International Annual GHG Emissions

Most of the world's greenhouse gas (GHG) emissions come from a relatively small number of countries. China, the United States, and the 27 nations that make up the European Union (EU-27) are the three largest emitters on an absolute basis.

Sources: International Energy Agency, World Energy Outlook (2012)
                 International Energy Agency, CO2 Highlights (2011)
                 U.S. Environmental Protection Agency, International Non CO2 Projections (2012) 


Historical Global CO2 Emissions

Carbon dioxide emissions, primarily from the combustion of fossil fuels, have risen dramatically since the start of the industrial revolution.

Sources: Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory (2012)
                  International Energy Agency, World Energy Outlook (2012)

International Emissions

This section highlights trends in historic global emissions and shows emissions by gas and by sector. Other figures illustrate annual and cumulative emissions of key countries.

Click on the images below to view additional information on each figure.

Historical Global CO2 Emissions
International Annual GHG Emissions
Per Capita GHG Emissions
GHG Intensity
Global Anthropogenic GHG Emissions by Sector
Global Anthropogenic GHG Emissions by Gas


Explore All the Facts & Figures Sections:

  1. Main Greenhouse Gases
  2. U.S. Emissions
  3. International Emissions


About Science and Impacts

The Earth’s average surface temperature has increased by 1.4°F (0.8°C) since the early years of the 20th century. The 10 warmest years on record (since 1850) have all occurred since 1998. The 10 warmest years to date are 2010, 2005, 1998, 2003, 2002, 2009, 2006, 2007, 2004, 2001.

The scientific consensus is that global warming is largely the result of increased atmospheric concentrations of carbon dioxide and other greenhouse gas emissions. The growth in emissions is caused by human activities, primarily fossil fuel combustion and changes in land use, such as agriculture and deforestation. The Intergovernmental Panel on Climate Change projects an increase of future average global surface temperature in the range of 2.0°F to 11.5°F (1.1°C to 6.4°C) by 2100, with warming in the United States expected to be about 50 percent greater. This warming, along with the associated changes in precipitation, drought, heat waves, and sea-level rise, will have important consequences for the U.S. environment and economy. Globally, climate change presents many challenges, particularly in poorer countries far less able to cope with a changing climate and in low-lying countries where sea level rise will cause severe damage to society and ecosystems.

C2ES's Science and Impacts Program educates the public and policymakers about the climate system and the environmental and economic implications of climate change for the United States, including impacts on natural ecosystems and resources, social systems, human health, and infrastructure. C2ES publishes a series of peer-reviewed scientific reports written by experts on the environmental impacts of climate change. Workshops sponsored by C2ES explore the implications of environmental impacts for climate change policy. C2ES constantly builds relationships with members of the scientific, policy, and business community to increase awareness regarding the science and impacts of climate change.

The Pew Center's Science and Impacts Program aims to advance policymakers' understanding of the climate system and the consequences of global warming for the United States.

Science & Impacts

Climate change poses an extraordinary challenge that demands immediate action. Human activity is a primary cause of the growth in emissions that results in global warming, and C2ES strives to increase awareness regarding the science and impacts of this critical problem. Read More

The scientific community has reached a strong consensus regarding the science of global climate change. The world is undoubtedly warming, and the warming is largely the result of emissions of carbon dioxide and other greenhouse gases from human activities. Read More

Regional Impacts of Climate Change: Four Case Studies in the United States

Regional Impacts Cover

Regional Impacts of Climate Change: Four Case Studies in the United States

Prepared for the Pew Center on Global Climate Change
December 2007 

Kristie L. Ebi, ESS
Gerald A. Meehl, National Center for Atmospheric Research
Dominique Bachelet, et al., Oregon State University
Robert R. Twilley, Louisiana State University
Donald F. Boesch, et al., University of Maryland Center for Environmental Science

Press Release

Download full report (pdf) or individual case studies:



Foreword Eileen Claussen, President, Pew Center on Global Climate Change

In 2007, the science of climate change achieved an unfortunate milestone: the Intergovernmental Panel on Climate Change reached a consensus position that human-induced global warming is already causing physical and biological impacts worldwide. The most recent scientific work demonstrates that changes in the climate system are occurring in the patterns that scientists had predicted, but the observed changes are happening earlier and faster than expected—again, unfortunate. Although serious reductions in manmade greenhouse gas emissions must be undertaken to reduce the extent of future impacts, climate change is already here and some impacts are clearly unavoidable. It is imperative, therefore, that we take stock of current and projected impacts so that we may begin to prepare for a future unlike the past we have known. 

The Pew Center has published a dozen previous reports on the environmental effects of climate change in various sectors across the United States. However, because climate impacts occur locally and can take many different forms in different places, Regional Impacts of Climate Change:Four Case Studies in the United States examines impacts of particular interest to different regions of the country. Although sections of the report examine different aspects of current and projected impacts, a look across the sections reveals common issues that decision makers and planners are likely to face in learning to cope with climate change. 

Kristie Ebi and Gerald Meehl find that Midwestern cities are very likely to experience more frequent, longer, and hotter heatwaves. According to Dominique Bachelet and her coauthors, wildfires are likely to increase in the West, continuing a dramatic trend already in progress. Robert Twilley explains that Gulf Coast wetlands provide critical ecosystems services to humanity, but sustaining these already fragile ecosystems will be increasingly difficult in the face of climate change. Finally, Donald Boesch and his colleagues warn that the Chesapeake Bay may respond to climate change with more frequent and larger low-oxygen “dead zone” events that damage fisheries and diminish tourist appeal. These authors are leading thinkers and practitioners in their respective fields and provide authoritative views on what must be done to adapt to climate change and diminish the threats to our environmental support systems. 

A key theme emerges from these four case studies: pre-existing problems caused by human activities are exacerbated by climate change, itself mostly a human-induced phenomenon. Fortunately, manmade problems are amenable to manmade solutions. Climate change cannot be stopped entirely, but it can be limited significantly through national and international action to reduce the amount of greenhouse gases emitted to the atmosphere over the next several decades and thereafter, thus limiting climate change impacts. Managing those impacts requires that we adapt other human activities so that crucial resources, such as Gulf Coast wetlands or public emergency systems, continue to function effectively. The papers in this volume offer insights into how we can adapt to a variety of major impacts that we can expect to face now and in decades to come. 

This report benefited from technical assistance, editing, and peer review. The Pew Center and the authors thank Joel Smith for project coordination as well as Ray Drapek, Anthony Janetos, BonnieNevel, James Morris, Steven Running, Don Scavia, Scott Sheridan, Peter Stott, Elizabeth Strange,Margaret Torn, Eugene Turner, John Wells, and Gary Yohe. 



The Pew Center on Global Climate Change has published many reports that address the impacts of climate change in a number of sectors and ecosystems across the United States, including agriculture, forests, coastal resources, water resources, and others. Results of previous studies in this series are summarized in a synthesis report (Smith, 2004).  

But differences in climate, topography, land use, and infrastructure result in different climate change impacts at the regional and local scales. As a complement to earlier the Pew Center reports focusing on the United States in general, this report presents four case studies of specific climate change impacts in different regions of the country:

  • The Heat is On: Climate Change & Heatwaves in the Midwest by Kristie L. Ebi of ESS and Gerald A. Meehl of the National Center for Atmospheric Research;
  • The Importance of Climate Change for Future Wildfire Scenarios in the Western United States by Dominique Bachelet of Oregon State University, and James M. Lenihan and Ronald P. Neilson of the U.S. Forest Service;
  • Gulf Coast Wetland Sustainability in a Changing Climate by Robert R. Twilley of Louisiana State University; and
  • Ramifications of Climate Change for Chesapeake Bay Hypoxia by Donald F. Boesch, Victoria J. Coles, David G. Kimmel and W. David Miller of the University of Maryland Center for Environmental Science.

Each case study focuses on a specific type of impact that is of particular concern for a region, but is not unique to that region. Each study also considers non-climatic factors, such as development and management practices, that are likely to interact with climate change. Consequently, cross-cutting themes emerge that are relevant to a wide array of regional and local climate change impacts beyond those examined here.


A. Individual Case Studies

Midwestern heatwaves. In coming decades heatwaves in the Midwest are likely to become more frequent, longer, and hotter than cities in the region have experienced in the past. This trend will result from a combination of general warming, which will raise temperatures more frequently above thresholds to which people have adapted, and more frequent and intense weather patterns that produce heatwaves. Studies projecting future mortality from heat foresee a substantial increase in health risks from heatwaves. Several factors contribute to increasing risk in Midwestern cities, including demographic shifts to more vulnerable populations and an infrastructure originally designed to withstand the less severe heat extremes of the past. The elderly living in inner cities are particularly vulnerable to stronger heatwaves; other groups, including children and the infirmed, are vulnerable as well. Adaptations of infrastructure and public health systems will be required to cope with increased heat stress in a warmer climate.  

Fire in the West. Wildfire is a natural part of the western landscape and is very sensitive to climate variability. In recent decades, a trend toward earlier spring snowmelt and hotter, drier summers has already increased the number and duration of large wildfires in the West (Westerling et al., 2006). Although total annual precipitation may increase in the Northwest, climate projections generally foresee less precipitation throughout the West during the summer when risk of fire is greatest. In Alaska and Canada, warming has accelerated the reproduction and increased the winter survival and geographic range of insect pests that may make forests more vulnerable to fire by killing more trees (Berg et al., 2006; Volney and Fleming, 2000). Development in the West has placed more people and assets in fire-prone areas, increasing the need to suppress wildfires (McKinley and Johnson, 2007). Ironically, suppression increases the risk of catastrophic fire by allowing vegetation to build up, providing more fuel for fires when they ignite. Humans have also introduced invasive plant species that consume limited soil moisture and burn readily. Careful attention to development decisions and human-induced ecosystem stressors may help with adapting to increased risk from fire in the West resulting from climate change.  

Gulf Coast wetlands. The coastline of the Gulf of Mexico offers a prototypical example of how human development patterns and climate change can interact to create high risks to human and natural systems. The combination of intense development in low-lying coastal areas, building levees along major rivers such as the Mississippi, high pollution levels, and extreme weather events, have degraded economically and culturally valuable coastal wetlands and made many human settlements in the Gulf region more vulnerable to rising seas and coastal storms. Accelerated sea-level rise and more intense hurricanes resulting from climate change would increase these risks. Therefore, plans to restore Gulf Coast wetlands and make them resilient to human activities and climate variability require careful consideration of how future climate change and human activities will degrade or enhance the natural processes that build and maintain coastal wetlands.  

Chesapeake Bay hypoxia. Hypoxia (inadequate levels of oxygen that can lead to dead zones) in the Chesapeake Bay is another example of a natural phenomenon made substantially worse by human development and that could also be exacerbated by climate change. Hypoxia occurs when nutrient runoff from land stimulates biological oxygen demand, reducing oxygen levels in the Chesapeake Bay. This condition adversely affects the bay ecosystem, including its fisheries, and recreational opportunities in the bay. Development within the Chesapeake Bay watershed has resulted in runoff of nutrients from farms and settlements, increasing the incidence and intensity of hypoxia in the bay. Increased regional rainfall, which washes nutrients into the bay, and higher summer temperatures, which accelerate oxygen depletion, are likely to increase the incidence and intensity of hypoxia in the Chesapeake Bay. These changes could alter the current assessment of nutrient reductions needed to meet water quality objectives.


B. Cross-cutting Themes

The case studies provide but a few diverse examples of potential climate change impacts. Many other impacts will occur far and wide and will affect many sectors in all regions of the country and the world in different ways. However, several key themes emerge from these studies that are likely to cut across many distinct impacts in many different regions:  

Impacts from climate change are already apparent. In all four of the case studies, there is growing evidence that climate change may already be increasing risks. To be sure, attribution of particular events either wholly or partially to climate change is a difficult process that can be controversial. But the literature linking climate change with the events discussed in this report is growing. Westerling et al. (2006) found that climate change over the 20th century is a key factor explaining the increase in fires in the American West after accounting for human settlements and fire management. Extreme heat events in the United States are on the rise. DeGaetano and Allen (2002) found that minimum and maximum temperatures increased in the latter half of the 20th century, with particularly large increases in urban areas. Multi-day extreme heat events are also increasing. Global sea levels have been rising for centuries, but recently the rate of sea-level rise has accelerated (IPCC, 2007). This rise is likely contributing to some loss of wetlands in places such as the Gulf of Mexico and the Chesapeake Bay. Finally, there is growing evidence that the intensity and possibly the number of hurricanes in the Atlantic have increased in recent decades as a result of rising sea surface temperatures (Emanuel, 2005; Hoyos et al., 2006).  

Multiple stressors exacerbate climate change impacts on natural systems. Enlarged pest populations, invasive species, and fire suppression all increase the vulnerability of ecosystems to fire. Nutrient inputs from farms and settlements increase the potential for hypoxia in coastal estuaries. Canals, flood-control structures, and pollution decrease the resilience of wetlands to rising sea levels and powerful storms. In many cases stressors that limit the ability of natural systems to resist stress from climate change are under human control, either directly (e.g., development) or indirectly (e.g., invasive species). Successful adaptation to climate change will likely require close attention to the many ways that human activities can be altered to increase ecosystem resilience to climate change.

Development patterns affect vulnerability to climate change impacts. In the four studies presented here, development and associated planning decisions and management practices exacerbate the impacts of climate change. The concentration of infrastructure and housing along with dense populations of the poor and elderly make inner cities more vulnerable to heatwaves than less developed areas. Increased population, building of impervious surfaces, and agriculture in the Chesapeake Bay watershed increase runoff of nutrients and risk of hypoxia. Development in low-lying coastal areas of the Gulf and Atlantic coasts places more people and property along the coastline and degrades buffering wetlands, putting people at greater risk from faster sea-level rise and more intense coastal storms. More development in wilderness areas in the West also increases the number of people and amount of property facing wildfire risk, as climate change increases the frequency and intensity of large fires. Adaptation to climate change will require closer attention to the implications of development patterns and land use decisions for climate change impacts.  

There are likely to be increasing risks and costs from future climate change. The impacts of future climate change are likely to become greater as climate continues to change. There will likely be more loss of wetlands, higher risk to human life and property from stronger storms and hurricanes in the Gulf of Mexico and the Atlantic, more potential for hypoxia in the Chesapeake Bay and other coastal waters, more frequent and more intense heatwaves with greater risks to human health, and more frequent and intense wildfires. Many impacts not examined here would likely follow similar trends. Droughts and flash floods, for example, will likely increase in the future, presenting greater risks in areas that are already prone to such events (IPCC, 2007).  

Climate change could have important consequences for the private insurance industry and for public disaster management and response. Many of the impacts discussed in these studies could affect lives and property, and therefore, are likely to affect insurance claims as well as government response to (and perhaps preparation for) disasters. For example, greater loss of life from more intense heatwaves and property damage from hurricanes and fires could well result in higher insurance payouts and insurance companies refusing coverage to more individuals and businesses. This effect would likely have further consequences for insurance rates, deductibles, and profits, which could affect other parts of the economy. Public disaster management and response will require increased resources and more funding in a future with more frequent and bigger fires, floods, and heatwaves.  

Adaptation will be important in determining future vulnerability. The climate is already changing and affecting society and nature. Significant reductions in greenhouse gas emissions leading to lower atmospheric concentrations would reduce the magnitude of climate change and its impacts. Nonetheless, even with the most optimistic emissions reductions, there will still be substantial additional climate change. Thus, adaptation is an important component of a response to climate change. Reducing the level of pollution in the Chesapeake Bay will most likely reduce the risks of hypoxia. Adoption of heatwave early warning systems and other measures such as improving access to air conditioning have been shown to reduce risks from extreme heat events (Ebi et al., 2004). Wisely managing development patterns and vegetation can reduce the risks of fire (Platt et al., 2006). Evacuation planning, adoption of certain building designs, and limiting development in coastal areas can reduce risks from hurricanes. Furthermore, limits on certain types of development can also reduce destruction of wetlands, which are important for their ecosystem services. 

C. Final Thoughts

Although climate change is a global problem, its impacts vary widely and are felt locally. With this report, the Pew Center on Global Climate Change endeavors to provide not just useful information about particular impacts in particular regions, but also a more general perspective on the types of challenges decision-makers everywhere will face in developing sustainable responses to varied climate impacts. Historically, risk management strategies have relied on the past as a guide to the future. But with global climate change, the future will no longer resemble the past. As illustrated by the four regional studies that follow, new strategies for developing resilience to climate variability and extreme weather events will be needed. Well-considered assumptions about regional climate change should be incorporated into development and management plans. Studying regions with different vulnerabilities will provide insights and methods for conducting assessments in other regions and sectors.    

Joel B. Smith

Jay Gulledge


Berg, E.E., J.D. Henry, C.L. Fastie, A.D. De Volderd, and S.M. Matsuoka. 2006. Spruce beetle outbreaks on the Kenai Peninsula, Alaska, and Kluane National Park and Reserve, Yukon Territory: Relationship to summer temperatures and regional differences in disturbance regimes. Forest Ecology and Management 227:219-232.

DeGaetano, A.T. and R.J. Allen. 2002. Trends in twentieth-century temperature extremes across the United States. Journal of Climate 15:3188-3205.

Ebi, K.L., T.J. Teisberg, L.S. Kalkstein, L. Robinson, and R.F. Weiher. 2004. Heat watch/warning systems save lives: estimated costs and benefits for Philadelphis 1995-1998. Bulletin of the American Meteorological Society 85:1067-1073.

Emanuel, K. 2005. Increasing destructiveness of tropical cyclones over the past 30 years. Nature 436:686-688.

Hoyos, C.D., P.A. Aguidelo, P.J. Webster, and J.A. Curry. 2006. Deconvolution of the factors contributing to the increase in global hurricane intensity. Science 312:94-97.

IPCC (Intergovernmental Panel on Climate Change). 2007. Climate Change 2007: The Physical Science Basis.  Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. S. Solomon, 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.

McKinley, J. and K. Johnson. 2007. On fringe of forests, homes and wildfires meet. New York Times, June 26.

Platt, R.V., T.T. Veblen, and R.L. Sherriff. 2006. Are wildfire mitigation and restoration of historic forest structure compatible? A spatial modeling assessment. Annals of the Association of American Geographers 96:455-470.

Smith, J.B. 2004. A Synthesis of the Potential Impacts of Climate Change on the United States. Pew Center on Global Climate Change, Arlington, VA.

Volney, W.J.A. and R.A. Fleming. 2000. Climate change and impacts of boreal forest insects. Agriculture, Ecosystems & Environment 82:283-294.

Westerling, A.L., H.G. Hidalgo, D.R. Cayan, and T.W. Swetnam. 2006. Warming and earlier spring increases western U.S. forest wildfire activity. Science 313:940-943.

About the Authors


Heatwaves in the Midwest. Dr. Kristie L. Ebi is an independent consultant (ESS, LLC) and has studied connections between climate change and human health for more than ten years. She is working with the World Health Organization, UN Development Program, and USAID on adaptation measures for developing countries, and with the Center for Climate Strategies on adaptation options for U.S. states. She is a Lead Author for the Human Health chapter of the Fourth Assessment Report of the Nobel Peace Prize-winning IPCC, and Lead Author for Human Health for the U.S. Climate Change Science Program’s assessment of global change effects on human health and welfare.  She has edited three books on climate change and health, and has more than 75 publications.  Dr. Ebi earned the M.S. degree in toxicology from MIT, and the Ph.D. and MPH degrees in epidemiology from the University of Michigan.

Dr. Gerald A. Meehl is a Senior Scientist in the Climate and Global Dynamics Division at the National Center for Atmospheric Research, where he has worked in various capacities since 1973, including his involvement in several large international climate experiments. He specializes in modeling climate dynamics, including the possible effects of increased carbon dioxide, sulfate aerosols, and other natural and manmade drivers of global climate. He was a Coordinating Lead Author for the Global Climate Projections chapter in the Fourth Assessment Report of the Nobel Peace Prize-winning IPCC. He also contributed to all of the previous IPCC assessment reports. Among other committee appointments, he is a member of National Research Council’s Climate Research Committee. He has published more than 150 peer-reviewed articles and contributed to several textbooks. Dr. Meehl earned the Ph.D. in climate dynamics from the University of Colorado in Boulder.

Fire in the West. Dr. Dominique Bachelet is an associate professor in the Department of Biological and Ecological Engineering at Oregon State University and Director of Climate Change Science at The Nature Conservancy. She uses models to study complex ecological systems and how they respond to climate variability and change. Over the past decade, she has worked with colleagues at Oregon State University, Colorado State University, and the U.S. Forest Service to develop the MC1 dynamic vegetation model which uses global climate model scenarios to project, among other things, future wildfire characteristics resulting from climate change. She has published more than 20 peer-reviewed scientific articles. Dr. Bachelet earned the Ph.D. from Colorado State University in 1983 and subsequently worked at the University of California Riverside, New Mexico State University, and the US Environmental Protection Agency.

Gulf Coast wetland sustainability. Dr. Robert R. Twilley is Distinguished Professor in Louisiana Environmental Studies and Associate Vice Chancellor of Research and Economic Development at Louisiana State University. He directs the Shell Coastal Environmental Modeling Laboratory and heads the Coastal Louisiana Ecosystem Assessment and Restoration program, which develops ecosystem models coupled with engineering designs to forecast the rehabilitation of coastal and wetland ecosystems. He edited a 64-author, two-volume report that is Louisiana’s official coastal restoration plan. Before moving to LSU, Dr. Twilley founded the Center for Ecology and Environmental Technology at University of Louisiana at Lafayette. He has published more than 80 peer-reviewed articles and co-edited the 1999 book, The Biogeochemistry of Gulf of Mexico Estuaries. Dr. Twilley received his PhD in plant and systems ecology from the University of Florida in 1982 and conducted postdoctoral research at University of Maryland on the Chesapeake Bay.

Chesapeake Bay Hypoxia. Dr. Donald F. Boesch is professor of marine science and President of the University of Maryland Center for Environmental Science. He has studied marine ecosystems of the Atlantic and Gulf coasts of the U.S. and in Australia and the East China Sea. He serves on the National Research Council’s Ocean Studies Board and the Board of Trustees of the Consortium for Ocean Leadership. He was a lead author on the U.S. National Assessment of the Potential Consequences of Climate Variability and Change, and is leading an impacts assessment for the Maryland Commission on Climate Change. He recently testified in the Senate on the impacts of global warming on the Chesapeake Bay and improving the Federal climate change research and information program. He has published two books and more than 85 research articles. He received his Ph.D. in marine science from the College of William and Mary in 1971.

Dominique Bachelet
Donald F. Boesch
Gerald A. Meehl
Kristie L. Ebi
Robert R. Twilley

Press Release: Report Examines U.S. Regional Impacts of Climate Change

Press Release
December 4, 2007

Contact: Tom Steinfeldt, (703) 516-4146


As the nations of the world gather this week in Bali, Indonesia, to work on a global agreement to reduce greenhouse gas emissions, the Pew Center on Global Climate Change today released a new report examining key impacts of climate change that are likely to affect different areas of the United States. The report, “Regional Impacts of Climate Change: Four Case Studies in the United States,” assesses particular climate vulnerabilities in the Midwest, West, Gulf Coast, and Chesapeake Bay regions.

The report provides useful information about particular impacts in different regions of the United States, as well as a more general perspective on the types of challenges decision-makers will face in developing workable responses to varied climate impacts. Each study also considers non-climatic factors, such as development and management practices that are likely to exacerbate our vulnerability to climate change.

The four studies are:

The Heat is On: Climate Change and Heatwaves in the Midwest by Kristie L. Ebi of ESS and Gerald A. Meehl of the National Center for Atmospheric Research;

The Importance of Climate Change for Future Wildfire Scenarios in the Western United States by Dominique Bachelet of Oregon State University and James M. Lenihan and Ronald P. Neilson of the U.S. Forest Service;

Gulf Coast Wetland Sustainability in a Changing Climate by Robert R. Twilley of Louisiana State University; and

Ramifications of Climate Change for Chesapeake Bay Hypoxia by Donald F. Boesch, Victoria J. Coles, David G. Kimmel and W. David Miller of the University of Maryland Center for Environmental Science.

The report’s four case studies offer key insights to issues that are likely to affect different regions in the U.S., including:

  • Midwestern cities are very likely to experience more frequent, longer, and hotter heatwaves
  • Wildfires are likely to increase in the West, continuing a dramatic trend already in progress.
  • Gulf Coast wetlands provide critical natural services to humanity, but sustaining these already fragile ecosystems will be increasingly difficult in the face of climate change.
  • The Chesapeake Bay may respond to climate change with more frequent and larger low-oxygen “dead zone” events that damage fisheries and diminish tourist appeal.

The authors find that well-considered assumptions about regional climate change should be incorporated into development and management plans based on a range of plausible projections. Studying regions with different vulnerabilities will provide insights and methods for conducting assessments in other regions and sectors.

“The degree to which we can adapt to the consequences of climate change will be determined in large part by the policies and management practices we put in place today,” said Pew Center President Eileen Claussen, “It is clear that we are already seeing changing conditions, and there is a real urgency for strong national and international policy action.” This report offers insights into how we can adapt to a variety of major impacts that we can expect to face now and in decades to come

Historically, risk management strategies have relied on the past as a guide to the future. But with global climate change, the future will no longer resemble the past. The report finds that adaptation measures will have to be a critical component of any long-term U.S. climate strategy. Managing the impacts of climate change requires that we adapt other human activities so that crucial resources, such as Gulf Coast wetlands or public emergency systems, continue to function effectively.

In a white paper released earlier this year, the Pew Center examines specific adaptation measures currently underway at the state level. This paper, Adaptation Planning – What U.S. States and Localities are Doing,” looks at state and local adaptation efforts and highlights five states with plans already in place and the six additional states considering such measures.

For more information about global climate change and the activities of the Pew Center, visit

IPCC Fourth Assessment Report, Summary for Policymakers

IPCC AR4 Summary for Policymakers

Released on November 17, 2007, the Summary for Policymakers of the IPCC Fourth Assessment Synthesis Report represents the IPCC’s most comprehensive and definitive statement to date on climate change. The report presents the key findings of the three Working Group reports released earlier this year by the Nobel Peace Prize winning-IPCC.

The following are some of the key highlights addressed in the Synthesis Report:

  • There is strong certainty that most of the observed warming of the past half-century is due to human influences, and a clear relationship between the growth in manmade greenhouse gas emissions and the observed impacts of climate change.
  • The climate system is more vulnerable to abrupt or irreversible changes than previously thought.
  • Avoiding the most serious impacts of climate change ­-- including irreversible changes – will require significant reductions in greenhouse gas emissions. 
  • Mitigation efforts must also be combined with adaptation measures to minimize the risks of climate change. 

The Synthesis Report is the fourth and final installment of the Fourth Assessment Report. The previous three installments published earlier this year examined the physical science basis for climate change, the impacts of global climate change, and the solutions to global climate change, particularly options for reducing greenhouse gas emissions.

For the full report, visit the IPCC website. For a summary of the synthesis report, click here (pdf). Relevant materials can be linked to from the "Related Content" box above.

Statement by Eileen Claussen, President, Pew Center on Global Climate Change

November 19, 2007

The latest IPCC report underscores the need for immediate and sustained action to reduce greenhouse gas emissions, both in the United States and globally. In the United States, many states are demonstrating strong leadership, and I am confident Congress is on the path to enacting a comprehensive mandatory policy in the near future.

Globally, 2008 marks a significant milestone as the Kyoto commitments take effect. But many already have their sights set on a post-Kyoto framework, and steps toward a new international agreement will be the key issue before negotiators next month in Bali.

The ideal outcome from Bali would be a clear mandate to negotiate a comprehensive post-2012 agreement establishing fair, effective, and binding commitments for all major economies.  Unfortunately, despite the latest wakeup call from the IPCC, it appears that the United States and some other key governments are not yet prepared to negotiate real commitments.  Even if a clear negotiating mandate isn’t possible, it is imperative that any process launched in Bali leave the door open to negotiating commitments.  That way, when a new U.S. administration takes office, governments can quickly get down to the business of forging an effective and durable post-2012 framework.

Back to coverage of the IPCC's Fourth Assessment Report

Current Understanding of Antarctic Climate Change

Current Understanding of Antarctic Climate Change
Fall 2007

At a time of dramatic warming and rapid sea ice decline in the Arctic, Antarctica has cooled slightly and sea ice has increased around it. Recent scientific progress in understanding how two distinct processes affect Antarctic climate reconciles these seemingly contradictory trends at the Earth’s poles. In a nutshell, the difference arises from (1) a weak response to increasing greenhouse gases and (2) a cooling effect of the stratospheric ozone hole—both unique to the southern hemisphere.

That is not to say that the southern hemisphere is exempt from global warming. As in the north, the southern hemisphere as a whole has warmed over the past half century, but at a slower rate than in the north (Trenberth et al. 2007). The southern hemisphere has much less land surface and more ocean surface than the northern hemisphere; ocean surfaces warm more slowly than land because more energy is required to heat water, and because ocean mixing transports much of the heat downward away from the surface (Parkinson 2004; Levitus et al. 2005). In fact, the signal of human-induced ocean warming has been detected to a depth of at least 700 meters (Barnett et al. 2005). As in the north, southern-hemisphere warming has been greater at mid-latitudes than at the equator, but the high latitudes around Antarctica have cooled over the past four decades (Chapman and Walsh 2007; Parkinson 2006). Because Antarctica occupies only five percent of the surface area of the southern hemisphere, there is no contradiction in this relatively small region cooling as the hemisphere warms overall. Antarctica is among a minority of regions with unique local climate conditions that currently override the global warming trend, although this situation is likely to change in the future if greenhouse gas concentrations continue to rise (Shindell and Schmidt 2004).

In spite of a moderate overall cooling trend, recent Antarctic climate change results from a mix of countervailing signals. A rapid net loss of sea ice occurred during the 1970s, followed by a slow gain. The geographic distribution of sea ice has changed, with the east gaining and the west losing sea ice. The gains and losses are each larger than the overall trend, indicating a high degree of variability and change in the Antarctic sea ice (Parkinson 2006). Scientists were surprised to discover recently that the land-based Antarctic ice sheet, which stores 60 percent of the earth’s fresh water—the equivalent of 70 meters (228 feet) of sea level rise—has been losing slightly more ice each year than it is gaining (Shepherd and Wingham 2007). Most of the ice loss is from the West Antarctic Ice Sheet, the margins of which lie in the ocean (Velicogna and Wahr 2006). Warming of the ocean appears to be eroding this ice sheet at its edges (Shepherd et al. 2004; Rignot and Kanagaratnam 2006). Reaching northward from West Antarctica into the mid-latitudes, the Antarctic Peninsula has experienced the most dramatic warming in the region (Chapman and Walsh 2007; Turner et al. 2005). In a preview of the possible consequences of ice sheet erosion by the warming Southern Ocean, the Larsen B ice shelf, which was attached to the peninsula, disintegrated suddenly in February 2002; as a result, the land-based ice behind the shelf began to flow more quickly into the sea (Scambos et al. 2004). Scientists infer that widespread warming in West Antarctica could lead to many such events in the future, potentially leading to dramatic acceleration of global sea level rise (Alley et al. 2005). Clearly, the Antarctic climate is not changing monotonically in a single direction.

Still, while every other continent on Earth has experienced a clear warming trend over the past five decades (Trenberth et al. 2007), Antarctica—the fifth largest continent—has shown no clear trend 
(Chapman and Walsh 2007). There are several key differences between the Arctic and the Antarctic that act in concert to explain the climatic departure between the two regions. Two of the most important factors are the predictably weak warming signal in the Antarctic compared to the Arctic, and the cooling effect of the human-induced stratospheric ozone hole above Antarctica.

As predicted by climate models, the southern hemisphere has warmed less than the northern hemisphere. The warming has occurred predominantly during the winter, and even Antarctica has warmed slightly during the winter, despite its average cooling across all seasons (Chapman and Walsh 2007). Winter is the time of year that climate models show the largest response to increasing greenhouse gas concentrations. So, even though the warming signal is weak, the seasonal pattern is consistent with the human-enhanced greenhouse effect. Since Antarctic winters are much colder than necessary to freeze seawater, a little wintertime warming is insufficient to induce large-scale losses of sea ice without concurrent warming during the summer. In an experiment using a climate model to simulate global sea ice change over a century as a result of increasing atmospheric greenhouse gases, antarctic sea ice decreased by only 10%, while arctic sea ice decreased by 60% (Parkinson 2004). It is not surprising, therefore, that Antarctic sea ice has not mirrored the rapid decline of arctic sea ice.

But Antarctica is cooling and antarctic sea ice is expanding—something more than regionally weak global warming is afoot. That other factor is the ozone hole in the upper atmosphere (stratosphere) above Antarctica. Over the past four decades, the southern Westerlies—a ring of wind that encircles the southern hemisphere between 30° and 60° latitude—have become more intense and have moved closer to the South Pole in an ever-tighter ring around Antarctica. Whenever the Westerlies intensify—regardless of the cause—Antarctica tends to cool because surface air pressure inside the ring decreases (Marshall 2006). This is called adiabatic cooling and is the same reason that the temperature drops as one climbs a mountain. Although scientists are just beginning to study the physical mechanisms by which changes in the stratosphere affect ground-level climate (Baldwin et al. 2007), observations and model results both indicate that the greater amount of stratospheric ozone depletion over the South Pole compared to mid-latitudes has caused the southern Westerlies to shift poleward and intensify (Gillett and Thompson 2003; Shindell and Schmidt 2004). Since ozone depletion is strong over Antarctica but weak over the Arctic (Solomon et al. 2007), this strong cooling effect is unique to Antarctica.

To summarize, surface warming from the greenhouse effect is weaker in the southern hemisphere than in the northern hemisphere, whereas cooling from stratospheric ozone depletion is stronger in the south than in the north. Consequently, the Arctic has warmed dramatically, even as the Antarctic has experienced a small cooling trend. Climate models reproduce this pattern when they are driven by both greenhouse gas increases and stratospheric ozone depletion (Gillett and Thompson 2003; Shindell and Schmidt 2004). Hence, the present cooling of Antarctica is consistent with the rest of the Earth’s surface warming in response to rising greenhouse gas concentrations.

The stratospheric ozone layer filters out harmful ultraviolet radiation from incoming sunlight. To protect public health and natural ecosystems, an international treaty—the Montreal Protocol—is phasing out the release of ozone-depleting chemicals to the atmosphere. According to climate models that correctly simulate the current cooling trend in Antarctica, if greenhouse gases continue to rise as the ozone layer recovers in future decades, the warming effect of greenhouse gases will begin to outweigh the cooling effect of ozone depletion (Shindell and Schmidt 2004). The result would be widespread warming in Antarctica, with attendant declines in sea ice and accelerated loss of land-based ice, with the latter contributing to accelerated sea level rise.


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The Science Behind the Shrinking Arctic Ice Cap

September 2007

The Arctic ice cap declined to a record minimum size in summer 2007. Studies indicate this accelerated shrinkage of Arctic sea ice may be in response to a strong warming trend and that the climate reacts more strongly to a given amount of global warming than generally believed.

The Arctic ice cap consists of a continent-sized sheet of sea ice that floats at the surface of the Arctic Ocean. During the dark of winter, the ice cap covers nearly the entire Arctic Ocean, but during the summer constant sunlight melts the edges of the ice cap, causing it to shrink in area. This annual shrinking begins in early spring and progresses into mid-September, when the extent of the ice cap reaches its summertime minimum and begins to grow again as the sun sets for the year and the chill of winter returns. Since 1979, the extent of the Arctic sea ice has been monitored using satellite observations. During this time, the September minimum extent has declined on average as the Arctic has warmed. Since 2000, there has been a series of record-breaking low annual minima, with 2002, 2005, and 2007 each establishing new records. View Graph

On August 17, 2007, the National Sea Ice Data Center (NSIDC) reported: “Arctic sea ice surpassed the previous single-day (absolute minimum) record for the lowest extent ever measured by satellite.” One month later, the NSIDC reported that the ice cap had reached it annual minimum size (Figure 1), which was “4.13 million square kilometers (1.59 million square miles), compared to 5.32 million square kilometers (2.05 million square miles) in 2005.” Compared to the long-term average between 1979 and 2000, The 2007 minimum “was lower by 2.61 million square kilometers (one million square miles), an area approximately equal to the size of Alaska and Texas combined, or the size of ten United Kingdoms.” (NSIDC, 2007b).

The wintertime maximum area of the ice cap occurs in March and has also been shrinking. The annual maximum sea ice extent reached record-breaking lows in three consecutive winters (2004-2006). In March 2007, the maximum extent was the second lowest on record after 2006. Regarding this observation, NSIDC scientist Walt Meier said, “This year's low wintertime extent is another milestone in a strong downward trend. We're still seeing near-record lows and higher-than-normal temperatures. We expect the downward trend to continue in future years” (NSIDC, 2007a).

In recent months, some important peer-reviewed studies have been published on the observed and projected shrinkage of the Arctic ice cap. In April, scientists from NSIDC and the National Center for Atmospheric Research published a study documenting from long-term observations that climate models underestimate the rate of Arctic sea ice loss (Stroeve et al., 2007). Observed loss of sea ice from 1953 to 2006 occurred three times faster than the average rate projected for the same period by 18 of the latest generation of climate models used by the Intergovernmental Panel on Climate Change (IPCC). According to a recent review of the scientific evidence, the observed ice loss “is best viewed as a combination of strong natural variability… and a growing radiative forcing associated with rising concentrations of atmospheric greenhouse gases…” (Serreze et al., 2007).

This summer, the initial rate of Arctic sea ice decline was similar to previous record-breaking years, but in late June and early July there was a dramatic surge in the rate of loss that led to the early arrival of the record-low sea ice extent reached in August (Figure 2). Regarding this surge, the NSIDC said, “…sea ice declined at a pace of up to 210,000 square kilometers (81,081 square miles) per day, or the equivalent of an area the size of Kansas each day. This rate was unprecedented in the satellite record…” (NSIDC, 2007b). View Graph

The cause of this surge is unclear, but is consistent with recent modeling research suggesting that sudden, extreme acceleration of shrinkage may be an inherent response of Arctic sea ice to a strong warming trend (Holland et al., 2006). In this study, about half of the model projections exhibited sudden accelerations in sea ice loss. In the model projections where such events occurred, trends in ice loss were four times faster than in projections without abrupt accelerations. If such accelerations are inherent to the response of sea ice to persistent warming, the Arctic could be ice free during the summer well before the end of this century (Serreze et al., 2007), a condition that has not existed for at least one million years and probably much longer (Overpeck et al., 2005).

The loss of Arctic sea ice is not the only aspect of climate change that has been underestimated by projections. Recent observations indicate that climate models have underestimated ice loss from the Greenland and Antarctic ice sheets (Shepherd & Wingham, 2007), ice loss from mountain glaciers (Meier et al., 2007), the rate of global sea level rise (Rahmstorf et al., 2007), change in global precipitation (Wentz et al., 2007; Zhang et al., 2007), and response of northern forests to warming (Soja et al., 2007). All of these changes were predicted before they were detected, but they are occurring sooner or more rapidly than expected (Engelhaupt, 2007). Although there are probably multiple reasons for underestimating climate change and ecosystem responses to it, inadequately treated positive feedbacks (amplifying factors within the climate system itself) are probably involved (Pittock, 2006).

The unexpectedly rapid change in Arctic sea ice and other climate processes suggests that the climate reacts more strongly to a given amount of global warming than scientists have calculated. As a result, risks from future climate change are likely greater than scientists have generally believed, and existing climate change projections might best be viewed as the minimum changes that humanity should expect.


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