Query: Doubling the concentration of atmospheric CO2 from its pre-industrial level, in the absence of other forcings and feedbacks, would likely cause a warming of ~0.3Â°C to 1.1Â°C

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Climate change feedback <=SEP=> Climate change feedback is important in the understanding of global warming because feedback processes may amplify or diminish the effect of each climate forcing , and so play an important part in determining the climate sensitivity and future climate state . Feedback in general is the process in which changing one quantity changes a second quantity , and the change in the second quantity in turn changes the first . Positive feedback amplifies the change in the first quantity while negative feedback reduces it .   The term `` forcing '' means a change which may `` push '' the climate system in the direction of warming or cooling . An example of a climate forcing is increased atmospheric concentrations of greenhouse gases . By definition , forcings are external to the climate system while feedbacks are internal ; in essence , feedbacks represent the internal processes of the system . Some feedbacks may act in relative isolation to the rest of the climate system ; others may be tightly coupled ; hence it may be difficult to tell just how much a particular process contributes . Forcings , feedbacks and the dynamics of the climate system determine how much and how fast the climate changes . The main positive feedback in global warming is the tendency of warming to increase the amount of water vapor in the atmosphere , which in turn leads to further warming . The main negative feedback comes from the Stefan -- Boltzmann law , the amount of heat radiated from the Earth into space changes with the fourth power of the temperature of Earth 's surface and atmosphere .   Some observed and potential effects of global warming are positive feedbacks , which contribute directly to further global warming . The Intergovernmental Panel on Climate Change 's ( IPCC ) Fourth Assessment Report states that `` Anthropogenic warming could lead to some effects that are abrupt or irreversible , depending upon the rate and magnitude of the climate change . ''

Climate sensitivity <=SEP=> Climate sensitivity is the equilibrium temperature change in response to changes of the radiative forcing . Therefore , climate sensitivity depends on the initial climate state , but potentially can be accurately inferred from precise palaeoclimate data . Slow climate feedbacks , especially changes of ice sheet size and atmospheric CO2 , amplify the total Earth system sensitivity by an amount that depends on the time scale considered .   Although climate sensitivity is usually used in the context of radiative forcing by carbon dioxide ( CO2 ) , it is thought of as a general property of the climate system : the change in surface air temperature ( ΔTs ) following a unit change in radiative forcing ( RF ) , and thus is expressed in units of ° C / ( W/m2 ) . For this to be useful , the measure must be independent of the nature of the forcing ( e.g. from greenhouse gases or solar variation ) ; to first order this is indeed found to be so .   The climate sensitivity specifically due to is often expressed as the temperature change in ° C associated with a doubling of the concentration of carbon dioxide in Earth 's atmosphere .   For coupled atmosphere-ocean global climate models ( e.g. CMIP5 ) the climate sensitivity is an emergent property : it is not a model parameter , but rather a result of a combination of model physics and parameters . By contrast , simpler energy-balance models may have climate sensitivity as an explicit parameter .   The terms represented in the equation relate radiative forcing ( RF ) to linear changes in global surface temperature change ( ΔTs ) via the climate sensitivity λ .   It is also possible to estimate climate sensitivity from observations ; however , this is difficult due to uncertainties in the forcing and temperature histories .

Carbon dioxide in Earth's atmosphere <=SEP=> Carbon dioxide is an important trace gas in Earth 's atmosphere . Currently it constitutes about 0.041 % ( equal to 410 parts per million ; ppm ) by volume of the atmosphere . Despite its relatively small concentration is a potent greenhouse gas and plays a vital role in regulating Earth 's surface temperature through radiative forcing and the greenhouse effect . Reconstructions show that concentrations of in the atmosphere have varied , ranging from as high as 7,000 ppm during the Cambrian period about 500 million years ago to as low as 180 ppm during the Quaternary glaciation of the last two million years .   Carbon dioxide is an integral part of the carbon cycle , a biogeochemical cycle in which carbon is exchanged between the Earth 's oceans , soil , rocks and the biosphere . Plants and other photoautotrophs use solar energy to produce carbohydrate from atmospheric carbon dioxide and water by photosynthesis . Almost all other organisms depend on carbohydrate derived from photosynthesis as their primary source of energy and carbon compounds .   The current episode of global warming is attributed to increasing emissions of and other greenhouse gases into Earth 's atmosphere . The global annual mean concentration of in the atmosphere has increased by more than 40 % since the start of the Industrial Revolution , from 280 ppm , the level it had for the last 10,000 years leading up to the mid-18th century , to 399 ppm as of 2015 . The present concentration is the highest in at least the past 800,000 years and likely the highest in the past 20 million years . The increase has been caused by anthropogenic sources , particularly the burning of fossil fuels and deforestation . The daily average concentration of atmospheric CO2 at Mauna Loa Observatory first exceeded 400 ppm on 10 May 2013 . It is currently rising at a rate of approximately 2 ppm/year and accelerating . An estimated 30 -- 40 % of the released by humans into the atmosphere dissolves into oceans , rivers and lakes , which contributes to ocean acidification .

Global warming <=SEP=> Global warming , also referred to as climate change , is the observed century-scale rise in the average temperature of the Earth 's climate system and its related effects . Multiple lines of scientific evidence show that the climate system is warming . Many of the observed changes since the 1950s are unprecedented in the instrumental temperature record which extends back to the mid 19th century , and in paleoclimate proxy records over thousands of years .   In 2013 , the Intergovernmental Panel on Climate Change ( IPCC ) Fifth Assessment Report concluded that `` It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century . '' The largest human influence has been emission of greenhouse gases such as carbon dioxide , methane and nitrous oxide . Climate model projections summarized in the report indicated that during the 21st century the global surface temperature is likely to rise a further 0.3 to for their lowest emissions scenario and 2.6 to for the highest emissions scenario . These findings have been recognized by the national science academies of the major industrialized nations and are not disputed by any scientific body of national or international standing .   Future climate change and associated impacts will differ from region to region around the globe . Anticipated effects include warming global temperature , rising sea levels , changing precipitation , and expansion of deserts in the subtropics . Warming is expected to be greater over land than over the oceans and greatest in the Arctic , with the continuing retreat of glaciers , permafrost and sea ice . Other likely changes include more frequent extreme weather events including heat waves , droughts , heavy rainfall with floods and heavy snowfall ; ocean acidification ; and species extinctions due to shifting temperature regimes . Effects significant to humans include the threat to food security from decreasing crop yields and the abandonment of populated areas due to rising sea levels . Because the climate system has a large `` inertia '' and greenhouse gases will stay in the atmosphere for a long time , many of these effects will not only exist for decades or centuries , but will persist for tens of thousands of years .   Possible societal responses to global warming include mitigation by emissions reduction , adaptation to its effects , building systems resilient to its effects , and possible future climate engineering . Most countries are parties to the United Nations Framework Convention on Climate Change ( UNFCCC ) ,  whose ultimate objective is to prevent dangerous anthropogenic climate change . Parties to the UNFCCC have agreed that deep cuts in emissions are required and that global warming should be limited to well below 2.0 C-change relative to pre-industrial levels , with efforts made to limit warming to 1.5 C-change .   Public reactions to global warming and concern about its effects are also increasing . A global 2015 Pew Research Center report showed a median of 54 % consider it `` a very serious problem '' . There are significant regional differences , with Americans and Chinese ( whose economies are responsible for the greatest annual CO2 emissions ) among the least concerned .