a limit would require that global net emissions of CO2 eventually decrease to zero. Reducing risks of climate change through mitigation would involve substantial cuts in GHG emissions over the next few decades. But some risks from residual damages are unavoidable, even with mitigation and adaptation. A subset of relevant climate change risks has been estimated using aggregate economic indicators. Such economic estimates have important limitations and are therefore a useful but insufficient basis for decision-making on long-term mitigation targets. Mitigation involves some level of co-benefits and risks, but these risks do not involve the same possibility of severe, widespread and irreversible impacts as risks from climate change . Scenarios that are likely to limit warming to below 2°C or even 3°C compared with pre-industrial temperatures involve large-scale changes in energy systems and potentially land use over the coming decades. Associated risks include those linked to large-scale deployment of technology options for producing low-carbon energy, the potential for high aggregate economic costs of mitigation and impacts on vulnerable countries and industries. Other risks and co-benefits are associated with human health, food security, energy security, poverty reduction, biodiversity conservation, water availability, income distribution, efficiency of taxation systems, labour supply and employment, urban sprawl, fossil fuel export revenues and the economic growth of developing countries. Inertia in the economic and climate systems and the possibility of irreversible impacts from climate change increase the benefits of near-term mitigation efforts. The actions taken today affect the options available in the future to reduce emissions, limit temperature change and adapt to climate change. Near-term choices can create, amplify or limit significant elements of lock-in that are important for decision-making. Lock-ins and irreversibilities occur in the climate system due to large inertia in some of its components such as heat transfer from the ocean surface to depth leading to continued ocean warming for centuries regardless of emission scenario and the irreversibility of a large fraction of anthropogenic climate change resulting from CO2 emissions on a multi-century to millennial timescale unless CO2 were to be removed from the atmosphere through large-scale human interventions over a sustained period . Irreversibilities in socio-economic and biological systems also result from infrastructure development and long-lived products and from climate change impacts, such as species extinction. The larger potential for irreversibility and pervasive impacts from climate change risks than from mitigation risks increases the benefit of shortterm mitigation efforts. Delays in additional mitigation or constraints on technological options limit the mitigation options and increase the long-term mitigation costs as well as other risks that would be incurred in the medium to long term to hold climate change impacts at a given level. - Characteristics of mitigation pathways There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to pre-industrial levels. These pathways would require substantial emissions reductions over the next few decades and near zero emissions of CO2 and other long-lived greenhouse gases by the end of the century. Implementing such reductions poses substantial technological, economic, social and institutional challenges, which increase with delays in additional mitigation and if key technologies are not available. Limiting warming to lower or higher levels involves similar challenges but on different timescales. Without additional efforts to reduce GHG emissions beyond those in place today, global emission growth is expected to persist driven by growth in global population and economic activities. Global GHG emissions under most scenarios without additional mitigation are between about 75 GtCO2-eq/yr and almost 140 GtCO2-eq/yr in 21003520which is approximately between the 2100 emission levels in the RCP6.0 and RCP8.5 pathways 3621 . Baseline scenarios exceed 450 ppm CO2-eq by 2030 and reach CO2-eq concentration levels between about 750 ppm CO2-eq and more than 1300 ppm CO2-eq by 2100. Global mean surface temperature increases in 2100 range from about 3.7°C to 4.8°C above the average for 1850–1900 for a median climate response. They range from 2.5°C to 7.8°C when including climate uncertainty 3722 . The future scenarios do not account for possible changes in natural forcings in the climate system. Many different combinations of technological, behavioural and policy options can be used to reduce emissions and limit temperature change. To evaluate possible pathways to long-term climate goals, about 900 mitigation scenarios were collected for this assessment, each of which describes different technological, socio-economic and institutional changes. Emission reductions under these scenarios lead to concentrations in 2100 from 430 ppm CO2-eq to above 720 ppm CO2-eq which is comparable to the 2100 forcing levels between RCP2.6 and RCP6.0. Scenarios with concentration levels of below 430 ppm CO2-eq by 2100 were also assessed. Scenarios leading to CO2-eq concentrations in 2100 of about 450 ppm or lower are likely to maintain warming below 2°C over the 21st century relative to pre-industrial levels. Mitigation scenarios reaching concentration levels of about 500 ppm CO2-eq by 2100 are more likely than not to limit warming to less than 2°C relative to pre-industrial levels, unless concentration levels temporarily exceed roughly 530 ppm CO2-eq before 2100. In this case, warming is about as likely as not to remain below 2°C relative to pre-industrial levels. Scenarios that exceed about 650 ppm CO2-eq by 2100 are unlikely to limit warming to below 2°C relative to pre-industrial levels. Mitigation scenarios in which warming is more likely than not to be less than 1.5°C relative to pre-industrial levels by 2100 are characterized by concentration levels by 2100 of below 430 ppm CO2-eq. In these scenarios, temperature peaks during the century and subsequently declines. Mitigation scenarios reaching about 450 ppm CO2-eq in 2100 typically involve temporary overshootof atmospheric concentrations, as do many scenarios reaching about 500 ppm CO2-eq to about 550 ppm CO2-eq by 2100. Depending on the level of overshoot, overshoot scenarios typically rely on the availability and widespread deployment of bioenergy with carbon dioxide capture and storage and afforestation in the second half of the century. The availability and scale of these and other Carbon Dioxide Removal technologies and methods are uncertain, and CDR technologies and methods are, to varying degrees, associated with challenges and risks 3924 . CDR is also prevalent in many scenarios without overshoot to compensate for residual emissions from sectors where mitigation is more expensive. Limiting warming with a likely chance to less than 2°C relative to pre-industrial levels would require substantial cuts in anthropogenic GHG emissions by mid-century through largescale changes in energy systems and possibly land use. Limiting warming to higher levels would require similar changes but less quickly. Limiting warming to lower levels would require these changes more quickly. Scenarios that are likely to maintain warming at below 2°C are characterized by a 40 to 70% reduction in GHG emissions by 2050, relative to 2010 levels, and emissions levels near zero or below in 2100. Scenarios with higher emissions in 2050 are characterized by a greater reliance on CDR technologies beyond mid-century, and vice versa. Scenarios that are likely to maintain warming at below 2°C include more rapid improvements in energy efficiency and a tripling to nearly a quadrupling of the share of zero- and low-carbon energy supply from renewable energy, nuclear energy and fossil energy with carbon dioxide capture and storage or BECCS by the year 2050. The scenarios describe a wide range of changes in land use, reflecting different assumptions about the scale of bioenergy production, afforestation and reduced deforestation. Scenarios leading to concentrations of 500 ppm CO2-eq by 2100 are characterized by a 25 to 55% reduction in GHG emissions by 2050, relative to 2010 levels. Scenarios that are likely to limit warming to 3°C relative to pre-industrial levels reduce emissions less rapidly than those limiting warming to 2°C. Only a limited number of studies provide scenarios that are more likely than not to limit warming to 1.5°C by 2100; these scenarios are characterized by concentrations below 430 ppm CO2-eq by 2100 and 2050 emission reduction between 70 and 95% below 2010. For a comprehensive overview of the characteristics of emissions scenarios, their CO2-equivalent concentrations and their likelihood to keep warming to below a range of temperature levels, see Table 3.1. Reducing emissions of non-CO2 climate forcing agents can be an important element of mitigation strategies. Emissions of nonCO2 gases, nitrous oxide, and fluorinated gases) contributed about 27% to the total emissions of Kyoto gases in 2010. For most non-CO2 gases, near-term, low-cost options are available to reduce their emissions. However, some sources of these non-CO2 gases are difficult to mitigate, such as N2O emissions from fertilizer use and CH4 emissions from livestock. As a result, emissions of most non-CO2 gases will not be reduced to zero, even under stringent mitigation scenarios. The differences in radiative properties and lifetimes of CO2 and non-CO2 climate forcing agents have important implications for mitigation strategies. All current GHG emissions and other climate forcing agents affect the rate and magnitude of climate change over the next few decades. Reducing the emissions of certain short-lived climate forcing agents can reduce the rate of warming in the short term but will have only a limited effect on long-term warming, which is driven mainly by CO2 emissions. There are large uncertainties related to the climate impacts of some of the short-lived climate forcing agents. Although the effects of CH4 emissions are well understood, there are large uncertainties related to the effects of black carbon. Co-emitted components with cooling effects may further complicate and reduce the climate impacts of emission reductions. Reducing emissions of sulfur dioxide would cause warming. Near-term reductions in short-lived climate forcing agents can have a relatively fast impact on climate change and possible co-benefits for air pollution. Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the 21st century to below 2°C relative to pre-industrial levels. GHG emissions in 2030 lie between about 30 GtCO2-eq/yr and 50 GtCO2-eq/yr in cost-effective scenarios that are likely to about as likely as not to limit warming to less than 2°C this century relative to pre-industrial levels . Scenarios with GHG emission levels of above 55 GtCO2-eq/yr require substantially higher rates of emissions reductions between 2030 and 2050 ; much more rapid scale-up of zero and low-carbon energy over this period ; a larger reliance on CDR technologies in the long term; and higher transitional and long-term economic impacts. Estimated global emission levels by 2020 based on the Cancún Pledges are not consistent with cost-effective long-term mitigation trajectories that are at least about as likely as not to limit warming to below 2°C relative to pre-industrial levels , but they do not preclude the option to meet this goal . The Cancún Pledges are broadly consistent with cost-effective scenarios that are likely to limit temperature change to below 3°C relative to pre-industrial levels. Estimates of the aggregate economic costs of mitigation vary widely depending on methodologies and assumptions but increase with the stringency of mitigation. Scenarios in which all countries of the world begin mitigation immediately, in which there is a single global carbon price, and in which all key technologies are available have been used as a cost-effective benchmark for estimating macroeconomic mitigation costs. Under these assumptions, mitigation scenarios that are likely to limit warming to below 2°C through the 21st century relative to pre-industrial levels entail losses in global consumption—not including benefits of reduced climate change as well as co-benefits and adverse side effects of mitigation —of 1 to 4% in 2030, 2 to 6% in 2050, and 3% to 11% in 2100, relative to consumption in baseline scenarios that grows anywhere from 300% to more than 900% over the century . These numbers correspond to an annualized reduction of consumption growth by 0.04 to 0.14 percentage points over the century relative to annualized consumption growth in the baseline that is between 1.6% and 3% per year. In the absence or under limited availability of mitigation technologies , mitigation costs can increase substantially depending on the technology considered . Delaying additional mitigation reduces near-term costs but increases mitigation costs in the medium- to long-term. Many models could not limit likely warming to below 2°C over the 21st century relative to pre-industrial levels, if additional mitigation is considerably delayed, or if availability of key technologies, such as bioenergy, CCS and their combination are limited . Mitigation efforts and associated cost are expected to vary across countries. The distribution of costs can differ from the distribution of the actions themselves. In globally cost-effective scenarios, the majority of mitigation efforts takes place in countries with the highest future GHG emissions in baseline scenarios. Some studies exploring particular effort-sharing frameworks, under the assumption of a global carbon market, have estimated substantial global financial flows associated with mitigation in scenarios that are likely to more unlikely than likely to limit warming during the 21st century to less than 2°C relative to pre-industrial levels. 3.2e: - Climate system responses Climate system properties that determine the response to external forcing have been estimated both from climate models and from analysis of past and recent climate change. The equilibrium climate sensitivity 3325is likely in the range 1.5°C to 4.5°C, extremely unlikely less than 1°C, and very unlikely greater than 6°C. Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond. Multiple lines of evidence indicate a strong and consistent near-linear relationship across all scenarios considered between net cumulative CO2 emissions and projected global temperature change to the year 2100. Past emissions and observed warming support this relationship within uncertainties. Any given level of warming is associated with a range of cumulative CO2 emissions , and therefore, for example, higher emissions in earlier decades imply lower emissions later. The global mean peak surface temperature change per trillion tonnes of carbon emitted as CO2 is likely in the range of 0.8°C to 2.5°C. This quantity, called the transient climate response to cumulative carbon emissions, is supported by both modelling and observational evidence and applies to cumulative emissions up to about 2000 GtC. Warming caused by CO2 emissions is effectively irreversible over multi-century timescales unless measures are taken to remove CO2 from the atmosphere. Ensuring CO2-induced warming remains likely less than 2°C requires cumulative CO2 emissions from all anthropogenic sources to remain below about 3650 GtCO2, over half of which were already emitted by 2011. Multi-model results show that limiting total human-induced warming to less than 2°C relative to the period 1861–1880 with a probability of >66% would require total CO2 emissions from all anthropogenic sources since 1870 to be limited to about 2900 GtCO2 when accounting for non-CO2 forcing as in the RCP2.6 scenario, with a range of 2550 to 3150 GtCO2 arising from variations in non-CO2 climate drivers across the scenarios considered by WGIII. About 1900 [1650 to 2150] GtCO2 were emitted by 2011, leaving about 1000 GtCO2 to be consistent with this temperature goal. Estimated total fossil carbon reserves exceed this remaining amount by a factor of 4 to 7, with resources much larger still. - Climate change risks reduced by adaptation and mitigation Without additional mitigation efforts beyond those in place today, and even with adaptation, warming by the end of the 21st century will lead to high to very high risk of severe, widespread and irreversible impacts globally. Mitigation involves some level of co-benefits and of risks due to adverse side effects, but these risks do not involve the same possibility of severe, widespread and irreversible impacts as risks from climate change, increasing the benefits from near-term mitigation efforts. The risks of climate change, adaptation and mitigation differ in nature, timescale, magnitude and persistence. Risks from adaptation include maladaptation and negative ancillary impacts. Risks from mitigation include possible adverse side effects of large-scale deployment of low-carbon technology options and economic costs. Climate change risks may persist for millennia and can involve very high risk of severe impacts and the presence of significant irreversibilities combined with limited adaptive capacity. In contrast, the stringency of climate policies can be adjusted much more quickly in response to observed consequences and costs and create lower risks of irreversible consequences. Mitigation and adaptation are complementary approaches for reducing risks of climate change impacts. They interact with one another and reduce risks over different timescales . Benefits from adaptation can already be realized in addressing current risks and can be realized in the future for addressing emerging risks. Adaptation has the potential to reduce climate change impacts over the next few decades, while mitigation has relatively little influence on climate outcomes over this timescale. Near-term and longerterm mitigation and adaptation, as well as development pathways, will determine the risks of climate change beyond mid-century. The potential for adaptation differs across sectors and will be limited by institutional and capacity constraints, increasing the long-term benefits of mitigation. The level of mitigation will influence the rate and magnitude of climate change, and greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits. Without additional mitigation efforts beyond those in place today, and even with adaptation, warming by the end of the 21st century will lead to high to very high risk of severe, widespread and irreversible impacts globally . Estimates of warming in 2100 without additional climate mitigation efforts are from 3.7°C to 4.8°C compared with pre-industrial levels; the range is 2.5°C to 7.8°C when using the 5th to 95th percentile range of the median climate response. The risks associated with temperatures at or above 4°C include severe and widespread impacts on unique and threatened systems, substantial species extinction, large risks to global and regional food security, consequential constraints on common human activities, increased likelihood of triggering tipping points and limited potential for adaptation in some cases . Some risks of climate change, such as risks to unique and threatened systems and risks associated with extreme weather events, are moderate to high at temperatures 1°C to 2°C above pre-industrial levels. Substantial cuts in GHG emissions over the next few decades can substantially reduce risks of climate change by limiting warming in the second half of the 21st century and beyond . Global mean surface warming is largely determined by cumulative emissions, which are, in turn, linked to emissions over different timescales. Limiting risks across Reasons For Concern would imply a limit for cumulative emissions of CO2. Such a limit would require that global net emissions of CO2 eventually decrease to zero. Reducing risks of climate change through mitigation would involve substantial cuts in GHG emissions over the next few decades. But some risks from residual damages are unavoidable, even with mitigation and adaptation. A subset of relevant climate change risks has been estimated using aggregate economic indicators. Such economic estimates have important limitations and are therefore a useful but insufficient basis for decision-making on long-term mitigation targets. Mitigation involves some level of co-benefits and risks, but these risks do not involve the same possibility of severe, widespread and irreversible impacts as risks from climate change . Scenarios that are likely to limit warming to below 2°C or even 3°C compared with pre-industrial temperatures involve large-scale changes in energy systems and potentially land use over the coming decades. Associated risks include those linked to large-scale deployment of technology options for producing low-carbon energy, the potential for high aggregate economic costs of mitigation and impacts on vulnerable countries and industries. Other risks and co-benefits are associated with human health, food security, energy security, poverty reduction, biodiversity conservation, water availability, income distribution, efficiency of taxation systems, labour supply and employment, urban sprawl, fossil fuel export revenues and the economic growth of developing countries. Inertia in the economic and climate systems and the possibility of irreversible impacts from climate change increase the benefits of near-term mitigation efforts. The actions taken today affect the options available in the future to reduce emissions, limit temperature change and adapt to climate change. Near-term choices can create, amplify or limit significant elements of lock-in that are important for decision-making. Lock-ins and irreversibilities occur in the climate system due to large inertia in some of its components such as heat transfer from the ocean surface to depth leading to continued ocean warming for centuries regardless of emission scenario and the irreversibility of a large fraction of anthropogenic climate change resulting from CO2 emissions on a multi-century to millennial timescale unless CO2 were to be removed from the atmosphere through large-scale human interventions over a sustained period . Irreversibilities in socio-economic and biological systems also result from infrastructure development and long-lived products and from climate change impacts, such as species extinction. The larger potential for irreversibility and pervasive impacts from climate change risks than from mitigation risks increases the benefit of shortterm mitigation efforts. Delays in additional mitigation or constraints on technological options limit the mitigation options and increase the long-term mitigation costs as well as other risks that would be incurred in the medium to long term to hold climate change impacts at a given level. - Characteristics of mitigation pathways There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to pre-industrial levels. These pathways would require substantial emissions reductions over the next few decades and near zero emissions of CO2 and other long-lived greenhouse gases by the end of the century. Implementing such reductions poses substantial technological, economic, social and institutional challenges, which increase with delays in additional mitigation and if key technologies are not available. Limiting warming to lower or higher levels involves similar challenges but on different timescales. Without additional efforts to reduce GHG emissions beyond those in place today, global emission growth is expected to persist driven by growth in global population and economic activities. Global GHG emissions under most scenarios without additional mitigation are between about 75 GtCO2-eq/yr and almost 140 GtCO2-eq/yr in 21003520which is approximately between the 2100 emission levels in the RCP6.0 and RCP8.5 pathways 3621 . Baseline scenarios exceed 450 ppm CO2-eq by 2030 and reach CO2-eq concentration levels between about 750 ppm CO2-eq and more than 1300 ppm CO2-eq by 2100. Global mean surface temperature increases in 2100 range from about 3.7°C to 4.8°C above the average for 1850–1900 for a median climate response. They range from 2.5°C to 7.8°C when including climate uncertainty 3722 . The future scenarios do not account for possible changes in natural forcings in the climate system. Many different combinations of technological, behavioural and policy options can be used to reduce emissions and limit temperature change. To evaluate possible pathways to long-term climate goals, about 900 mitigation scenarios were collected for this assessment, each of which describes different technological, socio-economic and institutional changes. Emission reductions under these scenarios lead to concentrations in 2100 from 430 ppm CO2-eq to above 720 ppm CO2-eq which is comparable to the 2100 forcing levels between RCP2.6 and RCP6.0. Scenarios with concentration levels of below 430 ppm CO2-eq by 2100 were also assessed. Scenarios leading to CO2-eq concentrations in 2100 of about 450 ppm or lower are likely to maintain warming below 2°C over the 21st century relative to pre-industrial levels. Mitigation scenarios reaching concentration levels of about 500 ppm CO2-eq by 2100 are more likely than not to limit warming to less than 2°C relative to pre-industrial levels, unless concentration levels temporarily exceed roughly 530 ppm CO2-eq before 2100. In this case, warming is about as likely as not to remain below 2°C relative to pre-industrial levels. Scenarios that exceed about 650 ppm CO2-eq by 2100 are unlikely to limit warming to below 2°C relative to pre-industrial levels. Mitigation scenarios in which warming is more likely than not to be less than 1.5°C relative to pre-industrial levels by 2100 are characterized by concentration levels by 2100 of below 430 ppm CO2-eq. In these scenarios, temperature peaks during the century and subsequently declines. Mitigation scenarios reaching about 450 ppm CO2-eq in 2100 typically involve temporary overshootof atmospheric concentrations, as do many scenarios reaching about 500 ppm CO2-eq to about 550 ppm CO2-eq by 2100. Depending on the level of overshoot, overshoot scenarios typically rely on the availability and widespread deployment of bioenergy with carbon dioxide capture and storage and afforestation in the second half of the century. The availability and scale of these and other Carbon Dioxide Removal technologies and methods are uncertain, and CDR technologies and methods are, to varying degrees, associated with challenges and risks 3924 . CDR is also prevalent in many scenarios without overshoot to compensate for residual emissions from sectors where mitigation is more expensive. Limiting warming with a likely chance to less than 2°C relative to pre-industrial levels would require substantial cuts in anthropogenic GHG emissions by mid-century through largescale changes in energy systems and possibly land use. Limiting warming to higher levels would require similar changes but less quickly. Limiting warming to lower levels would require these changes more quickly. Scenarios that are likely to maintain warming at below 2°C are characterized by a 40 to 70% reduction in GHG emissions by 2050, relative to 2010 levels, and emissions levels near zero or below in 2100. Scenarios with higher emissions in 2050 are characterized by a greater reliance on CDR technologies beyond mid-century, and vice versa. Scenarios that are likely to maintain warming at below 2°C include more rapid improvements in energy efficiency and a tripling to nearly a quadrupling of the share of zero- and low-carbon energy supply from renewable energy, nuclear energy and fossil energy with carbon dioxide capture and storage or BECCS by the year 2050. The scenarios describe a wide range of changes in land use, reflecting different assumptions about the scale of bioenergy production, afforestation and reduced deforestation. Scenarios leading to concentrations of 500 ppm CO2-eq by 2100 are characterized by a 25 to 55% reduction in GHG emissions by 2050, relative to 2010 levels. Scenarios that are likely to limit warming to 3°C relative to pre-industrial levels reduce emissions less rapidly than those limiting warming to 2°C. Only a limited number of studies provide scenarios that are more likely than not to limit warming to 1.5°C by 2100; these scenarios are characterized by concentrations below 430 ppm CO2-eq by 2100 and 2050 emission reduction between 70 and 95% below 2010. For a comprehensive overview of the characteristics of emissions scenarios, their CO2-equivalent concentrations and their likelihood to keep warming to below a range of temperature levels, see Table 3.1. Reducing emissions of non-CO2 climate forcing agents can be an important element of mitigation strategies. Emissions of nonCO2 gases, nitrous oxide, and fluorinated gases) contributed about 27% to the total emissions of Kyoto gases in 2010. For most non-CO2 gases, near-term, low-cost options are available to reduce their emissions. However, some sources of these non-CO2 gases are difficult to mitigate, such as N2O emissions from fertilizer use and CH4 emissions from livestock. As a result, emissions of most non-CO2 gases will not be reduced to zero, even under stringent mitigation scenarios. The differences in radiative properties and lifetimes of CO2 and non-CO2 climate forcing agents have important implications for mitigation strategies. All current GHG emissions and other climate forcing agents affect the rate and magnitude of climate change over the next few decades. Reducing the emissions of certain short-lived climate forcing agents can reduce the rate of warming in the short term but will have only a limited effect on long-term warming, which is driven mainly by CO2 emissions. There are large uncertainties related to the climate impacts of some of the short-lived climate forcing agents. Although the effects of CH4 emissions are well understood, there are large uncertainties related to the effects of black carbon. Co-emitted components with cooling effects may further complicate and reduce the climate impacts of emission reductions. Reducing emissions of sulfur dioxide would cause warming. Near-term reductions in short-lived climate forcing agents can have a relatively fast impact on climate change and possible co-benefits for air pollution. Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the 21st century to below 2°C relative to pre-industrial levels. GHG emissions in 2030 lie between about 30 GtCO2-eq/yr and 50 GtCO2-eq/yr in cost-effective scenarios that are likely to about as likely as not to limit warming to less than 2°C this century relative to pre-industrial levels . Scenarios with GHG emission levels of above 55 GtCO2-eq/yr require substantially higher rates of emissions reductions between 2030 and 2050 ; much more rapid scale-up of zero and low-carbon energy over this period ; a larger reliance on CDR technologies in the long term; and higher transitional and long-term economic impacts. Estimated global emission levels by 2020 based on the Cancún Pledges are not consistent with cost-effective long-term mitigation trajectories that are at least about as likely as not to limit warming to below 2°C relative to pre-industrial levels , but they do not preclude the option to meet this goal . The Cancún Pledges are broadly consistent with cost-effective scenarios that are likely to limit temperature change to below 3°C relative to pre-industrial levels. Estimates of the aggregate economic costs of mitigation vary widely depending on methodologies and assumptions but increase with the stringency of mitigation. Scenarios in which all countries of the world begin mitigation immediately, in which there is a single global carbon price, and in which all key technologies are available have been used as a cost-effective benchmark for estimating macroeconomic mitigation costs. Under these assumptions, mitigation scenarios that are likely to limit warming to below 2°C through the 21st century relative to pre-industrial levels entail losses in global consumption—not including benefits of reduced climate change as well as co-benefits and adverse side effects of mitigation —of 1 to 4% in 2030, 2 to 6% in 2050, and 3% to 11% in 2100, relative to consumption in baseline scenarios that grows anywhere from 300% to more than 900% over the century . These numbers correspond to an annualized reduction of consumption growth by 0.04 to 0.14 percentage points over the century relative to annualized consumption growth in the baseline that is between 1.6% and 3% per year. In the absence or under limited availability of mitigation technologies , mitigation costs can increase substantially depending on the technology considered . Delaying additional mitigation reduces near-term costs but increases mitigation costs in the medium- to long-term. Many models could not limit likely warming to below 2°C over the 21st century relative to pre-industrial levels, if additional mitigation is considerably delayed, or if availability of key technologies, such as bioenergy, CCS and their combination are limited . Mitigation efforts and associated cost are expected to vary across countries. The distribution of costs can differ from the distribution of the actions themselves. In globally cost-effective scenarios, the majority of mitigation efforts takes place in countries with the highest future GHG emissions in baseline scenarios. Some studies exploring particular effort-sharing frameworks, under the assumption of a global carbon market, have estimated substantial global financial flows associated with mitigation in scenarios that are likely to more unlikely than likely to limit warming during the 21st century to less than 2°C relative to pre-industrial levels. 3.2f: - Climate change risks reduced by adaptation and mitigation Without additional mitigation efforts beyond those in place today, and even with adaptation, warming by the end of the 21st century will lead to high to very high risk of severe, widespread and irreversible impacts globally. Mitigation involves some level of co-benefits and of risks due to adverse side effects, but these risks do not involve the same possibility of severe, widespread and irreversible impacts as risks from climate change, increasing the benefits from near-term mitigation efforts. The risks of climate change, adaptation and mitigation differ in nature, timescale, magnitude and persistence. Risks from adaptation include maladaptation and negative ancillary impacts. Risks from mitigation include possible adverse side effects of large-scale deployment of low-carbon technology options and economic costs. Climate change risks may persist for millennia and can involve very high risk of severe impacts and the presence of significant irreversibilities combined with limited adaptive capacity. In contrast, the stringency of climate policies can be adjusted much more quickly in response to observed consequences and costs and create lower risks of irreversible consequences. Mitigation and adaptation are complementary approaches for reducing risks of climate change impacts. They interact with one another and reduce risks over different timescales . Benefits from adaptation can already be realized in addressing current risks and can be realized in the future for addressing emerging risks. Adaptation has the potential to reduce climate change impacts over the next few decades, while mitigation has relatively little influence on climate outcomes over this timescale. Near-term and longerterm mitigation and adaptation, as well as development pathways, will determine the risks of climate change beyond mid-century. The potential for adaptation differs across sectors and will be limited by institutional and capacity constraints, increasing the long-term benefits of mitigation. The level of mitigation will influence the rate and magnitude of climate change, and greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits. Without additional mitigation efforts beyond those in place today, and even with adaptation, warming by the end of the 21st century will lead to high to very high risk of severe, widespread and irreversible impacts globally . Estimates of warming in 2100 without additional climate mitigation efforts are from 3.7°C to 4.8°C compared with pre-industrial levels; the range is 2.5°C to 7.8°C when using the 5th to 95th percentile range of the median climate response. The risks associated with temperatures at or above 4°C include severe and widespread impacts on unique and threatened systems, substantial species extinction, large risks to global and regional food security, consequential constraints on common human activities, increased likelihood of triggering tipping points and limited potential for adaptation in some cases . Some risks of climate change, such as risks to unique and threatened systems and risks associated with extreme weather events, are moderate to high at temperatures 1°C to 2°C above pre-industrial levels. Substantial cuts in GHG emissions over the next few decades can substantially reduce risks of climate change by limiting warming in the second half of the 21st century and beyond . Global mean surface warming is largely determined by cumulative emissions, which are, in turn, linked to emissions over different timescales. Limiting risks across Reasons For Concern would imply a limit for cumulative emissions of CO2. Such a limit would require that global net emissions of CO2 eventually decrease to zero. Reducing risks of climate change through mitigation would involve substantial cuts in GHG emissions over the next few decades. But some risks from residual damages are unavoidable, even with mitigation and adaptation. A subset of relevant climate change risks has been estimated using aggregate economic indicators. Such economic estimates have important limitations and are therefore a useful but insufficient basis for decision-making on long-term mitigation targets. Mitigation involves some level of co-benefits and risks, but these risks do not involve the same possibility of severe, widespread and irreversible impacts as risks from climate change . Scenarios that are likely to limit warming to below 2°C or even 3°C compared with pre-industrial temperatures involve large-scale changes in energy systems and potentially land use over the coming decades. Associated risks include those linked to large-scale deployment of technology options for producing low-carbon energy, the potential for high aggregate economic costs of mitigation and impacts on vulnerable countries and industries. Other risks and co-benefits are associated with human health, food security, energy security, poverty reduction, biodiversity conservation, water availability, income distribution, efficiency of taxation systems, labour supply and employment, urban sprawl, fossil fuel export revenues and the economic growth of developing countries. Inertia in the economic and climate systems and the possibility of irreversible impacts from climate change increase the benefits of near-term mitigation efforts. The actions taken today affect the options available in the future to reduce emissions, limit temperature change and adapt to climate change. Near-term choices can create, amplify or limit significant elements of lock-in that are important for decision-making. Lock-ins and irreversibilities occur in the climate system due to large inertia in some of its components such as heat transfer from the ocean surface to depth leading to continued ocean warming for centuries regardless of emission scenario and the irreversibility of a large fraction of anthropogenic climate change resulting from CO2 emissions on a multi-century to millennial timescale unless CO2 were to be removed from the atmosphere through large-scale human interventions over a sustained period . Irreversibilities in socio-economic and biological systems also result from infrastructure development and long-lived products and from climate change impacts, such as species extinction. The larger potential for irreversibility and pervasive impacts from climate change risks than from mitigation risks increases the benefit of shortterm mitigation efforts. Delays in additional mitigation or constraints on technological options limit the mitigation options and increase the long-term mitigation costs as well as other risks that would be incurred in the medium to long term to hold climate change impacts at a given level. - Characteristics of mitigation pathways There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to pre-industrial levels. These pathways would require substantial emissions reductions over the next few decades and near zero emissions of CO2 and other long-lived greenhouse gases by the end of the century. Implementing such reductions poses substantial technological, economic, social and institutional challenges, which increase with delays in additional mitigation and if key technologies are not available. Limiting warming to lower or higher levels involves similar challenges but on different timescales. Without additional efforts to reduce GHG emissions beyond those in place today, global emission growth is expected to persist driven by growth in global population and economic activities. Global GHG emissions under most scenarios without additional mitigation are between about 75 GtCO2-eq/yr and almost 140 GtCO2-eq/yr in 21003520which is approximately between the 2100 emission levels in the RCP6.0 and RCP8.5 pathways 3621 . Baseline scenarios exceed 450 ppm CO2-eq by 2030 and reach CO2-eq concentration levels between about 750 ppm CO2-eq and more than 1300 ppm CO2-eq by 2100. Global mean surface temperature increases in 2100 range from about 3.7°C to 4.8°C above the average for 1850–1900 for a median climate response. They range from 2.5°C to 7.8°C when including climate uncertainty 3722 . The future scenarios do not account for possible changes in natural forcings in the climate system. Many different combinations of technological, behavioural and policy options can be used to reduce emissions and limit temperature change. To evaluate possible pathways to long-term climate goals, about 900 mitigation scenarios were collected for this assessment, each of which describes different technological, socio-economic and institutional changes. Emission reductions under these scenarios lead to concentrations in 2100 from 430 ppm CO2-eq to above 720 ppm CO2-eq which is comparable to the 2100 forcing levels between RCP2.6 and RCP6.0. Scenarios with concentration levels of below 430 ppm CO2-eq by 2100 were also assessed. Scenarios leading to CO2-eq concentrations in 2100 of about 450 ppm or lower are likely to maintain warming below 2°C over the 21st century relative to pre-industrial levels. Mitigation scenarios reaching concentration levels of about 500 ppm CO2-eq by 2100 are more likely than not to limit warming to less than 2°C relative to pre-industrial levels, unless concentration levels temporarily exceed roughly 530 ppm CO2-eq before 2100. In this case, warming is about as likely as not to remain below 2°C relative to pre-industrial levels. Scenarios that exceed about 650 ppm CO2-eq by 2100 are unlikely to limit warming to below 2°C relative to pre-industrial levels. Mitigation scenarios in which warming is more likely than not to be less than 1.5°C relative to pre-industrial levels by 2100 are characterized by concentration levels by 2100 of below 430 ppm CO2-eq. In these scenarios, temperature peaks during the century and subsequently declines. Mitigation scenarios reaching about 450 ppm CO2-eq in 2100 typically involve temporary overshootof atmospheric concentrations, as do many scenarios reaching about 500 ppm CO2-eq to about 550 ppm CO2-eq by 2100. Depending on the level of overshoot, overshoot scenarios typically rely on the availability and widespread deployment of bioenergy with carbon dioxide capture and storage and afforestation in the second half of the century. The availability and scale of these and other Carbon Dioxide Removal technologies and methods are uncertain, and CDR technologies and methods are, to varying degrees, associated with challenges and risks 3924 . CDR is also prevalent in many scenarios without overshoot to compensate for residual emissions from sectors where mitigation is more expensive. Limiting warming with a likely chance to less than 2°C relative to pre-industrial levels would require substantial cuts in anthropogenic GHG emissions by mid-century through largescale changes in energy systems and possibly land use. Limiting warming to higher levels would require similar changes but less quickly. Limiting warming to lower levels would require these changes more quickly. Scenarios that are likely to maintain warming at below 2°C are characterized by a 40 to 70% reduction in GHG emissions by 2050, relative to 2010 levels, and emissions levels near zero or below in 2100. Scenarios with higher emissions in 2050 are characterized by a greater reliance on CDR technologies beyond mid-century, and vice versa. Scenarios that are likely to maintain warming at below 2°C include more rapid improvements in energy efficiency and a tripling to nearly a quadrupling of the share of zero- and low-carbon energy supply from renewable energy, nuclear energy and fossil energy with carbon dioxide capture and storage or BECCS by the year 2050. The scenarios describe a wide range of changes in land use, reflecting different assumptions about the scale of bioenergy production, afforestation and reduced deforestation. Scenarios leading to concentrations of 500 ppm CO2-eq by 2100 are characterized by a 25 to 55% reduction in GHG emissions by 2050, relative to 2010 levels. Scenarios that are likely to limit warming to 3°C relative to pre-industrial levels reduce emissions less rapidly than those limiting warming to 2°C. Only a limited number of studies provide scenarios that are more likely than not to limit warming to 1.5°C by 2100; these scenarios are characterized by concentrations below 430 ppm CO2-eq by 2100 and 2050 emission reduction between 70 and 95% below 2010. For a comprehensive overview of the characteristics of emissions scenarios, their CO2-equivalent concentrations and their likelihood to keep warming to below a range of temperature levels, see Table 3.1. Reducing emissions of non-CO2 climate forcing agents can be an important element of mitigation strategies. Emissions of nonCO2 gases, nitrous oxide, and fluorinated gases) contributed about 27% to the total emissions of Kyoto gases in 2010. For most non-CO2 gases, near-term, low-cost options are available to reduce their emissions. However, some sources of these non-CO2 gases are difficult to mitigate, such as N2O emissions from fertilizer use and CH4 emissions from livestock. As a result, emissions of most non-CO2 gases will not be reduced to zero, even under stringent mitigation scenarios. The differences in radiative properties and lifetimes of CO2 and non-CO2 climate forcing agents have important implications for mitigation strategies. All current GHG emissions and other climate forcing agents affect the rate and magnitude of climate change over the next few decades. Reducing the emissions of certain short-lived climate forcing agents can reduce the rate of warming in the short term but will have only a limited effect on long-term warming, which is driven mainly by CO2 emissions. There are large uncertainties related to the climate impacts of some of the short-lived climate forcing agents. Although the effects of CH4 emissions are well understood, there are large uncertainties related to the effects of black carbon. Co-emitted components with cooling effects may further complicate and reduce the climate impacts of emission reductions. Reducing emissions of sulfur dioxide would cause warming. Near-term reductions in short-lived climate forcing agents can have a relatively fast impact on climate change and possible co-benefits for air pollution. Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the 21st century to below 2°C relative to pre-industrial levels. GHG emissions in 2030 lie between about 30 GtCO2-eq/yr and 50 GtCO2-eq/yr in cost-effective scenarios that are likely to about as likely as not to limit warming to less than 2°C this century relative to pre-industrial levels . Scenarios with GHG emission levels of above 55 GtCO2-eq/yr require substantially higher rates of emissions reductions between 2030 and 2050 ; much more rapid scale-up of zero and low-carbon energy over this period ; a larger reliance on CDR technologies in the long term; and higher transitional and long-term economic impacts. Estimated global emission levels by 2020 based on the Cancún Pledges are not consistent with cost-effective long-term mitigation trajectories that are at least about as likely as not to limit warming to below 2°C relative to pre-industrial levels , but they do not preclude the option to meet this goal . The Cancún Pledges are broadly consistent with cost-effective scenarios that are likely to limit temperature change to below 3°C relative to pre-industrial levels. Estimates of the aggregate economic costs of mitigation vary widely depending on methodologies and assumptions but increase with the stringency of mitigation. Scenarios in which all countries of the world begin mitigation immediately, in which there is a single global carbon price, and in which all key technologies are available have been used as a cost-effective benchmark for estimating macroeconomic mitigation costs. Under these assumptions, mitigation scenarios that are likely to limit warming to below 2°C through the 21st century relative to pre-industrial levels entail losses in global consumption—not including benefits of reduced climate change as well as co-benefits and adverse side effects of mitigation —of 1 to 4% in 2030, 2 to 6% in 2050, and 3% to 11% in 2100, relative to consumption in baseline scenarios that grows anywhere from 300% to more than 900% over the century . These numbers correspond to an annualized reduction of consumption growth by 0.04 to 0.14 percentage points over the century relative to annualized consumption growth in the baseline that is between 1.6% and 3% per year. In the absence or under limited availability of mitigation technologies , mitigation costs can increase substantially depending on the technology considered . Delaying additional mitigation reduces near-term costs but increases mitigation costs in the medium- to long-term. Many models could not limit likely warming to below 2°C over the 21st century relative to pre-industrial levels, if additional mitigation is considerably delayed, or if availability of key technologies, such as bioenergy, CCS and their combination are limited . Mitigation efforts and associated cost are expected to vary across countries. The distribution of costs can differ from the distribution of the actions themselves. In globally cost-effective scenarios, the majority of mitigation efforts takes place in countries with the highest future GHG emissions in baseline scenarios. Some studies exploring particular effort-sharing frameworks, under the assumption of a global carbon market, have estimated substantial global financial flows associated with mitigation in scenarios that are likely to more unlikely than likely to limit warming during the 21st century to less than 2°C relative to pre-industrial levels. 3.3a: - 'Characteristics of adaptation pathways Adaptation can reduce the risks of climate change impacts, but there are limits to its effectiveness, especially with greater magnitudes and rates of climate change. Taking a longer-term perspective, in the context of sustainable development, increases the likelihood that more immediate adaptation actions will also enhance future options and preparedness. Adaptation can contribute to the well-being of current and future populations, the security of assets and the maintenance of ecosystem goods, functions and services now and in the future. Adaptation is place- and context-specific, with no single approach for reducing risks appropriate across all settings . Effective risk reduction and adaptation strategies consider vulnerability and exposure and their linkages with socio-economic processes, sustainable development, and climate change. Adaptation research since the IPCC Fourth Assessment Report has evolved from a dominant consideration of engineering and technological adaptation pathways to include more ecosystem-based, institutional and social measures. A previous focus on cost–benefit analysis, optimization and efficiency approaches has broadened with the development of multi-metric evaluations that include risk and uncertainty dimensions integrated within wider policy and ethical frameworks to assess tradeoffs and constraints. The range of specific adaptation measures has also expanded, as have the links to sustainable development. There are many studies on local and sectoral adaptation costs and benefits, but few global analyses and very low confidence in their results. Adaptation planning and implementation at all levels of governance are contingent on societal values, objectives and risk perceptions. Recognition of diverse interests, circumstances, social-cultural contexts and expectations can benefit decision-making processes. Indigenous, local and traditional knowledge systems and practices, including indigenous peoples’ holistic view of community and environment, are a major resource for adapting to climate change, but these have not been used consistently in existing adaptation efforts. Integrating such forms of knowledge into practices increases the effectiveness of adaptation as do effective decision support, engagement and policy processes. Adaptation planning and implementation can be enhanced through complementary actions across levels, from individuals to governments. National governments can coordinate adaptation efforts of local and sub-national governments, for example by protecting vulnerable groups, by supporting economic diversification and by providing information, policy and legal frameworks and financial support. Local government and the private sector are increasingly recognized as critical to progress in adaptation, given their roles in scaling up adaptation of communities, households and civil society and in managing risk information and financing. A first step towards adaptation to future climate change is reducing vulnerability and exposure to present climate variability, but some near-term responses to climate change may also limit future choices. Integration of adaptation into planning, including policy design, and decision-making can promote synergies with development and disaster risk reduction. However, poor planning or implementation, overemphasizing short-term outcomes or failing to sufficiently anticipate consequences can result in maladaptation, increasing the vulnerability or exposure of the target group in the future or the vulnerability of other people, places or sectors. For example, enhanced protection of exposed assets can lock in dependence on further protection measures. Appropriate adaptation options can be better assessed by including co-benefits and mitigation implications. Numerous interacting constraints can impede adaptation planning and implementation. Common constraints on implementation arise from the following: limited financial and human resources; limited integration or coordination of governance; uncertainties about projected impacts; different perceptions of risks; competing values; absence of key adaptation leaders and advocates; and limited tools to monitor adaptation effectiveness. Other constraints include insufficient research, monitoring and observation and the financial and other resources to maintain them. Underestimating the complexity of adaptation as a social process can create unrealistic expectations about achieving intended adaptation outcomes . Greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits. Limits to adaptation occur when adaptive actions to avoid intolerable risks for an actor’s objectives or for the needs of a system are not possible or are not currently available. Value-based judgments of what constitutes an intolerable risk may differ. Limits to adaptation emerge from the interaction among climate change and biophysical and/or socio-economic constraints. Opportunities to take advantage of positive synergies between adaptation and mitigation may decrease with time, particularly if limits to adaptation are exceeded. In some parts of the world, insufficient responses to emerging impacts are already eroding the basis for sustainable development. For most regions and sectors, empirical evidence is not sufficient to quantify magnitudes of climate change that would constitute a future adaptation limit. Furthermore, economic development, technology and cultural norms and values can change over time to enhance or reduce the capacity of systems to avoid limits. As a consequence, some limits are ‘soft’ in that they may be alleviated over time. Other limits are ‘hard’ in that there are no reasonable prospects for avoiding intolerable risks. Transformations in economic, social, technological and political decisions and actions can enhance adaptation and promote sustainable development. Restricting adaptation responses to incremental changes to existing systems and structures without considering transformational change may increase costs and losses and miss opportunities. For example, enhancing infrastructure to protect other built assets can be expensive and ultimately not defray increasing costs and risks, whereas options such as relocation or using ecosystem services to adapt may provide a range of benefits now and in the future. Transformational adaptation can include introduction of new technologies or practices, formation of new financial structures or systems of governance, adaptation at greater scales or magnitudes and shifts in the location of activities. Planning and implementation of transformational adaptation could reflect strengthened, altered or aligned paradigms and consequently may place new and increased demands on governance structures to reconcile different goals and visions for the future and to address possible equity and ethical implications: transformational adaptation pathways are enhanced by iterative learning, deliberative processes, and innovation. At the national level, transformation is considered most effective when it reflects a country’s own visions and approaches to achieving sustainable development in accordance with its national circumstances and priorities. Building adaptive capacity is crucial for effective selection and implementation of adaptation options . Successful adaptation requires not only identifying adaptation options and assessing their costs and benefits, but also increasing the adaptive capacity of human and natural systems . This can involve complex governance challenges and new institutions and institutional arrangements. Significant co-benefits, synergies and trade-offs exist between mitigation and adaptation and among different adaptation responses; interactions occur both within and across regions . Increasing efforts to mitigate and adapt to climate change imply an increasing complexity of interactions, particularly at the intersections among water, energy, land use and biodiversity, but tools to understand and manage these interactions remain limited. Examples of actions with co-benefits include improved energy efficiency and cleaner energy sources, leading to reduced emissions of health-damaging, climate-altering air pollutants; reduced energy and water consumption in urban areas through greening cities and recycling water; sustainable agriculture and forestry; and protection of ecosystems for carbon storage and other ecosystem services.' 3.3b: - 'Characteristics of adaptation pathways Adaptation can reduce the risks of climate change impacts, but there are limits to its effectiveness, especially with greater magnitudes and rates of climate change. Taking a longer-term perspective, in the context of sustainable development, increases the likelihood that more immediate adaptation actions will also enhance future options and preparedness. Adaptation can contribute to the well-being of current and future populations, the security of assets and the maintenance of ecosystem goods, functions and services now and in the future. Adaptation is place- and context-specific, with no single approach for reducing risks appropriate across all settings . Effective risk reduction and adaptation strategies consider vulnerability and exposure and their linkages with socio-economic processes, sustainable development, and climate change. Adaptation research since the IPCC Fourth Assessment Report has evolved from a dominant consideration of engineering and technological adaptation pathways to include more ecosystem-based, institutional and social measures. A previous focus on cost–benefit analysis, optimization and efficiency approaches has broadened with the development of multi-metric evaluations that include risk and uncertainty dimensions integrated within wider policy and ethical frameworks to assess tradeoffs and constraints. The range of specific adaptation measures has also expanded, as have the links to sustainable development. There are many studies on local and sectoral adaptation costs and benefits, but few global analyses and very low confidence in their results. Adaptation planning and implementation at all levels of governance are contingent on societal values, objectives and risk perceptions. Recognition of diverse interests, circumstances, social-cultural contexts and expectations can benefit decision-making processes. Indigenous, local and traditional knowledge systems and practices, including indigenous peoples’ holistic view of community and environment, are a major resource for adapting to climate change, but these have not been used consistently in existing adaptation efforts. Integrating such forms of knowledge into practices increases the effectiveness of adaptation as do effective decision support, engagement and policy processes. Adaptation planning and implementation can be enhanced through complementary actions across levels, from individuals to governments. National governments can coordinate adaptation efforts of local and sub-national governments, for example by protecting vulnerable groups, by supporting economic diversification and by providing information, policy and legal frameworks and financial support. Local government and the private sector are increasingly recognized as critical to progress in adaptation, given their roles in scaling up adaptation of communities, households and civil society and in managing risk information and financing. A first step towards adaptation to future climate change is reducing vulnerability and exposure to present climate variability, but some near-term responses to climate change may also limit future choices. Integration of adaptation into planning, including policy design, and decision-making can promote synergies with development and disaster risk reduction. However, poor planning or implementation, overemphasizing short-term outcomes or failing to sufficiently anticipate consequences can result in maladaptation, increasing the vulnerability or exposure of the target group in the future or the vulnerability of other people, places or sectors. For example, enhanced protection of exposed assets can lock in dependence on further protection measures. Appropriate adaptation options can be better assessed by including co-benefits and mitigation implications. Numerous interacting constraints can impede adaptation planning and implementation. Common constraints on implementation arise from the following: limited financial and human resources; limited integration or coordination of governance; uncertainties about projected impacts; different perceptions of risks; competing values; absence of key adaptation leaders and advocates; and limited tools to monitor adaptation effectiveness. Other constraints include insufficient research, monitoring and observation and the financial and other resources to maintain them. Underestimating the complexity of adaptation as a social process can create unrealistic expectations about achieving intended adaptation outcomes . Greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits. Limits to adaptation occur when adaptive actions to avoid intolerable risks for an actor’s objectives or for the needs of a system are not possible or are not currently available. Value-based judgments of what constitutes an intolerable risk may differ. Limits to adaptation emerge from the interaction among climate change and biophysical and/or socio-economic constraints. Opportunities to take advantage of positive synergies between adaptation and mitigation may decrease with time, particularly if limits to adaptation are exceeded. In some parts of the world, insufficient responses to emerging impacts are already eroding the basis for sustainable development. For most regions and sectors, empirical evidence is not sufficient to quantify magnitudes of climate change that would constitute a future adaptation limit. Furthermore, economic development, technology and cultural norms and values can change over time to enhance or reduce the capacity of systems to avoid limits. As a consequence, some limits are ‘soft’ in that they may be alleviated over time. Other limits are ‘hard’ in that there are no reasonable prospects for avoiding intolerable risks. Transformations in economic, social, technological and political decisions and actions can enhance adaptation and promote sustainable development. Restricting adaptation responses to incremental changes to existing systems and structures without considering transformational change may increase costs and losses and miss opportunities. For example, enhancing infrastructure to protect other built assets can be expensive and ultimately not defray increasing costs and risks, whereas options such as relocation or using ecosystem services to adapt may provide a range of benefits now and in the future. Transformational adaptation can include introduction of new technologies or practices, formation of new financial structures or systems of governance, adaptation at greater scales or magnitudes and shifts in the location of activities. Planning and implementation of transformational adaptation could reflect strengthened, altered or aligned paradigms and consequently may place new and increased demands on governance structures to reconcile different goals and visions for the future and to address possible equity and ethical implications: transformational adaptation pathways are enhanced by iterative learning, deliberative processes, and innovation. At the national level, transformation is considered most effective when it reflects a country’s own visions and approaches to achieving sustainable development in accordance with its national circumstances and priorities. Building adaptive capacity is crucial for effective selection and implementation of adaptation options . Successful adaptation requires not only identifying adaptation options and assessing their costs and benefits, but also increasing the adaptive capacity of human and natural systems . This can involve complex governance challenges and new institutions and institutional arrangements. Significant co-benefits, synergies and trade-offs exist between mitigation and adaptation and among different adaptation responses; interactions occur both within and across regions . Increasing efforts to mitigate and adapt to climate change imply an increasing complexity of interactions, particularly at the intersections among water, energy, land use and biodiversity, but tools to understand and manage these interactions remain limited. Examples of actions with co-benefits include improved energy efficiency and cleaner energy sources, leading to reduced emissions of health-damaging, climate-altering air pollutants; reduced energy and water consumption in urban areas through greening cities and recycling water; sustainable agriculture and forestry; and protection of ecosystems for carbon storage and other ecosystem services.' 3.3c: - 'Characteristics of adaptation pathways Adaptation can reduce the risks of climate change impacts, but there are limits to its effectiveness, especially with greater magnitudes and rates of climate change. Taking a longer-term perspective, in the context of sustainable development, increases the likelihood that more immediate adaptation actions will also enhance future options and preparedness. Adaptation can contribute to the well-being of current and future populations, the security of assets and the maintenance of ecosystem goods, functions and services now and in the future. Adaptation is place- and context-specific, with no single approach for reducing risks appropriate across all settings . Effective risk reduction and adaptation strategies consider vulnerability and exposure and their linkages with socio-economic processes, sustainable development, and climate change. Adaptation research since the IPCC Fourth Assessment Report has evolved from a dominant consideration of engineering and technological adaptation pathways to include more ecosystem-based, institutional and social measures. A previous focus on cost–benefit analysis, optimization and efficiency approaches has broadened with the development of multi-metric evaluations that include risk and uncertainty dimensions integrated within wider policy and ethical frameworks to assess tradeoffs and constraints. The range of specific adaptation measures has also expanded, as have the links to sustainable development. There are many studies on local and sectoral adaptation costs and benefits, but few global analyses and very low confidence in their results. Adaptation planning and implementation at all levels of governance are contingent on societal values, objectives and risk perceptions. Recognition of diverse interests, circumstances, social-cultural contexts and expectations can benefit decision-making processes. Indigenous, local and traditional knowledge systems and practices, including indigenous peoples’ holistic view of community and environment, are a major resource for adapting to climate change, but these have not been used consistently in existing adaptation efforts. Integrating such forms of knowledge into practices increases the effectiveness of adaptation as do effective decision support, engagement and policy processes. Adaptation planning and implementation can be enhanced through complementary actions across levels, from individuals to governments. National governments can coordinate adaptation efforts of local and sub-national governments, for example by protecting vulnerable groups, by supporting economic diversification and by providing information, policy and legal frameworks and financial support. Local government and the private sector are increasingly recognized as critical to progress in adaptation, given their roles in scaling up adaptation of communities, households and civil society and in managing risk information and financing. A first step towards adaptation to future climate change is reducing vulnerability and exposure to present climate variability, but some near-term responses to climate change may also limit future choices. Integration of adaptation into planning, including policy design, and decision-making can promote synergies with development and disaster risk reduction. However, poor planning or implementation, overemphasizing short-term outcomes or failing to sufficiently anticipate consequences can result in maladaptation, increasing the vulnerability or exposure of the target group in the future or the vulnerability of other people, places or sectors. For example, enhanced protection of exposed assets can lock in dependence on further protection measures. Appropriate adaptation options can be better assessed by including co-benefits and mitigation implications. Numerous interacting constraints can impede adaptation planning and implementation. Common constraints on implementation arise from the following: limited financial and human resources; limited integration or coordination of governance; uncertainties about projected impacts; different perceptions of risks; competing values; absence of key adaptation leaders and advocates; and limited tools to monitor adaptation effectiveness. Other constraints include insufficient research, monitoring and observation and the financial and other resources to maintain them. Underestimating the complexity of adaptation as a social process can create unrealistic expectations about achieving intended adaptation outcomes . Greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits. Limits to adaptation occur when adaptive actions to avoid intolerable risks for an actor’s objectives or for the needs of a system are not possible or are not currently available. Value-based judgments of what constitutes an intolerable risk may differ. Limits to adaptation emerge from the interaction among climate change and biophysical and/or socio-economic constraints. Opportunities to take advantage of positive synergies between adaptation and mitigation may decrease with time, particularly if limits to adaptation are exceeded. In some parts of the world, insufficient responses to emerging impacts are already eroding the basis for sustainable development. For most regions and sectors, empirical evidence is not sufficient to quantify magnitudes of climate change that would constitute a future adaptation limit. Furthermore, economic development, technology and cultural norms and values can change over time to enhance or reduce the capacity of systems to avoid limits. As a consequence, some limits are ‘soft’ in that they may be alleviated over time. Other limits are ‘hard’ in that there are no reasonable prospects for avoiding intolerable risks. Transformations in economic, social, technological and political decisions and actions can enhance adaptation and promote sustainable development. Restricting adaptation responses to incremental changes to existing systems and structures without considering transformational change may increase costs and losses and miss opportunities. For example, enhancing infrastructure to protect other built assets can be expensive and ultimately not defray increasing costs and risks, whereas options such as relocation or using ecosystem services to adapt may provide a range of benefits now and in the future. Transformational adaptation can include introduction of new technologies or practices, formation of new financial structures or systems of governance, adaptation at greater scales or magnitudes and shifts in the location of activities. Planning and implementation of transformational adaptation could reflect strengthened, altered or aligned paradigms and consequently may place new and increased demands on governance structures to reconcile different goals and visions for the future and to address possible equity and ethical implications: transformational adaptation pathways are enhanced by iterative learning, deliberative processes, and innovation. At the national level, transformation is considered most effective when it reflects a country’s own visions and approaches to achieving sustainable development in accordance with its national circumstances and priorities. Building adaptive capacity is crucial for effective selection and implementation of adaptation options . Successful adaptation requires not only identifying adaptation options and assessing their costs and benefits, but also increasing the adaptive capacity of human and natural systems . This can involve complex governance challenges and new institutions and institutional arrangements. Significant co-benefits, synergies and trade-offs exist between mitigation and adaptation and among different adaptation responses; interactions occur both within and across regions . Increasing efforts to mitigate and adapt to climate change imply an increasing complexity of interactions, particularly at the intersections among water, energy, land use and biodiversity, but tools to understand and manage these interactions remain limited. Examples of actions with co-benefits include improved energy efficiency and cleaner energy sources, leading to reduced emissions of health-damaging, climate-altering air pollutants; reduced energy and water consumption in urban areas through greening cities and recycling water; sustainable agriculture and forestry; and protection of ecosystems for carbon storage and other ecosystem services.' 3.3d: - 'Characteristics of adaptation pathways Adaptation can reduce the risks of climate change impacts, but there are limits to its effectiveness, especially with greater magnitudes and rates of climate change. Taking a longer-term perspective, in the context of sustainable development, increases the likelihood that more immediate adaptation actions will also enhance future options and preparedness. Adaptation can contribute to the well-being of current and future populations, the security of assets and the maintenance of ecosystem goods, functions and services now and in the future. Adaptation is place- and context-specific, with no single approach for reducing risks appropriate across all settings . Effective risk reduction and adaptation strategies consider vulnerability and exposure and their linkages with socio-economic processes, sustainable development, and climate change. Adaptation research since the IPCC Fourth Assessment Report has evolved from a dominant consideration of engineering and technological adaptation pathways to include more ecosystem-based, institutional and social measures. A previous focus on cost–benefit analysis, optimization and efficiency approaches has broadened with the development of multi-metric evaluations that include risk and uncertainty dimensions integrated within wider policy and ethical frameworks to assess tradeoffs and constraints. The range of specific adaptation measures has also expanded, as have the links to sustainable development. There are many studies on local and sectoral adaptation costs and benefits, but few global analyses and very low confidence in their results. Adaptation planning and implementation at all levels of governance are contingent on societal values, objectives and risk perceptions. Recognition of diverse interests, circumstances, social-cultural contexts and expectations can benefit decision-making processes. Indigenous, local and traditional knowledge systems and practices, including indigenous peoples’ holistic view of community and environment, are a major resource for adapting to climate change, but these have not been used consistently in existing adaptation efforts. Integrating such forms of knowledge into practices increases the effectiveness of adaptation as do effective decision support, engagement and policy processes. Adaptation planning and implementation can be enhanced through complementary actions across levels, from individuals to governments. National governments can coordinate adaptation efforts of local and sub-national governments, for example by protecting vulnerable groups, by supporting economic diversification and by providing information, policy and legal frameworks and financial support. Local government and the private sector are increasingly recognized as critical to progress in adaptation, given their roles in scaling up adaptation of communities, households and civil society and in managing risk information and financing. A first step towards adaptation to future climate change is reducing vulnerability and exposure to present climate variability, but some near-term responses to climate change may also limit future choices. Integration of adaptation into planning, including policy design, and decision-making can promote synergies with development and disaster risk reduction. However, poor planning or implementation, overemphasizing short-term outcomes or failing to sufficiently anticipate consequences can result in maladaptation, increasing the vulnerability or exposure of the target group in the future or the vulnerability of other people, places or sectors. For example, enhanced protection of exposed assets can lock in dependence on further protection measures. Appropriate adaptation options can be better assessed by including co-benefits and mitigation implications. Numerous interacting constraints can impede adaptation planning and implementation. Common constraints on implementation arise from the following: limited financial and human resources; limited integration or coordination of governance; uncertainties about projected impacts; different perceptions of risks; competing values; absence of key adaptation leaders and advocates; and limited tools to monitor adaptation effectiveness. Other constraints include insufficient research, monitoring and observation and the financial and other resources to maintain them. Underestimating the complexity of adaptation as a social process can create unrealistic expectations about achieving intended adaptation outcomes . Greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits. Limits to adaptation occur when adaptive actions to avoid intolerable risks for an actor’s objectives or for the needs of a system are not possible or are not currently available. Value-based judgments of what constitutes an intolerable risk may differ. Limits to adaptation emerge from the interaction among climate change and biophysical and/or socio-economic constraints. Opportunities to take advantage of positive synergies between adaptation and mitigation may decrease with time, particularly if limits to adaptation are exceeded. In some parts of the world, insufficient responses to emerging impacts are already eroding the basis for sustainable development. For most regions and sectors, empirical evidence is not sufficient to quantify magnitudes of climate change that would constitute a future adaptation limit. Furthermore, economic development, technology and cultural norms and values can change over time to enhance or reduce the capacity of systems to avoid limits. As a consequence, some limits are ‘soft’ in that they may be alleviated over time. Other limits are ‘hard’ in that there are no reasonable prospects for avoiding intolerable risks. Transformations in economic, social, technological and political decisions and actions can enhance adaptation and promote sustainable development. Restricting adaptation responses to incremental changes to existing systems and structures without considering transformational change may increase costs and losses and miss opportunities. For example, enhancing infrastructure to protect other built assets can be expensive and ultimately not defray increasing costs and risks, whereas options such as relocation or using ecosystem services to adapt may provide a range of benefits now and in the future. Transformational adaptation can include introduction of new technologies or practices, formation of new financial structures or systems of governance, adaptation at greater scales or magnitudes and shifts in the location of activities. Planning and implementation of transformational adaptation could reflect strengthened, altered or aligned paradigms and consequently may place new and increased demands on governance structures to reconcile different goals and visions for the future and to address possible equity and ethical implications: transformational adaptation pathways are enhanced by iterative learning, deliberative processes, and innovation. At the national level, transformation is considered most effective when it reflects a country’s own visions and approaches to achieving sustainable development in accordance with its national circumstances and priorities. Building adaptive capacity is crucial for effective selection and implementation of adaptation options . Successful adaptation requires not only identifying adaptation options and assessing their costs and benefits, but also increasing the adaptive capacity of human and natural systems . This can involve complex governance challenges and new institutions and institutional arrangements. Significant co-benefits, synergies and trade-offs exist between mitigation and adaptation and among different adaptation responses; interactions occur both within and across regions . Increasing efforts to mitigate and adapt to climate change imply an increasing complexity of interactions, particularly at the intersections among water, energy, land use and biodiversity, but tools to understand and manage these interactions remain limited. Examples of actions with co-benefits include improved energy efficiency and cleaner energy sources, leading to reduced emissions of health-damaging, climate-altering air pollutants; reduced energy and water consumption in urban areas through greening cities and recycling water; sustainable agriculture and forestry; and protection of ecosystems for carbon storage and other ecosystem services.' 3.3f: - 'Characteristics of adaptation pathways Adaptation can reduce the risks of climate change impacts, but there are limits to its effectiveness, especially with greater magnitudes and rates of climate change. Taking a longer-term perspective, in the context of sustainable development, increases the likelihood that more immediate adaptation actions will also enhance future options and preparedness. Adaptation can contribute to the well-being of current and future populations, the security of assets and the maintenance of ecosystem goods, functions and services now and in the future. Adaptation is place- and context-specific, with no single approach for reducing risks appropriate across all settings . Effective risk reduction and adaptation strategies consider vulnerability and exposure and their linkages with socio-economic processes, sustainable development, and climate change. Adaptation research since the IPCC Fourth Assessment Report has evolved from a dominant consideration of engineering and technological adaptation pathways to include more ecosystem-based, institutional and social measures. A previous focus on cost–benefit analysis, optimization and efficiency approaches has broadened with the development of multi-metric evaluations that include risk and uncertainty dimensions integrated within wider policy and ethical frameworks to assess tradeoffs and constraints. The range of specific adaptation measures has also expanded, as have the links to sustainable development. There are many studies on local and sectoral adaptation costs and benefits, but few global analyses and very low confidence in their results. Adaptation planning and implementation at all levels of governance are contingent on societal values, objectives and risk perceptions. Recognition of diverse interests, circumstances, social-cultural contexts and expectations can benefit decision-making processes. Indigenous, local and traditional knowledge systems and practices, including indigenous peoples’ holistic view of community and environment, are a major resource for adapting to climate change, but these have not been used consistently in existing adaptation efforts. Integrating such forms of knowledge into practices increases the effectiveness of adaptation as do effective decision support, engagement and policy processes. Adaptation planning and implementation can be enhanced through complementary actions across levels, from individuals to governments. National governments can coordinate adaptation efforts of local and sub-national governments, for example by protecting vulnerable groups, by supporting economic diversification and by providing information, policy and legal frameworks and financial support. Local government and the private sector are increasingly recognized as critical to progress in adaptation, given their roles in scaling up adaptation of communities, households and civil society and in managing risk information and financing. A first step towards adaptation to future climate change is reducing vulnerability and exposure to present climate variability, but some near-term responses to climate change may also limit future choices. Integration of adaptation into planning, including policy design, and decision-making can promote synergies with development and disaster risk reduction. However, poor planning or implementation, overemphasizing short-term outcomes or failing to sufficiently anticipate consequences can result in maladaptation, increasing the vulnerability or exposure of the target group in the future or the vulnerability of other people, places or sectors. For example, enhanced protection of exposed assets can lock in dependence on further protection measures. Appropriate adaptation options can be better assessed by including co-benefits and mitigation implications. Numerous interacting constraints can impede adaptation planning and implementation. Common constraints on implementation arise from the following: limited financial and human resources; limited integration or coordination of governance; uncertainties about projected impacts; different perceptions of risks; competing values; absence of key adaptation leaders and advocates; and limited tools to monitor adaptation effectiveness. Other constraints include insufficient research, monitoring and observation and the financial and other resources to maintain them. Underestimating the complexity of adaptation as a social process can create unrealistic expectations about achieving intended adaptation outcomes . Greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits. Limits to adaptation occur when adaptive actions to avoid intolerable risks for an actor’s objectives or for the needs of a system are not possible or are not currently available. Value-based judgments of what constitutes an intolerable risk may differ. Limits to adaptation emerge from the interaction among climate change and biophysical and/or socio-economic constraints. Opportunities to take advantage of positive synergies between adaptation and mitigation may decrease with time, particularly if limits to adaptation are exceeded. In some parts of the world, insufficient responses to emerging impacts are already eroding the basis for sustainable development. For most regions and sectors, empirical evidence is not sufficient to quantify magnitudes of climate change that would constitute a future adaptation limit. Furthermore, economic development, technology and cultural norms and values can change over time to enhance or reduce the capacity of systems to avoid limits. As a consequence, some limits are ‘soft’ in that they may be alleviated over time. Other limits are ‘hard’ in that there are no reasonable prospects for avoiding intolerable risks. Transformations in economic, social, technological and political decisions and actions can enhance adaptation and promote sustainable development. Restricting adaptation responses to incremental changes to existing systems and structures without considering transformational change may increase costs and losses and miss opportunities. For example, enhancing infrastructure to protect other built assets can be expensive and ultimately not defray increasing costs and risks, whereas options such as relocation or using ecosystem services to adapt may provide a range of benefits now and in the future. Transformational adaptation can include introduction of new technologies or practices, formation of new financial structures or systems of governance, adaptation at greater scales or magnitudes and shifts in the location of activities. Planning and implementation of transformational adaptation could reflect strengthened, altered or aligned paradigms and consequently may place new and increased demands on governance structures to reconcile different goals and visions for the future and to address possible equity and ethical implications: transformational adaptation pathways are enhanced by iterative learning, deliberative processes, and innovation. At the national level, transformation is considered most effective when it reflects a country’s own visions and approaches to achieving sustainable development in accordance with its national circumstances and priorities. Building adaptive capacity is crucial for effective selection and implementation of adaptation options . Successful adaptation requires not only identifying adaptation options and assessing their costs and benefits, but also increasing the adaptive capacity of human and natural systems . This can involve complex governance challenges and new institutions and institutional arrangements. Significant co-benefits, synergies and trade-offs exist between mitigation and adaptation and among different adaptation responses; interactions occur both within and across regions . Increasing efforts to mitigate and adapt to climate change imply an increasing complexity of interactions, particularly at the intersections among water, energy, land use and biodiversity, but tools to understand and manage these interactions remain limited. Examples of actions with co-benefits include improved energy efficiency and cleaner energy sources, leading to reduced emissions of health-damaging, climate-altering air pollutants; reduced energy and water consumption in urban areas through greening cities and recycling water; sustainable agriculture and forestry; and protection of ecosystems for carbon storage and other ecosystem services.' 3.3g: - 'Characteristics of adaptation pathways Adaptation can reduce the risks of climate change impacts, but there are limits to its effectiveness, especially with greater magnitudes and rates of climate change. Taking a longer-term perspective, in the context of sustainable development, increases the likelihood that more immediate adaptation actions will also enhance future options and preparedness. Adaptation can contribute to the well-being of current and future populations, the security of assets and the maintenance of ecosystem goods, functions and services now and in the future. Adaptation is place- and context-specific, with no single approach for reducing risks appropriate across all settings . Effective risk reduction and adaptation strategies consider vulnerability and exposure and their linkages with socio-economic processes, sustainable development, and climate change. Adaptation research since the IPCC Fourth Assessment Report has evolved from a dominant consideration of engineering and technological adaptation pathways to include more ecosystem-based, institutional and social measures. A previous focus on cost–benefit analysis, optimization and efficiency approaches has broadened with the development of multi-metric evaluations that include risk and uncertainty dimensions integrated within wider policy and ethical frameworks to assess tradeoffs and constraints. The range of specific adaptation measures has also expanded, as have the links to sustainable development. There are many studies on local and sectoral adaptation costs and benefits, but few global analyses and very low confidence in their results. Adaptation planning and implementation at all levels of governance are contingent on societal values, objectives and risk perceptions. Recognition of diverse interests, circumstances, social-cultural contexts and expectations can benefit decision-making processes. Indigenous, local and traditional knowledge systems and practices, including indigenous peoples’ holistic view of community and environment, are a major resource for adapting to climate change, but these have not been used consistently in existing adaptation efforts. Integrating such forms of knowledge into practices increases the effectiveness of adaptation as do effective decision support, engagement and policy processes. Adaptation planning and implementation can be enhanced through complementary actions across levels, from individuals to governments. National governments can coordinate adaptation efforts of local and sub-national governments, for example by protecting vulnerable groups, by supporting economic diversification and by providing information, policy and legal frameworks and financial support. Local government and the private sector are increasingly recognized as critical to progress in adaptation, given their roles in scaling up adaptation of communities, households and civil society and in managing risk information and financing. A first step towards adaptation to future climate change is reducing vulnerability and exposure to present climate variability, but some near-term responses to climate change may also limit future choices. Integration of adaptation into planning, including policy design, and decision-making can promote synergies with development and disaster risk reduction. However, poor planning or implementation, overemphasizing short-term outcomes or failing to sufficiently anticipate consequences can result in maladaptation, increasing the vulnerability or exposure of the target group in the future or the vulnerability of other people, places or sectors. For example, enhanced protection of exposed assets can lock in dependence on further protection measures. Appropriate adaptation options can be better assessed by including co-benefits and mitigation implications. Numerous interacting constraints can impede adaptation planning and implementation. Common constraints on implementation arise from the following: limited financial and human resources; limited integration or coordination of governance; uncertainties about projected impacts; different perceptions of risks; competing values; absence of key adaptation leaders and advocates; and limited tools to monitor adaptation effectiveness. Other constraints include insufficient research, monitoring and observation and the financial and other resources to maintain them. Underestimating the complexity of adaptation as a social process can create unrealistic expectations about achieving intended adaptation outcomes . Greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits. Limits to adaptation occur when adaptive actions to avoid intolerable risks for an actor’s objectives or for the needs of a system are not possible or are not currently available. Value-based judgments of what constitutes an intolerable risk may differ. Limits to adaptation emerge from the interaction among climate change and biophysical and/or socio-economic constraints. Opportunities to take advantage of positive synergies between adaptation and mitigation may decrease with time, particularly if limits to adaptation are exceeded. In some parts of the world, insufficient responses to emerging impacts are already eroding the basis for sustainable development. For most regions and sectors, empirical evidence is not sufficient to quantify magnitudes of climate change that would constitute a future adaptation limit. Furthermore, economic development, technology and cultural norms and values can change over time to enhance or reduce the capacity of systems to avoid limits. As a consequence, some limits are ‘soft’ in that they may be alleviated over time. Other limits are ‘hard’ in that there are no reasonable prospects for avoiding intolerable risks. Transformations in economic, social, technological and political decisions and actions can enhance adaptation and promote sustainable development. Restricting adaptation responses to incremental changes to existing systems and structures without considering transformational change may increase costs and losses and miss opportunities. For example, enhancing infrastructure to protect other built assets can be expensive and ultimately not defray increasing costs and risks, whereas options such as relocation or using ecosystem services to adapt may provide a range of benefits now and in the future. Transformational adaptation can include introduction of new technologies or practices, formation of new financial structures or systems of governance, adaptation at greater scales or magnitudes and shifts in the location of activities. Planning and implementation of transformational adaptation could reflect strengthened, altered or aligned paradigms and consequently may place new and increased demands on governance structures to reconcile different goals and visions for the future and to address possible equity and ethical implications: transformational adaptation pathways are enhanced by iterative learning, deliberative processes, and innovation. At the national level, transformation is considered most effective when it reflects a country’s own visions and approaches to achieving sustainable development in accordance with its national circumstances and priorities. Building adaptive capacity is crucial for effective selection and implementation of adaptation options . Successful adaptation requires not only identifying adaptation options and assessing their costs and benefits, but also increasing the adaptive capacity of human and natural systems . This can involve complex governance challenges and new institutions and institutional arrangements. Significant co-benefits, synergies and trade-offs exist between mitigation and adaptation and among different adaptation responses; interactions occur both within and across regions . Increasing efforts to mitigate and adapt to climate change imply an increasing complexity of interactions, particularly at the intersections among water, energy, land use and biodiversity, but tools to understand and manage these interactions remain limited. Examples of actions with co-benefits include improved energy efficiency and cleaner energy sources, leading to reduced emissions of health-damaging, climate-altering air pollutants; reduced energy and water consumption in urban areas through greening cities and recycling water; sustainable agriculture and forestry; and protection of ecosystems for carbon storage and other ecosystem services.' 3.4a: - Characteristics of mitigation pathways There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to pre-industrial levels. These pathways would require substantial emissions reductions over the next few decades and near zero emissions of CO2 and other long-lived greenhouse gases by the end of the century. Implementing such reductions poses substantial technological, economic, social and institutional challenges, which increase with delays in additional mitigation and if key technologies are not available. Limiting warming to lower or higher levels involves similar challenges but on different timescales. Without additional efforts to reduce GHG emissions beyond those in place today, global emission growth is expected to persist driven by growth in global population and economic activities. Global GHG emissions under most scenarios without additional mitigation are between about 75 GtCO2-eq/yr and almost 140 GtCO2-eq/yr in 21003520which is approximately between the 2100 emission levels in the RCP6.0 and RCP8.5 pathways 3621 . Baseline scenarios exceed 450 ppm CO2-eq by 2030 and reach CO2-eq concentration levels between about 750 ppm CO2-eq and more than 1300 ppm CO2-eq by 2100. Global mean surface temperature increases in 2100 range from about 3.7°C to 4.8°C above the average for 1850–1900 for a median climate response. They range from 2.5°C to 7.8°C when including climate uncertainty 3722 . The future scenarios do not account for possible changes in natural forcings in the climate system. Many different combinations of technological, behavioural and policy options can be used to reduce emissions and limit temperature change. To evaluate possible pathways to long-term climate goals, about 900 mitigation scenarios were collected for this assessment, each of which describes different technological, socio-economic and institutional changes. Emission reductions under these scenarios lead to concentrations in 2100 from 430 ppm CO2-eq to above 720 ppm CO2-eq which is comparable to the 2100 forcing levels between RCP2.6 and RCP6.0. Scenarios with concentration levels of below 430 ppm CO2-eq by 2100 were also assessed. Scenarios leading to CO2-eq concentrations in 2100 of about 450 ppm or lower are likely to maintain warming below 2°C over the 21st century relative to pre-industrial levels. Mitigation scenarios reaching concentration levels of about 500 ppm CO2-eq by 2100 are more likely than not to limit warming to less than 2°C relative to pre-industrial levels, unless concentration levels temporarily exceed roughly 530 ppm CO2-eq before 2100. In this case, warming is about as likely as not to remain below 2°C relative to pre-industrial levels. Scenarios that exceed about 650 ppm CO2-eq by 2100 are unlikely to limit warming to below 2°C relative to pre-industrial levels. Mitigation scenarios in which warming is more likely than not to be less than 1.5°C relative to pre-industrial levels by 2100 are characterized by concentration levels by 2100 of below 430 ppm CO2-eq. In these scenarios, temperature peaks during the century and subsequently declines. Mitigation scenarios reaching about 450 ppm CO2-eq in 2100 typically involve temporary overshootof atmospheric concentrations, as do many scenarios reaching about 500 ppm CO2-eq to about 550 ppm CO2-eq by 2100. Depending on the level of overshoot, overshoot scenarios typically rely on the availability and widespread deployment of bioenergy with carbon dioxide capture and storage and afforestation in the second half of the century. The availability and scale of these and other Carbon Dioxide Removal technologies and methods are uncertain, and CDR technologies and methods are, to varying degrees, associated with challenges and risks 3924 . CDR is also prevalent in many scenarios without overshoot to compensate for residual emissions from sectors where mitigation is more expensive. Limiting warming with a likely chance to less than 2°C relative to pre-industrial levels would require substantial cuts in anthropogenic GHG emissions by mid-century through largescale changes in energy systems and possibly land use. Limiting warming to higher levels would require similar changes but less quickly. Limiting warming to lower levels would require these changes more quickly. Scenarios that are likely to maintain warming at below 2°C are characterized by a 40 to 70% reduction in GHG emissions by 2050, relative to 2010 levels, and emissions levels near zero or below in 2100. Scenarios with higher emissions in 2050 are characterized by a greater reliance on CDR technologies beyond mid-century, and vice versa. Scenarios that are likely to maintain warming at below 2°C include more rapid improvements in energy efficiency and a tripling to nearly a quadrupling of the share of zero- and low-carbon energy supply from renewable energy, nuclear energy and fossil energy with carbon dioxide capture and storage or BECCS by the year 2050. The scenarios describe a wide range of changes in land use, reflecting different assumptions about the scale of bioenergy production, afforestation and reduced deforestation. Scenarios leading to concentrations of 500 ppm CO2-eq by 2100 are characterized by a 25 to 55% reduction in GHG emissions by 2050, relative to 2010 levels. Scenarios that are likely to limit warming to 3°C relative to pre-industrial levels reduce emissions less rapidly than those limiting warming to 2°C. Only a limited number of studies provide scenarios that are more likely than not to limit warming to 1.5°C by 2100; these scenarios are characterized by concentrations below 430 ppm CO2-eq by 2100 and 2050 emission reduction between 70 and 95% below 2010. For a comprehensive overview of the characteristics of emissions scenarios, their CO2-equivalent concentrations and their likelihood to keep warming to below a range of temperature levels, see Table 3.1. Reducing emissions of non-CO2 climate forcing agents can be an important element of mitigation strategies. Emissions of nonCO2 gases, nitrous oxide, and fluorinated gases) contributed about 27% to the total emissions of Kyoto gases in 2010. For most non-CO2 gases, near-term, low-cost options are available to reduce their emissions. However, some sources of these non-CO2 gases are difficult to mitigate, such as N2O emissions from fertilizer use and CH4 emissions from livestock. As a result, emissions of most non-CO2 gases will not be reduced to zero, even under stringent mitigation scenarios. The differences in radiative properties and lifetimes of CO2 and non-CO2 climate forcing agents have important implications for mitigation strategies. All current GHG emissions and other climate forcing agents affect the rate and magnitude of climate change over the next few decades. Reducing the emissions of certain short-lived climate forcing agents can reduce the rate of warming in the short term but will have only a limited effect on long-term warming, which is driven mainly by CO2 emissions. There are large uncertainties related to the climate impacts of some of the short-lived climate forcing agents. Although the effects of CH4 emissions are well understood, there are large uncertainties related to the effects of black carbon. Co-emitted components with cooling effects may further complicate and reduce the climate impacts of emission reductions. Reducing emissions of sulfur dioxide would cause warming. Near-term reductions in short-lived climate forcing agents can have a relatively fast impact on climate change and possible co-benefits for air pollution. Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the 21st century to below 2°C relative to pre-industrial levels. GHG emissions in 2030 lie between about 30 GtCO2-eq/yr and 50 GtCO2-eq/yr in cost-effective scenarios that are likely to about as likely as not to limit warming to less than 2°C this century relative to pre-industrial levels . Scenarios with GHG emission levels of above 55 GtCO2-eq/yr require substantially higher rates of emissions reductions between 2030 and 2050 ; much more rapid scale-up of zero and low-carbon energy over this period ; a larger reliance on CDR technologies in the long term; and higher transitional and long-term economic impacts. Estimated global emission levels by 2020 based on the Cancún Pledges are not consistent with cost-effective long-term mitigation trajectories that are at least about as likely as not to limit warming to below 2°C relative to pre-industrial levels , but they do not preclude the option to meet this goal . The Cancún Pledges are broadly consistent with cost-effective scenarios that are likely to limit temperature change to below 3°C relative to pre-industrial levels. Estimates of the aggregate economic costs of mitigation vary widely depending on methodologies and assumptions but increase with the stringency of mitigation. Scenarios in which all countries of the world begin mitigation immediately, in which there is a single global carbon price, and in which all key technologies are available have been used as a cost-effective benchmark for estimating macroeconomic mitigation costs. Under these assumptions, mitigation scenarios that are likely to limit warming to below 2°C through the 21st century relative to pre-industrial levels entail losses in global consumption—not including benefits of reduced climate change as well as co-benefits and adverse side effects of mitigation —of 1 to 4% in 2030, 2 to 6% in 2050, and 3% to 11% in 2100, relative to consumption in baseline scenarios that grows anywhere from 300% to more than 900% over the century . These numbers correspond to an annualized reduction of consumption growth by 0.04 to 0.14 percentage points over the century relative to annualized consumption growth in the baseline that is between 1.6% and 3% per year. In the absence or under limited availability of mitigation technologies , mitigation costs can increase substantially depending on the technology considered . Delaying additional mitigation reduces near-term costs but increases mitigation costs in the medium- to long-term. Many models could not limit likely warming to below 2°C over the 21st century relative to pre-industrial levels, if additional mitigation is considerably delayed, or if availability of key technologies, such as bioenergy, CCS and their combination are limited . Mitigation efforts and associated cost are expected to vary across countries. The distribution of costs can differ from the distribution of the actions themselves. In globally cost-effective scenarios, the majority of mitigation efforts takes place in countries with the highest future GHG emissions in baseline scenarios. Some studies exploring particular effort-sharing frameworks, under the assumption of a global carbon market, have estimated substantial global financial flows associated with mitigation in scenarios that are likely to more unlikely than likely to limit warming during the 21st century to less than 2°C relative to pre-industrial levels. 3.4b: - Characteristics of mitigation pathways There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to pre-industrial levels. These pathways would require substantial emissions reductions over the next few decades and near zero emissions of CO2 and other long-lived greenhouse gases by the end of the century. Implementing such reductions poses substantial technological, economic, social and institutional challenges, which increase with delays in additional mitigation and if key technologies are not available. Limiting warming to lower or higher levels involves similar challenges but on different timescales. Without additional efforts to reduce GHG emissions beyond those in place today, global emission growth is expected to persist driven by growth in global population and economic activities. Global GHG emissions under most scenarios without additional mitigation are between about 75 GtCO2-eq/yr and almost 140 GtCO2-eq/yr in 21003520which is approximately between the 2100 emission levels in the RCP6.0 and RCP8.5 pathways 3621 . Baseline scenarios exceed 450 ppm CO2-eq by 2030 and reach CO2-eq concentration levels between about 750 ppm CO2-eq and more than 1300 ppm CO2-eq by 2100. Global mean surface temperature increases in 2100 range from about 3.7°C to 4.8°C above the average for 1850–1900 for a median climate response. They range from 2.5°C to 7.8°C when including climate uncertainty 3722 . The future scenarios do not account for possible changes in natural forcings in the climate system. Many different combinations of technological, behavioural and policy options can be used to reduce emissions and limit temperature change. To evaluate possible pathways to long-term climate goals, about 900 mitigation scenarios were collected for this assessment, each of which describes different technological, socio-economic and institutional changes. Emission reductions under these scenarios lead to concentrations in 2100 from 430 ppm CO2-eq to above 720 ppm CO2-eq which is comparable to the 2100 forcing levels between RCP2.6 and RCP6.0. Scenarios with concentration levels of below 430 ppm CO2-eq by 2100 were also assessed. Scenarios leading to CO2-eq concentrations in 2100 of about 450 ppm or lower are likely to maintain warming below 2°C over the 21st century relative to pre-industrial levels. Mitigation scenarios reaching concentration levels of about 500 ppm CO2-eq by 2100 are more likely than not to limit warming to less than 2°C relative to pre-industrial levels, unless concentration levels temporarily exceed roughly 530 ppm CO2-eq before 2100. In this case, warming is about as likely as not to remain below 2°C relative to pre-industrial levels. Scenarios that exceed about 650 ppm CO2-eq by 2100 are unlikely to limit warming to below 2°C relative to pre-industrial levels. Mitigation scenarios in which warming is more likely than not to be less than 1.5°C relative to pre-industrial levels by 2100 are characterized by concentration levels by 2100 of below 430 ppm CO2-eq. In these scenarios, temperature peaks during the century and subsequently declines. Mitigation scenarios reaching about 450 ppm CO2-eq in 2100 typically involve temporary overshootof atmospheric concentrations, as do many scenarios reaching about 500 ppm CO2-eq to about 550 ppm CO2-eq by 2100. Depending on the level of overshoot, overshoot scenarios typically rely on the availability and widespread deployment of bioenergy with carbon dioxide capture and storage and afforestation in the second half of the century. The availability and scale of these and other Carbon Dioxide Removal technologies and methods are uncertain, and CDR technologies and methods are, to varying degrees, associated with challenges and risks 3924 . CDR is also prevalent in many scenarios without overshoot to compensate for residual emissions from sectors where mitigation is more expensive. Limiting warming with a likely chance to less than 2°C relative to pre-industrial levels would require substantial cuts in anthropogenic GHG emissions by mid-century through largescale changes in energy systems and possibly land use. Limiting warming to higher levels would require similar changes but less quickly. Limiting warming to lower levels would require these changes more quickly. Scenarios that are likely to maintain warming at below 2°C are characterized by a 40 to 70% reduction in GHG emissions by 2050, relative to 2010 levels, and emissions levels near zero or below in 2100. Scenarios with higher emissions in 2050 are characterized by a greater reliance on CDR technologies beyond mid-century, and vice versa. Scenarios that are likely to maintain warming at below 2°C include more rapid improvements in energy efficiency and a tripling to nearly a quadrupling of the share of zero- and low-carbon energy supply from renewable energy, nuclear energy and fossil energy with carbon dioxide capture and storage or BECCS by the year 2050. The scenarios describe a wide range of changes in land use, reflecting different assumptions about the scale of bioenergy production, afforestation and reduced deforestation. Scenarios leading to concentrations of 500 ppm CO2-eq by 2100 are characterized by a 25 to 55% reduction in GHG emissions by 2050, relative to 2010 levels. Scenarios that are likely to limit warming to 3°C relative to pre-industrial levels reduce emissions less rapidly than those limiting warming to 2°C. Only a limited number of studies provide scenarios that are more likely than not to limit warming to 1.5°C by 2100; these scenarios are characterized by concentrations below 430 ppm CO2-eq by 2100 and 2050 emission reduction between 70 and 95% below 2010. For a comprehensive overview of the characteristics of emissions scenarios, their CO2-equivalent concentrations and their likelihood to keep warming to below a range of temperature levels, see Table 3.1. Reducing emissions of non-CO2 climate forcing agents can be an important element of mitigation strategies. Emissions of nonCO2 gases, nitrous oxide, and fluorinated gases) contributed about 27% to the total emissions of Kyoto gases in 2010. For most non-CO2 gases, near-term, low-cost options are available to reduce their emissions. However, some sources of these non-CO2 gases are difficult to mitigate, such as N2O emissions from fertilizer use and CH4 emissions from livestock. As a result, emissions of most non-CO2 gases will not be reduced to zero, even under stringent mitigation scenarios. The differences in radiative properties and lifetimes of CO2 and non-CO2 climate forcing agents have important implications for mitigation strategies. All current GHG emissions and other climate forcing agents affect the rate and magnitude of climate change over the next few decades. Reducing the emissions of certain short-lived climate forcing agents can reduce the rate of warming in the short term but will have only a limited effect on long-term warming, which is driven mainly by CO2 emissions. There are large uncertainties related to the climate impacts of some of the short-lived climate forcing agents. Although the effects of CH4 emissions are well understood, there are large uncertainties related to the effects of black carbon. Co-emitted components with cooling effects may further complicate and reduce the climate impacts of emission reductions. Reducing emissions of sulfur dioxide would cause warming. Near-term reductions in short-lived climate forcing agents can have a relatively fast impact on climate change and possible co-benefits for air pollution. Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the 21st century to below 2°C relative to pre-industrial levels. GHG emissions in 2030 lie between about 30 GtCO2-eq/yr and 50 GtCO2-eq/yr in cost-effective scenarios that are likely to about as likely as not to limit warming to less than 2°C this century relative to pre-industrial levels . Scenarios with GHG emission levels of above 55 GtCO2-eq/yr require substantially higher rates of emissions reductions between 2030 and 2050 ; much more rapid scale-up of zero and low-carbon energy over this period ; a larger reliance on CDR technologies in the long term; and higher transitional and long-term economic impacts. Estimated global emission levels by 2020 based on the Cancún Pledges are not consistent with cost-effective long-term mitigation trajectories that are at least about as likely as not to limit warming to below 2°C relative to pre-industrial levels , but they do not preclude the option to meet this goal . The Cancún Pledges are broadly consistent with cost-effective scenarios that are likely to limit temperature change to below 3°C relative to pre-industrial levels. Estimates of the aggregate economic costs of mitigation vary widely depending on methodologies and assumptions but increase with the stringency of mitigation. Scenarios in which all countries of the world begin mitigation immediately, in which there is a single global carbon price, and in which all key technologies are available have been used as a cost-effective benchmark for estimating macroeconomic mitigation costs. Under these assumptions, mitigation scenarios that are likely to limit warming to below 2°C through the 21st century relative to pre-industrial levels entail losses in global consumption—not including benefits of reduced climate change as well as co-benefits and adverse side effects of mitigation —of 1 to 4% in 2030, 2 to 6% in 2050, and 3% to 11% in 2100, relative to consumption in baseline scenarios that grows anywhere from 300% to more than 900% over the century . These numbers correspond to an annualized reduction of consumption growth by 0.04 to 0.14 percentage points over the century relative to annualized consumption growth in the baseline that is between 1.6% and 3% per year. In the absence or under limited availability of mitigation technologies , mitigation costs can increase substantially depending on the technology considered . Delaying additional mitigation reduces near-term costs but increases mitigation costs in the medium- to long-term. Many models could not limit likely warming to below 2°C over the 21st century relative to pre-industrial levels, if additional mitigation is considerably delayed, or if availability of key technologies, such as bioenergy, CCS and their combination are limited . Mitigation efforts and associated cost are expected to vary across countries. The distribution of costs can differ from the distribution of the actions themselves. In globally cost-effective scenarios, the majority of mitigation efforts takes place in countries with the highest future GHG emissions in baseline scenarios. Some studies exploring particular effort-sharing frameworks, under the assumption of a global carbon market, have estimated substantial global financial flows associated with mitigation in scenarios that are likely to more unlikely than likely to limit warming during the 21st century to less than 2°C relative to pre-industrial levels. 3.4c: - Characteristics of mitigation pathways There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to pre-industrial levels. These pathways would require substantial emissions reductions over the next few decades and near zero emissions of CO2 and other long-lived greenhouse gases by the end of the century. Implementing such reductions poses substantial technological, economic, social and institutional challenges, which increase with delays in additional mitigation and if key technologies are not available. Limiting warming to lower or higher levels involves similar challenges but on different timescales. Without additional efforts to reduce GHG emissions beyond those in place today, global emission growth is expected to persist driven by growth in global population and economic activities. Global GHG emissions under most scenarios without additional mitigation are between about 75 GtCO2-eq/yr and almost 140 GtCO2-eq/yr in 21003520which is approximately between the 2100 emission levels in the RCP6.0 and RCP8.5 pathways 3621 . Baseline scenarios exceed 450 ppm CO2-eq by 2030 and reach CO2-eq concentration levels between about 750 ppm CO2-eq and more than 1300 ppm CO2-eq by 2100. Global mean surface temperature increases in 2100 range from about 3.7°C to 4.8°C above the average for 1850–1900 for a median climate response. They range from 2.5°C to 7.8°C when including climate uncertainty 3722 . The future scenarios do not account for possible changes in natural forcings in the climate system. Many different combinations of technological, behavioural and policy options can be used to reduce emissions and limit temperature change. To evaluate possible pathways to long-term climate goals, about 900 mitigation scenarios were collected for this assessment, each of which describes different technological, socio-economic and institutional changes. Emission reductions under these scenarios lead to concentrations in 2100 from 430 ppm CO2-eq to above 720 ppm CO2-eq which is comparable to the 2100 forcing levels between RCP2.6 and RCP6.0. Scenarios with concentration levels of below 430 ppm CO2-eq by 2100 were also assessed. Scenarios leading to CO2-eq concentrations in 2100 of about 450 ppm or lower are likely to maintain warming below 2°C over the 21st century relative to pre-industrial levels. Mitigation scenarios reaching concentration levels of about 500 ppm CO2-eq by 2100 are more likely than not to limit warming to less than 2°C relative to pre-industrial levels, unless concentration levels temporarily exceed roughly 530 ppm CO2-eq before 2100. In this case, warming is about as likely as not to remain below 2°C relative to pre-industrial levels. Scenarios that exceed about 650 ppm CO2-eq by 2100 are unlikely to limit warming to below 2°C relative to pre-industrial levels. Mitigation scenarios in which warming is more likely than not to be less than 1.5°C relative to pre-industrial levels by 2100 are characterized by concentration levels by 2100 of below 430 ppm CO2-eq. In these scenarios, temperature peaks during the century and subsequently declines. Mitigation scenarios reaching about 450 ppm CO2-eq in 2100 typically involve temporary overshootof atmospheric concentrations, as do many scenarios reaching about 500 ppm CO2-eq to about 550 ppm CO2-eq by 2100. Depending on the level of overshoot, overshoot scenarios typically rely on the availability and widespread deployment of bioenergy with carbon dioxide capture and storage and afforestation in the second half of the century. The availability and scale of these and other Carbon Dioxide Removal technologies and methods are uncertain, and CDR technologies and methods are, to varying degrees, associated with challenges and risks 3924 . CDR is also prevalent in many scenarios without overshoot to compensate for residual emissions from sectors where mitigation is more expensive. Limiting warming with a likely chance to less than 2°C relative to pre-industrial levels would require substantial cuts in anthropogenic GHG emissions by mid-century through largescale changes in energy systems and possibly land use. Limiting warming to higher levels would require similar changes but less quickly. Limiting warming to lower levels would require these changes more quickly. Scenarios that are likely to maintain warming at below 2°C are characterized by a 40 to 70% reduction in GHG emissions by 2050, relative to 2010 levels, and emissions levels near zero or below in 2100. Scenarios with higher emissions in 2050 are characterized by a greater reliance on CDR technologies beyond mid-century, and vice versa. Scenarios that are likely to maintain warming at below 2°C include more rapid improvements in energy efficiency and a tripling to nearly a quadrupling of the share of zero- and low-carbon energy supply from renewable energy, nuclear energy and fossil energy with carbon dioxide capture and storage or BECCS by the year 2050. The scenarios describe a wide range of changes in land use, reflecting different assumptions about the scale of bioenergy production, afforestation and reduced deforestation. Scenarios leading to concentrations of 500 ppm CO2-eq by 2100 are characterized by a 25 to 55% reduction in GHG emissions by 2050, relative to 2010 levels. Scenarios that are likely to limit warming to 3°C relative to pre-industrial levels reduce emissions less rapidly than those limiting warming to 2°C. Only a limited number of studies provide scenarios that are more likely than not to limit warming to 1.5°C by 2100; these scenarios are characterized by concentrations below 430 ppm CO2-eq by 2100 and 2050 emission reduction between 70 and 95% below 2010. For a comprehensive overview of the characteristics of emissions scenarios, their CO2-equivalent concentrations and their likelihood to keep warming to below a range of temperature levels, see Table 3.1. Reducing emissions of non-CO2 climate forcing agents can be an important element of mitigation strategies. Emissions of nonCO2 gases, nitrous oxide, and fluorinated gases) contributed about 27% to the total emissions of Kyoto gases in 2010. For most non-CO2 gases, near-term, low-cost options are available to reduce their emissions. However, some sources of these non-CO2 gases are difficult to mitigate, such as N2O emissions from fertilizer use and CH4 emissions from livestock. As a result, emissions of most non-CO2 gases will not be reduced to zero, even under stringent mitigation scenarios. The differences in radiative properties and lifetimes of CO2 and non-CO2 climate forcing agents have important implications for mitigation strategies. All current GHG emissions and other climate forcing agents affect the rate and magnitude of climate change over the next few decades. Reducing the emissions of certain short-lived climate forcing agents can reduce the rate of warming in the short term but will have only a limited effect on long-term warming, which is driven mainly by CO2 emissions. There are large uncertainties related to the climate impacts of some of the short-lived climate forcing agents. Although the effects of CH4 emissions are well understood, there are large uncertainties related to the effects of black carbon. Co-emitted components with cooling effects may further complicate and reduce the climate impacts of emission reductions. Reducing emissions of sulfur dioxide would cause warming. Near-term reductions in short-lived climate forcing agents can have a relatively fast impact on climate change and possible co-benefits for air pollution. Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the 21st century to below 2°C relative to pre-industrial levels. GHG emissions in 2030 lie between about 30 GtCO2-eq/yr and 50 GtCO2-eq/yr in cost-effective scenarios that are likely to about as likely as not to limit warming to less than 2°C this century relative to pre-industrial levels . Scenarios with GHG emission levels of above 55 GtCO2-eq/yr require substantially higher rates of emissions reductions between 2030 and 2050 ; much more rapid scale-up of zero and low-carbon energy over this period ; a larger reliance on CDR technologies in the long term; and higher transitional and long-term economic impacts. Estimated global emission levels by 2020 based on the Cancún Pledges are not consistent with cost-effective long-term mitigation trajectories that are at least about as likely as not to limit warming to below 2°C relative to pre-industrial levels , but they do not preclude the option to meet this goal . The Cancún Pledges are broadly consistent with cost-effective scenarios that are likely to limit temperature change to below 3°C relative to pre-industrial levels. Estimates of the aggregate economic costs of mitigation vary widely depending on methodologies and assumptions but increase with the stringency of mitigation. Scenarios in which all countries of the world begin mitigation immediately, in which there is a single global carbon price, and in which all key technologies are available have been used as a cost-effective benchmark for estimating macroeconomic mitigation costs. Under these assumptions, mitigation scenarios that are likely to limit warming to below 2°C through the 21st century relative to pre-industrial levels entail losses in global consumption—not including benefits of reduced climate change as well as co-benefits and adverse side effects of mitigation —of 1 to 4% in 2030, 2 to 6% in 2050, and 3% to 11% in 2100, relative to consumption in baseline scenarios that grows anywhere from 300% to more than 900% over the century . These numbers correspond to an annualized reduction of consumption growth by 0.04 to 0.14 percentage points over the century relative to annualized consumption growth in the baseline that is between 1.6% and 3% per year. In the absence or under limited availability of mitigation technologies , mitigation costs can increase substantially depending on the technology considered . Delaying additional mitigation reduces near-term costs but increases mitigation costs in the medium- to long-term. Many models could not limit likely warming to below 2°C over the 21st century relative to pre-industrial levels, if additional mitigation is considerably delayed, or if availability of key technologies, such as bioenergy, CCS and their combination are limited . Mitigation efforts and associated cost are expected to vary across countries. The distribution of costs can differ from the distribution of the actions themselves. In globally cost-effective scenarios, the majority of mitigation efforts takes place in countries with the highest future GHG emissions in baseline scenarios. Some studies exploring particular effort-sharing frameworks, under the assumption of a global carbon market, have estimated substantial global financial flows associated with mitigation in scenarios that are likely to more unlikely than likely to limit warming during the 21st century to less than 2°C relative to pre-industrial levels. 3.4d: - Characteristics of mitigation pathways There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to pre-industrial levels. These pathways would require substantial emissions reductions over the next few decades and near zero emissions of CO2 and other long-lived greenhouse gases by the end of the century. Implementing such reductions poses substantial technological, economic, social and institutional challenges, which increase with delays in additional mitigation and if key technologies are not available. Limiting warming to lower or higher levels involves similar challenges but on different timescales. Without additional efforts to reduce GHG emissions beyond those in place today, global emission growth is expected to persist driven by growth in global population and economic activities. Global GHG emissions under most scenarios without additional mitigation are between about 75 GtCO2-eq/yr and almost 140 GtCO2-eq/yr in 21003520which is approximately between the 2100 emission levels in the RCP6.0 and RCP8.5 pathways 3621 . Baseline scenarios exceed 450 ppm CO2-eq by 2030 and reach CO2-eq concentration levels between about 750 ppm CO2-eq and more than 1300 ppm CO2-eq by 2100. Global mean surface temperature increases in 2100 range from about 3.7°C to 4.8°C above the average for 1850–1900 for a median climate response. They range from 2.5°C to 7.8°C when including climate uncertainty 3722 . The future scenarios do not account for possible changes in natural forcings in the climate system. Many different combinations of technological, behavioural and policy options can be used to reduce emissions and limit temperature change. To evaluate possible pathways to long-term climate goals, about 900 mitigation scenarios were collected for this assessment, each of which describes different technological, socio-economic and institutional changes. Emission reductions under these scenarios lead to concentrations in 2100 from 430 ppm CO2-eq to above 720 ppm CO2-eq which is comparable to the 2100 forcing levels between RCP2.6 and RCP6.0. Scenarios with concentration levels of below 430 ppm CO2-eq by 2100 were also assessed. Scenarios leading to CO2-eq concentrations in 2100 of about 450 ppm or lower are likely to maintain warming below 2°C over the 21st century relative to pre-industrial levels. Mitigation scenarios reaching concentration levels of about 500 ppm CO2-eq by 2100 are more likely than not to limit warming to less than 2°C relative to pre-industrial levels, unless concentration levels temporarily exceed roughly 530 ppm CO2-eq before 2100. In this case, warming is about as likely as not to remain below 2°C relative to pre-industrial levels. Scenarios that exceed about 650 ppm CO2-eq by 2100 are unlikely to limit warming to below 2°C relative to pre-industrial levels. Mitigation scenarios in which warming is more likely than not to be less than 1.5°C relative to pre-industrial levels by 2100 are characterized by concentration levels by 2100 of below 430 ppm CO2-eq. In these scenarios, temperature peaks during the century and subsequently declines. Mitigation scenarios reaching about 450 ppm CO2-eq in 2100 typically involve temporary overshootof atmospheric concentrations, as do many scenarios reaching about 500 ppm CO2-eq to about 550 ppm CO2-eq by 2100. Depending on the level of overshoot, overshoot scenarios typically rely on the availability and widespread deployment of bioenergy with carbon dioxide capture and storage and afforestation in the second half of the century. The availability and scale of these and other Carbon Dioxide Removal technologies and methods are uncertain, and CDR technologies and methods are, to varying degrees, associated with challenges and risks 3924 . CDR is also prevalent in many scenarios without overshoot to compensate for residual emissions from sectors where mitigation is more expensive. Limiting warming with a likely chance to less than 2°C relative to pre-industrial levels would require substantial cuts in anthropogenic GHG emissions by mid-century through largescale changes in energy systems and possibly land use. Limiting warming to higher levels would require similar changes but less quickly. Limiting warming to lower levels would require these changes more quickly. Scenarios that are likely to maintain warming at below 2°C are characterized by a 40 to 70% reduction in GHG emissions by 2050, relative to 2010 levels, and emissions levels near zero or below in 2100. Scenarios with higher emissions in 2050 are characterized by a greater reliance on CDR technologies beyond mid-century, and vice versa. Scenarios that are likely to maintain warming at below 2°C include more rapid improvements in energy efficiency and a tripling to nearly a quadrupling of the share of zero- and low-carbon energy supply from renewable energy, nuclear energy and fossil energy with carbon dioxide capture and storage or BECCS by the year 2050. The scenarios describe a wide range of changes in land use, reflecting different assumptions about the scale of bioenergy production, afforestation and reduced deforestation. Scenarios leading to concentrations of 500 ppm CO2-eq by 2100 are characterized by a 25 to 55% reduction in GHG emissions by 2050, relative to 2010 levels. Scenarios that are likely to limit warming to 3°C relative to pre-industrial levels reduce emissions less rapidly than those limiting warming to 2°C. Only a limited number of studies provide scenarios that are more likely than not to limit warming to 1.5°C by 2100; these scenarios are characterized by concentrations below 430 ppm CO2-eq by 2100 and 2050 emission reduction between 70 and 95% below 2010. For a comprehensive overview of the characteristics of emissions scenarios, their CO2-equivalent concentrations and their likelihood to keep warming to below a range of temperature levels, see Table 3.1. Reducing emissions of non-CO2 climate forcing agents can be an important element of mitigation strategies. Emissions of nonCO2 gases, nitrous oxide, and fluorinated gases) contributed about 27% to the total emissions of Kyoto gases in 2010. For most non-CO2 gases, near-term, low-cost options are available to reduce their emissions. However, some sources of these non-CO2 gases are difficult to mitigate, such as N2O emissions from fertilizer use and CH4 emissions from livestock. As a result, emissions of most non-CO2 gases will not be reduced to zero, even under stringent mitigation scenarios. The differences in radiative properties and lifetimes of CO2 and non-CO2 climate forcing agents have important implications for mitigation strategies. All current GHG emissions and other climate forcing agents affect the rate and magnitude of climate change over the next few decades. Reducing the emissions of certain short-lived climate forcing agents can reduce the rate of warming in the short term but will have only a limited effect on long-term warming, which is driven mainly by CO2 emissions. There are large uncertainties related to the climate impacts of some of the short-lived climate forcing agents. Although the effects of CH4 emissions are well understood, there are large uncertainties related to the effects of black carbon. Co-emitted components with cooling effects may further complicate and reduce the climate impacts of emission reductions. Reducing emissions of sulfur dioxide would cause warming. Near-term reductions in short-lived climate forcing agents can have a relatively fast impact on climate change and possible co-benefits for air pollution. Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the 21st century to below 2°C relative to pre-industrial levels. GHG emissions in 2030 lie between about 30 GtCO2-eq/yr and 50 GtCO2-eq/yr in cost-effective scenarios that are likely to about as likely as not to limit warming to less than 2°C this century relative to pre-industrial levels . Scenarios with GHG emission levels of above 55 GtCO2-eq/yr require substantially higher rates of emissions reductions between 2030 and 2050 ; much more rapid scale-up of zero and low-carbon energy over this period ; a larger reliance on CDR technologies in the long term; and higher transitional and long-term economic impacts. Estimated global emission levels by 2020 based on the Cancún Pledges are not consistent with cost-effective long-term mitigation trajectories that are at least about as likely as not to limit warming to below 2°C relative to pre-industrial levels , but they do not preclude the option to meet this goal . The Cancún Pledges are broadly consistent with cost-effective scenarios that are likely to limit temperature change to below 3°C relative to pre-industrial levels. Estimates of the aggregate economic costs of mitigation vary widely depending on methodologies and assumptions but increase with the stringency of mitigation. Scenarios in which all countries of the world begin mitigation immediately, in which there is a single global carbon price, and in which all key technologies are available have been used as a cost-effective benchmark for estimating macroeconomic mitigation costs. Under these assumptions, mitigation scenarios that are likely to limit warming to below 2°C through the 21st century relative to pre-industrial levels entail losses in global consumption—not including benefits of reduced climate change as well as co-benefits and adverse side effects of mitigation —of 1 to 4% in 2030, 2 to 6% in 2050, and 3% to 11% in 2100, relative to consumption in baseline scenarios that grows anywhere from 300% to more than 900% over the century . These numbers correspond to an annualized reduction of consumption growth by 0.04 to 0.14 percentage points over the century relative to annualized consumption growth in the baseline that is between 1.6% and 3% per year. In the absence or under limited availability of mitigation technologies , mitigation costs can increase substantially depending on the technology considered . Delaying additional mitigation reduces near-term costs but increases mitigation costs in the medium- to long-term. Many models could not limit likely warming to below 2°C over the 21st century relative to pre-industrial levels, if additional mitigation is considerably delayed, or if availability of key technologies, such as bioenergy, CCS and their combination are limited . Mitigation efforts and associated cost are expected to vary across countries. The distribution of costs can differ from the distribution of the actions themselves. In globally cost-effective scenarios, the majority of mitigation efforts takes place in countries with the highest future GHG emissions in baseline scenarios. Some studies exploring particular effort-sharing frameworks, under the assumption of a global carbon market, have estimated substantial global financial flows associated with mitigation in scenarios that are likely to more unlikely than likely to limit warming during the 21st century to less than 2°C relative to pre-industrial levels. 3.4e: - Characteristics of mitigation pathways There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to pre-industrial levels. These pathways would require substantial emissions reductions over the next few decades and near zero emissions of CO2 and other long-lived greenhouse gases by the end of the century. Implementing such reductions poses substantial technological, economic, social and institutional challenges, which increase with delays in additional mitigation and if key technologies are not available. Limiting warming to lower or higher levels involves similar challenges but on different timescales. Without additional efforts to reduce GHG emissions beyond those in place today, global emission growth is expected to persist driven by growth in global population and economic activities. Global GHG emissions under most scenarios without additional mitigation are between about 75 GtCO2-eq/yr and almost 140 GtCO2-eq/yr in 21003520which is approximately between the 2100 emission levels in the RCP6.0 and RCP8.5 pathways 3621 . Baseline scenarios exceed 450 ppm CO2-eq by 2030 and reach CO2-eq concentration levels between about 750 ppm CO2-eq and more than 1300 ppm CO2-eq by 2100. Global mean surface temperature increases in 2100 range from about 3.7°C to 4.8°C above the average for 1850–1900 for a median climate response. They range from 2.5°C to 7.8°C when including climate uncertainty 3722 . The future scenarios do not account for possible changes in natural forcings in the climate system. Many different combinations of technological, behavioural and policy options can be used to reduce emissions and limit temperature change. To evaluate possible pathways to long-term climate goals, about 900 mitigation scenarios were collected for this assessment, each of which describes different technological, socio-economic and institutional changes. Emission reductions under these scenarios lead to concentrations in 2100 from 430 ppm CO2-eq to above 720 ppm CO2-eq which is comparable to the 2100 forcing levels between RCP2.6 and RCP6.0. Scenarios with concentration levels of below 430 ppm CO2-eq by 2100 were also assessed. Scenarios leading to CO2-eq concentrations in 2100 of about 450 ppm or lower are likely to maintain warming below 2°C over the 21st century relative to pre-industrial levels. Mitigation scenarios reaching concentration levels of about 500 ppm CO2-eq by 2100 are more likely than not to limit warming to less than 2°C relative to pre-industrial levels, unless concentration levels temporarily exceed roughly 530 ppm CO2-eq before 2100. In this case, warming is about as likely as not to remain below 2°C relative to pre-industrial levels. Scenarios that exceed about 650 ppm CO2-eq by 2100 are unlikely to limit warming to below 2°C relative to pre-industrial levels. Mitigation scenarios in which warming is more likely than not to be less than 1.5°C relative to pre-industrial levels by 2100 are characterized by concentration levels by 2100 of below 430 ppm CO2-eq. In these scenarios, temperature peaks during the century and subsequently declines. Mitigation scenarios reaching about 450 ppm CO2-eq in 2100 typically involve temporary overshootof atmospheric concentrations, as do many scenarios reaching about 500 ppm CO2-eq to about 550 ppm CO2-eq by 2100. Depending on the level of overshoot, overshoot scenarios typically rely on the availability and widespread deployment of bioenergy with carbon dioxide capture and storage and afforestation in the second half of the century. The availability and scale of these and other Carbon Dioxide Removal technologies and methods are uncertain, and CDR technologies and methods are, to varying degrees, associated with challenges and risks 3924 . CDR is also prevalent in many scenarios without overshoot to compensate for residual emissions from sectors where mitigation is more expensive. Limiting warming with a likely chance to less than 2°C relative to pre-industrial levels would require substantial cuts in anthropogenic GHG emissions by mid-century through largescale changes in energy systems and possibly land use. Limiting warming to higher levels would require similar changes but less quickly. Limiting warming to lower levels would require these changes more quickly. Scenarios that are likely to maintain warming at below 2°C are characterized by a 40 to 70% reduction in GHG emissions by 2050, relative to 2010 levels, and emissions levels near zero or below in 2100. Scenarios with higher emissions in 2050 are characterized by a greater reliance on CDR technologies beyond mid-century, and vice versa. Scenarios that are likely to maintain warming at below 2°C include more rapid improvements in energy efficiency and a tripling to nearly a quadrupling of the share of zero- and low-carbon energy supply from renewable energy, nuclear energy and fossil energy with carbon dioxide capture and storage or BECCS by the year 2050. The scenarios describe a wide range of changes in land use, reflecting different assumptions about the scale of bioenergy production, afforestation and reduced deforestation. Scenarios leading to concentrations of 500 ppm CO2-eq by 2100 are characterized by a 25 to 55% reduction in GHG emissions by 2050, relative to 2010 levels. Scenarios that are likely to limit warming to 3°C relative to pre-industrial levels reduce emissions less rapidly than those limiting warming to 2°C. Only a limited number of studies provide scenarios that are more likely than not to limit warming to 1.5°C by 2100; these scenarios are characterized by concentrations below 430 ppm CO2-eq by 2100 and 2050 emission reduction between 70 and 95% below 2010. For a comprehensive overview of the characteristics of emissions scenarios, their CO2-equivalent concentrations and their likelihood to keep warming to below a range of temperature levels, see Table 3.1. Reducing emissions of non-CO2 climate forcing agents can be an important element of mitigation strategies. Emissions of nonCO2 gases, nitrous oxide, and fluorinated gases) contributed about 27% to the total emissions of Kyoto gases in 2010. For most non-CO2 gases, near-term, low-cost options are available to reduce their emissions. However, some sources of these non-CO2 gases are difficult to mitigate, such as N2O emissions from fertilizer use and CH4 emissions from livestock. As a result, emissions of most non-CO2 gases will not be reduced to zero, even under stringent mitigation scenarios. The differences in radiative properties and lifetimes of CO2 and non-CO2 climate forcing agents have important implications for mitigation strategies. All current GHG emissions and other climate forcing agents affect the rate and magnitude of climate change over the next few decades. Reducing the emissions of certain short-lived climate forcing agents can reduce the rate of warming in the short term but will have only a limited effect on long-term warming, which is driven mainly by CO2 emissions. There are large uncertainties related to the climate impacts of some of the short-lived climate forcing agents. Although the effects of CH4 emissions are well understood, there are large uncertainties related to the effects of black carbon. Co-emitted components with cooling effects may further complicate and reduce the climate impacts of emission reductions. Reducing emissions of sulfur dioxide would cause warming. Near-term reductions in short-lived climate forcing agents can have a relatively fast impact on climate change and possible co-benefits for air pollution. Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the 21st century to below 2°C relative to pre-industrial levels. GHG emissions in 2030 lie between about 30 GtCO2-eq/yr and 50 GtCO2-eq/yr in cost-effective scenarios that are likely to about as likely as not to limit warming to less than 2°C this century relative to pre-industrial levels . Scenarios with GHG emission levels of above 55 GtCO2-eq/yr require substantially higher rates of emissions reductions between 2030 and 2050 ; much more rapid scale-up of zero and low-carbon energy over this period ; a larger reliance on CDR technologies in the long term; and higher transitional and long-term economic impacts. Estimated global emission levels by 2020 based on the Cancún Pledges are not consistent with cost-effective long-term mitigation trajectories that are at least about as likely as not to limit warming to below 2°C relative to pre-industrial levels , but they do not preclude the option to meet this goal . The Cancún Pledges are broadly consistent with cost-effective scenarios that are likely to limit temperature change to below 3°C relative to pre-industrial levels. Estimates of the aggregate economic costs of mitigation vary widely depending on methodologies and assumptions but increase with the stringency of mitigation. Scenarios in which all countries of the world begin mitigation immediately, in which there is a single global carbon price, and in which all key technologies are available have been used as a cost-effective benchmark for estimating macroeconomic mitigation costs. Under these assumptions, mitigation scenarios that are likely to limit warming to below 2°C through the 21st century relative to pre-industrial levels entail losses in global consumption—not including benefits of reduced climate change as well as co-benefits and adverse side effects of mitigation —of 1 to 4% in 2030, 2 to 6% in 2050, and 3% to 11% in 2100, relative to consumption in baseline scenarios that grows anywhere from 300% to more than 900% over the century . These numbers correspond to an annualized reduction of consumption growth by 0.04 to 0.14 percentage points over the century relative to annualized consumption growth in the baseline that is between 1.6% and 3% per year. In the absence or under limited availability of mitigation technologies , mitigation costs can increase substantially depending on the technology considered . Delaying additional mitigation reduces near-term costs but increases mitigation costs in the medium- to long-term. Many models could not limit likely warming to below 2°C over the 21st century relative to pre-industrial levels, if additional mitigation is considerably delayed, or if availability of key technologies, such as bioenergy, CCS and their combination are limited . Mitigation efforts and associated cost are expected to vary across countries. The distribution of costs can differ from the distribution of the actions themselves. In globally cost-effective scenarios, the majority of mitigation efforts takes place in countries with the highest future GHG emissions in baseline scenarios. Some studies exploring particular effort-sharing frameworks, under the assumption of a global carbon market, have estimated substantial global financial flows associated with mitigation in scenarios that are likely to more unlikely than likely to limit warming during the 21st century to less than 2°C relative to pre-industrial levels. 3.4f: - Characteristics of mitigation pathways There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to pre-industrial levels. These pathways would require substantial emissions reductions over the next few decades and near zero emissions of CO2 and other long-lived greenhouse gases by the end of the century. Implementing such reductions poses substantial technological, economic, social and institutional challenges, which increase with delays in additional mitigation and if key technologies are not available. Limiting warming to lower or higher levels involves similar challenges but on different timescales. Without additional efforts to reduce GHG emissions beyond those in place today, global emission growth is expected to persist driven by growth in global population and economic activities. Global GHG emissions under most scenarios without additional mitigation are between about 75 GtCO2-eq/yr and almost 140 GtCO2-eq/yr in 21003520which is approximately between the 2100 emission levels in the RCP6.0 and RCP8.5 pathways 3621 . Baseline scenarios exceed 450 ppm CO2-eq by 2030 and reach CO2-eq concentration levels between about 750 ppm CO2-eq and more than 1300 ppm CO2-eq by 2100. Global mean surface temperature increases in 2100 range from about 3.7°C to 4.8°C above the average for 1850–1900 for a median climate response. They range from 2.5°C to 7.8°C when including climate uncertainty 3722 . The future scenarios do not account for possible changes in natural forcings in the climate system. Many different combinations of technological, behavioural and policy options can be used to reduce emissions and limit temperature change. To evaluate possible pathways to long-term climate goals, about 900 mitigation scenarios were collected for this assessment, each of which describes different technological, socio-economic and institutional changes. Emission reductions under these scenarios lead to concentrations in 2100 from 430 ppm CO2-eq to above 720 ppm CO2-eq which is comparable to the 2100 forcing levels between RCP2.6 and RCP6.0. Scenarios with concentration levels of below 430 ppm CO2-eq by 2100 were also assessed. Scenarios leading to CO2-eq concentrations in 2100 of about 450 ppm or lower are likely to maintain warming below 2°C over the 21st century relative to pre-industrial levels. Mitigation scenarios reaching concentration levels of about 500 ppm CO2-eq by 2100 are more likely than not to limit warming to less than 2°C relative to pre-industrial levels, unless concentration levels temporarily exceed roughly 530 ppm CO2-eq before 2100. In this case, warming is about as likely as not to remain below 2°C relative to pre-industrial levels. Scenarios that exceed about 650 ppm CO2-eq by 2100 are unlikely to limit warming to below 2°C relative to pre-industrial levels. Mitigation scenarios in which warming is more likely than not to be less than 1.5°C relative to pre-industrial levels by 2100 are characterized by concentration levels by 2100 of below 430 ppm CO2-eq. In these scenarios, temperature peaks during the century and subsequently declines. Mitigation scenarios reaching about 450 ppm CO2-eq in 2100 typically involve temporary overshootof atmospheric concentrations, as do many scenarios reaching about 500 ppm CO2-eq to about 550 ppm CO2-eq by 2100. Depending on the level of overshoot, overshoot scenarios typically rely on the availability and widespread deployment of bioenergy with carbon dioxide capture and storage and afforestation in the second half of the century. The availability and scale of these and other Carbon Dioxide Removal technologies and methods are uncertain, and CDR technologies and methods are, to varying degrees, associated with challenges and risks 3924 . CDR is also prevalent in many scenarios without overshoot to compensate for residual emissions from sectors where mitigation is more expensive. Limiting warming with a likely chance to less than 2°C relative to pre-industrial levels would require substantial cuts in anthropogenic GHG emissions by mid-century through largescale changes in energy systems and possibly land use. Limiting warming to higher levels would require similar changes but less quickly. Limiting warming to lower levels would require these changes more quickly. Scenarios that are likely to maintain warming at below 2°C are characterized by a 40 to 70% reduction in GHG emissions by 2050, relative to 2010 levels, and emissions levels near zero or below in 2100. Scenarios with higher emissions in 2050 are characterized by a greater reliance on CDR technologies beyond mid-century, and vice versa. Scenarios that are likely to maintain warming at below 2°C include more rapid improvements in energy efficiency and a tripling to nearly a quadrupling of the share of zero- and low-carbon energy supply from renewable energy, nuclear energy and fossil energy with carbon dioxide capture and storage or BECCS by the year 2050. The scenarios describe a wide range of changes in land use, reflecting different assumptions about the scale of bioenergy production, afforestation and reduced deforestation. Scenarios leading to concentrations of 500 ppm CO2-eq by 2100 are characterized by a 25 to 55% reduction in GHG emissions by 2050, relative to 2010 levels. Scenarios that are likely to limit warming to 3°C relative to pre-industrial levels reduce emissions less rapidly than those limiting warming to 2°C. Only a limited number of studies provide scenarios that are more likely than not to limit warming to 1.5°C by 2100; these scenarios are characterized by concentrations below 430 ppm CO2-eq by 2100 and 2050 emission reduction between 70 and 95% below 2010. For a comprehensive overview of the characteristics of emissions scenarios, their CO2-equivalent concentrations and their likelihood to keep warming to below a range of temperature levels, see Table 3.1. Reducing emissions of non-CO2 climate forcing agents can be an important element of mitigation strategies. Emissions of nonCO2 gases, nitrous oxide, and fluorinated gases) contributed about 27% to the total emissions of Kyoto gases in 2010. For most non-CO2 gases, near-term, low-cost options are available to reduce their emissions. However, some sources of these non-CO2 gases are difficult to mitigate, such as N2O emissions from fertilizer use and CH4 emissions from livestock. As a result, emissions of most non-CO2 gases will not be reduced to zero, even under stringent mitigation scenarios. The differences in radiative properties and lifetimes of CO2 and non-CO2 climate forcing agents have important implications for mitigation strategies. All current GHG emissions and other climate forcing agents affect the rate and magnitude of climate change over the next few decades. Reducing the emissions of certain short-lived climate forcing agents can reduce the rate of warming in the short term but will have only a limited effect on long-term warming, which is driven mainly by CO2 emissions. There are large uncertainties related to the climate impacts of some of the short-lived climate forcing agents. Although the effects of CH4 emissions are well understood, there are large uncertainties related to the effects of black carbon. Co-emitted components with cooling effects may further complicate and reduce the climate impacts of emission reductions. Reducing emissions of sulfur dioxide would cause warming. Near-term reductions in short-lived climate forcing agents can have a relatively fast impact on climate change and possible co-benefits for air pollution. Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the 21st century to below 2°C relative to pre-industrial levels. GHG emissions in 2030 lie between about 30 GtCO2-eq/yr and 50 GtCO2-eq/yr in cost-effective scenarios that are likely to about as likely as not to limit warming to less than 2°C this century relative to pre-industrial levels . Scenarios with GHG emission levels of above 55 GtCO2-eq/yr require substantially higher rates of emissions reductions between 2030 and 2050 ; much more rapid scale-up of zero and low-carbon energy over this period ; a larger reliance on CDR technologies in the long term; and higher transitional and long-term economic impacts. Estimated global emission levels by 2020 based on the Cancún Pledges are not consistent with cost-effective long-term mitigation trajectories that are at least about as likely as not to limit warming to below 2°C relative to pre-industrial levels , but they do not preclude the option to meet this goal . The Cancún Pledges are broadly consistent with cost-effective scenarios that are likely to limit temperature change to below 3°C relative to pre-industrial levels. Estimates of the aggregate economic costs of mitigation vary widely depending on methodologies and assumptions but increase with the stringency of mitigation. Scenarios in which all countries of the world begin mitigation immediately, in which there is a single global carbon price, and in which all key technologies are available have been used as a cost-effective benchmark for estimating macroeconomic mitigation costs. Under these assumptions, mitigation scenarios that are likely to limit warming to below 2°C through the 21st century relative to pre-industrial levels entail losses in global consumption—not including benefits of reduced climate change as well as co-benefits and adverse side effects of mitigation —of 1 to 4% in 2030, 2 to 6% in 2050, and 3% to 11% in 2100, relative to consumption in baseline scenarios that grows anywhere from 300% to more than 900% over the century . These numbers correspond to an annualized reduction of consumption growth by 0.04 to 0.14 percentage points over the century relative to annualized consumption growth in the baseline that is between 1.6% and 3% per year. In the absence or under limited availability of mitigation technologies , mitigation costs can increase substantially depending on the technology considered . Delaying additional mitigation reduces near-term costs but increases mitigation costs in the medium- to long-term. Many models could not limit likely warming to below 2°C over the 21st century relative to pre-industrial levels, if additional mitigation is considerably delayed, or if availability of key technologies, such as bioenergy, CCS and their combination are limited . Mitigation efforts and associated cost are expected to vary across countries. The distribution of costs can differ from the distribution of the actions themselves. In globally cost-effective scenarios, the majority of mitigation efforts takes place in countries with the highest future GHG emissions in baseline scenarios. Some studies exploring particular effort-sharing frameworks, under the assumption of a global carbon market, have estimated substantial global financial flows associated with mitigation in scenarios that are likely to more unlikely than likely to limit warming during the 21st century to less than 2°C relative to pre-industrial levels. 3.4g: - Characteristics of mitigation pathways There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to pre-industrial levels. These pathways would require substantial emissions reductions over the next few decades and near zero emissions of CO2 and other long-lived greenhouse gases by the end of the century. Implementing such reductions poses substantial technological, economic, social and institutional challenges, which increase with delays in additional mitigation and if key technologies are not available. Limiting warming to lower or higher levels involves similar challenges but on different timescales. Without additional efforts to reduce GHG emissions beyond those in place today, global emission growth is expected to persist driven by growth in global population and economic activities. Global GHG emissions under most scenarios without additional mitigation are between about 75 GtCO2-eq/yr and almost 140 GtCO2-eq/yr in 21003520which is approximately between the 2100 emission levels in the RCP6.0 and RCP8.5 pathways 3621 . Baseline scenarios exceed 450 ppm CO2-eq by 2030 and reach CO2-eq concentration levels between about 750 ppm CO2-eq and more than 1300 ppm CO2-eq by 2100. Global mean surface temperature increases in 2100 range from about 3.7°C to 4.8°C above the average for 1850–1900 for a median climate response. They range from 2.5°C to 7.8°C when including climate uncertainty 3722 . The future scenarios do not account for possible changes in natural forcings in the climate system. Many different combinations of technological, behavioural and policy options can be used to reduce emissions and limit temperature change. To evaluate possible pathways to long-term climate goals, about 900 mitigation scenarios were collected for this assessment, each of which describes different technological, socio-economic and institutional changes. Emission reductions under these scenarios lead to concentrations in 2100 from 430 ppm CO2-eq to above 720 ppm CO2-eq which is comparable to the 2100 forcing levels between RCP2.6 and RCP6.0. Scenarios with concentration levels of below 430 ppm CO2-eq by 2100 were also assessed. Scenarios leading to CO2-eq concentrations in 2100 of about 450 ppm or lower are likely to maintain warming below 2°C over the 21st century relative to pre-industrial levels. Mitigation scenarios reaching concentration levels of about 500 ppm CO2-eq by 2100 are more likely than not to limit warming to less than 2°C relative to pre-industrial levels, unless concentration levels temporarily exceed roughly 530 ppm CO2-eq before 2100. In this case, warming is about as likely as not to remain below 2°C relative to pre-industrial levels. Scenarios that exceed about 650 ppm CO2-eq by 2100 are unlikely to limit warming to below 2°C relative to pre-industrial levels. Mitigation scenarios in which warming is more likely than not to be less than 1.5°C relative to pre-industrial levels by 2100 are characterized by concentration levels by 2100 of below 430 ppm CO2-eq. In these scenarios, temperature peaks during the century and subsequently declines. Mitigation scenarios reaching about 450 ppm CO2-eq in 2100 typically involve temporary overshootof atmospheric concentrations, as do many scenarios reaching about 500 ppm CO2-eq to about 550 ppm CO2-eq by 2100. Depending on the level of overshoot, overshoot scenarios typically rely on the availability and widespread deployment of bioenergy with carbon dioxide capture and storage and afforestation in the second half of the century. The availability and scale of these and other Carbon Dioxide Removal technologies and methods are uncertain, and CDR technologies and methods are, to varying degrees, associated with challenges and risks 3924 . CDR is also prevalent in many scenarios without overshoot to compensate for residual emissions from sectors where mitigation is more expensive. Limiting warming with a likely chance to less than 2°C relative to pre-industrial levels would require substantial cuts in anthropogenic GHG emissions by mid-century through largescale changes in energy systems and possibly land use. Limiting warming to higher levels would require similar changes but less quickly. Limiting warming to lower levels would require these changes more quickly. Scenarios that are likely to maintain warming at below 2°C are characterized by a 40 to 70% reduction in GHG emissions by 2050, relative to 2010 levels, and emissions levels near zero or below in 2100. Scenarios with higher emissions in 2050 are characterized by a greater reliance on CDR technologies beyond mid-century, and vice versa. Scenarios that are likely to maintain warming at below 2°C include more rapid improvements in energy efficiency and a tripling to nearly a quadrupling of the share of zero- and low-carbon energy supply from renewable energy, nuclear energy and fossil energy with carbon dioxide capture and storage or BECCS by the year 2050. The scenarios describe a wide range of changes in land use, reflecting different assumptions about the scale of bioenergy production, afforestation and reduced deforestation. Scenarios leading to concentrations of 500 ppm CO2-eq by 2100 are characterized by a 25 to 55% reduction in GHG emissions by 2050, relative to 2010 levels. Scenarios that are likely to limit warming to 3°C relative to pre-industrial levels reduce emissions less rapidly than those limiting warming to 2°C. Only a limited number of studies provide scenarios that are more likely than not to limit warming to 1.5°C by 2100; these scenarios are characterized by concentrations below 430 ppm CO2-eq by 2100 and 2050 emission reduction between 70 and 95% below 2010. For a comprehensive overview of the characteristics of emissions scenarios, their CO2-equivalent concentrations and their likelihood to keep warming to below a range of temperature levels, see Table 3.1. Reducing emissions of non-CO2 climate forcing agents can be an important element of mitigation strategies. Emissions of nonCO2 gases, nitrous oxide, and fluorinated gases) contributed about 27% to the total emissions of Kyoto gases in 2010. For most non-CO2 gases, near-term, low-cost options are available to reduce their emissions. However, some sources of these non-CO2 gases are difficult to mitigate, such as N2O emissions from fertilizer use and CH4 emissions from livestock. As a result, emissions of most non-CO2 gases will not be reduced to zero, even under stringent mitigation scenarios. The differences in radiative properties and lifetimes of CO2 and non-CO2 climate forcing agents have important implications for mitigation strategies. All current GHG emissions and other climate forcing agents affect the rate and magnitude of climate change over the next few decades. Reducing the emissions of certain short-lived climate forcing agents can reduce the rate of warming in the short term but will have only a limited effect on long-term warming, which is driven mainly by CO2 emissions. There are large uncertainties related to the climate impacts of some of the short-lived climate forcing agents. Although the effects of CH4 emissions are well understood, there are large uncertainties related to the effects of black carbon. Co-emitted components with cooling effects may further complicate and reduce the climate impacts of emission reductions. Reducing emissions of sulfur dioxide would cause warming. Near-term reductions in short-lived climate forcing agents can have a relatively fast impact on climate change and possible co-benefits for air pollution. Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the 21st century to below 2°C relative to pre-industrial levels. GHG emissions in 2030 lie between about 30 GtCO2-eq/yr and 50 GtCO2-eq/yr in cost-effective scenarios that are likely to about as likely as not to limit warming to less than 2°C this century relative to pre-industrial levels . Scenarios with GHG emission levels of above 55 GtCO2-eq/yr require substantially higher rates of emissions reductions between 2030 and 2050 ; much more rapid scale-up of zero and low-carbon energy over this period ; a larger reliance on CDR technologies in the long term; and higher transitional and long-term economic impacts. Estimated global emission levels by 2020 based on the Cancún Pledges are not consistent with cost-effective long-term mitigation trajectories that are at least about as likely as not to limit warming to below 2°C relative to pre-industrial levels , but they do not preclude the option to meet this goal . The Cancún Pledges are broadly consistent with cost-effective scenarios that are likely to limit temperature change to below 3°C relative to pre-industrial levels. Estimates of the aggregate economic costs of mitigation vary widely depending on methodologies and assumptions but increase with the stringency of mitigation. Scenarios in which all countries of the world begin mitigation immediately, in which there is a single global carbon price, and in which all key technologies are available have been used as a cost-effective benchmark for estimating macroeconomic mitigation costs. Under these assumptions, mitigation scenarios that are likely to limit warming to below 2°C through the 21st century relative to pre-industrial levels entail losses in global consumption—not including benefits of reduced climate change as well as co-benefits and adverse side effects of mitigation —of 1 to 4% in 2030, 2 to 6% in 2050, and 3% to 11% in 2100, relative to consumption in baseline scenarios that grows anywhere from 300% to more than 900% over the century . These numbers correspond to an annualized reduction of consumption growth by 0.04 to 0.14 percentage points over the century relative to annualized consumption growth in the baseline that is between 1.6% and 3% per year. In the absence or under limited availability of mitigation technologies , mitigation costs can increase substantially depending on the technology considered . Delaying additional mitigation reduces near-term costs but increases mitigation costs in the medium- to long-term. Many models could not limit likely warming to below 2°C over the 21st century relative to pre-industrial levels, if additional mitigation is considerably delayed, or if availability of key technologies, such as bioenergy, CCS and their combination are limited . Mitigation efforts and associated cost are expected to vary across countries. The distribution of costs can differ from the distribution of the actions themselves. In globally cost-effective scenarios, the majority of mitigation efforts takes place in countries with the highest future GHG emissions in baseline scenarios. Some studies exploring particular effort-sharing frameworks, under the assumption of a global carbon market, have estimated substantial global financial flows associated with mitigation in scenarios that are likely to more unlikely than likely to limit warming during the 21st century to less than 2°C relative to pre-industrial levels. 3.4h: - Characteristics of mitigation pathways There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to pre-industrial levels. These pathways would require substantial emissions reductions over the next few decades and near zero emissions of CO2 and other long-lived greenhouse gases by the end of the century. Implementing such reductions poses substantial technological, economic, social and institutional challenges, which increase with delays in additional mitigation and if key technologies are not available. Limiting warming to lower or higher levels involves similar challenges but on different timescales. Without additional efforts to reduce GHG emissions beyond those in place today, global emission growth is expected to persist driven by growth in global population and economic activities. Global GHG emissions under most scenarios without additional mitigation are between about 75 GtCO2-eq/yr and almost 140 GtCO2-eq/yr in 21003520which is approximately between the 2100 emission levels in the RCP6.0 and RCP8.5 pathways 3621 . Baseline scenarios exceed 450 ppm CO2-eq by 2030 and reach CO2-eq concentration levels between about 750 ppm CO2-eq and more than 1300 ppm CO2-eq by 2100. Global mean surface temperature increases in 2100 range from about 3.7°C to 4.8°C above the average for 1850–1900 for a median climate response. They range from 2.5°C to 7.8°C when including climate uncertainty 3722 . The future scenarios do not account for possible changes in natural forcings in the climate system. Many different combinations of technological, behavioural and policy options can be used to reduce emissions and limit temperature change. To evaluate possible pathways to long-term climate goals, about 900 mitigation scenarios were collected for this assessment, each of which describes different technological, socio-economic and institutional changes. Emission reductions under these scenarios lead to concentrations in 2100 from 430 ppm CO2-eq to above 720 ppm CO2-eq which is comparable to the 2100 forcing levels between RCP2.6 and RCP6.0. Scenarios with concentration levels of below 430 ppm CO2-eq by 2100 were also assessed. Scenarios leading to CO2-eq concentrations in 2100 of about 450 ppm or lower are likely to maintain warming below 2°C over the 21st century relative to pre-industrial levels. Mitigation scenarios reaching concentration levels of about 500 ppm CO2-eq by 2100 are more likely than not to limit warming to less than 2°C relative to pre-industrial levels, unless concentration levels temporarily exceed roughly 530 ppm CO2-eq before 2100. In this case, warming is about as likely as not to remain below 2°C relative to pre-industrial levels. Scenarios that exceed about 650 ppm CO2-eq by 2100 are unlikely to limit warming to below 2°C relative to pre-industrial levels. Mitigation scenarios in which warming is more likely than not to be less than 1.5°C relative to pre-industrial levels by 2100 are characterized by concentration levels by 2100 of below 430 ppm CO2-eq. In these scenarios, temperature peaks during the century and subsequently declines. Mitigation scenarios reaching about 450 ppm CO2-eq in 2100 typically involve temporary overshootof atmospheric concentrations, as do many scenarios reaching about 500 ppm CO2-eq to about 550 ppm CO2-eq by 2100. Depending on the level of overshoot, overshoot scenarios typically rely on the availability and widespread deployment of bioenergy with carbon dioxide capture and storage and afforestation in the second half of the century. The availability and scale of these and other Carbon Dioxide Removal technologies and methods are uncertain, and CDR technologies and methods are, to varying degrees, associated with challenges and risks 3924 . CDR is also prevalent in many scenarios without overshoot to compensate for residual emissions from sectors where mitigation is more expensive. Limiting warming with a likely chance to less than 2°C relative to pre-industrial levels would require substantial cuts in anthropogenic GHG emissions by mid-century through largescale changes in energy systems and possibly land use. Limiting warming to higher levels would require similar changes but less quickly. Limiting warming to lower levels would require these changes more quickly. Scenarios that are likely to maintain warming at below 2°C are characterized by a 40 to 70% reduction in GHG emissions by 2050, relative to 2010 levels, and emissions levels near zero or below in 2100. Scenarios with higher emissions in 2050 are characterized by a greater reliance on CDR technologies beyond mid-century, and vice versa. Scenarios that are likely to maintain warming at below 2°C include more rapid improvements in energy efficiency and a tripling to nearly a quadrupling of the share of zero- and low-carbon energy supply from renewable energy, nuclear energy and fossil energy with carbon dioxide capture and storage or BECCS by the year 2050. The scenarios describe a wide range of changes in land use, reflecting different assumptions about the scale of bioenergy production, afforestation and reduced deforestation. Scenarios leading to concentrations of 500 ppm CO2-eq by 2100 are characterized by a 25 to 55% reduction in GHG emissions by 2050, relative to 2010 levels. Scenarios that are likely to limit warming to 3°C relative to pre-industrial levels reduce emissions less rapidly than those limiting warming to 2°C. Only a limited number of studies provide scenarios that are more likely than not to limit warming to 1.5°C by 2100; these scenarios are characterized by concentrations below 430 ppm CO2-eq by 2100 and 2050 emission reduction between 70 and 95% below 2010. For a comprehensive overview of the characteristics of emissions scenarios, their CO2-equivalent concentrations and their likelihood to keep warming to below a range of temperature levels, see Table 3.1. Reducing emissions of non-CO2 climate forcing agents can be an important element of mitigation strategies. Emissions of nonCO2 gases, nitrous oxide, and fluorinated gases) contributed about 27% to the total emissions of Kyoto gases in 2010. For most non-CO2 gases, near-term, low-cost options are available to reduce their emissions. However, some sources of these non-CO2 gases are difficult to mitigate, such as N2O emissions from fertilizer use and CH4 emissions from livestock. As a result, emissions of most non-CO2 gases will not be reduced to zero, even under stringent mitigation scenarios. The differences in radiative properties and lifetimes of CO2 and non-CO2 climate forcing agents have important implications for mitigation strategies. All current GHG emissions and other climate forcing agents affect the rate and magnitude of climate change over the next few decades. Reducing the emissions of certain short-lived climate forcing agents can reduce the rate of warming in the short term but will have only a limited effect on long-term warming, which is driven mainly by CO2 emissions. There are large uncertainties related to the climate impacts of some of the short-lived climate forcing agents. Although the effects of CH4 emissions are well understood, there are large uncertainties related to the effects of black carbon. Co-emitted components with cooling effects may further complicate and reduce the climate impacts of emission reductions. Reducing emissions of sulfur dioxide would cause warming. Near-term reductions in short-lived climate forcing agents can have a relatively fast impact on climate change and possible co-benefits for air pollution. Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the 21st century to below 2°C relative to pre-industrial levels. GHG emissions in 2030 lie between about 30 GtCO2-eq/yr and 50 GtCO2-eq/yr in cost-effective scenarios that are likely to about as likely as not to limit warming to less than 2°C this century relative to pre-industrial levels . Scenarios with GHG emission levels of above 55 GtCO2-eq/yr require substantially higher rates of emissions reductions between 2030 and 2050 ; much more rapid scale-up of zero and low-carbon energy over this period ; a larger reliance on CDR technologies in the long term; and higher transitional and long-term economic impacts. Estimated global emission levels by 2020 based on the Cancún Pledges are not consistent with cost-effective long-term mitigation trajectories that are at least about as likely as not to limit warming to below 2°C relative to pre-industrial levels , but they do not preclude the option to meet this goal . The Cancún Pledges are broadly consistent with cost-effective scenarios that are likely to limit temperature change to below 3°C relative to pre-industrial levels. Estimates of the aggregate economic costs of mitigation vary widely depending on methodologies and assumptions but increase with the stringency of mitigation. Scenarios in which all countries of the world begin mitigation immediately, in which there is a single global carbon price, and in which all key technologies are available have been used as a cost-effective benchmark for estimating macroeconomic mitigation costs. Under these assumptions, mitigation scenarios that are likely to limit warming to below 2°C through the 21st century relative to pre-industrial levels entail losses in global consumption—not including benefits of reduced climate change as well as co-benefits and adverse side effects of mitigation —of 1 to 4% in 2030, 2 to 6% in 2050, and 3% to 11% in 2100, relative to consumption in baseline scenarios that grows anywhere from 300% to more than 900% over the century . These numbers correspond to an annualized reduction of consumption growth by 0.04 to 0.14 percentage points over the century relative to annualized consumption growth in the baseline that is between 1.6% and 3% per year. In the absence or under limited availability of mitigation technologies , mitigation costs can increase substantially depending on the technology considered . Delaying additional mitigation reduces near-term costs but increases mitigation costs in the medium- to long-term. Many models could not limit likely warming to below 2°C over the 21st century relative to pre-industrial levels, if additional mitigation is considerably delayed, or if availability of key technologies, such as bioenergy, CCS and their combination are limited . Mitigation efforts and associated cost are expected to vary across countries. The distribution of costs can differ from the distribution of the actions themselves. In globally cost-effective scenarios, the majority of mitigation efforts takes place in countries with the highest future GHG emissions in baseline scenarios. Some studies exploring particular effort-sharing frameworks, under the assumption of a global carbon market, have estimated substantial global financial flows associated with mitigation in scenarios that are likely to more unlikely than likely to limit warming during the 21st century to less than 2°C relative to pre-industrial levels. 3.4i: - 'Mitigation There has been a considerable increase in national and subnational mitigation plans and strategies since AR4. In 2012, 67% of global GHG emissions were subject to national legislation or strategies versus 45% in 2007. However, there has not yet been a substantial deviation in global emissions from the past trend. These plans and strategies are in their early stages of development and implementation in many countries, making it difficult to assess their aggregate impact on future global emissions. Since AR4, there has been an increased focus on policies designed to integrate multiple objectives, increase co-benefits and reduce adverse side effects. Governments often explicitly reference co-benefits in climate and sectoral plans and strategies. Sector-specific policies have been more widely used than economy-wide policies . Although most economic theory suggests that economy-wide policies for mitigation would be more cost-effective than sector-specific policies, administrative and political barriers may make economy-wide policies harder to design and implement than sector-specific policies. The latter may be better suited to address barriers or market failures specific to certain sectors and may be bundled in packages of complementary policies In principle, mechanisms that set a carbon price, including cap and trade systems and carbon taxes, can achieve mitigation in a cost-effective way, but have been implemented with diverse effects due in part to national circumstances as well as policy design. The short-run environmental effects of cap and trade systems have been limited as a result of loose caps or caps that have not proved to be constraining. In some countries, tax-based policies specifically aimed at reducing GHG emissions—alongside technology and other policies—have helped to weaken the link between GHG emissions and gross domestic product . In addition, in a large group of countries, fuel taxes have had effects that are akin to sectoral carbon taxes . Revenues from carbon taxes or auctioned emission allowances are used in some countries to reduce other taxes and/or to provide transfers to low-income groups. This illustrates the general principle that mitigation policies that raise government revenue generally have lower social costs than approaches which do not. Economic instruments in the form of subsidies may be applied across sectors, and include a variety of policy designs, such as tax rebates or exemptions, grants, loans and credit lines. An increasing number and variety of RE policies including subsidies—motivated by many factors—have driven escalated growth of RE technologies in recent years. Government policies play a crucial role in accelerating the deployment of RE technologies. Energy access and social and economic development have been the primary drivers in most developing countries whereas secure energy supply and environmental concerns have been most important in developed countries. The focus of policies is broadening from a concentration primarily on RE electricity to include RE heating and cooling and transportation. The reduction of subsidies for GHG-related activities in various sectors can achieve emission reductions, depending on the social and economic context. While subsidies can affect emissions in many sectors, most of the recent literature has focused on subsidies for fossil fuels. Since AR4 a small but growing literature based on economy-wide models has projected that complete removal of subsidies to fossil fuels in all countries could result in reductions in global aggregate emissions by mid-century . Studies vary in methodology, the type and definition of subsidies and the time frame for phase out considered. In particular, the studies assess the impacts of complete removal of all fossil fuel subsides without seeking to assess which subsidies are wasteful and inefficient, keeping in mind national circumstances. Regulatory approaches and information measures are widely used and are often environmentally effective . Examples of regulatory approaches include energy efficiency standards; examples of information programmes include labelling programmes that can help consumers make better-informed decisions. Mitigation policy could devalue fossil fuel assets and reduce revenues for fossil fuel exporters, but differences between regions and fuels exist. Most mitigation scenarios are associated with reduced revenues from coal and oil trade for major exporters. The effect on natural gas export revenues is more uncertain. The availability of CCS would reduce the adverse effect of mitigation on the value of fossil fuel assets. Interactions between or among mitigation policies may be synergistic or may have no additive effect on reducing emissions . For instance, a carbon tax can have an additive environmental effect to policies such as subsidies for the supply of RE. By contrast, if a cap and trade system has a sufficiently stringent cap to affect emission-related decisions, then other policies have no further impact on reducing emissions . In either case, additional policies may be needed to address market failures relating to innovation and technology diffusion. Sub-national climate policies are increasingly prevalent, both in countries with national policies and in those without. These policies include state and provincial climate plans combining market, regulatory and information instruments, and sub-national cap-and-trade systems. In addition, transnational cooperation has arisen among sub-national actors, notably among institutional investors, NGOs seeking to govern carbon offset markets, and networks of cities seeking to collaborate in generating low-carbon urban development. Co-benefits and adverse side effects of mitigation could affect achievement of other objectives such as those related to human health, food security, biodiversity, local environmental quality, energy access, livelihoods and equitable sustainable development: • Mitigation scenarios reaching about 450 or 500 ppm CO2-equivalent by 2100 show reduced costs for achieving air quality and energy security objectives, with significant co-benefits for human health, ecosystem impacts and sufficiency of resources and resilience of the energy system. • Some mitigation policies raise the prices for some energy services and could hamper the ability of societies to expand access to modern energy services to underserved populations . These potential adverse side effects can be avoided with the adoption of complementary policies such as income tax rebates or other benefit transfer mechanisms. The costs of achieving nearly universal access to electricity and clean fuels for cooking and heating are projected to be between USD 72 to 95 billion per year until 2030 with minimal effects on GHG emissions and multiple benefits in health and air pollutant reduction. Whether or not side effects materialize, and to what extent side effects materialize, will be case- and site-specific, and depend on local circumstances and the scale, scope and pace of implementation. Many co-benefits and adverse side effects have not been well-quantified.' 3.4l: - 'Mitigation There has been a considerable increase in national and subnational mitigation plans and strategies since AR4. In 2012, 67% of global GHG emissions were subject to national legislation or strategies versus 45% in 2007. However, there has not yet been a substantial deviation in global emissions from the past trend. These plans and strategies are in their early stages of development and implementation in many countries, making it difficult to assess their aggregate impact on future global emissions. Since AR4, there has been an increased focus on policies designed to integrate multiple objectives, increase co-benefits and reduce adverse side effects. Governments often explicitly reference co-benefits in climate and sectoral plans and strategies. Sector-specific policies have been more widely used than economy-wide policies . Although most economic theory suggests that economy-wide policies for mitigation would be more cost-effective than sector-specific policies, administrative and political barriers may make economy-wide policies harder to design and implement than sector-specific policies. The latter may be better suited to address barriers or market failures specific to certain sectors and may be bundled in packages of complementary policies In principle, mechanisms that set a carbon price, including cap and trade systems and carbon taxes, can achieve mitigation in a cost-effective way, but have been implemented with diverse effects due in part to national circumstances as well as policy design. The short-run environmental effects of cap and trade systems have been limited as a result of loose caps or caps that have not proved to be constraining. In some countries, tax-based policies specifically aimed at reducing GHG emissions—alongside technology and other policies—have helped to weaken the link between GHG emissions and gross domestic product . In addition, in a large group of countries, fuel taxes have had effects that are akin to sectoral carbon taxes . Revenues from carbon taxes or auctioned emission allowances are used in some countries to reduce other taxes and/or to provide transfers to low-income groups. This illustrates the general principle that mitigation policies that raise government revenue generally have lower social costs than approaches which do not. Economic instruments in the form of subsidies may be applied across sectors, and include a variety of policy designs, such as tax rebates or exemptions, grants, loans and credit lines. An increasing number and variety of RE policies including subsidies—motivated by many factors—have driven escalated growth of RE technologies in recent years. Government policies play a crucial role in accelerating the deployment of RE technologies. Energy access and social and economic development have been the primary drivers in most developing countries whereas secure energy supply and environmental concerns have been most important in developed countries. The focus of policies is broadening from a concentration primarily on RE electricity to include RE heating and cooling and transportation. The reduction of subsidies for GHG-related activities in various sectors can achieve emission reductions, depending on the social and economic context. While subsidies can affect emissions in many sectors, most of the recent literature has focused on subsidies for fossil fuels. Since AR4 a small but growing literature based on economy-wide models has projected that complete removal of subsidies to fossil fuels in all countries could result in reductions in global aggregate emissions by mid-century . Studies vary in methodology, the type and definition of subsidies and the time frame for phase out considered. In particular, the studies assess the impacts of complete removal of all fossil fuel subsides without seeking to assess which subsidies are wasteful and inefficient, keeping in mind national circumstances. Regulatory approaches and information measures are widely used and are often environmentally effective . Examples of regulatory approaches include energy efficiency standards; examples of information programmes include labelling programmes that can help consumers make better-informed decisions. Mitigation policy could devalue fossil fuel assets and reduce revenues for fossil fuel exporters, but differences between regions and fuels exist. Most mitigation scenarios are associated with reduced revenues from coal and oil trade for major exporters. The effect on natural gas export revenues is more uncertain. The availability of CCS would reduce the adverse effect of mitigation on the value of fossil fuel assets. Interactions between or among mitigation policies may be synergistic or may have no additive effect on reducing emissions . For instance, a carbon tax can have an additive environmental effect to policies such as subsidies for the supply of RE. By contrast, if a cap and trade system has a sufficiently stringent cap to affect emission-related decisions, then other policies have no further impact on reducing emissions . In either case, additional policies may be needed to address market failures relating to innovation and technology diffusion. Sub-national climate policies are increasingly prevalent, both in countries with national policies and in those without. These policies include state and provincial climate plans combining market, regulatory and information instruments, and sub-national cap-and-trade systems. In addition, transnational cooperation has arisen among sub-national actors, notably among institutional investors, NGOs seeking to govern carbon offset markets, and networks of cities seeking to collaborate in generating low-carbon urban development. Co-benefits and adverse side effects of mitigation could affect achievement of other objectives such as those related to human health, food security, biodiversity, local environmental quality, energy access, livelihoods and equitable sustainable development: • Mitigation scenarios reaching about 450 or 500 ppm CO2-equivalent by 2100 show reduced costs for achieving air quality and energy security objectives, with significant co-benefits for human health, ecosystem impacts and sufficiency of resources and resilience of the energy system. • Some mitigation policies raise the prices for some energy services and could hamper the ability of societies to expand access to modern energy services to underserved populations . These potential adverse side effects can be avoided with the adoption of complementary policies such as income tax rebates or other benefit transfer mechanisms. The costs of achieving nearly universal access to electricity and clean fuels for cooking and heating are projected to be between USD 72 to 95 billion per year until 2030 with minimal effects on GHG emissions and multiple benefits in health and air pollutant reduction. Whether or not side effects materialize, and to what extent side effects materialize, will be case- and site-specific, and depend on local circumstances and the scale, scope and pace of implementation. Many co-benefits and adverse side effects have not been well-quantified.' 4.1a: - Common enabling factors and constraints for adaptation and mitigation responses Adaptation and mitigation responses are underpinned by common enabling factors. These include effective institutions and governance, innovation and investments in environmentally sound technologies and infrastructure, sustainable livelihoods and behavioural and lifestyle choices. Innovation and investments in environmentally sound infrastructure and technologies can reduce greenhouse gas emissions and enhance resilience to climate change . Innovation and change can expand the availability and/ or effectiveness of adaptation and mitigation options. For example, investments in low-carbon and carbon-neutral energy technologies can reduce the energy intensity of economic development, the carbon intensity of energy, GHG emissions, and the long-term costs of mitigation. Similarly, new technologies and infrastructure can increase the resilience of human systems while reducing adverse impacts on natural systems. Investments in technology and infrastructure rely on an enabling policy environment, access to finance and technology and broader economic development that builds capacity . Adaptation and mitigation are constrained by the inertia of global and regional trends in economic development, GHG emissions, resource consumption, infrastructure and settlement patterns, institutional behaviour and technology . Such inertia may limit the capacity to reduce GHG emissions, remain below particular climate thresholds or avoid adverse impacts. Some constraints may be overcome through new technologies, financial resources, increased institutional effectiveness and governance or changes in social and cultural attitudes and behaviours. Vulnerability to climate change, GHG emissions, and the capacity for adaptation and mitigation are strongly influenced by livelihoods, lifestyles, behaviour and culture . Shifts toward more energy-intensive lifestyles can contribute to higher energy and resource consumption, driving greater energy production and GHG emissions and increasing mitigation costs. In contrast, emissions can be substantially lowered through changes in consumption patterns. The social acceptability and/or effectiveness of climate policies are influenced by the extent to which they incentivize or depend on regionally appropriate changes in lifestyles or behaviours. Similarly, livelihoods that depend on climate-sensitive sectors or resources may be particularly vulnerable to climate change and climate change policies. Economic development and urbanization of landscapes exposed to climate hazards may increase the exposure of human settlements and reduce the resilience of natural systems. For many regions and sectors, enhanced capacities to mitigate and adapt are part of the foundation essential for managing climate change risks. Such capacities are place- and context-specific and therefore there is no single approach for reducing risk that is appropriate across all settings. For example, developing nations with low income levels have the lowest financial, technological and institutional capacities to pursue low-carbon, climate-resilient development pathways. Although developed nations generally have greater relative capacity to manage the risks of climate change, such capacity does not necessarily translate into the implementation of adaptation and mitigation options. Improving institutions as well as enhancing coordination and cooperation in governance can help overcome regional constraints associated with mitigation, adaptation and disaster risk reduction. Despite the presence of a wide array of multilateral, national and sub-national institutions focused on adaptation and mitigation, global GHG emissions continue to increase and identified adaptation needs have not been adequately addressed. The implementation of effective adaptation and mitigation options may necessitate new institutions and institutional arrangements that span multiple scales . 4.1b: - Common enabling factors and constraints for adaptation and mitigation responses Adaptation and mitigation responses are underpinned by common enabling factors. These include effective institutions and governance, innovation and investments in environmentally sound technologies and infrastructure, sustainable livelihoods and behavioural and lifestyle choices. Innovation and investments in environmentally sound infrastructure and technologies can reduce greenhouse gas emissions and enhance resilience to climate change . Innovation and change can expand the availability and/ or effectiveness of adaptation and mitigation options. For example, investments in low-carbon and carbon-neutral energy technologies can reduce the energy intensity of economic development, the carbon intensity of energy, GHG emissions, and the long-term costs of mitigation. Similarly, new technologies and infrastructure can increase the resilience of human systems while reducing adverse impacts on natural systems. Investments in technology and infrastructure rely on an enabling policy environment, access to finance and technology and broader economic development that builds capacity . Adaptation and mitigation are constrained by the inertia of global and regional trends in economic development, GHG emissions, resource consumption, infrastructure and settlement patterns, institutional behaviour and technology . Such inertia may limit the capacity to reduce GHG emissions, remain below particular climate thresholds or avoid adverse impacts. Some constraints may be overcome through new technologies, financial resources, increased institutional effectiveness and governance or changes in social and cultural attitudes and behaviours. Vulnerability to climate change, GHG emissions, and the capacity for adaptation and mitigation are strongly influenced by livelihoods, lifestyles, behaviour and culture . Shifts toward more energy-intensive lifestyles can contribute to higher energy and resource consumption, driving greater energy production and GHG emissions and increasing mitigation costs. In contrast, emissions can be substantially lowered through changes in consumption patterns. The social acceptability and/or effectiveness of climate policies are influenced by the extent to which they incentivize or depend on regionally appropriate changes in lifestyles or behaviours. Similarly, livelihoods that depend on climate-sensitive sectors or resources may be particularly vulnerable to climate change and climate change policies. Economic development and urbanization of landscapes exposed to climate hazards may increase the exposure of human settlements and reduce the resilience of natural systems. For many regions and sectors, enhanced capacities to mitigate and adapt are part of the foundation essential for managing climate change risks. Such capacities are place- and context-specific and therefore there is no single approach for reducing risk that is appropriate across all settings. For example, developing nations with low income levels have the lowest financial, technological and institutional capacities to pursue low-carbon, climate-resilient development pathways. Although developed nations generally have greater relative capacity to manage the risks of climate change, such capacity does not necessarily translate into the implementation of adaptation and mitigation options. Improving institutions as well as enhancing coordination and cooperation in governance can help overcome regional constraints associated with mitigation, adaptation and disaster risk reduction. Despite the presence of a wide array of multilateral, national and sub-national institutions focused on adaptation and mitigation, global GHG emissions continue to increase and identified adaptation needs have not been adequately addressed. The implementation of effective adaptation and mitigation options may necessitate new institutions and institutional arrangements that span multiple scales . 4.1c: - Common enabling factors and constraints for adaptation and mitigation responses Adaptation and mitigation responses are underpinned by common enabling factors. These include effective institutions and governance, innovation and investments in environmentally sound technologies and infrastructure, sustainable livelihoods and behavioural and lifestyle choices. Innovation and investments in environmentally sound infrastructure and technologies can reduce greenhouse gas emissions and enhance resilience to climate change . Innovation and change can expand the availability and/ or effectiveness of adaptation and mitigation options. For example, investments in low-carbon and carbon-neutral energy technologies can reduce the energy intensity of economic development, the carbon intensity of energy, GHG emissions, and the long-term costs of mitigation. Similarly, new technologies and infrastructure can increase the resilience of human systems while reducing adverse impacts on natural systems. Investments in technology and infrastructure rely on an enabling policy environment, access to finance and technology and broader economic development that builds capacity . Adaptation and mitigation are constrained by the inertia of global and regional trends in economic development, GHG emissions, resource consumption, infrastructure and settlement patterns, institutional behaviour and technology . Such inertia may limit the capacity to reduce GHG emissions, remain below particular climate thresholds or avoid adverse impacts. Some constraints may be overcome through new technologies, financial resources, increased institutional effectiveness and governance or changes in social and cultural attitudes and behaviours. Vulnerability to climate change, GHG emissions, and the capacity for adaptation and mitigation are strongly influenced by livelihoods, lifestyles, behaviour and culture . Shifts toward more energy-intensive lifestyles can contribute to higher energy and resource consumption, driving greater energy production and GHG emissions and increasing mitigation costs. In contrast, emissions can be substantially lowered through changes in consumption patterns. The social acceptability and/or effectiveness of climate policies are influenced by the extent to which they incentivize or depend on regionally appropriate changes in lifestyles or behaviours. Similarly, livelihoods that depend on climate-sensitive sectors or resources may be particularly vulnerable to climate change and climate change policies. Economic development and urbanization of landscapes exposed to climate hazards may increase the exposure of human settlements and reduce the resilience of natural systems. For many regions and sectors, enhanced capacities to mitigate and adapt are part of the foundation essential for managing climate change risks. Such capacities are place- and context-specific and therefore there is no single approach for reducing risk that is appropriate across all settings. For example, developing nations with low income levels have the lowest financial, technological and institutional capacities to pursue low-carbon, climate-resilient development pathways. Although developed nations generally have greater relative capacity to manage the risks of climate change, such capacity does not necessarily translate into the implementation of adaptation and mitigation options. Improving institutions as well as enhancing coordination and cooperation in governance can help overcome regional constraints associated with mitigation, adaptation and disaster risk reduction. Despite the presence of a wide array of multilateral, national and sub-national institutions focused on adaptation and mitigation, global GHG emissions continue to increase and identified adaptation needs have not been adequately addressed. The implementation of effective adaptation and mitigation options may necessitate new institutions and institutional arrangements that span multiple scales . 4.2a: - 'Response options for adaptation Adaptation options exist in all sectors, but their context for implementation and potential to reduce climate-related risks differs across sectors and regions. Some adaptation responses involve significant co-benefits, synergies and trade-offs. Increasing climate change will increase challenges for many adaptation options. People, governments and the private sector are starting to adapt to a changing climate. Since the IPCC Fourth Assessment Report , understanding of response options has increased, with improved knowledge of their benefits, costs and links to sustainable development. Adaptation can take a variety of approaches depending on its context in vulnerability reduction, disaster risk management or proactive adaptation planning. These include : • Social, ecological asset and infrastructure development • Technological process optimization • Integrated natural resources management • Institutional, educational and behavioural change or reinforcement • Financial services, including risk transfer • Information systems to support early warning and proactive planning There is increasing recognition of the value of social , institutional, and ecosystem-based measures and of the extent of constraints to adaptation. Effective strategies and actions consider the potential for co-benefits and opportunities within wider strategic goals and development plans. Opportunities to enable adaptation planning and implementation exist in all sectors and regions, with diverse potential and approaches depending on context. The need for adaptation along with associated challenges is expected to increase with climate change . Examples of key adaptation approaches for particular sectors, including constraints and limits, are summarized below. Freshwater resources Adaptive water management techniques, including scenario planning, learning-based approaches and flexible and low-regret solutions, can help adjust to uncertain hydrological changes due to climate change and their impacts . Strategies include adopting integrated water management, augmenting supply, reducing the mismatch between water supply and demand, reducing non-climate stressors, strengthening institutional capacities and adopting more water-efficient technologies and water-saving strategies. Terrestrial and freshwater ecosystems Management actions can reduce but not eliminate risks of impacts to terrestrial and freshwater ecosystems due to climate change. Actions include maintenance of genetic diversity, assisted species migration and dispersal, manipulation of disturbance regimes and reduction of other stressors. Management options that reduce non-climatic stressors, such as habitat modification, overexploitation, pollution and invasive species, increase the inherent capacity of ecosystems and their species to adapt to a changing climate. Other options include improving early warning systems and associated response systems. Enhanced connectivity of vulnerable ecosystems may also assist autonomous adaptation. Translocation of species is controversial and is expected to become less feasible where whole ecosystems are at risk. Coastal systems and low-lying areas Increasingly, coastal adaptation options include those based on integrated coastal zone management, local community participation, ecosystems-based approaches and disaster risk reduction, mainstreamed into relevant strategies and management plans. The analysis and implementation of coastal adaptation has progressed more significantly in developed countries than in developing countries. The relative costs of coastal adaptation are expected to vary strongly among and within regions and countries. Marine systems and oceans Marine forecasting and early warning systems as well as reducing non-climatic stressors have the potential to reduce risks for some fisheries and aquaculture industries, but options for unique ecosystems such as coral reefs are limited. Fisheries and some aquaculture industries with high-technology and/or large investments have high capacities for adaptation due to greater development of environmental monitoring, modelling and resource assessments. Adaptation options include large-scale translocation of industrial fishing activities and flexible management that can react to variability and change. For smaller-scale fisheries and nations with limited adaptive capacities, building social resilience, alternative livelihoods and occupational flexibility are important strategies. Adaptation options for coral reef systems are generally limited to reducing other stressors, mainly by enhancing water quality and limiting pressures from tourism and fishing, but their efficacy will be severely reduced as thermal stress and ocean acidification increase. Food production system/Rural areas Adaptation options for agriculture include technological responses, enhancing smallholder access to credit and other critical production resources, strengthening institutions at local to regional levels and improving market access through trade reform. Responses to decreased food production and quality include: developing new crop varieties adapted to changes in CO2, temperature, and drought; enhancing the capacity for climate risk management; and offsetting economic impacts of land use change. Improving financial support and investing in the production of small-scale farms can also provide benefits. Expanding agricultural markets and improving the predictability and reliability of the world trading system could result in reduced market volatility and help manage food supply shortages caused by climate change. Urban areas/Key economic sectors and services Urban adaptation benefits from effective multi-level governance, alignment of policies and incentives, strengthened local government and community adaptation capacity, synergies with the private sector and appropriate financing and institutional development. Enhancing the capacity of low-income groups and vulnerable communities and their partnerships with local governments can also be an effective urban climate adaptation strategy. Examples of adaptation mechanisms include large-scale public-private risk reduction initiatives and economic diversification and government insurance for the non-diversifiable portion of risk. In some locations, especially at the upper end of projected climate changes, responses could also require transformational changes such as managed retreat. Human health, security and livelihoods Adaptation options that focus on strengthening existing delivery systems and institutions, as well as insurance and social protection strategies, can improve health, security and livelihoods in the near term. The most effective vulnerability reduction measures for health in the near term are programmes that implement and improve basic public health measures such as provision of clean water and sanitation, secure essential health care including vaccination and child health services, increase capacity for disaster preparedness and response and alleviate poverty. Options to address heat related mortality include health warning systems linked to response strategies, urban planning and improvements to the built environment to reduce heat stress. Robust institutions can manage many transboundary impacts of climate change to reduce risk of conflicts over shared natural resources. Insurance programmes, social protection measures and disaster risk management may enhance long-term livelihood resilience among the poor and marginalized people, if policies address multi-dimensional poverty. Significant co-benefits, synergies and trade-offs exist between adaptation and mitigation and among different adaptation responses; interactions occur both within and across regions and sectors. For example, investments in crop varieties adapted to climate change can increase the capacity to cope with drought, and public health measures to address vector-borne diseases can enhance the capacity of health systems to address other challenges. Similarly, locating infrastructure away from low-lying coastal areas helps settlements and ecosystems adapt to sea level rise while also protecting against tsunamis. However, some adaptation options may have adverse side effects that imply real or perceived trade-offs with other adaptation objectives, mitigation objectives or broader development goals. For example, while protection of ecosystems can assist adaptation to climate change and enhance carbon storage, increased use of air conditioning to maintain thermal comfort in buildings or the use of desalination to enhance water resource security can increase energy demand, and therefore, GHG emissions.' 4.2b: - 'Human responses to climate change: adaptation and mitigation Adaptation and mitigation experience is accumulating across regions and scales, even while global anthropogenic greenhouse gas emissions have continued to increase. Throughout history, people and societies have adjusted to and coped with climate, climate variability and extremes, with varying degrees of success. In today’s changing climate, accumulating experience with adaptation and mitigation efforts can provide opportunities for learning and refinement. Adaptation is becoming embedded in some planning processes, with more limited implementation of responses . Engineered and technological options are commonly implemented adaptive responses, often integrated within existing programmes, such as disaster risk management and water management. There is increasing recognition of the value of social, institutional and ecosystem-based measures and of the extent of constraints to adaptation. Governments at various levels have begun to develop adaptation plans and policies and integrate climate change considerations into broader development plans. Examples of adaptation are now available from all regions of the world . Global increases in anthropogenic emissions and climate impacts have occurred, even while mitigation activities have taken place in many parts of the world. Though various mitigation initiatives between the sub-national and global scales have been developed or implemented, a full assessment of their impact may be premature.' - 'Response options for adaptation Adaptation options exist in all sectors, but their context for implementation and potential to reduce climate-related risks differs across sectors and regions. Some adaptation responses involve significant co-benefits, synergies and trade-offs. Increasing climate change will increase challenges for many adaptation options. People, governments and the private sector are starting to adapt to a changing climate. Since the IPCC Fourth Assessment Report , understanding of response options has increased, with improved knowledge of their benefits, costs and links to sustainable development. Adaptation can take a variety of approaches depending on its context in vulnerability reduction, disaster risk management or proactive adaptation planning. These include : • Social, ecological asset and infrastructure development • Technological process optimization • Integrated natural resources management • Institutional, educational and behavioural change or reinforcement • Financial services, including risk transfer • Information systems to support early warning and proactive planning There is increasing recognition of the value of social , institutional, and ecosystem-based measures and of the extent of constraints to adaptation. Effective strategies and actions consider the potential for co-benefits and opportunities within wider strategic goals and development plans. Opportunities to enable adaptation planning and implementation exist in all sectors and regions, with diverse potential and approaches depending on context. The need for adaptation along with associated challenges is expected to increase with climate change . Examples of key adaptation approaches for particular sectors, including constraints and limits, are summarized below. Freshwater resources Adaptive water management techniques, including scenario planning, learning-based approaches and flexible and low-regret solutions, can help adjust to uncertain hydrological changes due to climate change and their impacts . Strategies include adopting integrated water management, augmenting supply, reducing the mismatch between water supply and demand, reducing non-climate stressors, strengthening institutional capacities and adopting more water-efficient technologies and water-saving strategies. Terrestrial and freshwater ecosystems Management actions can reduce but not eliminate risks of impacts to terrestrial and freshwater ecosystems due to climate change. Actions include maintenance of genetic diversity, assisted species migration and dispersal, manipulation of disturbance regimes and reduction of other stressors. Management options that reduce non-climatic stressors, such as habitat modification, overexploitation, pollution and invasive species, increase the inherent capacity of ecosystems and their species to adapt to a changing climate. Other options include improving early warning systems and associated response systems. Enhanced connectivity of vulnerable ecosystems may also assist autonomous adaptation. Translocation of species is controversial and is expected to become less feasible where whole ecosystems are at risk. Coastal systems and low-lying areas Increasingly, coastal adaptation options include those based on integrated coastal zone management, local community participation, ecosystems-based approaches and disaster risk reduction, mainstreamed into relevant strategies and management plans. The analysis and implementation of coastal adaptation has progressed more significantly in developed countries than in developing countries. The relative costs of coastal adaptation are expected to vary strongly among and within regions and countries. Marine systems and oceans Marine forecasting and early warning systems as well as reducing non-climatic stressors have the potential to reduce risks for some fisheries and aquaculture industries, but options for unique ecosystems such as coral reefs are limited. Fisheries and some aquaculture industries with high-technology and/or large investments have high capacities for adaptation due to greater development of environmental monitoring, modelling and resource assessments. Adaptation options include large-scale translocation of industrial fishing activities and flexible management that can react to variability and change. For smaller-scale fisheries and nations with limited adaptive capacities, building social resilience, alternative livelihoods and occupational flexibility are important strategies. Adaptation options for coral reef systems are generally limited to reducing other stressors, mainly by enhancing water quality and limiting pressures from tourism and fishing, but their efficacy will be severely reduced as thermal stress and ocean acidification increase. Food production system/Rural areas Adaptation options for agriculture include technological responses, enhancing smallholder access to credit and other critical production resources, strengthening institutions at local to regional levels and improving market access through trade reform. Responses to decreased food production and quality include: developing new crop varieties adapted to changes in CO2, temperature, and drought; enhancing the capacity for climate risk management; and offsetting economic impacts of land use change. Improving financial support and investing in the production of small-scale farms can also provide benefits. Expanding agricultural markets and improving the predictability and reliability of the world trading system could result in reduced market volatility and help manage food supply shortages caused by climate change. Urban areas/Key economic sectors and services Urban adaptation benefits from effective multi-level governance, alignment of policies and incentives, strengthened local government and community adaptation capacity, synergies with the private sector and appropriate financing and institutional development. Enhancing the capacity of low-income groups and vulnerable communities and their partnerships with local governments can also be an effective urban climate adaptation strategy. Examples of adaptation mechanisms include large-scale public-private risk reduction initiatives and economic diversification and government insurance for the non-diversifiable portion of risk. In some locations, especially at the upper end of projected climate changes, responses could also require transformational changes such as managed retreat. Human health, security and livelihoods Adaptation options that focus on strengthening existing delivery systems and institutions, as well as insurance and social protection strategies, can improve health, security and livelihoods in the near term. The most effective vulnerability reduction measures for health in the near term are programmes that implement and improve basic public health measures such as provision of clean water and sanitation, secure essential health care including vaccination and child health services, increase capacity for disaster preparedness and response and alleviate poverty. Options to address heat related mortality include health warning systems linked to response strategies, urban planning and improvements to the built environment to reduce heat stress. Robust institutions can manage many transboundary impacts of climate change to reduce risk of conflicts over shared natural resources. Insurance programmes, social protection measures and disaster risk management may enhance long-term livelihood resilience among the poor and marginalized people, if policies address multi-dimensional poverty. Significant co-benefits, synergies and trade-offs exist between adaptation and mitigation and among different adaptation responses; interactions occur both within and across regions and sectors. For example, investments in crop varieties adapted to climate change can increase the capacity to cope with drought, and public health measures to address vector-borne diseases can enhance the capacity of health systems to address other challenges. Similarly, locating infrastructure away from low-lying coastal areas helps settlements and ecosystems adapt to sea level rise while also protecting against tsunamis. However, some adaptation options may have adverse side effects that imply real or perceived trade-offs with other adaptation objectives, mitigation objectives or broader development goals. For example, while protection of ecosystems can assist adaptation to climate change and enhance carbon storage, increased use of air conditioning to maintain thermal comfort in buildings or the use of desalination to enhance water resource security can increase energy demand, and therefore, GHG emissions.' - 'Adaptation Adaptation experience is accumulating across regions in the public and private sector and within communities . Adaptation options adopted to date emphasize incremental adjustments and co-benefits and are starting to emphasize flexibility and learning. Most assessments of adaptation have been restricted to impacts, vulnerability and adaptation planning, with very few assessing the processes of implementation or the effects of adaptation actions . National governments play key roles in adaptation planning and implementation. There has been substantial progress since the AR4 in the development of national adaptation strategies and plans. This includes National Adaptation Programmes of Action by least developed countries, the National Adaptation Plan process, and strategic frameworks for national adaptation in Organisation for Economic Co-operation and Development countries. National governments can coordinate adaptation efforts of local and sub-national governments, for example by protecting vulnerable groups, by supporting economic diversification, and by providing information, policy and legal frameworks and financial support. While local government and the private sector have different functions, which vary regionally, they are increasingly recognized as critical to progress in adaptation, given their roles in scaling up adaptation of communities, households and civil society and in managing risk information and financing . There is a significant increase in the number of planned adaptation responses at the local level in rural and urban communities of developed and developing countries since the AR4. However, local councils and planners are often confronted by the complexity of adaptation without adequate access to guiding information or data on local vulnerabilities and potential impacts. Steps for mainstreaming adaptation into local decision-making have been identified but challenges remain in their implementation. Hence, scholars stress the important role of linkages with national and sub-national levels of government as well as partnerships among public, civic and private sectors in implementing local adaptation responses. Institutional dimensions of adaptation governance, including the integration of adaptation into planning and decision-making, play a key role in promoting the transition from planning to implementation of adaptation . The most commonly emphasized institutional barriers or enablers for adaptation planning and implementation are: 1) multilevel institutional co-ordination between different political and administrative levels in society; 2) key actors, advocates and champions initiating, mainstreaming and sustaining momentum for climate adaptation; 3) horizontal interplay between sectors, actors and policies operating at similar administrative levels; 4) political dimensions in planning and implementation; and 5) coordination between formal governmental, administrative agencies and private sectors and stakeholders to increase efficiency, representation and support for climate adaptation measures. Existing and emerging economic instruments can foster adaptation by providing incentives for anticipating and reducing impacts. Instruments include public-private finance partnerships, loans, payments for environmental services, improved resource pricing, charges and subsidies, norms and regulations and risk sharing and transfer mechanisms. Risk financing mechanisms in the public and private sector, such as insurance and risk pools, can contribute to increasing resilience, but without attention to major design challenges, they can also provide disincentives, cause market failure and decrease equity. Governments often play key roles as regulators, providers or insurers of last resort.' 4.2c: - 'Response options for adaptation Adaptation options exist in all sectors, but their context for implementation and potential to reduce climate-related risks differs across sectors and regions. Some adaptation responses involve significant co-benefits, synergies and trade-offs. Increasing climate change will increase challenges for many adaptation options. People, governments and the private sector are starting to adapt to a changing climate. Since the IPCC Fourth Assessment Report , understanding of response options has increased, with improved knowledge of their benefits, costs and links to sustainable development. Adaptation can take a variety of approaches depending on its context in vulnerability reduction, disaster risk management or proactive adaptation planning. These include : • Social, ecological asset and infrastructure development • Technological process optimization • Integrated natural resources management • Institutional, educational and behavioural change or reinforcement • Financial services, including risk transfer • Information systems to support early warning and proactive planning There is increasing recognition of the value of social , institutional, and ecosystem-based measures and of the extent of constraints to adaptation. Effective strategies and actions consider the potential for co-benefits and opportunities within wider strategic goals and development plans. Opportunities to enable adaptation planning and implementation exist in all sectors and regions, with diverse potential and approaches depending on context. The need for adaptation along with associated challenges is expected to increase with climate change . Examples of key adaptation approaches for particular sectors, including constraints and limits, are summarized below. Freshwater resources Adaptive water management techniques, including scenario planning, learning-based approaches and flexible and low-regret solutions, can help adjust to uncertain hydrological changes due to climate change and their impacts . Strategies include adopting integrated water management, augmenting supply, reducing the mismatch between water supply and demand, reducing non-climate stressors, strengthening institutional capacities and adopting more water-efficient technologies and water-saving strategies. Terrestrial and freshwater ecosystems Management actions can reduce but not eliminate risks of impacts to terrestrial and freshwater ecosystems due to climate change. Actions include maintenance of genetic diversity, assisted species migration and dispersal, manipulation of disturbance regimes and reduction of other stressors. Management options that reduce non-climatic stressors, such as habitat modification, overexploitation, pollution and invasive species, increase the inherent capacity of ecosystems and their species to adapt to a changing climate. Other options include improving early warning systems and associated response systems. Enhanced connectivity of vulnerable ecosystems may also assist autonomous adaptation. Translocation of species is controversial and is expected to become less feasible where whole ecosystems are at risk. Coastal systems and low-lying areas Increasingly, coastal adaptation options include those based on integrated coastal zone management, local community participation, ecosystems-based approaches and disaster risk reduction, mainstreamed into relevant strategies and management plans. The analysis and implementation of coastal adaptation has progressed more significantly in developed countries than in developing countries. The relative costs of coastal adaptation are expected to vary strongly among and within regions and countries. Marine systems and oceans Marine forecasting and early warning systems as well as reducing non-climatic stressors have the potential to reduce risks for some fisheries and aquaculture industries, but options for unique ecosystems such as coral reefs are limited. Fisheries and some aquaculture industries with high-technology and/or large investments have high capacities for adaptation due to greater development of environmental monitoring, modelling and resource assessments. Adaptation options include large-scale translocation of industrial fishing activities and flexible management that can react to variability and change. For smaller-scale fisheries and nations with limited adaptive capacities, building social resilience, alternative livelihoods and occupational flexibility are important strategies. Adaptation options for coral reef systems are generally limited to reducing other stressors, mainly by enhancing water quality and limiting pressures from tourism and fishing, but their efficacy will be severely reduced as thermal stress and ocean acidification increase. Food production system/Rural areas Adaptation options for agriculture include technological responses, enhancing smallholder access to credit and other critical production resources, strengthening institutions at local to regional levels and improving market access through trade reform. Responses to decreased food production and quality include: developing new crop varieties adapted to changes in CO2, temperature, and drought; enhancing the capacity for climate risk management; and offsetting economic impacts of land use change. Improving financial support and investing in the production of small-scale farms can also provide benefits. Expanding agricultural markets and improving the predictability and reliability of the world trading system could result in reduced market volatility and help manage food supply shortages caused by climate change. Urban areas/Key economic sectors and services Urban adaptation benefits from effective multi-level governance, alignment of policies and incentives, strengthened local government and community adaptation capacity, synergies with the private sector and appropriate financing and institutional development. Enhancing the capacity of low-income groups and vulnerable communities and their partnerships with local governments can also be an effective urban climate adaptation strategy. Examples of adaptation mechanisms include large-scale public-private risk reduction initiatives and economic diversification and government insurance for the non-diversifiable portion of risk. In some locations, especially at the upper end of projected climate changes, responses could also require transformational changes such as managed retreat. Human health, security and livelihoods Adaptation options that focus on strengthening existing delivery systems and institutions, as well as insurance and social protection strategies, can improve health, security and livelihoods in the near term. The most effective vulnerability reduction measures for health in the near term are programmes that implement and improve basic public health measures such as provision of clean water and sanitation, secure essential health care including vaccination and child health services, increase capacity for disaster preparedness and response and alleviate poverty. Options to address heat related mortality include health warning systems linked to response strategies, urban planning and improvements to the built environment to reduce heat stress. Robust institutions can manage many transboundary impacts of climate change to reduce risk of conflicts over shared natural resources. Insurance programmes, social protection measures and disaster risk management may enhance long-term livelihood resilience among the poor and marginalized people, if policies address multi-dimensional poverty. Significant co-benefits, synergies and trade-offs exist between adaptation and mitigation and among different adaptation responses; interactions occur both within and across regions and sectors. For example, investments in crop varieties adapted to climate change can increase the capacity to cope with drought, and public health measures to address vector-borne diseases can enhance the capacity of health systems to address other challenges. Similarly, locating infrastructure away from low-lying coastal areas helps settlements and ecosystems adapt to sea level rise while also protecting against tsunamis. However, some adaptation options may have adverse side effects that imply real or perceived trade-offs with other adaptation objectives, mitigation objectives or broader development goals. For example, while protection of ecosystems can assist adaptation to climate change and enhance carbon storage, increased use of air conditioning to maintain thermal comfort in buildings or the use of desalination to enhance water resource security can increase energy demand, and therefore, GHG emissions.' 4.3a: - Response options for mitigation Mitigation options are available in every major sector. Mitigation can be more cost-effective if using an integrated approach that combines measures to reduce energy use and the greenhouse gas intensity of end-use sectors, decarbonize energy supply, reduce net emissions and enhance carbon sinks in land-based sectors. A broad range of sectoral mitigation options is available that can reduce GHG emission intensity, improve energy intensity through enhancements of technology, behaviour, production and resource efficiency and enable structural changes or changes in activity. In addition, direct options in agriculture, forestry and other land use involve reducing CO2 emissions by reducing deforestation, forest degradation and forest fires; storing carbon in terrestrial systems; and providing bioenergy feedstocks. Options to reduce non-CO2 emissions exist across all sectors but most notably in agriculture, energy supply and industry. An overview of sectoral mitigation options and potentials is provided in Table 4.4. Well-designed systemic and cross-sectoral mitigation strategies are more cost-effective in cutting emissions than a focus on individual technologies and sectors with efforts in one sector affecting the need for mitigation in others . In baseline scenarios without new mitigation policies, GHG emissions are projected to grow in all sectors, except for net CO2 emissions in the AFOLU sector. Mitigation scenarios reaching around 450 ppm CO2-eq concentration by 21004328 show largescale global changes in the energy supply sector . While rapid decarbonization of energy supply generally entails more flexibility for end-use and AFOLU sectors, stronger demand reductions lessen the mitigation challenge for the supply side of the energy system. There are thus strong interdependencies across sectors and the resulting distribution of the mitigation effort is strongly influenced by the availability and performance of future technologies, particularly BECCS and large scale afforestation . The next two decades present a window of opportunity for mitigation in urban areas, as a large portion of the world’s urban areas will be developed during this period. Decarbonizing electricity generation is a key component of cost-effective mitigation strategies in achieving low stabilization levels . In most integrated modelling scenarios, decarbonization happens more rapidly in electricity generation than in the industry, buildings and transport sectors. In scenarios reaching 450 ppm CO2-eq concentrations by 2100, global CO2 emissions from the energy supply sector are projected to decline over the next decade and are characterized by reductions of 90% or more below 2010 levels between 2040 and 2070. Efficiency enhancements and behavioural changes, in order to reduce energy demand compared to baseline scenarios without compromising development, are a key mitigation strategy in scenarios reaching atmospheric CO2-eq concentrations of about 450 to about 500 ppm by 2100 . Near-term reductions in energy demand are an important element of cost-effective mitigation strategies, provide more flexibility for reducing carbon intensity in the energy supply sector, hedge against related supply-side risks, avoid lock-in to carbon-intensive infrastructures and are associated with important co-benefits . Emissions can be substantially lowered through changes in consumption patterns and dietary change and reduction in food wastes. A number of options including monetary and non-monetary incentives as well as information measures may facilitate behavioural changes. Decarbonization of the energy supply sector requires upscaling of low- and zero-carbon electricity generation technologies. In the majority of low-concentration stabilization scenarios , the share of low-carbon electricity supply, nuclear and CCS, including BECCS) increases from the current share of approximately 30% to more than 80% by 2050 and 90% by 2100, and fossil fuel power generation without CCS is phased out almost entirely by 2100. Among these low-carbon technologies, a growing number of RE technologies have achieved a level of maturity to enable deployment at significant scale since AR4 and nuclear energy is a mature low-GHG emission source of baseload power, but its share of global electricity generation has been declining. GHG emissions from energy supply can be reduced significantly by replacing current world average coal-fired power plants with modern, highly efficient natural gas combined-cycle power plants or combined heat and power plants, provided that natural gas is available and the fugitive emissions associated with extraction and supply are low or mitigated. Behaviour, lifestyle and culture have a considerable influence on energy use and associated emissions, with high mitigation potential in some sectors, in particular when complementing technological and structural change . In the transport sector, technical and behavioural mitigation measures for all modes, plus new infrastructure and urban redevelopment investments, could reduce final energy demand significantly below baseline levels. While opportunities for switching to low-carbon fuels exist, the rate of decarbonization in the transport sector might be constrained by challenges associated with energy storage and the relatively low energy density of low-carbon transport fuels. In the building sector, recent advances in technologies, know-how and policies provide opportunities to stabilize or reduce global energy use to about current levels by mid-century. In addition, recent improvements in performance and costs make very low energy construction and retrofits of buildings economically attractive, sometimes even at net negative costs. In the industry sector, improvements in GHG emission efficiency and in the efficiency of material use, recycling and reuse of materials and products, and overall reductions in product demand and service demand could, in addition to energy efficiency, help reduce GHG emissions below the baseline level. Prevalent approaches for promoting energy efficiency in industry include information programmes followed by economic instruments, regulatory approaches and voluntary actions. Important options for mitigation in waste management are waste reduction, followed by re-use, recycling and energy recovery. The most cost-effective mitigation options in forestry are afforestation, sustainable forest management and reducing deforestation, with large differences in their relative importance across regions. In agriculture, the most cost-effective mitigation options are cropland management, grazing land management and restoration of organic soils . About a third of mitigation potential in forestry can be achieved at a cost <20 USD/tCO2-eq emission. Demand-side measures, such as changes in diet and reductions of losses in the food supply chain, have a significant, but uncertain, potential to reduce GHG emissions from food production . Bioenergy can play a critical role for mitigation, but there are issues to consider, such as the sustainability of practices and the efficiency of bioenergy systems . Evidence suggests that bioenergy options with low lifecycle emissions, some already available, can reduce GHG emissions; outcomes are site-specific and rely on efficient integrated ‘biomassto-bioenergy systems’, and sustainable land use management and governance. Barriers to large-scale deployment of bioenergy include concerns about GHG emissions from land, food security, water resources, biodiversity conservation and livelihoods. Mitigation measures intersect with other societal goals, creating the possibility of co-benefits or adverse side-effects. These intersections, if well-managed, can strengthen the basis for undertaking climate mitigation actions . Mitigation can positively or negatively influence the achievement of other societal goals, such as those related to human health, food security, biodiversity, local environmental quality, energy access, livelihoods and equitable sustainable development . On the other hand, policies towards other societal goals can influence the achievement of mitigation and adaptation objectives. These influences can be substantial, although sometimes difficult to quantify, especially in welfare terms. This multi-objective perspective is important in part because it helps to identify areas where support for policies that advance multiple goals will be robust. Potential co-benefits and adverse side effects of the main sectoral mitigation measures are summarized in Table 4.5. Overall, the potential for co-benefits for energy end-use measures outweigh the potential for adverse side effects, whereas the evidence suggests this may not be the case for all energy supply and AFOLU measures. 4.3b: - Response options for mitigation Mitigation options are available in every major sector. Mitigation can be more cost-effective if using an integrated approach that combines measures to reduce energy use and the greenhouse gas intensity of end-use sectors, decarbonize energy supply, reduce net emissions and enhance carbon sinks in land-based sectors. A broad range of sectoral mitigation options is available that can reduce GHG emission intensity, improve energy intensity through enhancements of technology, behaviour, production and resource efficiency and enable structural changes or changes in activity. In addition, direct options in agriculture, forestry and other land use involve reducing CO2 emissions by reducing deforestation, forest degradation and forest fires; storing carbon in terrestrial systems; and providing bioenergy feedstocks. Options to reduce non-CO2 emissions exist across all sectors but most notably in agriculture, energy supply and industry. An overview of sectoral mitigation options and potentials is provided in Table 4.4. Well-designed systemic and cross-sectoral mitigation strategies are more cost-effective in cutting emissions than a focus on individual technologies and sectors with efforts in one sector affecting the need for mitigation in others . In baseline scenarios without new mitigation policies, GHG emissions are projected to grow in all sectors, except for net CO2 emissions in the AFOLU sector. Mitigation scenarios reaching around 450 ppm CO2-eq concentration by 21004328 show largescale global changes in the energy supply sector . While rapid decarbonization of energy supply generally entails more flexibility for end-use and AFOLU sectors, stronger demand reductions lessen the mitigation challenge for the supply side of the energy system. There are thus strong interdependencies across sectors and the resulting distribution of the mitigation effort is strongly influenced by the availability and performance of future technologies, particularly BECCS and large scale afforestation . The next two decades present a window of opportunity for mitigation in urban areas, as a large portion of the world’s urban areas will be developed during this period. Decarbonizing electricity generation is a key component of cost-effective mitigation strategies in achieving low stabilization levels . In most integrated modelling scenarios, decarbonization happens more rapidly in electricity generation than in the industry, buildings and transport sectors. In scenarios reaching 450 ppm CO2-eq concentrations by 2100, global CO2 emissions from the energy supply sector are projected to decline over the next decade and are characterized by reductions of 90% or more below 2010 levels between 2040 and 2070. Efficiency enhancements and behavioural changes, in order to reduce energy demand compared to baseline scenarios without compromising development, are a key mitigation strategy in scenarios reaching atmospheric CO2-eq concentrations of about 450 to about 500 ppm by 2100 . Near-term reductions in energy demand are an important element of cost-effective mitigation strategies, provide more flexibility for reducing carbon intensity in the energy supply sector, hedge against related supply-side risks, avoid lock-in to carbon-intensive infrastructures and are associated with important co-benefits . Emissions can be substantially lowered through changes in consumption patterns and dietary change and reduction in food wastes. A number of options including monetary and non-monetary incentives as well as information measures may facilitate behavioural changes. Decarbonization of the energy supply sector requires upscaling of low- and zero-carbon electricity generation technologies. In the majority of low-concentration stabilization scenarios , the share of low-carbon electricity supply, nuclear and CCS, including BECCS) increases from the current share of approximately 30% to more than 80% by 2050 and 90% by 2100, and fossil fuel power generation without CCS is phased out almost entirely by 2100. Among these low-carbon technologies, a growing number of RE technologies have achieved a level of maturity to enable deployment at significant scale since AR4 and nuclear energy is a mature low-GHG emission source of baseload power, but its share of global electricity generation has been declining. GHG emissions from energy supply can be reduced significantly by replacing current world average coal-fired power plants with modern, highly efficient natural gas combined-cycle power plants or combined heat and power plants, provided that natural gas is available and the fugitive emissions associated with extraction and supply are low or mitigated. Behaviour, lifestyle and culture have a considerable influence on energy use and associated emissions, with high mitigation potential in some sectors, in particular when complementing technological and structural change . In the transport sector, technical and behavioural mitigation measures for all modes, plus new infrastructure and urban redevelopment investments, could reduce final energy demand significantly below baseline levels. While opportunities for switching to low-carbon fuels exist, the rate of decarbonization in the transport sector might be constrained by challenges associated with energy storage and the relatively low energy density of low-carbon transport fuels. In the building sector, recent advances in technologies, know-how and policies provide opportunities to stabilize or reduce global energy use to about current levels by mid-century. In addition, recent improvements in performance and costs make very low energy construction and retrofits of buildings economically attractive, sometimes even at net negative costs. In the industry sector, improvements in GHG emission efficiency and in the efficiency of material use, recycling and reuse of materials and products, and overall reductions in product demand and service demand could, in addition to energy efficiency, help reduce GHG emissions below the baseline level. Prevalent approaches for promoting energy efficiency in industry include information programmes followed by economic instruments, regulatory approaches and voluntary actions. Important options for mitigation in waste management are waste reduction, followed by re-use, recycling and energy recovery. The most cost-effective mitigation options in forestry are afforestation, sustainable forest management and reducing deforestation, with large differences in their relative importance across regions. In agriculture, the most cost-effective mitigation options are cropland management, grazing land management and restoration of organic soils . About a third of mitigation potential in forestry can be achieved at a cost <20 USD/tCO2-eq emission. Demand-side measures, such as changes in diet and reductions of losses in the food supply chain, have a significant, but uncertain, potential to reduce GHG emissions from food production . Bioenergy can play a critical role for mitigation, but there are issues to consider, such as the sustainability of practices and the efficiency of bioenergy systems . Evidence suggests that bioenergy options with low lifecycle emissions, some already available, can reduce GHG emissions; outcomes are site-specific and rely on efficient integrated ‘biomassto-bioenergy systems’, and sustainable land use management and governance. Barriers to large-scale deployment of bioenergy include concerns about GHG emissions from land, food security, water resources, biodiversity conservation and livelihoods. Mitigation measures intersect with other societal goals, creating the possibility of co-benefits or adverse side-effects. These intersections, if well-managed, can strengthen the basis for undertaking climate mitigation actions . Mitigation can positively or negatively influence the achievement of other societal goals, such as those related to human health, food security, biodiversity, local environmental quality, energy access, livelihoods and equitable sustainable development . On the other hand, policies towards other societal goals can influence the achievement of mitigation and adaptation objectives. These influences can be substantial, although sometimes difficult to quantify, especially in welfare terms. This multi-objective perspective is important in part because it helps to identify areas where support for policies that advance multiple goals will be robust. Potential co-benefits and adverse side effects of the main sectoral mitigation measures are summarized in Table 4.5. Overall, the potential for co-benefits for energy end-use measures outweigh the potential for adverse side effects, whereas the evidence suggests this may not be the case for all energy supply and AFOLU measures. 4.3d: - Common enabling factors and constraints for adaptation and mitigation responses Adaptation and mitigation responses are underpinned by common enabling factors. These include effective institutions and governance, innovation and investments in environmentally sound technologies and infrastructure, sustainable livelihoods and behavioural and lifestyle choices. Innovation and investments in environmentally sound infrastructure and technologies can reduce greenhouse gas emissions and enhance resilience to climate change . Innovation and change can expand the availability and/ or effectiveness of adaptation and mitigation options. For example, investments in low-carbon and carbon-neutral energy technologies can reduce the energy intensity of economic development, the carbon intensity of energy, GHG emissions, and the long-term costs of mitigation. Similarly, new technologies and infrastructure can increase the resilience of human systems while reducing adverse impacts on natural systems. Investments in technology and infrastructure rely on an enabling policy environment, access to finance and technology and broader economic development that builds capacity . Adaptation and mitigation are constrained by the inertia of global and regional trends in economic development, GHG emissions, resource consumption, infrastructure and settlement patterns, institutional behaviour and technology . Such inertia may limit the capacity to reduce GHG emissions, remain below particular climate thresholds or avoid adverse impacts. Some constraints may be overcome through new technologies, financial resources, increased institutional effectiveness and governance or changes in social and cultural attitudes and behaviours. Vulnerability to climate change, GHG emissions, and the capacity for adaptation and mitigation are strongly influenced by livelihoods, lifestyles, behaviour and culture . Shifts toward more energy-intensive lifestyles can contribute to higher energy and resource consumption, driving greater energy production and GHG emissions and increasing mitigation costs. In contrast, emissions can be substantially lowered through changes in consumption patterns. The social acceptability and/or effectiveness of climate policies are influenced by the extent to which they incentivize or depend on regionally appropriate changes in lifestyles or behaviours. Similarly, livelihoods that depend on climate-sensitive sectors or resources may be particularly vulnerable to climate change and climate change policies. Economic development and urbanization of landscapes exposed to climate hazards may increase the exposure of human settlements and reduce the resilience of natural systems. For many regions and sectors, enhanced capacities to mitigate and adapt are part of the foundation essential for managing climate change risks. Such capacities are place- and context-specific and therefore there is no single approach for reducing risk that is appropriate across all settings. For example, developing nations with low income levels have the lowest financial, technological and institutional capacities to pursue low-carbon, climate-resilient development pathways. Although developed nations generally have greater relative capacity to manage the risks of climate change, such capacity does not necessarily translate into the implementation of adaptation and mitigation options. Improving institutions as well as enhancing coordination and cooperation in governance can help overcome regional constraints associated with mitigation, adaptation and disaster risk reduction. Despite the presence of a wide array of multilateral, national and sub-national institutions focused on adaptation and mitigation, global GHG emissions continue to increase and identified adaptation needs have not been adequately addressed. The implementation of effective adaptation and mitigation options may necessitate new institutions and institutional arrangements that span multiple scales . - Response options for mitigation Mitigation options are available in every major sector. Mitigation can be more cost-effective if using an integrated approach that combines measures to reduce energy use and the greenhouse gas intensity of end-use sectors, decarbonize energy supply, reduce net emissions and enhance carbon sinks in land-based sectors. A broad range of sectoral mitigation options is available that can reduce GHG emission intensity, improve energy intensity through enhancements of technology, behaviour, production and resource efficiency and enable structural changes or changes in activity. In addition, direct options in agriculture, forestry and other land use involve reducing CO2 emissions by reducing deforestation, forest degradation and forest fires; storing carbon in terrestrial systems; and providing bioenergy feedstocks. Options to reduce non-CO2 emissions exist across all sectors but most notably in agriculture, energy supply and industry. An overview of sectoral mitigation options and potentials is provided in Table 4.4. Well-designed systemic and cross-sectoral mitigation strategies are more cost-effective in cutting emissions than a focus on individual technologies and sectors with efforts in one sector affecting the need for mitigation in others . In baseline scenarios without new mitigation policies, GHG emissions are projected to grow in all sectors, except for net CO2 emissions in the AFOLU sector. Mitigation scenarios reaching around 450 ppm CO2-eq concentration by 21004328 show largescale global changes in the energy supply sector . While rapid decarbonization of energy supply generally entails more flexibility for end-use and AFOLU sectors, stronger demand reductions lessen the mitigation challenge for the supply side of the energy system. There are thus strong interdependencies across sectors and the resulting distribution of the mitigation effort is strongly influenced by the availability and performance of future technologies, particularly BECCS and large scale afforestation . The next two decades present a window of opportunity for mitigation in urban areas, as a large portion of the world’s urban areas will be developed during this period. Decarbonizing electricity generation is a key component of cost-effective mitigation strategies in achieving low stabilization levels . In most integrated modelling scenarios, decarbonization happens more rapidly in electricity generation than in the industry, buildings and transport sectors. In scenarios reaching 450 ppm CO2-eq concentrations by 2100, global CO2 emissions from the energy supply sector are projected to decline over the next decade and are characterized by reductions of 90% or more below 2010 levels between 2040 and 2070. Efficiency enhancements and behavioural changes, in order to reduce energy demand compared to baseline scenarios without compromising development, are a key mitigation strategy in scenarios reaching atmospheric CO2-eq concentrations of about 450 to about 500 ppm by 2100 . Near-term reductions in energy demand are an important element of cost-effective mitigation strategies, provide more flexibility for reducing carbon intensity in the energy supply sector, hedge against related supply-side risks, avoid lock-in to carbon-intensive infrastructures and are associated with important co-benefits . Emissions can be substantially lowered through changes in consumption patterns and dietary change and reduction in food wastes. A number of options including monetary and non-monetary incentives as well as information measures may facilitate behavioural changes. Decarbonization of the energy supply sector requires upscaling of low- and zero-carbon electricity generation technologies. In the majority of low-concentration stabilization scenarios , the share of low-carbon electricity supply, nuclear and CCS, including BECCS) increases from the current share of approximately 30% to more than 80% by 2050 and 90% by 2100, and fossil fuel power generation without CCS is phased out almost entirely by 2100. Among these low-carbon technologies, a growing number of RE technologies have achieved a level of maturity to enable deployment at significant scale since AR4 and nuclear energy is a mature low-GHG emission source of baseload power, but its share of global electricity generation has been declining. GHG emissions from energy supply can be reduced significantly by replacing current world average coal-fired power plants with modern, highly efficient natural gas combined-cycle power plants or combined heat and power plants, provided that natural gas is available and the fugitive emissions associated with extraction and supply are low or mitigated. Behaviour, lifestyle and culture have a considerable influence on energy use and associated emissions, with high mitigation potential in some sectors, in particular when complementing technological and structural change . In the transport sector, technical and behavioural mitigation measures for all modes, plus new infrastructure and urban redevelopment investments, could reduce final energy demand significantly below baseline levels. While opportunities for switching to low-carbon fuels exist, the rate of decarbonization in the transport sector might be constrained by challenges associated with energy storage and the relatively low energy density of low-carbon transport fuels. In the building sector, recent advances in technologies, know-how and policies provide opportunities to stabilize or reduce global energy use to about current levels by mid-century. In addition, recent improvements in performance and costs make very low energy construction and retrofits of buildings economically attractive, sometimes even at net negative costs. In the industry sector, improvements in GHG emission efficiency and in the efficiency of material use, recycling and reuse of materials and products, and overall reductions in product demand and service demand could, in addition to energy efficiency, help reduce GHG emissions below the baseline level. Prevalent approaches for promoting energy efficiency in industry include information programmes followed by economic instruments, regulatory approaches and voluntary actions. Important options for mitigation in waste management are waste reduction, followed by re-use, recycling and energy recovery. The most cost-effective mitigation options in forestry are afforestation, sustainable forest management and reducing deforestation, with large differences in their relative importance across regions. In agriculture, the most cost-effective mitigation options are cropland management, grazing land management and restoration of organic soils . About a third of mitigation potential in forestry can be achieved at a cost <20 USD/tCO2-eq emission. Demand-side measures, such as changes in diet and reductions of losses in the food supply chain, have a significant, but uncertain, potential to reduce GHG emissions from food production . Bioenergy can play a critical role for mitigation, but there are issues to consider, such as the sustainability of practices and the efficiency of bioenergy systems . Evidence suggests that bioenergy options with low lifecycle emissions, some already available, can reduce GHG emissions; outcomes are site-specific and rely on efficient integrated ‘biomassto-bioenergy systems’, and sustainable land use management and governance. Barriers to large-scale deployment of bioenergy include concerns about GHG emissions from land, food security, water resources, biodiversity conservation and livelihoods. Mitigation measures intersect with other societal goals, creating the possibility of co-benefits or adverse side-effects. These intersections, if well-managed, can strengthen the basis for undertaking climate mitigation actions . Mitigation can positively or negatively influence the achievement of other societal goals, such as those related to human health, food security, biodiversity, local environmental quality, energy access, livelihoods and equitable sustainable development . On the other hand, policies towards other societal goals can influence the achievement of mitigation and adaptation objectives. These influences can be substantial, although sometimes difficult to quantify, especially in welfare terms. This multi-objective perspective is important in part because it helps to identify areas where support for policies that advance multiple goals will be robust. Potential co-benefits and adverse side effects of the main sectoral mitigation measures are summarized in Table 4.5. Overall, the potential for co-benefits for energy end-use measures outweigh the potential for adverse side effects, whereas the evidence suggests this may not be the case for all energy supply and AFOLU measures. 4.4b: - 'International and regional cooperation on adaptation and mitigation Because climate change has the characteristics of a collective action problem at the global scale, effective mitigation will not be achieved if individual agents advance their own interests independently, even though mitigation can also have local co-benefits. Cooperative responses, including international cooperation, are therefore required to effectively mitigate GHG emissions and address other climate change issues. While adaptation focuses primarily on local to national scale outcomes, its effectiveness can be enhanced through coordination across governance scales, including international cooperation. In fact, international cooperation has helped to facilitate the creation of adaptation strategies, plans, and actions at national, sub-national, and local levels. A variety of climate policy instruments have been employed, and even more could be employed, at international and regional levels to address mitigation and to support and promote adaptation at national and sub-national scales. Evidence suggests that outcomes seen as equitable can lead to more effective cooperation. The United Nations Framework Convention on Climate Change is the main multilateral forum focused on addressing climate change, with nearly universal participation. UNFCCC activities since 2007, which include the 2010 Cancún Agreements and the 2011 Durban Platform for Enhanced Action, have sought to enhance actions under the Convention, and have led to an increasing number of institutions and other arrangements for international climate change cooperation. Other institutions organized at different levels of governance have resulted in diversifying international climate change cooperation. Existing and proposed international climate change cooperation arrangements vary in their focus and degree of centralization and coordination. They span: multilateral agreements, harmonized national policies and decentralized but coordinated national policies, as well as regional and regionally-coordinated policies . While a number of new institutions are focused on adaptation funding and coordination, adaptation has historically received less attention than mitigation in international climate policy . Inclusion of adaptation is increasingly important to reduce the risk from climate change impacts and may engage a greater number of countries. The Kyoto Protocol offers lessons towards achieving the ultimate objective of the UNFCCC, particularly with respect to participation, implementation, flexibility mechanisms, and environmental effectiveness. The Protocol was the first binding step toward implementing the principles and goals provided by the UNFCCC. According to national GHG inventories through 2012 submitted to the UNFCCC by October 2013, Annex B Parties with quantified emission limitations in aggregate may have bettered their collective emission reduction target in the first commitment period, but some emissions reductions that would have occurred even in its absence were also counted. The Protocol’s Clean Development Mechanism created a market for emissions offsets from developing countries, the purpose being two-fold: to help Annex I countries fulfill their commitments and to assist non-Annex I countries achieve sustainable development. The CDM generated Certified Emission Reductions equivalent to emissions of over 1.4 GtCO2-eqby October 2013, led to significant project investments, and generated investment flows for a variety of functions, including the UNFCCC Adaptation Fund. However, its environmental effectiveness has been questioned by some, particularly in regard to its early years, due to concerns about the additionality of projects , the validity of baselines, and the possibility of emissions leakage . Such concerns about additionality are common to any emission-reduction-credit program, and are not specific to the CDM. Due to market forces, the majority of single CDM projects have been concentrated in a limited number of countries, while Programmes of Activities, though less frequent, have been more evenly distributed. In addition, the Kyoto Protocol created two other ‘flexibility mechanisms’: Joint Implementation and International Emissions Trading. Several conceptual models for effort-sharing have been identified in research. However, realized distributional impacts from actual international cooperative agreements depend not only on the approach taken but also on criteria applied to operationalize equity and the manner in which developing countries’ emissions reduction plans are financed. Policy linkages among regional, national and sub-national climate policies offer potential climate change mitigation benefits. Linkages have been established between carbon markets and in principle could also be established between and among a heterogeneous set of policy instruments including non-market-based policies, such as performance standards. Potential advantages include lower mitigation costs, decreased emission leakage and increased market liquidity. Regional initiatives between national and global scales are being developed and implemented, but their impact on global mitigation has been limited to date. Some climate policies could be more environmentally and economically effective if implemented across broad regions, such as by embodying mitigation objectives in trade agreements or jointly constructing infrastructures that facilitate reduction in carbon emissions. International cooperation for supporting adaptation planning and implementation has assisted in the creation of adaptation strategies, plans and actions at national, sub-national and local levels. For example, a range of multilateral and regionally targeted funding mechanisms have been established for adaptation; UN agencies, international development organizations and non-governmental organisations have provided information, methodologies and guidelines; and global and regional initiatives supported and promoted the creation of national adaptation strategies in both developing and developed countries. Closer integration of disaster risk reduction and climate change adaptation at the international level, and the mainstreaming of both into international development assistance, may foster greater efficiency in the use of resources and capacity. However, stronger efforts at the international level do not necessarily lead to substantive and rapid results at the local level.' 4.4g: - 'Adaptation Adaptation experience is accumulating across regions in the public and private sector and within communities . Adaptation options adopted to date emphasize incremental adjustments and co-benefits and are starting to emphasize flexibility and learning. Most assessments of adaptation have been restricted to impacts, vulnerability and adaptation planning, with very few assessing the processes of implementation or the effects of adaptation actions . National governments play key roles in adaptation planning and implementation. There has been substantial progress since the AR4 in the development of national adaptation strategies and plans. This includes National Adaptation Programmes of Action by least developed countries, the National Adaptation Plan process, and strategic frameworks for national adaptation in Organisation for Economic Co-operation and Development countries. National governments can coordinate adaptation efforts of local and sub-national governments, for example by protecting vulnerable groups, by supporting economic diversification, and by providing information, policy and legal frameworks and financial support. While local government and the private sector have different functions, which vary regionally, they are increasingly recognized as critical to progress in adaptation, given their roles in scaling up adaptation of communities, households and civil society and in managing risk information and financing . There is a significant increase in the number of planned adaptation responses at the local level in rural and urban communities of developed and developing countries since the AR4. However, local councils and planners are often confronted by the complexity of adaptation without adequate access to guiding information or data on local vulnerabilities and potential impacts. Steps for mainstreaming adaptation into local decision-making have been identified but challenges remain in their implementation. Hence, scholars stress the important role of linkages with national and sub-national levels of government as well as partnerships among public, civic and private sectors in implementing local adaptation responses. Institutional dimensions of adaptation governance, including the integration of adaptation into planning and decision-making, play a key role in promoting the transition from planning to implementation of adaptation . The most commonly emphasized institutional barriers or enablers for adaptation planning and implementation are: 1) multilevel institutional co-ordination between different political and administrative levels in society; 2) key actors, advocates and champions initiating, mainstreaming and sustaining momentum for climate adaptation; 3) horizontal interplay between sectors, actors and policies operating at similar administrative levels; 4) political dimensions in planning and implementation; and 5) coordination between formal governmental, administrative agencies and private sectors and stakeholders to increase efficiency, representation and support for climate adaptation measures. Existing and emerging economic instruments can foster adaptation by providing incentives for anticipating and reducing impacts. Instruments include public-private finance partnerships, loans, payments for environmental services, improved resource pricing, charges and subsidies, norms and regulations and risk sharing and transfer mechanisms. Risk financing mechanisms in the public and private sector, such as insurance and risk pools, can contribute to increasing resilience, but without attention to major design challenges, they can also provide disincentives, cause market failure and decrease equity. Governments often play key roles as regulators, providers or insurers of last resort.' - 'Mitigation There has been a considerable increase in national and subnational mitigation plans and strategies since AR4. In 2012, 67% of global GHG emissions were subject to national legislation or strategies versus 45% in 2007. However, there has not yet been a substantial deviation in global emissions from the past trend. These plans and strategies are in their early stages of development and implementation in many countries, making it difficult to assess their aggregate impact on future global emissions. Since AR4, there has been an increased focus on policies designed to integrate multiple objectives, increase co-benefits and reduce adverse side effects. Governments often explicitly reference co-benefits in climate and sectoral plans and strategies. Sector-specific policies have been more widely used than economy-wide policies . Although most economic theory suggests that economy-wide policies for mitigation would be more cost-effective than sector-specific policies, administrative and political barriers may make economy-wide policies harder to design and implement than sector-specific policies. The latter may be better suited to address barriers or market failures specific to certain sectors and may be bundled in packages of complementary policies In principle, mechanisms that set a carbon price, including cap and trade systems and carbon taxes, can achieve mitigation in a cost-effective way, but have been implemented with diverse effects due in part to national circumstances as well as policy design. The short-run environmental effects of cap and trade systems have been limited as a result of loose caps or caps that have not proved to be constraining. In some countries, tax-based policies specifically aimed at reducing GHG emissions—alongside technology and other policies—have helped to weaken the link between GHG emissions and gross domestic product . In addition, in a large group of countries, fuel taxes have had effects that are akin to sectoral carbon taxes . Revenues from carbon taxes or auctioned emission allowances are used in some countries to reduce other taxes and/or to provide transfers to low-income groups. This illustrates the general principle that mitigation policies that raise government revenue generally have lower social costs than approaches which do not. Economic instruments in the form of subsidies may be applied across sectors, and include a variety of policy designs, such as tax rebates or exemptions, grants, loans and credit lines. An increasing number and variety of RE policies including subsidies—motivated by many factors—have driven escalated growth of RE technologies in recent years. Government policies play a crucial role in accelerating the deployment of RE technologies. Energy access and social and economic development have been the primary drivers in most developing countries whereas secure energy supply and environmental concerns have been most important in developed countries. The focus of policies is broadening from a concentration primarily on RE electricity to include RE heating and cooling and transportation. The reduction of subsidies for GHG-related activities in various sectors can achieve emission reductions, depending on the social and economic context. While subsidies can affect emissions in many sectors, most of the recent literature has focused on subsidies for fossil fuels. Since AR4 a small but growing literature based on economy-wide models has projected that complete removal of subsidies to fossil fuels in all countries could result in reductions in global aggregate emissions by mid-century . Studies vary in methodology, the type and definition of subsidies and the time frame for phase out considered. In particular, the studies assess the impacts of complete removal of all fossil fuel subsides without seeking to assess which subsidies are wasteful and inefficient, keeping in mind national circumstances. Regulatory approaches and information measures are widely used and are often environmentally effective . Examples of regulatory approaches include energy efficiency standards; examples of information programmes include labelling programmes that can help consumers make better-informed decisions. Mitigation policy could devalue fossil fuel assets and reduce revenues for fossil fuel exporters, but differences between regions and fuels exist. Most mitigation scenarios are associated with reduced revenues from coal and oil trade for major exporters. The effect on natural gas export revenues is more uncertain. The availability of CCS would reduce the adverse effect of mitigation on the value of fossil fuel assets. Interactions between or among mitigation policies may be synergistic or may have no additive effect on reducing emissions . For instance, a carbon tax can have an additive environmental effect to policies such as subsidies for the supply of RE. By contrast, if a cap and trade system has a sufficiently stringent cap to affect emission-related decisions, then other policies have no further impact on reducing emissions . In either case, additional policies may be needed to address market failures relating to innovation and technology diffusion. Sub-national climate policies are increasingly prevalent, both in countries with national policies and in those without. These policies include state and provincial climate plans combining market, regulatory and information instruments, and sub-national cap-and-trade systems. In addition, transnational cooperation has arisen among sub-national actors, notably among institutional investors, NGOs seeking to govern carbon offset markets, and networks of cities seeking to collaborate in generating low-carbon urban development. Co-benefits and adverse side effects of mitigation could affect achievement of other objectives such as those related to human health, food security, biodiversity, local environmental quality, energy access, livelihoods and equitable sustainable development: • Mitigation scenarios reaching about 450 or 500 ppm CO2-equivalent by 2100 show reduced costs for achieving air quality and energy security objectives, with significant co-benefits for human health, ecosystem impacts and sufficiency of resources and resilience of the energy system. • Some mitigation policies raise the prices for some energy services and could hamper the ability of societies to expand access to modern energy services to underserved populations . These potential adverse side effects can be avoided with the adoption of complementary policies such as income tax rebates or other benefit transfer mechanisms. The costs of achieving nearly universal access to electricity and clean fuels for cooking and heating are projected to be between USD 72 to 95 billion per year until 2030 with minimal effects on GHG emissions and multiple benefits in health and air pollutant reduction. Whether or not side effects materialize, and to what extent side effects materialize, will be case- and site-specific, and depend on local circumstances and the scale, scope and pace of implementation. Many co-benefits and adverse side effects have not been well-quantified.' 4.4l: - 'Mitigation There has been a considerable increase in national and subnational mitigation plans and strategies since AR4. In 2012, 67% of global GHG emissions were subject to national legislation or strategies versus 45% in 2007. However, there has not yet been a substantial deviation in global emissions from the past trend. These plans and strategies are in their early stages of development and implementation in many countries, making it difficult to assess their aggregate impact on future global emissions. Since AR4, there has been an increased focus on policies designed to integrate multiple objectives, increase co-benefits and reduce adverse side effects. Governments often explicitly reference co-benefits in climate and sectoral plans and strategies. Sector-specific policies have been more widely used than economy-wide policies . Although most economic theory suggests that economy-wide policies for mitigation would be more cost-effective than sector-specific policies, administrative and political barriers may make economy-wide policies harder to design and implement than sector-specific policies. The latter may be better suited to address barriers or market failures specific to certain sectors and may be bundled in packages of complementary policies In principle, mechanisms that set a carbon price, including cap and trade systems and carbon taxes, can achieve mitigation in a cost-effective way, but have been implemented with diverse effects due in part to national circumstances as well as policy design. The short-run environmental effects of cap and trade systems have been limited as a result of loose caps or caps that have not proved to be constraining. In some countries, tax-based policies specifically aimed at reducing GHG emissions—alongside technology and other policies—have helped to weaken the link between GHG emissions and gross domestic product . In addition, in a large group of countries, fuel taxes have had effects that are akin to sectoral carbon taxes . Revenues from carbon taxes or auctioned emission allowances are used in some countries to reduce other taxes and/or to provide transfers to low-income groups. This illustrates the general principle that mitigation policies that raise government revenue generally have lower social costs than approaches which do not. Economic instruments in the form of subsidies may be applied across sectors, and include a variety of policy designs, such as tax rebates or exemptions, grants, loans and credit lines. An increasing number and variety of RE policies including subsidies—motivated by many factors—have driven escalated growth of RE technologies in recent years. Government policies play a crucial role in accelerating the deployment of RE technologies. Energy access and social and economic development have been the primary drivers in most developing countries whereas secure energy supply and environmental concerns have been most important in developed countries. The focus of policies is broadening from a concentration primarily on RE electricity to include RE heating and cooling and transportation. The reduction of subsidies for GHG-related activities in various sectors can achieve emission reductions, depending on the social and economic context. While subsidies can affect emissions in many sectors, most of the recent literature has focused on subsidies for fossil fuels. Since AR4 a small but growing literature based on economy-wide models has projected that complete removal of subsidies to fossil fuels in all countries could result in reductions in global aggregate emissions by mid-century . Studies vary in methodology, the type and definition of subsidies and the time frame for phase out considered. In particular, the studies assess the impacts of complete removal of all fossil fuel subsides without seeking to assess which subsidies are wasteful and inefficient, keeping in mind national circumstances. Regulatory approaches and information measures are widely used and are often environmentally effective . Examples of regulatory approaches include energy efficiency standards; examples of information programmes include labelling programmes that can help consumers make better-informed decisions. Mitigation policy could devalue fossil fuel assets and reduce revenues for fossil fuel exporters, but differences between regions and fuels exist. Most mitigation scenarios are associated with reduced revenues from coal and oil trade for major exporters. The effect on natural gas export revenues is more uncertain. The availability of CCS would reduce the adverse effect of mitigation on the value of fossil fuel assets. Interactions between or among mitigation policies may be synergistic or may have no additive effect on reducing emissions . For instance, a carbon tax can have an additive environmental effect to policies such as subsidies for the supply of RE. By contrast, if a cap and trade system has a sufficiently stringent cap to affect emission-related decisions, then other policies have no further impact on reducing emissions . In either case, additional policies may be needed to address market failures relating to innovation and technology diffusion. Sub-national climate policies are increasingly prevalent, both in countries with national policies and in those without. These policies include state and provincial climate plans combining market, regulatory and information instruments, and sub-national cap-and-trade systems. In addition, transnational cooperation has arisen among sub-national actors, notably among institutional investors, NGOs seeking to govern carbon offset markets, and networks of cities seeking to collaborate in generating low-carbon urban development. Co-benefits and adverse side effects of mitigation could affect achievement of other objectives such as those related to human health, food security, biodiversity, local environmental quality, energy access, livelihoods and equitable sustainable development: • Mitigation scenarios reaching about 450 or 500 ppm CO2-equivalent by 2100 show reduced costs for achieving air quality and energy security objectives, with significant co-benefits for human health, ecosystem impacts and sufficiency of resources and resilience of the energy system. • Some mitigation policies raise the prices for some energy services and could hamper the ability of societies to expand access to modern energy services to underserved populations . These potential adverse side effects can be avoided with the adoption of complementary policies such as income tax rebates or other benefit transfer mechanisms. The costs of achieving nearly universal access to electricity and clean fuels for cooking and heating are projected to be between USD 72 to 95 billion per year until 2030 with minimal effects on GHG emissions and multiple benefits in health and air pollutant reduction. Whether or not side effects materialize, and to what extent side effects materialize, will be case- and site-specific, and depend on local circumstances and the scale, scope and pace of implementation. Many co-benefits and adverse side effects have not been well-quantified.' 4.4m: - 'Mitigation There has been a considerable increase in national and subnational mitigation plans and strategies since AR4. In 2012, 67% of global GHG emissions were subject to national legislation or strategies versus 45% in 2007. However, there has not yet been a substantial deviation in global emissions from the past trend. These plans and strategies are in their early stages of development and implementation in many countries, making it difficult to assess their aggregate impact on future global emissions. Since AR4, there has been an increased focus on policies designed to integrate multiple objectives, increase co-benefits and reduce adverse side effects. Governments often explicitly reference co-benefits in climate and sectoral plans and strategies. Sector-specific policies have been more widely used than economy-wide policies . Although most economic theory suggests that economy-wide policies for mitigation would be more cost-effective than sector-specific policies, administrative and political barriers may make economy-wide policies harder to design and implement than sector-specific policies. The latter may be better suited to address barriers or market failures specific to certain sectors and may be bundled in packages of complementary policies In principle, mechanisms that set a carbon price, including cap and trade systems and carbon taxes, can achieve mitigation in a cost-effective way, but have been implemented with diverse effects due in part to national circumstances as well as policy design. The short-run environmental effects of cap and trade systems have been limited as a result of loose caps or caps that have not proved to be constraining. In some countries, tax-based policies specifically aimed at reducing GHG emissions—alongside technology and other policies—have helped to weaken the link between GHG emissions and gross domestic product . In addition, in a large group of countries, fuel taxes have had effects that are akin to sectoral carbon taxes . Revenues from carbon taxes or auctioned emission allowances are used in some countries to reduce other taxes and/or to provide transfers to low-income groups. This illustrates the general principle that mitigation policies that raise government revenue generally have lower social costs than approaches which do not. Economic instruments in the form of subsidies may be applied across sectors, and include a variety of policy designs, such as tax rebates or exemptions, grants, loans and credit lines. An increasing number and variety of RE policies including subsidies—motivated by many factors—have driven escalated growth of RE technologies in recent years. Government policies play a crucial role in accelerating the deployment of RE technologies. Energy access and social and economic development have been the primary drivers in most developing countries whereas secure energy supply and environmental concerns have been most important in developed countries. The focus of policies is broadening from a concentration primarily on RE electricity to include RE heating and cooling and transportation. The reduction of subsidies for GHG-related activities in various sectors can achieve emission reductions, depending on the social and economic context. While subsidies can affect emissions in many sectors, most of the recent literature has focused on subsidies for fossil fuels. Since AR4 a small but growing literature based on economy-wide models has projected that complete removal of subsidies to fossil fuels in all countries could result in reductions in global aggregate emissions by mid-century . Studies vary in methodology, the type and definition of subsidies and the time frame for phase out considered. In particular, the studies assess the impacts of complete removal of all fossil fuel subsides without seeking to assess which subsidies are wasteful and inefficient, keeping in mind national circumstances. Regulatory approaches and information measures are widely used and are often environmentally effective . Examples of regulatory approaches include energy efficiency standards; examples of information programmes include labelling programmes that can help consumers make better-informed decisions. Mitigation policy could devalue fossil fuel assets and reduce revenues for fossil fuel exporters, but differences between regions and fuels exist. Most mitigation scenarios are associated with reduced revenues from coal and oil trade for major exporters. The effect on natural gas export revenues is more uncertain. The availability of CCS would reduce the adverse effect of mitigation on the value of fossil fuel assets. Interactions between or among mitigation policies may be synergistic or may have no additive effect on reducing emissions . For instance, a carbon tax can have an additive environmental effect to policies such as subsidies for the supply of RE. By contrast, if a cap and trade system has a sufficiently stringent cap to affect emission-related decisions, then other policies have no further impact on reducing emissions . In either case, additional policies may be needed to address market failures relating to innovation and technology diffusion. Sub-national climate policies are increasingly prevalent, both in countries with national policies and in those without. These policies include state and provincial climate plans combining market, regulatory and information instruments, and sub-national cap-and-trade systems. In addition, transnational cooperation has arisen among sub-national actors, notably among institutional investors, NGOs seeking to govern carbon offset markets, and networks of cities seeking to collaborate in generating low-carbon urban development. Co-benefits and adverse side effects of mitigation could affect achievement of other objectives such as those related to human health, food security, biodiversity, local environmental quality, energy access, livelihoods and equitable sustainable development: • Mitigation scenarios reaching about 450 or 500 ppm CO2-equivalent by 2100 show reduced costs for achieving air quality and energy security objectives, with significant co-benefits for human health, ecosystem impacts and sufficiency of resources and resilience of the energy system. • Some mitigation policies raise the prices for some energy services and could hamper the ability of societies to expand access to modern energy services to underserved populations . These potential adverse side effects can be avoided with the adoption of complementary policies such as income tax rebates or other benefit transfer mechanisms. The costs of achieving nearly universal access to electricity and clean fuels for cooking and heating are projected to be between USD 72 to 95 billion per year until 2030 with minimal effects on GHG emissions and multiple benefits in health and air pollutant reduction. Whether or not side effects materialize, and to what extent side effects materialize, will be case- and site-specific, and depend on local circumstances and the scale, scope and pace of implementation. Many co-benefits and adverse side effects have not been well-quantified.' 4.4n: - 'Mitigation There has been a considerable increase in national and subnational mitigation plans and strategies since AR4. In 2012, 67% of global GHG emissions were subject to national legislation or strategies versus 45% in 2007. However, there has not yet been a substantial deviation in global emissions from the past trend. These plans and strategies are in their early stages of development and implementation in many countries, making it difficult to assess their aggregate impact on future global emissions. Since AR4, there has been an increased focus on policies designed to integrate multiple objectives, increase co-benefits and reduce adverse side effects. Governments often explicitly reference co-benefits in climate and sectoral plans and strategies. Sector-specific policies have been more widely used than economy-wide policies . Although most economic theory suggests that economy-wide policies for mitigation would be more cost-effective than sector-specific policies, administrative and political barriers may make economy-wide policies harder to design and implement than sector-specific policies. The latter may be better suited to address barriers or market failures specific to certain sectors and may be bundled in packages of complementary policies In principle, mechanisms that set a carbon price, including cap and trade systems and carbon taxes, can achieve mitigation in a cost-effective way, but have been implemented with diverse effects due in part to national circumstances as well as policy design. The short-run environmental effects of cap and trade systems have been limited as a result of loose caps or caps that have not proved to be constraining. In some countries, tax-based policies specifically aimed at reducing GHG emissions—alongside technology and other policies—have helped to weaken the link between GHG emissions and gross domestic product . In addition, in a large group of countries, fuel taxes have had effects that are akin to sectoral carbon taxes . Revenues from carbon taxes or auctioned emission allowances are used in some countries to reduce other taxes and/or to provide transfers to low-income groups. This illustrates the general principle that mitigation policies that raise government revenue generally have lower social costs than approaches which do not. Economic instruments in the form of subsidies may be applied across sectors, and include a variety of policy designs, such as tax rebates or exemptions, grants, loans and credit lines. An increasing number and variety of RE policies including subsidies—motivated by many factors—have driven escalated growth of RE technologies in recent years. Government policies play a crucial role in accelerating the deployment of RE technologies. Energy access and social and economic development have been the primary drivers in most developing countries whereas secure energy supply and environmental concerns have been most important in developed countries. The focus of policies is broadening from a concentration primarily on RE electricity to include RE heating and cooling and transportation. The reduction of subsidies for GHG-related activities in various sectors can achieve emission reductions, depending on the social and economic context. While subsidies can affect emissions in many sectors, most of the recent literature has focused on subsidies for fossil fuels. Since AR4 a small but growing literature based on economy-wide models has projected that complete removal of subsidies to fossil fuels in all countries could result in reductions in global aggregate emissions by mid-century . Studies vary in methodology, the type and definition of subsidies and the time frame for phase out considered. In particular, the studies assess the impacts of complete removal of all fossil fuel subsides without seeking to assess which subsidies are wasteful and inefficient, keeping in mind national circumstances. Regulatory approaches and information measures are widely used and are often environmentally effective . Examples of regulatory approaches include energy efficiency standards; examples of information programmes include labelling programmes that can help consumers make better-informed decisions. Mitigation policy could devalue fossil fuel assets and reduce revenues for fossil fuel exporters, but differences between regions and fuels exist. Most mitigation scenarios are associated with reduced revenues from coal and oil trade for major exporters. The effect on natural gas export revenues is more uncertain. The availability of CCS would reduce the adverse effect of mitigation on the value of fossil fuel assets. Interactions between or among mitigation policies may be synergistic or may have no additive effect on reducing emissions . For instance, a carbon tax can have an additive environmental effect to policies such as subsidies for the supply of RE. By contrast, if a cap and trade system has a sufficiently stringent cap to affect emission-related decisions, then other policies have no further impact on reducing emissions . In either case, additional policies may be needed to address market failures relating to innovation and technology diffusion. Sub-national climate policies are increasingly prevalent, both in countries with national policies and in those without. These policies include state and provincial climate plans combining market, regulatory and information instruments, and sub-national cap-and-trade systems. In addition, transnational cooperation has arisen among sub-national actors, notably among institutional investors, NGOs seeking to govern carbon offset markets, and networks of cities seeking to collaborate in generating low-carbon urban development. Co-benefits and adverse side effects of mitigation could affect achievement of other objectives such as those related to human health, food security, biodiversity, local environmental quality, energy access, livelihoods and equitable sustainable development: • Mitigation scenarios reaching about 450 or 500 ppm CO2-equivalent by 2100 show reduced costs for achieving air quality and energy security objectives, with significant co-benefits for human health, ecosystem impacts and sufficiency of resources and resilience of the energy system. • Some mitigation policies raise the prices for some energy services and could hamper the ability of societies to expand access to modern energy services to underserved populations . These potential adverse side effects can be avoided with the adoption of complementary policies such as income tax rebates or other benefit transfer mechanisms. The costs of achieving nearly universal access to electricity and clean fuels for cooking and heating are projected to be between USD 72 to 95 billion per year until 2030 with minimal effects on GHG emissions and multiple benefits in health and air pollutant reduction. Whether or not side effects materialize, and to what extent side effects materialize, will be case- and site-specific, and depend on local circumstances and the scale, scope and pace of implementation. Many co-benefits and adverse side effects have not been well-quantified.' 4.4p: - Response options for mitigation Mitigation options are available in every major sector. Mitigation can be more cost-effective if using an integrated approach that combines measures to reduce energy use and the greenhouse gas intensity of end-use sectors, decarbonize energy supply, reduce net emissions and enhance carbon sinks in land-based sectors. A broad range of sectoral mitigation options is available that can reduce GHG emission intensity, improve energy intensity through enhancements of technology, behaviour, production and resource efficiency and enable structural changes or changes in activity. In addition, direct options in agriculture, forestry and other land use involve reducing CO2 emissions by reducing deforestation, forest degradation and forest fires; storing carbon in terrestrial systems; and providing bioenergy feedstocks. Options to reduce non-CO2 emissions exist across all sectors but most notably in agriculture, energy supply and industry. An overview of sectoral mitigation options and potentials is provided in Table 4.4. Well-designed systemic and cross-sectoral mitigation strategies are more cost-effective in cutting emissions than a focus on individual technologies and sectors with efforts in one sector affecting the need for mitigation in others . In baseline scenarios without new mitigation policies, GHG emissions are projected to grow in all sectors, except for net CO2 emissions in the AFOLU sector. Mitigation scenarios reaching around 450 ppm CO2-eq concentration by 21004328 show largescale global changes in the energy supply sector . While rapid decarbonization of energy supply generally entails more flexibility for end-use and AFOLU sectors, stronger demand reductions lessen the mitigation challenge for the supply side of the energy system. There are thus strong interdependencies across sectors and the resulting distribution of the mitigation effort is strongly influenced by the availability and performance of future technologies, particularly BECCS and large scale afforestation . The next two decades present a window of opportunity for mitigation in urban areas, as a large portion of the world’s urban areas will be developed during this period. Decarbonizing electricity generation is a key component of cost-effective mitigation strategies in achieving low stabilization levels . In most integrated modelling scenarios, decarbonization happens more rapidly in electricity generation than in the industry, buildings and transport sectors. In scenarios reaching 450 ppm CO2-eq concentrations by 2100, global CO2 emissions from the energy supply sector are projected to decline over the next decade and are characterized by reductions of 90% or more below 2010 levels between 2040 and 2070. Efficiency enhancements and behavioural changes, in order to reduce energy demand compared to baseline scenarios without compromising development, are a key mitigation strategy in scenarios reaching atmospheric CO2-eq concentrations of about 450 to about 500 ppm by 2100 . Near-term reductions in energy demand are an important element of cost-effective mitigation strategies, provide more flexibility for reducing carbon intensity in the energy supply sector, hedge against related supply-side risks, avoid lock-in to carbon-intensive infrastructures and are associated with important co-benefits . Emissions can be substantially lowered through changes in consumption patterns and dietary change and reduction in food wastes. A number of options including monetary and non-monetary incentives as well as information measures may facilitate behavioural changes. Decarbonization of the energy supply sector requires upscaling of low- and zero-carbon electricity generation technologies. In the majority of low-concentration stabilization scenarios , the share of low-carbon electricity supply, nuclear and CCS, including BECCS) increases from the current share of approximately 30% to more than 80% by 2050 and 90% by 2100, and fossil fuel power generation without CCS is phased out almost entirely by 2100. Among these low-carbon technologies, a growing number of RE technologies have achieved a level of maturity to enable deployment at significant scale since AR4 and nuclear energy is a mature low-GHG emission source of baseload power, but its share of global electricity generation has been declining. GHG emissions from energy supply can be reduced significantly by replacing current world average coal-fired power plants with modern, highly efficient natural gas combined-cycle power plants or combined heat and power plants, provided that natural gas is available and the fugitive emissions associated with extraction and supply are low or mitigated. Behaviour, lifestyle and culture have a considerable influence on energy use and associated emissions, with high mitigation potential in some sectors, in particular when complementing technological and structural change . In the transport sector, technical and behavioural mitigation measures for all modes, plus new infrastructure and urban redevelopment investments, could reduce final energy demand significantly below baseline levels. While opportunities for switching to low-carbon fuels exist, the rate of decarbonization in the transport sector might be constrained by challenges associated with energy storage and the relatively low energy density of low-carbon transport fuels. In the building sector, recent advances in technologies, know-how and policies provide opportunities to stabilize or reduce global energy use to about current levels by mid-century. In addition, recent improvements in performance and costs make very low energy construction and retrofits of buildings economically attractive, sometimes even at net negative costs. In the industry sector, improvements in GHG emission efficiency and in the efficiency of material use, recycling and reuse of materials and products, and overall reductions in product demand and service demand could, in addition to energy efficiency, help reduce GHG emissions below the baseline level. Prevalent approaches for promoting energy efficiency in industry include information programmes followed by economic instruments, regulatory approaches and voluntary actions. Important options for mitigation in waste management are waste reduction, followed by re-use, recycling and energy recovery. The most cost-effective mitigation options in forestry are afforestation, sustainable forest management and reducing deforestation, with large differences in their relative importance across regions. In agriculture, the most cost-effective mitigation options are cropland management, grazing land management and restoration of organic soils . About a third of mitigation potential in forestry can be achieved at a cost <20 USD/tCO2-eq emission. Demand-side measures, such as changes in diet and reductions of losses in the food supply chain, have a significant, but uncertain, potential to reduce GHG emissions from food production . Bioenergy can play a critical role for mitigation, but there are issues to consider, such as the sustainability of practices and the efficiency of bioenergy systems . Evidence suggests that bioenergy options with low lifecycle emissions, some already available, can reduce GHG emissions; outcomes are site-specific and rely on efficient integrated ‘biomassto-bioenergy systems’, and sustainable land use management and governance. Barriers to large-scale deployment of bioenergy include concerns about GHG emissions from land, food security, water resources, biodiversity conservation and livelihoods. Mitigation measures intersect with other societal goals, creating the possibility of co-benefits or adverse side-effects. These intersections, if well-managed, can strengthen the basis for undertaking climate mitigation actions . Mitigation can positively or negatively influence the achievement of other societal goals, such as those related to human health, food security, biodiversity, local environmental quality, energy access, livelihoods and equitable sustainable development . On the other hand, policies towards other societal goals can influence the achievement of mitigation and adaptation objectives. These influences can be substantial, although sometimes difficult to quantify, especially in welfare terms. This multi-objective perspective is important in part because it helps to identify areas where support for policies that advance multiple goals will be robust. Potential co-benefits and adverse side effects of the main sectoral mitigation measures are summarized in Table 4.5. Overall, the potential for co-benefits for energy end-use measures outweigh the potential for adverse side effects, whereas the evidence suggests this may not be the case for all energy supply and AFOLU measures. - 'Mitigation There has been a considerable increase in national and subnational mitigation plans and strategies since AR4. In 2012, 67% of global GHG emissions were subject to national legislation or strategies versus 45% in 2007. However, there has not yet been a substantial deviation in global emissions from the past trend. These plans and strategies are in their early stages of development and implementation in many countries, making it difficult to assess their aggregate impact on future global emissions. Since AR4, there has been an increased focus on policies designed to integrate multiple objectives, increase co-benefits and reduce adverse side effects. Governments often explicitly reference co-benefits in climate and sectoral plans and strategies. Sector-specific policies have been more widely used than economy-wide policies . Although most economic theory suggests that economy-wide policies for mitigation would be more cost-effective than sector-specific policies, administrative and political barriers may make economy-wide policies harder to design and implement than sector-specific policies. The latter may be better suited to address barriers or market failures specific to certain sectors and may be bundled in packages of complementary policies In principle, mechanisms that set a carbon price, including cap and trade systems and carbon taxes, can achieve mitigation in a cost-effective way, but have been implemented with diverse effects due in part to national circumstances as well as policy design. The short-run environmental effects of cap and trade systems have been limited as a result of loose caps or caps that have not proved to be constraining. In some countries, tax-based policies specifically aimed at reducing GHG emissions—alongside technology and other policies—have helped to weaken the link between GHG emissions and gross domestic product . In addition, in a large group of countries, fuel taxes have had effects that are akin to sectoral carbon taxes . Revenues from carbon taxes or auctioned emission allowances are used in some countries to reduce other taxes and/or to provide transfers to low-income groups. This illustrates the general principle that mitigation policies that raise government revenue generally have lower social costs than approaches which do not. Economic instruments in the form of subsidies may be applied across sectors, and include a variety of policy designs, such as tax rebates or exemptions, grants, loans and credit lines. An increasing number and variety of RE policies including subsidies—motivated by many factors—have driven escalated growth of RE technologies in recent years. Government policies play a crucial role in accelerating the deployment of RE technologies. Energy access and social and economic development have been the primary drivers in most developing countries whereas secure energy supply and environmental concerns have been most important in developed countries. The focus of policies is broadening from a concentration primarily on RE electricity to include RE heating and cooling and transportation. The reduction of subsidies for GHG-related activities in various sectors can achieve emission reductions, depending on the social and economic context. While subsidies can affect emissions in many sectors, most of the recent literature has focused on subsidies for fossil fuels. Since AR4 a small but growing literature based on economy-wide models has projected that complete removal of subsidies to fossil fuels in all countries could result in reductions in global aggregate emissions by mid-century . Studies vary in methodology, the type and definition of subsidies and the time frame for phase out considered. In particular, the studies assess the impacts of complete removal of all fossil fuel subsides without seeking to assess which subsidies are wasteful and inefficient, keeping in mind national circumstances. Regulatory approaches and information measures are widely used and are often environmentally effective . Examples of regulatory approaches include energy efficiency standards; examples of information programmes include labelling programmes that can help consumers make better-informed decisions. Mitigation policy could devalue fossil fuel assets and reduce revenues for fossil fuel exporters, but differences between regions and fuels exist. Most mitigation scenarios are associated with reduced revenues from coal and oil trade for major exporters. The effect on natural gas export revenues is more uncertain. The availability of CCS would reduce the adverse effect of mitigation on the value of fossil fuel assets. Interactions between or among mitigation policies may be synergistic or may have no additive effect on reducing emissions . For instance, a carbon tax can have an additive environmental effect to policies such as subsidies for the supply of RE. By contrast, if a cap and trade system has a sufficiently stringent cap to affect emission-related decisions, then other policies have no further impact on reducing emissions . In either case, additional policies may be needed to address market failures relating to innovation and technology diffusion. Sub-national climate policies are increasingly prevalent, both in countries with national policies and in those without. These policies include state and provincial climate plans combining market, regulatory and information instruments, and sub-national cap-and-trade systems. In addition, transnational cooperation has arisen among sub-national actors, notably among institutional investors, NGOs seeking to govern carbon offset markets, and networks of cities seeking to collaborate in generating low-carbon urban development. Co-benefits and adverse side effects of mitigation could affect achievement of other objectives such as those related to human health, food security, biodiversity, local environmental quality, energy access, livelihoods and equitable sustainable development: • Mitigation scenarios reaching about 450 or 500 ppm CO2-equivalent by 2100 show reduced costs for achieving air quality and energy security objectives, with significant co-benefits for human health, ecosystem impacts and sufficiency of resources and resilience of the energy system. • Some mitigation policies raise the prices for some energy services and could hamper the ability of societies to expand access to modern energy services to underserved populations . These potential adverse side effects can be avoided with the adoption of complementary policies such as income tax rebates or other benefit transfer mechanisms. The costs of achieving nearly universal access to electricity and clean fuels for cooking and heating are projected to be between USD 72 to 95 billion per year until 2030 with minimal effects on GHG emissions and multiple benefits in health and air pollutant reduction. Whether or not side effects materialize, and to what extent side effects materialize, will be case- and site-specific, and depend on local circumstances and the scale, scope and pace of implementation. Many co-benefits and adverse side effects have not been well-quantified.' 4.4q: - Technology development and transfer Technology policy complements other mitigation policies across all scales from international to sub-national, but worldwide investment in research in support of GHG mitigation is small relative to overall public research spending. Technology policy includes technology-push and demand-pull . Such policies address a pervasive market failure because, in the absence of government policy such as patent protection, the invention of new technologies and practices from R&D efforts has aspects of a public good and thus tends to be under-provided by market forces alone. Technology support policies have promoted substantial innovation and diffusion of new technologies, but the cost-effectiveness of such policies is often difficult to assess. Technology policy can increase incentives for participation and compliance with international cooperative efforts, particularly in the long run. Many adaptation efforts also critically rely on diffusion and transfer of technologies and management practices, but their effective use depends on a suitable institutional, regulatory, social and cultural context. Adaptation technologies are often familiar and already applied elsewhere. However, the success of technology transfer may involve not only the provision of finance and information, but also strengthening of policy and regulatory environments and capacities to absorb, employ and improve technologies appropriate to local circumstances. 4.4r: - Investment and finance Substantial reductions in emissions would require large changes in investment patterns. Mitigation scenarios in which policies stabilize atmospheric concentrations in the range from 430 to 530 ppm CO2-eq by 21004530 lead to substantial shifts in annual investment flows during the period 2010–2029 compared to baseline scenarios. Over the next two decades , annual investments in conventional fossil fuel technologies associated with the electricity supply sector are projected to decline in the scenarios by about USD 30 billion while annual investment in low carbon electricity supply is projected to rise in the scenarios by about USD 147 billion . In addition, annual incremental energy efficiency investments in transport, industry and buildings is projected to rise in the scenarios by about USD 336 billion. Global total annual investment in the energy system is presently about USD 1, billion. This number includes only energy supply of electricity and heat and respective upstream and downstream activities. Energy efficiency investment or underlying sector investment is not included. There is no widely agreed definition of what constitutes climate finance, but estimates of the financial flows associated with climate change mitigation and adaptation are available. See Figure 4.5 for an overview of climate finance flows. Published assessments of all current annual financial flows whose expected effect is to reduce net GHG emissions and/or to enhance resilience to climate change and climate variability show USD 343 to 385 billion per year globally. Out of this, total public climate finance that flowed to developing countries is estimated to be between USD 35 and 49 billion per year in 2011 and 2012. Estimates of international private climate finance flowing to developing countries range from USD 10 to 72 billion per year including foreign direct investment as equity and loans in the range of USD 10 to 37 billion per year over the period of 2008–2011. In many countries, the private sector plays central roles in the processes that lead to emissions as well as to mitigation and adaptation. Within appropriate enabling environments, the private sector, along with the public sector, can play an important role in financing mitigation and adaptation . The share of total mitigation finance from the private sector, acknowledging data limitations, is estimated to be on average between two-thirds and three-fourths on the global level . In many countries, public finance interventions by governments and international development banks encourage climate investments by the private sector and provide finance where private sector investment is limited. The quality of a country’s enabling environment includes the effectiveness of its institutions, regulations and guidelines regarding the private sector, security of property rights, credibility of policies and other factors that have a substantial impact on whether private firms invest in new technologies and infrastructures. Dedicated policy instruments and financial arrangements, for example, credit insurance, feed-in tariffs, concessional finance or rebates provide an incentive for mitigation investment by improving the return adjusted for the risk for private actors. Public-private risk reduction initiatives and economic diversification are examples of adaptation action enabling and relying on private sector participation. Financial resources for adaptation have become available more slowly than for mitigation in both developed and developing countries. Limited evidence indicates that there is a gap between global adaptation needs and the funds available for adaptation. Potential synergies between international finance for disaster risk management and adaptation to climate change have not yet been fully realized. There is a need for better assessment of global adaptation costs, funding and investment. Studies estimating the global cost of adaptation are characterized by shortcomings in data, methods and coverage . 4.5a: - Interaction among mitigation, adaptation and sustainable development Climate change is a threat to equitable and sustainable development. Adaptation, mitigation and sustainable development are closely related, with potential for synergies and trade-offs. Climate change poses an increasing threat to equitable and sustainable development. Some climate-related impacts on development are already being observed. Climate change is a threat multiplier. It exacerbates other threats to social and natural systems, placing additional burdens particularly on the poor and constraining possible development paths for all. Development along current global pathways can contribute to climate risk and vulnerability, further eroding the basis for sustainable development. Aligning climate policy with sustainable development requires attention to both adaptation and mitigation. Interaction among adaptation, mitigation and sustainable development occurs both within and across regions and scales, often in the context of multiple stressors. Some options for responding to climate change could impose risks of other environmental and social costs, have adverse distributional effects and draw resources away from other development priorities, including poverty eradication. Both adaptation and mitigation can bring substantial co-benefits . Examples of actions with co-benefits include improved air quality; enhanced energy security, reduced energy and water consumption in urban areas through greening cities and recycling water; sustainable agriculture and forestry; and protection of ecosystems for carbon storage and other ecosystem services. Strategies and actions can be pursued now that will move towards climate-resilient pathways for sustainable development, while at the same time helping to improve livelihoods, social and economic well-being and effective environmental management. Prospects for climate-resilient pathways are related fundamentally to what the world accomplishes with climate change mitigation. Since mitigation reduces the rate as well as the magnitude of warming, it also increases the time available for adaptation to a particular level of climate change, potentially by several decades. Delaying mitigation actions may reduce options for climate-resilient pathways in the future. - Trade-offs, synergies and integrated responses There are many opportunities to link mitigation, adaptation and the pursuit of other societal objectives through integrated responses. Successful implementation relies on relevant tools, suitable governance structures and enhanced capacity to respond. A growing evidence base indicates close links between adaptation and mitigation, their co-benefits and adverse side effects, and recognizes sustainable development as the overarching context for climate policy . Developing tools to address these linkages is critical to the success of climate policy in the context of sustainable development. This section presents examples of integrated responses in specific policy arenas, as well as some of the factors that promote or impede policies aimed at multiple objectives. Increasing efforts to mitigate and adapt to climate change imply an increasing complexity of interactions, encompassing connections among human health, water, energy, land use and biodiversity. Mitigation can support the achievement of other societal goals, such as those related to human health, food security, environmental quality, energy access, livelihoods and sustainable development, although there can also be negative effects. Adaptation measures also have the potential to deliver mitigation co-benefits, and vice versa, and support other societal goals, though trade-offs can also arise. Integration of adaptation and mitigation into planning and decision-making can create synergies with sustainable development. Synergies and trade-offs among mitigation and adaptation policies and policies advancing other societal goals can be substantial, although sometimes difficult to quantify especially in welfare terms. A multi-objective approach to policy-making can help manage these synergies and trade-offs. Policies advancing multiple goals may also attract greater support. Effective integrated responses depend on suitable tools and governance structures, as well as adequate capacity . Managing trade-offs and synergies is challenging and requires tools to help understand interactions and support decision-making at local and regional scales. Integrated responses also depend on governance that enables coordination across scales and sectors, supported by appropriate institutions. Developing and implementing suitable tools and governance structures often requires upgrading the human and institutional capacity to design and deploy integrated responses. An integrated approach to energy planning and implementation that explicitly assesses the potential for co-benefits and the presence of adverse side effects can capture complementarities across multiple climate, social and environmental objectives . There are strong interactive effects across various energy policy objectives, such as energy security, air quality, health and energy access and between a range of social and environmental objectives and climate mitigation objectives . An integrated approach can be assisted by tools such as cost-benefit analysis, cost-effectiveness analysis, multi-criteria analysis and expected utility theory. It also requires appropriate coordinating institutions. Explicit consideration of interactions among water, food, energy and biological carbon sequestration plays an important role in supporting effective decisions for climate resilient pathways . Both biofuel-based power generation and large-scale afforestation designed to mitigate climate change can reduce catchment run-off, which may conflict with alternative water uses for food production, human consumption or the maintenance of ecosystem function and services. Conversely, irrigation can increase the climate resilience of food and fibre production but reduces water availability for other uses. An integrated response to urbanization provides substantial opportunities for enhanced resilience, reduced emissions and more sustainable development. Urban areas account for more than half of global primary energy use and energy-related CO2 emissions and contain a high proportion of the population and economic activities at risk from climate change. In rapidly growing and urbanizing regions, mitigation strategies based on spatial planning and efficient infrastructure supply can avoid the lock-in of high emission patterns. Mixed-use zoning, transport-oriented development, increased density and co-located jobs and homes can reduce direct and indirect energy use across sectors. Compact development of urban spaces and intelligent densification can preserve land carbon stocks and land for agriculture and bioenergy. Reduced energy and water consumption in urban areas through greening cities and recycling water are examples of mitigation actions with adaptation benefits. Building resilient infrastructure systems can reduce vulnerability of urban settlements and cities to coastal flooding, sea level rise and other climate-induced stresses. 4.5b: - 'Foundations of decision-making about climate change Effective decision-making to limit climate change and its effects can be informed by a wide range of analytical approaches for evaluating expected risks and benefits, recognizing the importance of governance, ethical dimensions, equity, value judgments, economic assessments and diverse perceptions and responses to risk and uncertainty. Sustainable development and equity provide a basis for assessing climate policies. Limiting the effects of climate change is necessary to achieve sustainable development and equity, including poverty eradication. Countries’ past and future contributions to the accumulation of GHGs in the atmosphere are different, and countries also face varying challenges and circumstances and have different capacities to address mitigation and adaptation. Mitigation and adaptation raise issues of equity, justice and fairness and are necessary to achieve sustainable development and poverty eradication. Many of those most vulnerable to climate change have contributed and contribute little to GHG emissions. Delaying mitigation shifts burdens from the present to the future, and insufficient adaptation responses to emerging impacts are already eroding the basis for sustainable development. Both adaptation and mitigation can have distributional effects locally, nationally and internationally, depending on who pays and who benefits. The process of decision-making about climate change, and the degree to which it respects the rights and views of all those affected, is also a concern of justice. Effective mitigation will not be achieved if individual agents advance their own interests independently. Climate change has the characteristics of a collective action problem at the global scale, because most GHGs accumulate over time and mix globally, and emissions by any agent affect other agents. Cooperative responses, including international cooperation, are therefore required to effectively mitigate GHG emissions and address other climate change issues. The effectiveness of adaptation can be enhanced through complementary actions across levels, including international cooperation. The evidence suggests that outcomes seen as equitable can lead to more effective cooperation. Decision-making about climate change involves valuation and mediation among diverse values and may be aided by the analytic methods of several normative disciplines. Ethics analyses the different values involved and the relations between them. Recent political philosophy has investigated the question of responsibility for the effects of emissions. Economics and decision analysis provide quantitative methods of valuation which can be used for estimating the social cost of carbon, in cost–benefit and costeffectiveness analyses, for optimization in integrated models and elsewhere. Economic methods can reflect ethical principles, and take account of non-marketed goods, equity, behavioural biases, ancillary benefits and costs and the differing values of money to different people. They are, however, subject to well-documented limitations. Analytical methods of valuation cannot identify a single best balance between mitigation, adaptation and residual climate impacts. Important reasons for this are that climate change involves extremely complex natural and social processes, there is extensive disagreement about the values concerned, and climate change impacts and mitigation approaches have important distributional effects. Nevertheless, information on the consequences of emissions pathways to alternative climate goals and risk levels can be a useful input into decision-making processes. Evaluating responses to climate change involves assessment of the widest possible range of impacts, including low-probability outcomes with large consequences. Effective decision-making and risk management in the complex environment of climate change may be iterative: strategies can often be adjusted as new information and understanding develops during implementation. However, adaptation and mitigation choices in the near term will affect the risks of climate change throughout the 21st century and beyond, and prospects for climate-resilient pathways for sustainable development depend on what is achieved through mitigation. Opportunities to take advantage of positive synergies between adaptation and mitigation may decrease with time, particularly if mitigation is delayed too long. Decision-making about climate change is influenced by how individuals and organizations perceive risks and uncertainties and take them into account. They sometimes use simplified decision rules, overestimate or underestimate risks and are biased towards the status quo. They differ in their degree of risk aversion and the relative importance placed on near-term versus long-term ramifications of specific actions. Formalized analytical methods for decision-making under uncertainty can account accurately for risk, and focus attention on both short- and long-term consequences.' - Interaction among mitigation, adaptation and sustainable development Climate change is a threat to equitable and sustainable development. Adaptation, mitigation and sustainable development are closely related, with potential for synergies and trade-offs. Climate change poses an increasing threat to equitable and sustainable development. Some climate-related impacts on development are already being observed. Climate change is a threat multiplier. It exacerbates other threats to social and natural systems, placing additional burdens particularly on the poor and constraining possible development paths for all. Development along current global pathways can contribute to climate risk and vulnerability, further eroding the basis for sustainable development. Aligning climate policy with sustainable development requires attention to both adaptation and mitigation. Interaction among adaptation, mitigation and sustainable development occurs both within and across regions and scales, often in the context of multiple stressors. Some options for responding to climate change could impose risks of other environmental and social costs, have adverse distributional effects and draw resources away from other development priorities, including poverty eradication. Both adaptation and mitigation can bring substantial co-benefits . Examples of actions with co-benefits include improved air quality; enhanced energy security, reduced energy and water consumption in urban areas through greening cities and recycling water; sustainable agriculture and forestry; and protection of ecosystems for carbon storage and other ecosystem services. Strategies and actions can be pursued now that will move towards climate-resilient pathways for sustainable development, while at the same time helping to improve livelihoods, social and economic well-being and effective environmental management. Prospects for climate-resilient pathways are related fundamentally to what the world accomplishes with climate change mitigation. Since mitigation reduces the rate as well as the magnitude of warming, it also increases the time available for adaptation to a particular level of climate change, potentially by several decades. Delaying mitigation actions may reduce options for climate-resilient pathways in the future. - Trade-offs, synergies and integrated responses There are many opportunities to link mitigation, adaptation and the pursuit of other societal objectives through integrated responses. Successful implementation relies on relevant tools, suitable governance structures and enhanced capacity to respond. A growing evidence base indicates close links between adaptation and mitigation, their co-benefits and adverse side effects, and recognizes sustainable development as the overarching context for climate policy . Developing tools to address these linkages is critical to the success of climate policy in the context of sustainable development. This section presents examples of integrated responses in specific policy arenas, as well as some of the factors that promote or impede policies aimed at multiple objectives. Increasing efforts to mitigate and adapt to climate change imply an increasing complexity of interactions, encompassing connections among human health, water, energy, land use and biodiversity. Mitigation can support the achievement of other societal goals, such as those related to human health, food security, environmental quality, energy access, livelihoods and sustainable development, although there can also be negative effects. Adaptation measures also have the potential to deliver mitigation co-benefits, and vice versa, and support other societal goals, though trade-offs can also arise. Integration of adaptation and mitigation into planning and decision-making can create synergies with sustainable development. Synergies and trade-offs among mitigation and adaptation policies and policies advancing other societal goals can be substantial, although sometimes difficult to quantify especially in welfare terms. A multi-objective approach to policy-making can help manage these synergies and trade-offs. Policies advancing multiple goals may also attract greater support. Effective integrated responses depend on suitable tools and governance structures, as well as adequate capacity . Managing trade-offs and synergies is challenging and requires tools to help understand interactions and support decision-making at local and regional scales. Integrated responses also depend on governance that enables coordination across scales and sectors, supported by appropriate institutions. Developing and implementing suitable tools and governance structures often requires upgrading the human and institutional capacity to design and deploy integrated responses. An integrated approach to energy planning and implementation that explicitly assesses the potential for co-benefits and the presence of adverse side effects can capture complementarities across multiple climate, social and environmental objectives . There are strong interactive effects across various energy policy objectives, such as energy security, air quality, health and energy access and between a range of social and environmental objectives and climate mitigation objectives . An integrated approach can be assisted by tools such as cost-benefit analysis, cost-effectiveness analysis, multi-criteria analysis and expected utility theory. It also requires appropriate coordinating institutions. Explicit consideration of interactions among water, food, energy and biological carbon sequestration plays an important role in supporting effective decisions for climate resilient pathways . Both biofuel-based power generation and large-scale afforestation designed to mitigate climate change can reduce catchment run-off, which may conflict with alternative water uses for food production, human consumption or the maintenance of ecosystem function and services. Conversely, irrigation can increase the climate resilience of food and fibre production but reduces water availability for other uses. An integrated response to urbanization provides substantial opportunities for enhanced resilience, reduced emissions and more sustainable development. Urban areas account for more than half of global primary energy use and energy-related CO2 emissions and contain a high proportion of the population and economic activities at risk from climate change. In rapidly growing and urbanizing regions, mitigation strategies based on spatial planning and efficient infrastructure supply can avoid the lock-in of high emission patterns. Mixed-use zoning, transport-oriented development, increased density and co-located jobs and homes can reduce direct and indirect energy use across sectors. Compact development of urban spaces and intelligent densification can preserve land carbon stocks and land for agriculture and bioenergy. Reduced energy and water consumption in urban areas through greening cities and recycling water are examples of mitigation actions with adaptation benefits. Building resilient infrastructure systems can reduce vulnerability of urban settlements and cities to coastal flooding, sea level rise and other climate-induced stresses. paragraph_topics: 1.1a: Observed changes in the climate system 1.1b: Observed changes in the climate system 1.1c: Observed changes in the climate system 1.1d: Observed changes in the climate system 1.1e: Observed changes in the climate system 1.1g: Observed changes in the climate system 1.2a: Causes of climate change 1.2c: Causes of climate change 1.3a: Impacts of climate change 1.4a: Extreme events 2.1b: Key drivers of future climate 2.1c: Key drivers of future climate 2.1d: Key drivers of future climate 2.2e: Projected changes in the climate system 2.2f: Projected changes in the climate system 2.2g: Projected changes in the climate system 2.2h: Projected changes in the climate system 2.2m: Projected changes in the climate system 2.3a: Future risks and impacts caused by a changing climate 2.3b: Future risks and impacts caused by a changing climate 2.3c: Future risks and impacts caused by a changing climate 2.3e: Future risks and impacts caused by a changing climate 2.3f: Future risks and impacts caused by a changing climate 2.4a: Climate change beyond 2010, irreversibility and abrupt changes 2.4b: Climate change beyond 2010, irreversibility and abrupt changes 2.4c: Climate change beyond 2010, irreversibility and abrupt changes 2.4d: Climate change beyond 2010, irreversibility and abrupt changes 3.1a: Foundations of decion-making about climate change 3.1b: Foundations of decion-making about climate change 3.1c: Foundations of decion-making about climate change 3.1d: Foundations of decion-making about climate change 3.2b: Climate change risks reduced by mitigation and adaptation 3.2d: Climate change risks reduced by mitigation and adaptation 3.2e: Climate change risks reduced by mitigation and adaptation 3.2f: Climate change risks reduced by mitigation and adaptation 3.3a: Characteristics of adaptation pathways 3.3b: Characteristics of adaptation pathways 3.3c: Characteristics of adaptation pathways 3.3d: Characteristics of adaptation pathways 3.3f: Characteristics of adaptation pathways 3.3g: Characteristics of adaptation pathways 3.4a: Characteristics of mitigation pathways 3.4b: Characteristics of mitigation pathways 3.4c: Characteristics of mitigation pathways 3.4d: Characteristics of mitigation pathways 3.4e: Characteristics of mitigation pathways 3.4f: Characteristics of mitigation pathways 3.4g: Characteristics of mitigation pathways 3.4h: Characteristics of mitigation pathways 3.4i: Characteristics of mitigation pathways 3.4l: Characteristics of mitigation pathways 4.1a: Common enabling factors and constraints for adaptation and mitigation responses 4.1b: Common enabling factors and constraints for adaptation and mitigation responses 4.1c: Common enabling factors and constraints for adaptation and mitigation responses 4.2a: Response options for adaptation 4.2b: Response options for adaptation 4.2c: Response options for adaptation 4.3a: Response options for mitigations 4.3b: Response options for mitigations 4.3d: Response options for mitigations 4.4b: Policy approaches for adaptation and mitigation, technology and finance 4.4g: Policy approaches for adaptation and mitigation, technology and finance 4.4l: Policy approaches for adaptation and mitigation, technology and finance 4.4m: Policy approaches for adaptation and mitigation, technology and finance 4.4n: Policy approaches for adaptation and mitigation, technology and finance 4.4p: Policy approaches for adaptation and mitigation, technology and finance 4.4q: Policy approaches for adaptation and mitigation, technology and finance 4.4r: Policy approaches for adaptation and mitigation, technology and finance 4.5a: Trade-offs, synergies and interactions with sustainable development 4.5b: Trade-offs, synergies and interactions with sustainable development pointers: 1.1a: - 1.1.1 1.1b: - 1.1.2 1.1c: - 1.1.1 - 1.1.2 1.1d: - 1.1.2 1.1e: - 1.1.3 1.1g: - 1.1.4 1.2a: - 1.2.1 - 1.2.2 1.2c: - '1.3' 1.3a: - 1.3.2 1.4a: - '1.4' 2.1b: - '2.1' - '4.3' 2.1c: - 2.2.5 2.1d: - 2.2.5 2.2e: - 2.2.2 2.2f: - 2.2.3 2.2g: - 2.2.4 2.2h: - 2.2.3 2.2m: - 2.2.3 2.3a: - '1.5' - '2.3' - '2.4' - '3.3' 2.3b: - '2.3' - '2.4' 2.3c: - 2.3.1 - 2.3.2 2.3e: - 2.3.2 2.3f: - 2.3.2 2.4a: - '2.4' 2.4b: - '2.4' 2.4c: - '2.4' 2.4d: - '2.4' 3.1a: - '3.1' 3.1b: - '3.1' - '3.5' 3.1c: - '3.1' 3.1d: - '3.1' 3.2b: - '3.2' - '4.5' 3.2d: - '2.3' - '3.2' - '3.4' 3.2e: - 2.2.5 - '3.2' - '3.4' 3.2f: - '3.2' - '3.4' 3.3a: - '3.3' 3.3b: - '3.3' 3.3c: - '3.3' 3.3d: - '3.3' 3.3f: - '3.3' 3.3g: - '3.3' 3.4a: - '3.4' 3.4b: - '3.4' 3.4c: - '3.4' 3.4d: - '3.4' 3.4e: - '3.4' 3.4f: - '3.4' 3.4g: - '3.4' 3.4h: - '3.4' 3.4i: - 4.4.2.2 3.4l: - 4.4.2.2 4.1a: - '4.1' 4.1b: - '4.1' 4.1c: - '4.1' 4.2a: - '4.2' 4.2b: - '1.6' - '4.2' - 4.4.2.1 4.2c: - '4.2' 4.3a: - '4.3' 4.3b: - '4.3' 4.3d: - '4.1' - '4.3' 4.4b: - 4.4.1 4.4g: - 4.4.2.1 - 4.4.2.2 4.4l: - 4.4.2.2 4.4m: - 4.4.2.2 4.4n: - 4.4.2.2 4.4p: - '4.3' - 4.4.2.2 4.4q: - 4.4.3 4.4r: - 4.4.4 4.5a: - '3.5' - '4.5' 4.5b: - '3.1' - '3.5' - '4.5' section_topics: 1.1a: Observed Changes amd their Causes 1.1b: Observed Changes amd their Causes 1.1c: Observed Changes amd their Causes 1.1d: Observed Changes amd their Causes 1.1e: Observed Changes amd their Causes 1.1g: Observed Changes amd their Causes 1.2a: Observed Changes amd their Causes 1.2c: Observed Changes amd their Causes 1.3a: Observed Changes amd their Causes 1.4a: Observed Changes amd their Causes 2.1b: Future Climate Changes, Risks and Impacts 2.1c: Future Climate Changes, Risks and Impacts 2.1d: Future Climate Changes, Risks and Impacts 2.2e: Future Climate Changes, Risks and Impacts 2.2f: Future Climate Changes, Risks and Impacts 2.2g: Future Climate Changes, Risks and Impacts 2.2h: Future Climate Changes, Risks and Impacts 2.2m: Future Climate Changes, Risks and Impacts 2.3a: Future Climate Changes, Risks and Impacts 2.3b: Future Climate Changes, Risks and Impacts 2.3c: Future Climate Changes, Risks and Impacts 2.3e: Future Climate Changes, Risks and Impacts 2.3f: Future Climate Changes, Risks and Impacts 2.4a: Future Climate Changes, Risks and Impacts 2.4b: Future Climate Changes, Risks and Impacts 2.4c: Future Climate Changes, Risks and Impacts 2.4d: Future Climate Changes, Risks and Impacts 3.1a: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.1b: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.1c: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.1d: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.2b: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.2d: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.2e: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.2f: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.3a: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.3b: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.3c: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.3d: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.3f: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.3g: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.4a: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.4b: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.4c: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.4d: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.4e: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.4f: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.4g: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.4h: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.4i: Future Pathways for Adaptation, Mitigation and Sustainable Development 3.4l: Future Pathways for Adaptation, Mitigation and Sustainable Development 4.1a: Adaptation and Mitigation 4.1b: Adaptation and Mitigation 4.1c: Adaptation and Mitigation 4.2a: Adaptation and Mitigation 4.2b: Adaptation and Mitigation 4.2c: Adaptation and Mitigation 4.3a: Adaptation and Mitigation 4.3b: Adaptation and Mitigation 4.3d: Adaptation and Mitigation 4.4b: Adaptation and Mitigation 4.4g: Adaptation and Mitigation 4.4l: Adaptation and Mitigation 4.4m: Adaptation and Mitigation 4.4n: Adaptation and Mitigation 4.4p: Adaptation and Mitigation 4.4q: Adaptation and Mitigation 4.4r: Adaptation and Mitigation 4.5a: Adaptation and Mitigation 4.5b: Adaptation and Mitigation summaries: 1.1a: Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. The period from 1983 to 2012 was likely the warmest 30-year period of the last 1400 years in the Northern Hemisphere, where such assessment is possible. The globally averaged combined land and ocean surface temperature data as calculated by a linear trend show a warming of 0.85 [0.65 to 1.06] °C 2 over the period 1880 to 2012, when multiple independently produced datasets exist. In addition to robust multi-decadal warming, the globally averaged surface temperature exhibits substantial decadal and interannual variability. Due to this natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over the past 15 years, which begins with a strong El Niño, is smaller than the rate calculated since 1951. 1.1b: Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010, with only about 1% stored in the atmosphere. On a global scale, the ocean warming is largest near the surface, and the upper 75 m warmed by 0.11 [0.09 to 0.13] °C per decade over the period 1971 to 2010. It is virtually certain that the upper ocean warmed from 1971 to 2010, and it likely warmed between the 1870s and 1971. 1.1c: Averaged over the mid-latitude land areas of the Northern Hemisphere, precipitation has increased since 1901 . For other latitudes, area-averaged long-term positive or negative trends have low confidence. Observations of changes in ocean surface salinity also provide indirect evidence for changes in the global water cycle over the ocean. It is very likely that regions of high salinity, where evaporation dominates, have become more saline, while regions of low salinity, where precipitation dominates, have become fresher since the 1950s. 1.1d: Since the beginning of the industrial era, oceanic uptake of CO2 has resulted in acidification of the ocean; the pH of ocean surface water has decreased by 0.1, corresponding to a 26% increase in acidity, measured as hydrogen ion concentration. 1.1e: Over the period 1992 to 2011, the Greenland and Antarctic ice sheets have been losing mass , likely at a larger rate over 2002 to 2011. Glaciers have continued to shrink almost worldwide. Northern Hemisphere spring snow cover has continued to decrease in extent. There is high confidence that permafrost temperatures have increased in most regions since the early 1980s in response to increased surface temperature and changing snow cover. The annual mean Arctic sea-ice extent decreased over the period 1979 to 2012, with a rate that was very likely in the range 3.5 to 4.1% per decade. Arctic sea-ice extent has decreased in every season and in every successive decade since 1979, with the most rapid decrease in decadal mean extent in summer. It is very likely that the annual mean Antarctic sea-ice extent increased in the range of 1.2 to 1.8% per decade between 1979 and 2012. However, there is high confidence that there are strong regional differences in Antarctica, with extent increasing in some regions and decreasing in others. 1.1g: Over the period 1901 to 2010, global mean sea level rose by 0.19 [0.17 to 0.21] m . The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia. 1.2a: Anthropogenic greenhouse gas emissions since the pre-industrial era have driven large increases in the atmospheric concentrations of carbon dioxide, methane and nitrous oxide. Between 1750 and 2011, cumulative anthropogenic CO2 emissions to the atmosphere were 2040 ± 310 GtCO2. About 40% of these emissions have remained in the atmosphere; the rest was removed from the atmosphere and stored on land and in the ocean. The ocean has absorbed about 30% of the emitted anthropogenic CO2, causing ocean acidification. About half of the anthropogenic CO2 emissions between 1750 and 2011 have occurred in the last 40 years . Total anthropogenic GHG emissions have continued to increase over 1970 to 2010 with larger absolute increases between 2000 and 2010, despite a growing number of climate change mitigation policies. Anthropogenic GHG emissions in 2010 have reached 49 ± 4.5 GtCO2-eq/yr 3. Emissions of CO2 from fossil fuel combustion and industrial processes contributed about 78% of the total GHG emissions increase from 1970 to 2010, with a similar percentage contribution for the increase during the period 2000 to 2010. Globally, economic and population growth continued to be the most important drivers of increases in CO2 emissions from fossil fuel combustion. The contribution of population growth between 2000 and 2010 remained roughly identical to the previous three decades, while the contribution of economic growth has risen sharply. Increased use of coal has reversed the long-standing trend of gradual decarbonization of the world’s energy supply. 1.2c: The evidence for human influence on the climate system has grown since the IPCC Fourth Assessment Report . It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in GHG concentrations and other anthropogenic forcings together. The best estimate of the human-induced contribution to warming is similar to the observed warming over this period. Anthropogenic forcings have likely made a substantial contribution to surface temperature increases since the mid-20th century over every continental region except Antarctica. Anthropogenic influences have likely affected the global water cycle since 1960 and contributed to the retreat of glaciers since the 1960s and to the increased surface melting of the Greenland ice sheet since 1993. Anthropogenic influences have very likely contributed to Arctic sea-ice loss since 1979 and have very likely made a substantial contribution to increases in global upper ocean heat content and to global mean sea level rise observed since the 1970s. 1.3a: Evidence of observed climate change impacts is strongest and most comprehensive for natural systems. In many regions, changing precipitation or melting snow and ice are altering hydrological systems, affecting water resources in terms of quantity and quality. Many terrestrial, freshwater and marine species have shifted their geographic ranges, seasonal activities, migration patterns, abundances and species interactions in response to ongoing climate change . Some impacts on human systems have also been attributed to climate change, with a major or minor contribution of climate change distinguishable from other influences. Assessment of many studies covering a wide range of regions and crops shows that negative impacts of climate change on crop yields have been more common than positive impacts. Some impacts of ocean acidification on marine organisms have been attributed to human influence. 1.4a: It is very likely that the number of cold days and nights has decreased and the number of warm days and nights has increased on the global scale. It is likely that the frequency of heat waves has increased in large parts of Europe, Asia and Australia. It is very likely that human influence has contributed to the observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-20th century. It is likely that human influence has more than doubled the probability of occurrence of heat waves in some locations. There is medium confidence that the observed warming has increased heat-related human mortality and decreased cold-related human mortality in some regions. There are likely more land regions where the number of heavy precipitation events has increased than where it has decreased. Recent detection of increasing trends in extreme precipitation and discharge in some catchments implies greater risks of flooding at regional scale. It is likely that extreme sea levels have increased since 1970, being mainly a result of rising mean sea level. Impacts from recent climate-related extremes, such as heat waves, droughts, floods, cyclones and wildfires, reveal significant vulnerability and exposure of some ecosystems and many human systems to current climate variability . 2.1b: Anthropogenic GHG emissions are mainly driven by population size, economic activity, lifestyle, energy use, land use patterns, technology and climate policy. The Representative Concentration Pathways, which are used for making projections based on these factors, describe four different 21st century pathways of GHG emissions and atmospheric concentrations, air pollutant emissions and land use. The RCPs include a stringent mitigation scenario, two intermediate scenarios and one scenario with very high GHG emissions. Scenarios without additional efforts to constrain emissions lead to pathways ranging between RCP6.0 and RCP8.5. RCP2.6 is representative of a scenario that aims to keep global warming likely below 2°C above pre-industrial temperatures. The RCPs are consistent with the wide range of scenarios in the literature as assessed by WGIII5. 2.1c: Multiple lines of evidence indicate a strong, consistent, almost linear relationship between cumulative CO2 emissions and projected global temperature change to the year 2100 in both the RCPs and the wider set of mitigation scenarios analysed in WGIII. Any given level of warming is associated with a range of cumulative CO2 emissions, and therefore, e.g., higher emissions in earlier decades imply lower emissions later. 2.1d: Multi-model results show that limiting total human-induced warming to less than 2°C relative to the period 1861–1880 with a probability of >66%7 would require cumulative CO2 emissions from all anthropogenic sources since 1870 to remain below about 2900 GtCO2. About 1900 GtCO28 had already been emitted by 2011. For additional context see Table 2.2. 2.2e: Changes in precipitation will not be uniform. The high latitudes and the equatorial Pacific are likely to experience an increase in annual mean precipitation under the RCP8.5 scenario. In many mid-latitude and subtropical dry regions, mean precipitation will likely decrease, while in many mid-latitude wet regions, mean precipitation will likely increase under the RCP8.5 scenario. Extreme precipitation events over most of the mid-latitude land masses and over wet tropical regions will very likely become more intense and more frequent. 2.2f: The global ocean will continue to warm during the 21st century, with the strongest warming projected for the surface in tropical and Northern Hemisphere subtropical regions. 2.2g: Earth System Models project a global increase in ocean acidification for all RCP scenarios by the end of the 21st century, with a slow recovery after mid-century under RCP2.6. The decrease in surface ocean pH is in the range of 0.06 to 0.07 for RCP2.6, 0.14 to 0.15 for RCP4.5, 0.20 to 0.21 for RCP6.0 and 0.30 to 0.32 for RCP8.5. 2.2h: Year-round reductions in Arctic sea ice are projected for all RCP scenarios. A nearly ice-free11 Arctic Ocean in the summer sea- ice minimum in September before mid-century is likely for RCP8.512. It is virtually certain that near-surface permafrost extent at high northern latitudes will be reduced as global mean surface temperature increases, with the area of permafrost near the surface projected to decrease by 37% to 81% for the multi-model average. The global glacier volume, excluding glaciers on the periphery of Antarctica , is projected to decrease by 15 to 55% for RCP2.6 and by 35 to 85% for RCP8.5. 2.2m: Summary for Policymakers13SPM There has been significant improvement in understanding and projection of sea level change since the AR4. Global mean sea level rise will continue during the 21st century, very likely at a faster rate than observed from 1971 to 2010. For the period 2081–2100 relative to 1986–2005, the rise will likely be in the ranges of 0.26 to 0.55 m for RCP2.6, and of 0.45 to 0.82 m for RCP8.5 10. Sea level rise will not be uniform across regions. By the end of the 21st century, it is very likely that sea level will rise in more than about 95% of the ocean area. About 70% of the coastlines worldwide are projected to experience a sea level change within ±20% of the global mean. 2.3a: Risk of climate-related impacts results from the interaction of climate-related hazards with the vulnerability and exposure of human and natural systems, including their ability to adapt. Rising rates and magnitudes of warming and other changes in the climate system, accompanied by ocean acidification, increase the risk of severe, pervasive and in some cases irreversible detrimental impacts. Some risks are particularly relevant for individual regions, while others are global. The overall risks of future climate change impacts can be reduced by limiting the rate and magnitude of climate change, including ocean acidification. The precise levels of climate change sufficient to trigger abrupt and irreversible change remain uncertain, but the risk associated with crossing such thresholds increases with rising temperature. For risk assessment, it is important to evaluate the widest possible range of impacts, including low-probability outcomes with large consequences. 2.3b: A large fraction of species faces increased extinction risk due to climate change during and beyond the 21st century, espe- cially as climate change interacts with other stressors. Most plant species cannot naturally shift their geographical ranges sufficiently fast to keep up with current and high projected rates of climate change in most landscapes; most small mammals and freshwater molluscs will not be able to keep up at the rates projected under RCP4.5 and above in flat landscapes in this century. Future risk is indicated to be high by the observation that natural global climate change at rates lower than current anthropogenic climate change caused significant ecosystem shifts and species extinctions during the past millions of years. Marine organisms will face progressively lower oxygen levels and high rates and magnitudes of ocean acidification, with associated risks exacerbated by rising ocean temperature extremes . Coral reefs and polar ecosystems are highly vulnerable. Coastal systems and low-lying areas are at risk from sea level rise, which will continue for centuries even if the global mean temperature is stabilized. 2.3c: Climate change is projected to undermine food security . Due to projected climate change by the mid-21st century and beyond, global marine species redistribution and marine biodiversity reduction in sensitive regions will challenge the sustained provision of fisheries productivity and other ecosystem services. For wheat, rice and maize in tropical and temperate regions, climate change without adaptation is projected to negatively impact production for local temperature increases of 2°C or more above late 20th century levels, although individual locations may benefit. Global temperature increases of ~4°C or more above late 20th century levels, combined with increasing food demand, would pose large risks to food security globally. Climate change is projected to reduce renewable surface water and groundwater resources in most dry subtropical regions, intensifying competition for water among sectors. Rural areas are expected to experience major impacts on water availability and supply, food security, infrastructure and agricultural incomes, including shifts in the production areas of food and non-food crops around the world. 2.3e: Aggregate economic losses accelerate with increasing temperature , but global economic impacts from climate change are currently difficult to estimate. From a poverty perspective, climate change impacts are projected to slow down economic growth, make poverty reduction more difficult, further erode food security and prolong existing and create new poverty traps, the latter particularly in urban areas and emerging hotspots of hunger . International dimensions such as trade and relations among states are also important for understanding the risks of climate change at regional scales. 2.3f: Climate change is projected to increase displacement of people . Populations that lack the resources for planned migration experience higher exposure to extreme weather events, particularly in developing countries with low income. Climate change can indirectly increase risks of violent conflicts by amplifying well-documented drivers of these conflicts such as poverty and economic shocks. 2.4a: Warming will continue beyond 2100 under all RCP scenarios except RCP2.6. Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions. A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial timescale, except in the case of a large net removal of CO2 from the atmosphere over a sustained period. 2.4b: Stabilization of global average surface temperature does not imply stabilization for all aspects of the climate system. Shifting biomes, soil carbon, ice sheets, ocean temperatures and associated sea level rise all have their own intrinsic long timescales which will result in changes lasting hundreds to thousands of years after global surface temperature is stabilized. There is high confidence that ocean acidification will increase for centuries if CO2 emissions continue, and will strongly affect marine ecosystems. 2.4c: It is virtually certain that global mean sea level rise will continue for many centuries beyond 2100, with the amount of rise dependent on future emissions. The threshold for the loss of the Greenland ice sheet over a millennium or more, and an associated sea level rise of up to 7 m, is greater than about 1°C but less than about 4°C of global warming with respect to pre-industrial temperatures. Abrupt and irreversible ice loss from the Antarctic ice sheet is possible, but current evidence and understanding is insufficient to make a quantitative assessment. 2.4d: Magnitudes and rates of climate change associated with medium- to high-emission scenarios pose an increased risk of abrupt and irreversible regional-scale change in the composition, structure and function of marine, terrestrial and freshwater ecosystems, including wetlands. A reduction in permafrost extent is virtually certain with continued rise in global temperatures. 3.1a: Effective decision-making to limit climate change and its effects can be informed by a wide range of analytical approaches for evaluating expected risks and benefits, recognizing the importance of governance, ethical dimensions, equity, value judgments, economic assessments and diverse perceptions and responses to risk and uncertainty. 3.1b: Sustainable development and equity provide a basis for assessing climate policies. Limiting the effects of climate change is necessary to achieve sustainable development and equity, including poverty eradication. Countries’ past and future contributions to the accumulation of GHGs in the atmosphere are different, and countries also face varying challenges and circumstances and have different capacities to address mitigation and adaptation. Mitigation and adaptation raise issues of equity, justice and fairness. Many of those most vulnerable to climate change have contributed and contribute little to GHG emissions. Delaying mitigation shifts burdens from the present to the future, and insufficient adaptation responses to emerging impacts are already eroding the basis for sustainable development. Comprehensive strategies in response to climate change that are consistent with sustainable development take into account the co-benefits, adverse side effects and risks that may arise from both adaptation and mitigation options. 3.1c: The design of climate policy is influenced by how individuals and organizations perceive risks and uncertainties and take them into account. Methods of valuation from economic, social and ethical analysis are available to assist decision-making. These methods can take account of a wide range of possible impacts, including low-probability outcomes with large consequences. But they cannot identify a single best balance between mitigation, adaptation and residual climate impacts. 3.1d: Climate change has the characteristics of a collective action problem at the global scale, because most GHGs accumulate over time and mix globally, and emissions by any agent affect other agents. Effective mitigation will not be achieved if individual agents advance their own interests independently. Cooperative responses, including international cooperation, are therefore required to effectively mitigate GHG emissions and address other climate change issues. The effectiveness of adaptation can be enhanced through complementary actions across levels, including international cooperation. The evidence suggests that outcomes seen as equitable can lead to more effective cooperation. 3.2b: Mitigation and adaptation are complementary approaches for reducing risks of climate change impacts over different time- scales. Mitigation, in the near term and through the century, can substantially reduce climate change impacts in the latter decades of the 21st century and beyond. Benefits from adaptation can already be realized in addressing current risks, and can be realized in the future for addressing emerging risks. 3.2d: Without additional mitigation efforts beyond those in place today, and even with adaptation, warming by the end of the 21st century will lead to high to very high risk of severe, widespread and irreversible impacts globally . In most scenarios without additional mitigation efforts, warming is more likely than not to exceed 4°C above pre-industrial levels by 2100. The risks associated with temperatures at or above 4°C include substantial species extinction, global and regional food insecurity, consequential constraints on common human activities and limited potential for adaptation in some cases. Some risks of climate change, such as risks to unique and threatened systems and risks associated with extreme weather events, are moderate to high at temperatures 1°C to 2°C above pre-industrial levels. 3.2e: Substantial cuts in GHG emissions over the next few decades can substantially reduce risks of climate change by limiting warming in the second half of the 21st century and beyond. Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond. Limiting risks across RFCs would imply a limit for cumulative emissions of CO2. Such a limit would require that global net emissions of CO2 eventually decrease to zero and would constrain annual emissions over the next few decades. But some risks from climate damages are unavoidable, even with mitigation and adaptation. 3.2f: Mitigation involves some level of co-benefits and risks, but these risks do not involve the same possibility of severe, wide- spread and irreversible impacts as risks from climate change. Inertia in the economic and climate system and the possibility of irreversible impacts from climate change increase the benefits from near-term mitigation efforts. Delays in additional mitigation or constraints on technological options increase the longer-term mitigation costs to hold climate change risks at a given level. 3.3a: Adaptation can contribute to the well-being of populations, the security of assets and the maintenance of ecosystem goods, functions and services now and in the future. Adaptation is place- and context-specific. A first step towards adaptation to future climate change is reducing vulnerability and exposure to present climate variability. Integration of adaptation into planning, including policy design, and decision-making can promote synergies with development and disaster risk reduction. Building adaptive capacity is crucial for effective selection and implementation of adaptation options. 3.3b: Adaptation planning and implementation can be enhanced through complementary actions across levels, from individuals to governments. National governments can coordinate adaptation efforts of local and sub-national governments, for example by protecting vulnerable groups, by supporting economic diversification and by providing information, policy and legal frameworks and financial support. Local government and the private sector are increasingly recognized as critical to progress in adaptation, given their roles in scaling up adaptation of communities, households and civil society and in managing risk information and financing. 3.3c: Adaptation planning and implementation at all levels of governance are contingent on societal values, objectives and risk perceptions. Recognition of diverse interests, circumstances, social-cultural contexts and expectations can benefit decision-making processes. Indigenous, local and traditional knowledge systems and practices, including indigenous peoples’ holistic view of community and environment, are a major resource for adapting to climate change, but these have not been used consistently in existing adaptation efforts. Integrating such forms of knowledge with existing practices increases the effectiveness of adaptation. 3.3d: 'Constraints can interact to impede adaptation planning and implementation . Common constraints on implementation arise from the following: limited financial and human resources; limited integration or coordination of governance; uncertainties about projected impacts; different perceptions of risks; competing values; absence of key adaptation leaders and advocates; and limited tools to monitor adaptation effectiveness. Another constraint includes insufficient research, monitoring, and observation and the finance to maintain them. Greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits . Limits to adaptation emerge from the interaction among climate change and biophysical and/or socio-economic constraints. Further, poor planning or implementation, overemphasizing short-term outcomes or failing to sufficiently anticipate consequences can result in maladaptation, increasing the vulnerability or exposure of the target group in the future or the vulnerability of other people, places or sectors. Underestimating the complexity of adaptation as a social process can create unrealistic expectations about achieving intended adaptation outcomes.' 3.3f: Significant co-benefits, synergies and trade-offs exist between mitigation and adaptation and among different adap- tation responses; interactions occur both within and across regions. Increasing efforts to mitigate and adapt to climate change imply an increasing complexity of interactions, particularly at the intersections among water, energy, land use and biodiversity, but tools to understand and manage these interactions remain limited. Examples of actions with co-benefits include improved energy efficiency and cleaner energy sources, leading to reduced emissions of health-damaging, climate-altering air pollutants; reduced energy and water consumption in urban areas through greening cities and recycling water; sustainable agriculture and forestry; and protection of ecosystems for carbon storage and other ecosystem services. 3.3g: Transformations in economic, social, technological and political decisions and actions can enhance adaptation and promote sustainable development. At the national level, transformation is considered most effective when it reflects a country’s own visions and approaches to achieving sustainable development in accordance with its national circumstances and priorities. Restricting adaptation responses to incremental changes to existing systems and structures, without considering transformational change, may increase costs and losses and miss opportunities. Planning and implementation of transformational adaptation could reflect strengthened, altered or aligned paradigms and may place new and increased demands on governance structures to reconcile different goals and visions for the future and to address possible equity and ethical implications. Adaptation pathways are enhanced by iterative learning, deliberative processes and innovation. 3.4a: There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to pre-industrial levels. These pathways would require substantial emissions reductions over the next few decades and near zero emissions of CO2 and other long-lived greenhouse gases by the end of the century. Implementing such reductions poses substantial technological, economic, social and institutional challenges, which increase with delays in additional mitigation and if key technologies are not available. Limiting warming to lower or higher levels involves similar challenges but on different timescales. 3.4b: Without additional efforts to reduce GHG emissions beyond those in place today, global emissions growth is expected to persist, driven by growth in global population and economic activities. Global mean surface temperature increases in 2100 in baseline scenarios—those without additional mitigation—range from 3.7°C to 4.8°C above the average for 1850–1900 for a median climate response. They range from 2.5°C to 7.8°C when including climate uncertainty . 3.4c: Emissions scenarios leading to CO2-equivalent concentrations in 2100 of about 450 ppm or lower are likely to maintain warming below 2°C over the 21st century relative to pre-industrial levels. These scenarios are characterized by 40 to 70% global anthropogenic GHG emissions reductions by 2050 compared to 201016, and emissions levels near zero or below in 2100. Mitigation scenarios reaching concentration levels of about 500 ppm CO2-eq by 2100 are more likely than not to limit temperature change to less than 2°C, unless they temporarily overshoot concentration levels of roughly 530 ppm CO2-eq before 2100, in which case they are about as likely as not to achieve that goal. In these 500 ppm CO2-eq scenarios, global 2050 emissions levels are 25 to 55% lower than in 2010. Scenarios with higher emissions in 2050 are characterized by a greater reliance on Carbon Dioxide Removal technologies beyond mid-century. Trajectories that are likely to limit warming to 3°C relative to pre-industrial levels reduce emissions less rapidly than those limiting warming to 2°C. A limited number of studies provide scenarios that are more likely than not to limit warming to 1.5°C by 2100; these scenarios are characterized by concentrations below 430 ppm CO2-eq by 2100 and 2050 emission reduction between 70% and 95% below 2010. For a comprehensive overview of the characteristics of emissions scenarios, their CO2-equivalent concentrations and their likelihood to keep warming to below a range of temperature levels, see Figure SPM.11 and Table SPM.1. 3.4d: Mitigation scenarios reaching about 450 ppm CO2-eq in 2100 typically involve temporary overshoot of atmospheric concentrations, as do many scenarios reaching about 500 ppm CO2-eq to about 550 ppm CO2-eq in 2100. Depending on the level of overshoot, overshoot scenarios typically rely on the availability and widespread deployment of bioenergy with carbon dioxide capture and storage and afforestation in the second half of the century. The availability and scale of these and other CDR technologies and methods are uncertain and CDR technologies are, to varying degrees, associated with challenges and risks. CDR is also prevalent in many scenarios without overshoot to compensate for residual emissions from sectors where mitigation is more expensive. 3.4e: Reducing emissions of non-CO2 agents can be an important element of mitigation strategies. All current GHG emissions and other forcing agents affect the rate and magnitude of climate change over the next few decades, although long-term warming is mainly driven by CO2 emissions. Emissions of non-CO2 forcers are often expressed as ‘CO2-equivalent emissions’, but the choice of metric to calculate these emissions, and the implications for the emphasis and timing of abatement of the various climate forcers, depends on application and policy context and contains value judgments. 3.4f: Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the 21st century to below 2°C relative to pre-industrial levels. It will require substantially higher rates of emissions reductions from 2030 to 2050; a much more rapid scale-up of low-carbon energy over this period; a larger reliance on CDR in the long term; and higher transitional and long-term economic impacts. Estimated global emissions levels in 2020 based on the Cancún Pledges are not consistent with cost-effective mitigation trajectories that are at least about as likely as not to limit warming to below 2°C relative to pre-industrial levels, but they do not preclude the option to meet this goal . 3.4g: Estimates of the aggregate economic costs of mitigation vary widely depending on methodologies and assumptions, but increase with the stringency of mitigation. Scenarios in which all countries of the world begin mitigation immediately, in which there is a single global carbon price, and in which all key technologies are available have been used as a cost-effective benchmark for estimating macro-economic mitigation costs. Under these assumptions mitigation scenarios that are likely to limit warming to below 2°C through the 21st century relative to pre-industrial levels entail losses in global consumption—not including benefits of reduced climate change as well as co-benefits and adverse side effects of mitigation—of 1 to 4% in 2030, 2 to 6% in 2050 and 3 to 11% in 2100 relative to consumption in baseline scenarios that grows anywhere from 300% to more than 900% over the century. These numbers correspond to an annualized reduction of consumption growth by 0.04 to 0.14 percentage points over the century relative to annualized consumption growth in the baseline that is between 1.6 and 3% per year . 3.4h: In the absence or under limited availability of mitigation technologies , mitigation costs can increase substantially depending on the technology considered. Delaying additional mitigation increases mitigation costs in the medium to long term. Many models could not limit likely warming to below 2°C over the 21st century relative to pre-industrial levels if additional mitigation is considerably delayed. Many models could not limit likely warming to below 2°C if bioenergy, CCS and their combination are limited . 3.4i: Mitigation scenarios reaching about 450 or 500 ppm CO2-eq by 2100 show reduced costs for achieving air quality and energy security objectives, with significant co-benefits for human health, ecosystem impacts and sufficiency of resources and resilience of the energy system. 3.4l: Mitigation policy could devalue fossil fuel assets and reduce revenues for fossil fuel exporters, but differences between regions and fuels exist. Most mitigation scenarios are associated with reduced revenues from coal and oil trade for major exporters. The availability of CCS would reduce the adverse effects of mitigation on the value of fossil fuel assets. 4.1a: Adaptation and mitigation responses are underpinned by common enabling factors. These include effective institutions and governance, innovation and investments in environmentally sound technologies and infrastructure, sustainable livelihoods and behavioural and lifestyle choices. For many regions and sectors, enhanced capacities to mitigate and adapt are part of the foundation essential for managing climate change risks. Improving institutions as well as coordination and cooperation in governance can help overcome regional constraints associated with mitigation, adaptation and disaster risk reduction. 4.1b: Inertia in many aspects of the socio-economic system constrains adaptation and mitigation options . Innovation and investments in environmentally sound infrastructure and technologies can reduce GHG emissions and enhance resilience to climate change. 4.1c: Vulnerability to climate change, GHG emissions and the capacity for adaptation and mitigation are strongly influenced by livelihoods, lifestyles, behaviour and culture. Also, the social acceptability and/or effectiveness of climate policies are influenced by the extent to which they incentivize or depend on regionally appropriate changes in lifestyles or behaviours. 4.2a: Adaptation options exist in all sectors, but their context for implementation and potential to reduce climate-related risks differs across sectors and regions. Some adaptation responses involve significant co-benefits, synergies and trade-offs. Increasing climate change will increase challenges for many adaptation options. 4.2b: Adaptation experience is accumulating across regions in the public and private sectors and within communities. There is increasing recognition of the value of social, institutional, and ecosystem-based measures and of the extent of constraints to adaptation. Adaptation is becoming embedded in some planning processes, with more limited implementation of responses. 4.2c: The need for adaptation along with associated challenges is expected to increase with climate change . Adaptation options exist in all sectors and regions, with diverse potential and approaches depending on their context in vulnerability reduction, disaster risk management or proactive adaptation planning. Effective strategies and actions consider the potential for co-benefits and opportunities within wider strategic goals and development plans. 4.3a: Mitigation options are available in every major sector. Mitigation can be more cost-effective if using an integrated approach that combines measures to reduce energy use and the greenhouse gas intensity of end-use sectors, decarbonize energy supply, reduce net emissions and enhance carbon sinks in land-based sectors. Well-designed systemic and cross-sectoral mitigation strategies are more cost-effective in cutting emissions than a focus on individual technologies and sectors, with efforts in one sector affecting the need for mitigation in others . Mitigation measures intersect with other societal goals, creating the possibility of co-benefits or adverse side effects. These intersections, if well-managed, can strengthen the basis for undertaking climate action. Near-term reductions in energy demand are an important element of cost-effective mitigation strategies, provide more flexibility for reducing carbon intensity in the energy supply sector, hedge against related supply-side risks, avoid lock-in to carbon-intensive infrastructures, and are associated with important co-benefits. The most cost-effective mitigation options in forestry are afforestation, sustainable forest management and reducing deforestation, with large differences in their relative importance across regions; and in agriculture, cropland management, grazing land management and restoration of organic soils. 4.3b: Emissions ranges for baseline scenarios and mitigation scenarios that limit CO2-equivalent concentrations to low levels are shown for different sectors and gases in Figure SPM.14. Key measures to achieve such mitigation goals include decarbonizing electricity generation as well as efficiency enhancements and behavioural changes, in order to reduce energy demand compared to baseline scenarios without compromising development . In scenarios reaching 450 ppm CO2-eq concentrations by 2100, global CO2 emissions from the energy supply sector are projected to decline over the next decade and are characterized by reductions of 90% or more below 2010 levels between 2040 and 2070. In the majority of low-concentration stabilization scenarios , the share of low-carbon electricity supply , nuclear and carbon dioxide capture and storage including bioenergy with carbon dioxide capture and storage ) increases from the current share of approximately 30% to more than 80% by 2050, and fossil fuel power generation without CCS is phased out almost entirely by 2100. 4.3d: Behaviour, lifestyle and culture have a considerable influence on energy use and associated emissions, with high mitigation potential in some sectors, in particular when complementing technological and structural change . Emissions can be substantially lowered through changes in consumption patterns, adoption of energy savings measures, dietary change and reduction in food wastes. 4.4b: 'International cooperation is critical for effective mitigation, even though mitigation can also have local co-benefits. Adapta- tion focuses primarily on local to national scale outcomes, but its effectiveness can be enhanced through coordination across governance scales, including international cooperation: • The United Nations Framework Convention on Climate Change is the main multilateral forum focused on addressing climate change, with nearly universal participation. Other institutions organized at different levels of governance have resulted in diversifying international climate change cooperation. • The Kyoto Protocol offers lessons towards achieving the ultimate objective of the UNFCCC, particularly with respect to participation, implementation, flexibility mechanisms and environmental effectiveness . • Policy linkages among regional, national and sub-national climate policies offer potential climate change mitigation ben- efits. Potential advantages include lower mitigation costs, decreased emission leakage and increased market liquidity. International cooperation for supporting adaptation planning and implementation has received less attention histori- cally than mitigation but is increasing and has assisted in the creation of adaptation strategies, plans and actions at the national, sub-national and local level.' 4.4g: 'There has been a considerable increase in national and sub-national plans and strategies on both adaptation and mitigation since the AR4, with an increased focus on policies designed to integrate multiple objectives, increase co-benefits and reduce adverse side effects: • National governments play key roles in adaptation planning and implementation through coordinating actions and providing frameworks and support. While local government and the private sector have different functions, which vary regionally, they are increasingly recognized as critical to progress in adaptation, given their roles in scaling up adaptation of communities, households and civil society and in managing risk information and financing. • Institutional dimensions of adaptation governance, including the integration of adaptation into planning and decision- making, play a key role in promoting the transition from planning to implementation of adaptation. Examples of institutional approaches to adaptation involving multiple actors include economic options , laws and regulations and national and government policies and programmes.' 4.4l: • In principle, mechanisms that set a carbon price, including cap and trade systems and carbon taxes, can achieve mitiga- tion in a cost-effective way but have been implemented with diverse effects due in part to national circumstances as well as policy design. The short-run effects of cap and trade systems have been limited as a result of loose caps or caps that have not proved to be constraining. In some countries, tax-based policies specifically aimed at reducing GHG emissions—alongside technology and other policies—have helped to weaken the link between GHG emissions and GDP. In addition, in a large group of countries, fuel taxes have had effects that are akin to sectoral carbon taxes. 4.4m: • Regulatory approaches and information measures are widely used and are often environmentally effective . Examples of regulatory approaches include energy efficiency standards; examples of information programmes include labelling programmes that can help consumers make better-informed decisions. 4.4n: • Sector-specific mitigation policies have been more widely used than economy-wide policies . Sector-specific policies may be better suited to address sector-specific barriers or market failures and may be bundled in packages of complementary policies. Although theoretically more cost-effective, administrative and political barriers may make economy-wide policies harder to implement. Interactions between or among mitigation policies may be synergistic or may have no additive effect on reducing emissions. • Economic instruments in the form of subsidies may be applied across sectors, and include a variety of policy designs, such as tax rebates or exemptions, grants, loans and credit lines. An increasing number and variety of renewable energy policies including subsidies—motivated by many factors—have driven escalated growth of RE technologies in recent years. At the same time, reducing subsidies for GHG-related activities in various sectors can achieve emission reductions, depending on the social and economic context. 4.4p: Co-benefits and adverse side effects of mitigation could affect achievement of other objectives such as those related to human health, food security, biodiversity, local environmental quality, energy access, livelihoods and equitable sustainable development. The potential for co-benefits for energy end-use measures outweighs the potential for adverse side effects whereas the evidence suggests this may not be the case for all energy supply and agriculture, forestry and other land use measures. Some mitigation policies raise the prices for some energy services and could hamper the ability of societies to expand access to modern energy services to underserved populations. These potential adverse side effects on energy access can be avoided with the adoption of complementary policies such as income tax rebates or other benefit transfer mechanisms. Whether or not side effects materialize, and to what extent side effects materialize, will be case- and site-specific, and depend on local circumstances and the scale, scope and pace of implementation. Many co-benefits and adverse side effects have not been well-quantified. 4.4q: Technology policy complements other mitigation policies across all scales, from international to sub-national; many adaptation efforts also critically rely on diffusion and transfer of technologies and management practices. Policies exist to address market failures in R&D, but the effective use of technologies can also depend on capacities to adopt technologies appropriate to local circumstances. 4.4r: Substantial reductions in emissions would require large changes in investment patterns . For mitigation scenarios that stabilize concentrations in the range of 430 to 530 ppm CO2-eq by 210019, annual investments in low carbon electricity supply and energy efficiency in key sectors are projected in the scenarios to rise by several hundred billion dollars per year before 2030. Within appropriate enabling environments, the private sector, along with the public sector, can play important roles in financing mitigation and adaptation . Financial resources for adaptation have become available more slowly than for mitigation in both developed and developing countries. Limited evidence indicates that there is a gap between global adaptation needs and the funds available for adaptation. There is a need for better assessment of global adaptation costs, funding and investment. Potential synergies between international finance for disaster risk management and adaptation have not yet been fully realized . 4.5a: Climate change is a threat to sustainable development. Nonetheless, there are many opportu- nities to link mitigation, adaptation and the pursuit of other societal objectives through integrated responses. Successful implementation relies on relevant tools, suitable governance structures and enhanced capacity to respond. 4.5b: Climate change exacerbates other threats to social and natural systems, placing additional burdens particularly on the poor . Aligning climate policy with sustainable development requires attention to both adaptation and mitigation . Delaying global mitigation actions may reduce options for climate-resilient pathways and adaptation in the future. Opportunities to take advantage of positive synergies between adaptation and mitigation may decrease with time, particularly if limits to adaptation are exceeded. Increasing efforts to mitigate and adapt to climate change imply an increasing complexity of interactions, encompassing connections among human health, water, energy, land use and biodiversity . summary_topics: 1.1a: Climate Change Trend 1.1b: Ocean Warming 1.1c: Precipitations and Ocean Salinity 1.1d: Acidification 1.1e: Ice Melting 1.1g: Sea Level 1.2a: Anthropogenic Greenhouse Gas Emissions 1.2c: Human Influence on Climate Change 1.3a: Impacts of Climate Change 1.4a: Extreme Events 2.1b: Anthropogenic Emissions Causes and Projections 2.1c: CO2 Correlation with Global Warming 2.1d: Reducing Global Warming 2.2e: Changes in Precipitations 2.2f: Ocean Warming 2.2g: Acidification 2.2h: Ice Melting 2.2m: Sea Level 2.3a: Main Risks of Climate Change 2.3b: Animal Extiction 2.3c: Food Security 2.3e: Economic Impact 2.3f: Societal Impact 2.4a: Surface Temperature 2.4b: Stabilization 2.4c: Sea Level 2.4d: Overall Risks 3.1a: Decision-making 3.1b: Sustainable Development and Equity 3.1c: Designing Climate Policies 3.1d: Cooperation and Collective Action 3.2b: Mitigation and Adaptation 3.2d: Risks of Inaction 3.2e: Actions for Mitigation 3.2f: Mitigation Risks 3.3a: Adaptation Advantages 3.3b: Adaptation Implementation 3.3c: Adaptation and Sciety 3.3d: Adaptation Limits 3.3f: Co-benefits 3.3g: Transformational Change 3.4a: Mitigation Pathways 3.4b: Risks of Inaction 3.4c: Emission Scenarios 3.4d: Mitigation Scenarios 3.4e: Reducing Emissions 3.4f: Effects of Mitigation Delay 3.4g: Economic Cost of Mitigation 3.4h: Mitigation Technologies 3.4i: Mitigation Co-benefits 3.4l: Fossil Fuel 4.1a: Enabling Mitigation and Adaptation 4.1b: Innovation and Investments 4.1c: Factors of Vulnerability 4.2a: Adaptation Options 4.2b: Adaptation Adoption 4.2c: Need for Adaptation 4.3a: Mitigation Options 4.3b: Mitigation Scenarios 4.3d: Impact of Behaviour 4.4b: International Cooperation 4.4g: National Strategies 4.4l: Carbon Tax 4.4m: Regulatory Approaches and Information Measures 4.4n: Sector Policies 4.4p: Co-benefits and Side Effects 4.4q: Technological Solutions 4.4r: Financial Solutions 4.5a: Climate Change Projections 4.5b: Tackling Climate Change titles: 1.1a: - 'Atmosphere ' 1.1b: - 'Ocean ' 1.1c: - 'Atmosphere ' - 'Ocean ' 1.1d: - 'Ocean ' 1.1e: - 'Cryosphere ' 1.1g: - 'Sea level ' 1.2a: - 'Natural and anthropogenic radiative forcings ' - 'Human activities affecting emission drivers ' 1.2c: - 'Attribution of climate ' 1.3a: - 'Observed impacts attributed to climate change ' 1.4a: - 'Extreme events ' 2.1b: - 'Key drivers of future climate and the ' - 'Response options for mitigation ' 2.1c: - 'Climate system responses ' 2.1d: - 'Climate system responses ' 2.2e: - 'Water cycle ' 2.2f: - 'Ocean, cryosphere and sea level ' 2.2g: - 'Carbon cycle and biogeochemistry ' 2.2h: - 'Ocean, cryosphere and sea level ' 2.2m: - 'Ocean, cryosphere and sea level ' 2.3a: - 'Exposure and vulnerability ' - 'Future risks and impacts caused ' - 'Climate change beyond 2100, ' - 'Characteristics of adaptation pathways ' 2.3b: - 'Future risks and impacts caused ' - 'Climate change beyond 2100, ' 2.3c: - 'Ecosystems and their services in the oceans, ' - 'Water, food and urban systems, human ' 2.3e: - 'Water, food and urban systems, human ' 2.3f: - 'Water, food and urban systems, human ' 2.4a: - 'Climate change beyond 2100, ' 2.4b: - 'Climate change beyond 2100, ' 2.4c: - 'Climate change beyond 2100, ' 2.4d: - 'Climate change beyond 2100, ' 3.1a: - 'Foundations of decision-making ' 3.1b: - 'Foundations of decision-making ' - 'Interaction among mitigation, adaptation ' 3.1c: - 'Foundations of decision-making ' 3.1d: - 'Foundations of decision-making ' 3.2b: - 'Climate change risks reduced by ' - 'Trade-offs, synergies and integrated responses ' 3.2d: - 'Future risks and impacts caused ' - 'Climate change risks reduced by ' - 'Characteristics of mitigation pathways ' 3.2e: - 'Climate system responses ' - 'Climate change risks reduced by ' - 'Characteristics of mitigation pathways ' 3.2f: - 'Climate change risks reduced by ' - 'Characteristics of mitigation pathways ' 3.3a: - 'Characteristics of adaptation pathways ' 3.3b: - 'Characteristics of adaptation pathways ' 3.3c: - 'Characteristics of adaptation pathways ' 3.3d: - 'Characteristics of adaptation pathways ' 3.3f: - 'Characteristics of adaptation pathways ' 3.3g: - 'Characteristics of adaptation pathways ' 3.4a: - 'Characteristics of mitigation pathways ' 3.4b: - 'Characteristics of mitigation pathways ' 3.4c: - 'Characteristics of mitigation pathways ' 3.4d: - 'Characteristics of mitigation pathways ' 3.4e: - 'Characteristics of mitigation pathways ' 3.4f: - 'Characteristics of mitigation pathways ' 3.4g: - 'Characteristics of mitigation pathways ' 3.4h: - 'Characteristics of mitigation pathways ' 3.4i: - 'Mitigation ' 3.4l: - 'Mitigation ' 4.1a: - 'Common enabling factors and constraints ' 4.1b: - 'Common enabling factors and constraints ' 4.1c: - 'Common enabling factors and constraints ' 4.2a: - 'Response options for adaptation ' 4.2b: - 'Human responses to climate change: ' - 'Response options for adaptation ' - 'Adaptation ' 4.2c: - 'Response options for adaptation ' 4.3a: - 'Response options for mitigation ' 4.3b: - 'Response options for mitigation ' 4.3d: - 'Common enabling factors and constraints ' - 'Response options for mitigation ' 4.4b: - 'International and regional cooperation ' 4.4g: - 'Adaptation ' - 'Mitigation ' 4.4l: - 'Mitigation ' 4.4m: - 'Mitigation ' 4.4n: - 'Mitigation ' 4.4p: - 'Response options for mitigation ' - 'Mitigation ' 4.4q: - 'Technology development and transfer ' 4.4r: - 'Investment and finance ' 4.5a: - 'Interaction among mitigation, adaptation ' - 'Trade-offs, synergies and integrated responses ' 4.5b: - 'Foundations of decision-making ' - 'Interaction among mitigation, adaptation ' - 'Trade-offs, synergies and integrated responses '