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CHAPTER 3

DEFINING THE CLIMATE CHANGE PROBLEM AND RATIONALE FOR RULEMAKING

3.1 Overview of Climate Change Impacts from GHG Emissions

In 2009, the EPA Administrator found that elevated concentrations of greenhouse gases 

in the atmosphere may reasonably be anticipated both to endanger public health and to 

endanger public welfare. It is these adverse impacts that make it necessary for EPA to regulate 

greenhouse gases from EGU sources. This proposed rule is designed to minimize emissions of 

greenhouse gases, minimize the rate of increase of concentrations of these gases, and 

therefore reduce the risk of adverse effects. As the analysis presented in Chapter 5 shows, 

existing and anticipated economic conditions in the marketplace will lead electricity generators 

to choose technologies that meet the proposed standards. However, rather than relying solely 

on changeable energy market conditions to provide low emissions from new power plants in 

the future, though this rule, EPA ensures that new electricity generation will be clean 

generation. 

This chapter summarizes the adverse effects on public health and public welfare 

detailed in the 2009 Endangerment Finding.1The major assessments by the U.S. Global Change

Research Program (USGCRP), the Intergovernmental Panel on Climate Change (IPCC), and the 

National Research Council (NRC) served as the primary scientific basis for these effects.

3.1.1 Public Health

Climate change threatens public health in a number of ways: direct temperature effects, 

air quality effects, the potential for changes in vector-borne diseases, and the potential for 

changes in the severity and frequency of extreme weather events. Additionally, susceptible 

populations may be particularly at risk. Each of these effects will be addressed in turn in this 

section, based on the 2009 Endangerment Finding.

Regarding direct temperature changes, it has already been estimated that unusually hot 

days and heat waves are becoming more frequent, and that unusually cold days are becoming 

less frequent. Heat is already the leading cause of weather-related deaths in the United States. 

In the future, severe heat waves are projected to intensify in magnitude and duration over the 

portions of the United States where these events already occur. Heat waves are associated with 

marked short-term increases in mortality. Hot temperatures have also been associated with 

 

1

Endangerment and Cause or Contribute Findings for Greenhouse Gases Under Section 202(a) of the Clean Air Act, 

74 Fed. Reg. 66,496 (Dec. 15, 2009).
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increased morbidity. The projected warming is therefore projected to increase heat related 

mortality and morbidity, especially among the elderly, young, and frail. The populations most 

sensitive to hot temperatures are older adults, the chronically sick, the very young, citydwellers, those taking medications that disrupt thermoregulation, the mentally ill, those lacking 

access to air conditioning, those working or playing outdoors, and socially isolated persons. As

warming increases over time, these adverse effects would be expected to increase as the 

serious heat events become more serious. 

Increases in temperature are also expected to lead to some reduction in the risk of 

death related to extreme cold. It is not clear whether reduced mortality in the United States 

from cold would be greater or less than increased heat-related mortality in the United States 

due to climate change. However, there is a risk that projections of cold-related deaths, and the 

potential for decreasing their numbers due to warmer winters, can be overestimated unless 

they take into account the effects of season and influenza, which is not strongly associated with 

monthly winter temperature. In addition, the latest USGCRP report (2009) refers to a study

(Medina-Ramon and Schwartz, 2007) that analyzed daily mortality and weather data in 50 U.S. 

cities from 1989 to 2000 and found that, on average, cold snaps in the United States increased 

death rates by 1.6 percent, while heat waves triggered a 5.7 percent increase in death rates. 

The study concludes that increases in heat-related mortality due to global warming in the 

United States are unlikely to be compensated for by decreases in cold-related mortality. 

Regarding air quality effects, increases in regional ozone pollution relative to ozone 

levels without climate change are expected due to higher temperatures and weaker circulation 

in the United States relative to air quality levels without climate change. Climate change is 

expected to increase regional ozone pollution, with associated risks in respiratory illnesses and 

premature death. In addition to human health effects, tropospheric ozone has significant 

adverse effects on crop yields, pasture and forest growth, and species composition. 

Modeling studies discussed in EPA’s Interim Assessment (2009) show that simulated 

climate change causes increases in summertime ozone concentrations over substantial regions 

of the country, though this was not uniform, and some areas showed little change or decreases, 

though the decreases tend to be less pronounced than the increases. For those regions that 

showed climate-induced increases, the increase in maximum daily 8-hour average ozone 

concentration, a key metric for regulating U.S. air quality, was in the range of 2 to 8 ppb, 

averaged over the summer season. The increases were substantially greater than this during 

the peak pollution episodes that tend to occur over a number of days each summer. The overall 

effect of climate change was projected to increase ozone levels, compared to what would occur 
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without this climate change, over broad areas of the country, especially on the highest ozone 

days and in the largest metropolitan areas with the worst ozone problems. Ozone decreases are 

projected to be less pronounced, and generally to be limited to some regions of the country 

with smaller population. 

In addition to impacts on heat-related mortality and air quality, there is also the 

potential for increased deaths, injuries, infectious diseases, and stress-related disorders and 

other adverse effects associated with social disruption and migration from more frequent 

extreme weather. Vulnerability to these disasters depends on the attributes of the people at 

risk and on broader social and environmental factors. 

Increases in the frequency of heavy precipitation events are associated with increased 

risk of deaths and injuries as well as infectious, respiratory, and skin diseases. Floods are lowprobability, high-impact events that can overwhelm physical infrastructure, human resilience, 

and social organization. Flood health impacts include deaths, injuries, infectious diseases, 

intoxications, and mental health problems. 

Increases in tropical cyclone intensity are linked to increases in the risk of deaths, 

injuries, waterborne and food borne diseases, as well as post-traumatic stress disorders. 

Drowning by storm surge, heightened by rising sea levels and more intense storms (as 

projected by IPCC), is the major killer in coastal storms where there are large numbers of 

deaths. Flooding can cause health impacts including direct injuries as well as increased 

incidence of waterborne diseases.

According to the assessment literature, there will also likely be an increase in the spread 

of several food and water-borne pathogens among susceptible populations depending on the 

pathogens’ survival, persistence, habitat range and transmission under changing climate and 

environmental conditions. Food borne diseases show some relationship with temperature, and 

the range of some zoonotic disease carriers such as the Lyme disease carrying tick may increase 

with temperature. 

Climate change, including changes in carbon dioxide concentrations, could impact the 

production, distribution, dispersion, and allergenicity of aeroallergens and the growth and 

distribution of weeds, grasses, and trees that produce them. These changes in aeroallergens 

and subsequent human exposures could affect the prevalence and severity of allergy 

symptoms. However, the scientific literature does not provide definitive data or conclusions on 

how climate change might impact aeroallergens and subsequently the prevalence of allergenic 
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illnesses in the United States. It has generally been observed that the presence of elevated 

carbon dioxide concentrations and temperatures stimulate plants to increase photosynthesis, 

biomass, water use efficiency, and reproductive effort. The IPCC concluded that pollens are 

likely to increase with elevated temperature and carbon dioxide. 

3.1.2 Public Welfare

As with public health, there are multiple pathways in which the greenhouse gas air 

pollution and resultant climate change affect climate-sensitive sectors. These sectors include 

food production and agriculture; forestry; water resources; sea level rise and coastal areas; 

energy, infrastructure, and settlements; and ecosystems and wildlife. Impacts also arise from 

climate change occurring outside of the United States, such as national security concerns for 

the United States that may arise as a result of climate change impacts in other regions of the 

world. Each of these effects will be addressed in turn in this section, based on the 2009 Finding.

Regarding food production and agriculture, elevated carbon dioxide concentrations can 

have a stimulatory effect, as may modest temperature increases and a longer growing season 

that results. However, elevated carbon dioxide concentrations may also enhance pest and 

weed growth. In addition, higher temperature increases, changing precipitation patterns and 

variability, and any increases in ground-level ozone induced by higher temperatures, can work 

to counteract any direct stimulatory carbon dioxide effect, as well as lead to their own adverse 

impacts. A USGCRP report (2009) concluded that while for some crops such as grain and oilseed 

crops there may be a beneficial effect overall in the next couple decades, as temperature rises, 

these crops will increasingly begin to experience failure, especially if climate variability 

increases and precipitation lessens or becomes more variable. Changes in the intensity and 

frequency of extreme weather events such as droughts and heavy storms have the potential to 

have serious negative impact on U.S. food production and agriculture. Changing precipitation 

patterns, in addition to increasing temperatures and longer growing seasons, can change the 

demand for irrigation requirements, potentially increasing irrigation demand. 

With respect to livestock, higher temperatures will very likely reduce livestock 

production during the summer season in some areas, but these losses will very likely be 

partially offset by warmer temperatures during the winter season. The impact on livestock 

productivity due to increased variability in weather patterns will likely be far greater than 

effects associated with the average change in climatic conditions.

For the forestry sector there are similar factors to consider. There is the potential for 

beneficial effects due to elevated concentrations of carbon dioxide, increased temperatures,
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and nitrogen deposition, but there is also the potential for adverse effects from increasing 

temperatures, changing precipitation patterns, increased insects and disease, and the potential 

for more frequent and severe extreme weather events. According to the science assessment 

reports on which the Administrator relied for the 2009 Finding, climate change has very likely 

increased the size and number of wildfires, insect outbreaks, and tree mortality in the Interior 

West, the Southwest, and Alaska, and will continue to do so. 

If existing trends in precipitation continue, it is expected that forest productivity will 

likely decrease in the Interior West, the Southwest, eastern portions of the Southeast, and 

Alaska, and that forest productivity will likely increase in the northeastern United States, the 

Lake States, and in western portions of the Southeast. An increase in drought events will very 

likely reduce forest productivity wherever such events occur. 

The sensitivity of water resources to climate change is very important given the 

increasing demand for adequate water supplies and services for agricultural, municipal, and 

energy and industrial uses, and the current strains on this resource in many parts of the 

country. According to the assessment literature, climate change has already altered, and will

likely continue to alter, the water cycle, affecting where, when, and how much water is 

available for all uses. With higher temperatures, the water-holding capacity of the atmosphere 

and evaporation into the atmosphere increase, and this favors increased climate variability, 

with more intense precipitation and more droughts. 

Climate change is causing and will increasingly cause shrinking snowpack induced by 

increasing temperature. In the western United States, there is already well-documented 

evidence of shrinking snowpack due to warming. Earlier meltings, with increased runoff in the 

winter and early spring, increase flood concerns and also result in substantially decreased 

summer flows. This pattern of reduced snowpack and changes to the flow regime pose very 

serious risks to major population regions, such as California, that rely on snowmelt-dominated 

watersheds for their water supply. While increased precipitation is expected to increase water 

flow levels in some eastern areas, this may be tempered by increased variability in the 

precipitation and the accompanying increased risk of floods and other concerns such as water 

pollution. 

Climate change will likely further constrain already over-allocated water resources in 

some regions of the United States, increasing competition among agricultural, municipal, 

industrial, and ecological uses. Although water management practices in the United States are 

generally advanced, particularly in the West, the reliance on past conditions as the basis for 
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current and future planning may no longer be appropriate, as climate change increasingly 

creates conditions well outside of historical observations. Increased incidence of extreme 

weather and floods may also overwhelm or damage water treatment and management 

systems, resulting in water quality impairments. 

According to the assessment literature, sea level is rising along much of the U.S. coast, 

and the rate of change will very likely increase in the future, exacerbating the impacts of 

progressive inundation, storm-surge flooding, and shoreline erosion. A large percentage of the 

U.S. population lives in these coastal areas. The most vulnerable areas are the Atlantic and Gulf 

Coasts, the Pacific Islands, and parts of Alaska. Cities such as New Orleans, Miami, and New 

York are particularly at risk, and could have difficulty coping with the sea level rise projected by 

the end of the century under a higher emissions scenario. Population growth and the rising 

value of infrastructure increases the vulnerability to climate variability and future climate 

change in coastal areas. Adverse impacts on islands present concerns for Hawaii and the U.S. 

territories. Reductions in Arctic sea ice increases extreme coastal erosion in Alaska, due to the 

increased exposure of the coastline to strong wave action. In the Great Lakes, where sea level 

rise is not a concern, both extremely high and low water levels resulting from changes to the 

hydrological cycle have been damaging and disruptive to shoreline communities. 

Coastal wetland loss is being observed in the United States where these ecosystems are 

squeezed between natural and artificial landward boundaries and rising sea levels. Up to 21 

percent of the remaining coastal wetlands in the U.S. mid-Atlantic region are potentially at risk 

of inundation between 2000 and 2100. Coastal habitats will likely be increasingly stressed by 

climate change impacts interacting with development and pollution. 

Although increases in mean sea level over the 21st century and beyond will inundate 

unprotected, low-lying areas, the most devastating impacts are likely to be associated with 

storm surge. Superimposed on expected rates of sea level rise, projected storm intensity, wave 

height, and storm surge suggest more severe coastal flooding and erosion hazards. Higher sea 

level provides an elevated base for storm surges to build upon and diminishes the rate at which 

low-lying areas drain, thereby increasing the risk of flooding from rainstorms. In New York City 

and Long Island, flooding from a combination of sea level rise and storm surge could be several 

meters deep. Projections suggest that the return period of a 100-year flood event in this area 

might be reduced to 4–60 years by the 2080s. Additionally, some major urban centers in the 

United States, such as areas of New Orleans are situated in low-lying flood plains, presenting 

increased risk from storm surges. 
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With respect to infrastructure, climate change vulnerabilities of industry, settlement,

and society are mainly related to changes in intensity and frequency of extreme weather events 

rather than to gradual climate change. Extreme weather events could threaten U.S. energy 

infrastructure (transmission and distribution), transportation infrastructure (roads, bridges, 

airports and seaports), water infrastructure, and other built aspects of human settlements. 

Moreover, soil subsidence caused by the melting of permafrost in the Arctic region is a risk to 

gas and oil pipelines, electrical transmission towers, roads, and water systems. 

Within settlements experiencing climate change stressors, certain parts of the 

population may be especially vulnerable based on their circumstances. These include the poor, 

the elderly, the very young, those already in poor health, the disabled, those living alone, 

and/or indigenous populations dependent on one or a few resources. In Alaska, indigenous 

communities are likely to experience disruptive impacts, including shifts in the range or 

abundance of wild species crucial to their livelihoods and well-being. 

Climate change is exerting major influences on natural environments and biodiversity, 

and these influences are generally expected to grow with increased warming. Observed 

changes in the life cycles of plants and animals include shifts in habitat ranges, timing of 

migration patterns, and changes in reproductive timing and behavior. 

The underlying assessment literature finds with high confidence that substantial 

changes in the structure and functioning of terrestrial ecosystems are very likely to occur with a 

global warming greater than 2 to 3 °C above pre-industrial levels, with predominantly negative 

consequences for biodiversity and the provisioning of ecosystem goods and services. With 

global average temperature changes above 2 °C, many terrestrial, freshwater, and marine 

species (particularly endemic species) are at a far greater risk of extinction than in the 

geological past. Climate change and ocean acidification will likely impair a wide range of 

planktonic and other marine calcifiers such as corals. Even without ocean acidification effects, 

increases in sea surface temperature of about 1–3 °C are projected to result in more frequent 

coral bleaching events and widespread mortality. In the Arctic, wildlife faces great challenges 

from the effects of climatic warming, as projected reductions in sea ice will drastically shrink 

marine habitat for polar bears, ice-inhabiting seals, and other animals. 

Some common forest types are projected to expand, others are projected to contract, 

and others, such as spruce-fir, are likely to disappear from the contiguous United States. 

Changes in plant species composition in response to climate change can increase ecosystem 

vulnerability to other disturbances, including wildfires and biological invasion. Disturbances 
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such as wildfires and insect outbreaks are increasing in the United States and are likely to 

intensify in a warmer future with warmer winters, drier soils and longer growing seasons. The 

areal extent of drought-limited ecosystems is projected to increase 11 percent per °C warming 

in the United States. In California, temperature increases greater than 2°C may lead to 

conversion of shrubland into desert and grassland ecosystems and evergreen conifer forests 

into mixed deciduous forests. Greater intensity of extreme events may alter disturbance 

regimes in coastal ecosystems leading to changes in diversity and ecosystem functioning. 

Species inhabiting salt marshes, mangroves, and coral reefs are likely to be particularly 

vulnerable to these effects. 

According to the USGCRP report of June 2009 and other sources, climate change 

impacts in certain regions of the world may exacerbate problems that raise humanitarian, 

trade, and national security issues for the United States.2The IPCC identifies the most 

vulnerable world regions as the Arctic, because of the effects of high rates of projected 

warming on natural systems; Africa, especially the sub-Saharan region, because of current low 

adaptive capacity as well as climate change; small islands, due to high exposure of population 

and infrastructure to risk of sea-level rise and increased storm surge; and Asian mega-deltas 

due to large populations and high exposure to sea level rise, storm surge, and river flooding. 

Climate change has been described as a potential threat multiplier with regard to national 

security issues. While some of these international risks do not readily lend themselves to 

precise analyses or future projections, given the unavoidable global nature of the climate 

change problem it is appropriate and prudent to consider how impacts in other world regions 

may present risks to the U.S. population.

3.1.3 Recent Assessments

Since the Endangerment Finding was released, more recent assessments have produced 

similar conclusions to those of the assessments upon which the Finding was based. In May 

2010, the NRC published its comprehensive assessment, “Advancing the Science of Climate 

Change” (2010). It concluded that “climate change is occurring, is caused largely by human 

activities, and poses significant risks for — and in many cases is already affecting — a broad 

range of human and natural systems.” Furthermore, the NRC stated that this conclusion is 

based on findings that are “consistent with the conclusions of recent assessments by the U.S. 

 

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‘‘In an increasingly interdependent world, U.S. vulnerability to climate change is linked to the fates of other 

nations. For example, conflicts or mass migrations of people resulting from food scarcity and other resource limits, 

health impacts or environmental stresses in other parts of the world could threaten U.S. national security.’’ (Karl et 

al., 2009).
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Global Change Research Program, the Intergovernmental Panel on Climate Change’s Fourth 

Assessment Report, and other assessments of the state of scientific knowledge on climate 

change.” These are the same assessments that served as the primary scientific references 

underlying the Administrator’s Endangerment Finding. Another NRC assessment, “Climate 

Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia,” was 

published in 2011. This report found that climate change due to carbon dioxide emissions will 

persist for many centuries. The report also estimate a number of specific climate change 

impacts, finding that every degree Celsius of warming could lead to increases in the heaviest 15 

percent of daily rainfalls of 3 to 10 percent, decreases of 5 to 15 percent in yields for a number 

of crops (absent adaptation measures that do not presently exist), decreases of Arctic sea ice 

extent of 25 percent in September and 15 percent annually averaged, along with changes in 

precipitation and stream flow of 5 to 10 percent in many regions and river basins. The 

assessment also found that for an increase of 4 degrees C nearly all land areas would 

experience average summers warmer than all but 5 percent of summers in the 20th century, 

that for an increase of 1 to 2 degrees C the area burnt by wildfires in western North America 

will likely more than double, that coral bleaching and erosion will increase due both to warming 

and ocean acidification, and that sea level will rise 1.6 to 3.3 feet by 2100 in a 3 degree C 

scenario. The assessment notes that many important aspects of climate change are difficult to 

quantify but that the risk of adverse impacts is likely to increase with increasing temperature, 

and that the risk of surprises can be expected to increase with the duration and magnitude of 

warming.

In the 2010 report cited above, the NRC stated that some of the largest potential risks 

associated with future climate change may come not from relatively smooth changes that are 

reasonably well understood, but from extreme events, abrupt changes, and surprises that 

might occur when climate or environmental system thresholds are crossed. Examples cited as

warranting more research include the release of large quantities of GHGs stored in permafrost 

(frozen soils) across the Arctic, rapid disintegration of the major ice sheets, irreversible drying 

and desertification in the subtropics, changes in ocean circulation, and the rapid release of 

destabilized methane hydrates in the oceans.

On ocean acidification, the same report noted the potential for broad, “catastrophic” 

impacts on marine ecosystems. Ocean acidity has increased 25 percent since pre-industrial 

times, and is projected to continue increasing. By the time atmospheric CO2 content doubles

over its preindustrial value, there would be virtually no place left in the ocean that can sustain 
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coral reef growth. Ocean acidification could have dramatic consequences for polar food webs 

including salmon, the report said.

Importantly, these recent NRC assessments represent another independent and critical 

inquiry of the state of climate change science, separate and apart from the previous IPCC, NRC,

and USGCRP assessments.

3.2 References

40 CFR Chapter I [EPA–HQ–OAR–2009–0171; FRL–9091–8] RIN 2060–ZA14, “Endangerment and 

Cause or Contribute Findings for Greenhouse Gases Under Section 202(a) of the Clean 

Air Act,”Federal Register / Vol. 74, No. 239 / Tuesday, December 15, 2009 / Rules and 

Regulations .

Karl, T., J. Melillo, and T. Peterson (Eds.) (2009). Global Climate Change Impacts in the United 

States. Cambridge University Press, Cambridge, United Kingdom.

Medina-Ramon, M. and J. Schwartz, 2007: Temperature, temperature extremes, and mortality: 

a study of acclimatization and effect modification in 50 U.S. cities. Occupational and 

Environmental Medicine, 64(12), 827-833.

National Research Council (2010). Advancing the Science of Climate Change. National Academy 

Press. Washington, DC.

National Research Council (2011). Climate Stabilization Targets: Emissions, Concentrations, and 

Impacts over Decades to Millennia. National Academy Press, Washington, DC.

U.S. Environmental Protection Agency (2009). Assessment of the Impacts of Global Change on

Regional U.S. Air Quality: A Synthesis of Climate Change Impacts on Ground-Level Ozone. 

An Interim Report of the U.S. EPA Global Change Research Program. U.S. Environmental 

Protection Agency, Washington, DC, EPA/600/R–07/094.

U.S. Global Change Research Program (USGCRP). Global Climate Change Impacts in the United 

States. Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson, (eds.). Cambridge 

University Press, 2009.