EPA - Report on Future Climate Change

Increasing greenhouse gas concentrations will have many effects
Greenhouse gas concentrations in the atmosphere will continue to increase unless the billions of tons of our annual emissions decrease substantially. Increased concentrations are expected to:

• Increase Earth's average temperature
  
• Influence the patterns and amounts of precipitation
  
• Reduce ice and snow cover, as well as permafrost
  
• Raise sea level
  
• Increase the acidity of the oceans
  

These changes will impact our food supply, water resources, infrastructure, ecosystems, and even our own health .

Future changes will depend on many factors
The magnitude and rate of future climate change will primarily depend on the following factors:

• The rate at which levels of greenhouse gas concentrations in our atmosphere continue to increase
  
• How strongly features of the climate (e.g., temperature, precipitation, and sea level) respond to the expected increase in greenhouse gas concentrations
  
• Natural influences on climate (e.g., from volcanic activity and changes in the sun's intensity) and natural processes within the climate system (e.g., changes in ocean circulation patterns)
  

Scientists use computer models of the climate system to better understand these issues and project future climate changes.
Past and present-day greenhouse gas emissions will affect climate far into the future
Many greenhouse gases stay in the atmosphere for long periods of time. As a result, even if emissions stopped increasing, atmospheric greenhouse gas concentrations would continue to increase and remain elevated for hundreds of years. Moreover, if we stabilized concentrations and the composition of today's atmosphere remained steady (which would require a dramatic reduction in current greenhouse gas emissions), surface air temperatures would continue to warm. This is because the oceans, which store heat, take many decades to fully respond to higher greenhouse gas concentrations. The ocean's response to higher greenhouse gas concentrations and higher temperatures will continue to impact climate over the next several decades to hundreds of years. ^{[1] [2]}
To learn more about greenhouse gases, please visit the Greenhouse Gas Emissions page and the Greenhouse Effect section of the Causes of Climate Change page.
Because it is difficult to project far-off future emissions and other human factors that influence climate, scientists use a range of scenarios using various assumptions about future economic, social, technological, and environmental conditions. The slideshow above provides more information on these scenarios in the "Estimating the Future" section.




Future Temperature Changes
We have already observed global warming over the last several decades. Future temperatures are expected to change further. Climate models project the following key temperature-related changes.
Key Global Projections

• Average global temperatures are expected to increase by 2°F to 11.5°F by 2100, depending on the level of future greenhouse gas emissions, and the outcomes from various climate models. ^[3]
  
• By 2100, global average temperature is expected to warm at least twice as much as it has during the last 100 years. ^[2]
  
• Ground-level air temperatures are expected to continue to warm more rapidly over land than oceans. ^[2]
  
• Some parts of the world are projected to see larger temperature increases than the global average. ^[2]
  

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Projected changes in global average temperatures under three emissions scenarios (rows) for three different time periods (columns). Changes in temperatures are relative to 1961-1990 averages. The scenarios come from the IPCC Special Report on Emissions Scenarios: B1 is a low emissions scenario, A1B is a medium-high emissions scenario, and A2 is a high emissions scenario. Source: NRC 2010
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Observed and projected changes in global average temperature under three no-policy emissions scenarios. The shaded areas show the likely ranges while the lines show the central projections from a set of climate models. A wider range of model types shows outcomes from 2 to 11.5°F. Changes are relative to the 1960-1979 average.
Source: USGCRP 2009
Key U.S. Projections

• By 2100, the average U.S. temperature is projected to increase by about 4°F to 11°F, depending on emissions scenario and climate model. ^[1]
  
• An increase in average temperatures worldwide implies more frequent and intense extreme heat events, or heat waves. The number of days with high temperatures above 90°F is expected to increase throughout the United States, especially in areas that already experience heat waves. For example, areas of the Southeast and Southwest currently experience an average of 60 days per year with a high temperature above 90°F. These areas are projected to experience 150 or more days a year above 90°F by the end of the century, under a higher emissions scenario. In addition to occurring more frequently, these very hot days are projected to be about 10°F hotter at the end of this century than they are today, under a higher emissions scenario. ^[1]
  

Future Precipitation and Storm Events
Patterns of precipitation and storm events, including both rain and snowfall are also likely to change. However, some of these changes are less certain than the changes associated with temperature. Projections show that future precipitation and storm changes will vary by season and region. Some regions may have less precipitation, some may have more precipitation, and some may have little or no change. The amount of rain falling in heavy precipitation events is likely to increase in most regions, while storm tracks are projected to shift poleward. ^[4] Climate models project the following precipitation and storm changes.
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Global precipitation projections for December, January, and February (top map) and June, July, and August (bottom map.) Blue and green areas are projected to experience increases in precipitation by the end of the century, while yellow and pink areas are projected to experience decreases.
Source: Christensen et al. 2007
Key Global Projections

• Global average annual precipitation through the end of the century is expected to increase, although changes in the amount and intensity of precipitation will vary by region. ^[4]
  
• The intensity of precipitation events will likely increase on average. This will be particularly pronounced in tropical and high-latitude regions, which are also expected to experience overall increases in precipitation. ^[4]
  
• The strength of the winds associated with tropical storms is likely to increase. The amount of precipitation falling in tropical storms is also likely to increase. ^[5]
  
• Annual average precipitation is projected to increase in some areas and decrease in others. The figure to the right shows projected regional differences in precipitation for summer and winter. ^[6]
  

Key U.S. Projections

• Northern areas are projected to become wetter, especially in the winter and spring. Southern areas, especially in the West, are projected to become drier. ^[1]
  
• Heavy precipitation events will likely be more frequent. Heavy downpours that currently occur about once every 20 years are projected to occur about every four to 15 years by 2100, depending on location. ^[1]
  
• More precipitation is expected to fall as rain rather than snow, particularly in some northern areas. ^[1]
  
• The intensity of Atlantic hurricanes is likely to increase as the ocean warms. Climate models project that for each 1.8°F increase in tropical sea surface temperatures the rainfall rates of hurricanes could increase by 6-18% and the wind speeds of the strongest hurricanes could increase by about 1-8%. ^[1] There is less confidence in projections of the frequency of hurricanes, but the global frequency of tropical hurricanes is likely to decrease or remain essentially unchanged. ^[5]
  
• Cold-season storm tracks are expected to continue to shift northward. The strongest cold-season storms are projected to become stronger and more frequent. ^[1]
  

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The maps show projected future changes in precipitation relative to the recent past as simulated by 15 climate models. The simulations are for late this century, under a higher emissions scenario. For example, in the spring, climate models agree that northern areas are likely to get wetter and southern areas drier. There is less confidence in exactly where the transition between wetter and drier areas will occur. Confidence in the projected changes is highest in the areas marked with diagonal lines.
Source: USGCRP 2009
Future Ice, Snowpack, and Permafrost
Arctic sea ice is already declining. ^[7] The area of snow cover in the Northern Hemisphere has decreased since about 1970. ^[7] Permafrost temperature has increased over the last century. ^[7]
These are just three of the many forms of snow and ice found on Earth. To learn more about the different forms of snow and ice and how they affect the global climate system, visit the Snow and Ice page of the Indicators section.
Over the next century, it is expected that sea ice will continue to decline, glaciers will continue to shrink, snow cover will continue to decrease, and permafrost will continue to thaw. Potential changes to ice, snow, and permafrost are described below.

These maps show projected losses of sea ice. A and B show climate model simulations of sea ice thickness in March (A) and September (B) under current conditions. C and D show climate model simulations of sea ice thickness in March (C) and September (D) near the end of the 21st century. In the future, March is projected to have thinner ice (more blue in panel C); September is projected to be nearly ice-free (almost all blue in panel D).
Source: NRC 2011
Key Global Projections

• For every 2°F of warming, models project about a 15% decrease in the extent of annually averaged sea ice and a 25% decrease in September Arctic sea ice. ^[7]
  
• The coastal sections of the Greenland and Antarctic ice sheets are expected to continue to melt or slide into the ocean. If the rate of this ice melting increases in the 21st century, the ice sheets could add significantly to global sea level rise. ^[7]
  
• Glaciers are expected to continue to decrease in size. The rate of melting is expected to continue to increase, which will contribute to sea level rise. ^[7]
  

Key U.S. Projections

• Northern Hemisphere snow cover is expected to decrease by approximately 15% by 2100. ^[7]
  
• Models project the snow season will continue to shorten, with snow accumulation beginning later and melting starting earlier. Snowpack is expected to decrease in many regions. ^[7]
  
• Permafrost is expected to continue to thaw in northern latitudes. This would have large impacts in Alaska. ^[7]
  

Future Sea Level Change

Meltwater flowing from the Greenland ice sheet Source: NASA
Warming temperatures contribute to sea level rise by: expanding ocean water; melting mountain glaciers and ice caps; and causing portions of the Greenland and Antarctic ice sheets to melt or flow into the ocean. ^[7]
Since 1870, global sea level has risen by about 8 inches. ^[5] Estimates of future sea level rise vary for different regions, but global sea level for the next century is expected to rise at a greater rate than during the past 50 years. ^[8]
The contribution of thermal expansion, ice caps, and small glaciers to sea level rise is relatively well-studied, but the impacts of climate change on ice sheets are less understood and represent an active area of research. Thus it is more difficult to predict how much changes in ice sheets will contribute to sea level rise. ^[7]
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Projection of sea level rise from 1990 to 2100, based on three different emissions scenarios. Also shown: observations of annual global sea level rise over the past half century (red line), relative to 1990.
Source: NRC 2010
Ice loss from the Greenland and Antarctic ice sheets could contribute an additional 1 foot of sea level rise, depending on how the ice sheets respond. ^[7]
Regional and local factors will influence future relative sea level rise for specific coastlines around the world. For example, relative sea level rise depends on land elevation changes that occur as a result of subsidence (sinking) or uplift (rising). Assuming that these historical geological forces continue, a 2-foot rise in global sea level by 2100 would result in the following relative sea level rise: ^[1]

• 2.3 feet at New York City
  
• 2.9 feet at Hampton Roads, Virginia
  
• 3.5 feet at Galveston, Texas
  
• 1 foot at Neah Bay in Washington state
  

Relative sea level rise also depends on local changes in currents, winds, salinity, and water temperatures, as well as proximity to thinning ice sheets. ^[1]
Future Ocean Acidification
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Corals require the right combination of temperature, light, and the presence of calcium carbonate (which they use to build their skeletons). As atmospheric carbon dioxide (CO₂) levels rise, some of the excess CO₂ dissolves into ocean water, reducing its calcium carbonate saturation. As the maps indicate, calcium carbonate saturation has already been reduced considerably from its pre-industrial level, and model projections suggest much greater reductions in the future. The blue dots indicate current coral reefs. Note that under projections for the future, it is very unlikely that calcium carbonate saturation levels will be adequate to support coral reefs in any U.S. waters. Source: USGCRP 2009
Oceans become more acidic as carbon dioxide (CO₂) emissions in the atmosphere dissolve in the ocean. This change is measured on the pH scale, with lower values being more acidic. The pH level of the oceans has decreased by approximately 0.1 pH units since pre-industrial times, which is equivalent to a 25% increase in acidity. The pH level of the oceans is projected to decrease even more by the end of the century as CO₂ concentrations are expected to increase for the foreseeable future. ^{[1] [3]}
Ocean acidification adversely affects many marine species, including plankton, mollusks, shellfish, and corals. As ocean acidification increases, the availability of calcium carbonate will decline. Calcium carbonate is a key building block for the shells and skeletons of many marine organisms. If atmospheric CO₂ concentrations double, coral calcification rates are projected to decline by more than 30%. If CO₂ concentrations continue to rise at their current rate, corals could become rare on tropical and subtropical reefs by 2050. ^{[1] [3] [9]}
References
[1] USGCRP (2009). Global Climate Change Impacts in the United States . Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson (eds.). United States Global Change Research Program. Cambridge University Press, New York, NY, USA.
[2] IPCC (2007). Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis . Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
[3] NRC (2010). Advancing the Science of Climate Change . National Research Council. The National Academies Press, Washington, DC, USA.
[4] Meehl, G.A., T.F. Stocker, W.D. Collins, P. Friedlingstein, A.T. Gaye, J.M. Gregory, A. Kitoh, R. Knutti, J.M. Murphy, A. Noda, S.C.B. Raper, I.G. Watterson, A.J. Weaver and Z.-C. Zhao (2007). Global Climate Projections. In: Climate Change 2007: The Physical Science Basis . Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
[5] IPCC (2012).Summary for Policymakersin: Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation [Field, C.B., V. Barros, T.F. Stocker, D. Qin, D.J. Dokken, K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen, M. Tignor and P.M. Midgley (eds.)]. A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
[6] Christensen, J.H., B. Hewitson, A. Busuioc, A. Chen, X. Gao, I. Held, R. Jones, R.K. Kolli, W.-T. Kwon, R. Laprise, V. Magaa Rueda, L. Mearns, C.G. Menndez, J. Risnen, A. Rinke, A. Sarr and P. Whetton (2007). Regional Climate Projections. In: Climate Change 2007: The Physical Science Basis . Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
[7] NRC (2011). Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia . National Research Council. The National Academies Press, Washington, DC, USA.
[8] Nicholls, R.J., P.P. Wong, V.R. Burkett, J.O. Codignotto, J.E. Hay, R.F. McLean, S. Ragoonaden and C.D. Woodroffe (2007). Coastal systems and low-lying areas. In: Climate Change 2007: Impacts, Adaptation, and Vulnerability . Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Parry, M.L., O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson (eds.)]. Cambridge University Press, Cambridge, United Kingdom.
[9] Fischlin, A., G.F. Midgley, J.T. Price, R. Leemans, B. Gopal, C. Turley, M.D.A. Rounsevell, O.P. Dube, J. Tarazona, A.A. Velichko (2007) Ecosystems, their Properties, Goods, and Services. In: Climate Change 2007: Impacts, Adaptation and Vulnerability . Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Parry, M.L., O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson (eds.)]. Cambridge University Press, Cambridge, United Kingdom.