Statement: IPCC Fourth Assessment Report, Working Group II

Intergovernmental Panel on Climate Change (IPCC) Releases New Assessment Report on the Impacts of Climate Change

Statement by the Pew Center on Global Climate Change

April 6, 2007

The IPCC Fourth Assessment “Summary for Policymakers” Working Group II report represents the IPCC’s strongest statement to date on the impacts of global climate change. Because of a dramatic increase in the number and quality of observations, this report concludes that, “it is likely [better than 2:1 odds] that anthropogenic warming has had a discernible influence on many physical and biological systems.” The report also projects with greater confidence than in the past that many regions, including North America, will experience severe impacts in the future, even for moderate warming scenarios. Particularly vulnerable are low-lying coastal regions worldwide. Many poor countries at low latitudes are also particularly vulnerable because of a combination of strong climate impacts, low capacity for adaptation, and heavy reliance on climate-impacted resources, such as local food and water supplies.

The assessment is based on extensive published, peer-reviewed scientific literature.  Today’s report is the second of three major studies that comprise the Fourth Assessment with input from more than 1,200 authors and 2,500 scientific expert reviewers from more than 130 countries. The first report, released in February 2007, examined the physical science basis for climate change. The third report, to be released in May 2007, will explore the solutions to global climate change, particularly options for reducing greenhouse gas emissions.


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

April 6, 2007

This week began with a landmark decision by the US Supreme Court and ended with the release of the IPCC's 4th Assessment on climate change impacts.  Following the Supreme Court's decision, it's clear that EPA has the authority – and should -- regulate CO2, and the IPCC report delivered the strongest statement to date on the consequences of climate change. Taken together with increasing calls from CEOs, states, and the public, the message is loud and clear: Read our lips - We need mandatory climate policy in the United States. 


Trends in CO2 Emissions

This figure shows emissions of carbon dioxide (CO2) by fuel source across all sectors of the economy. The fuels shown are coal, natural gas, petroleum, as well as the total emissions.

Overall, coal and petroleum consumption are down since 2007, while natural gas use has increased. In the electric power and indutrial sectors, natural gas, which emits about half the amount of CO2 as coal, is being used more extensively due to its lower price. In the transportation sector, petroleum consumption is down due to an increase in car and light truck fuel economy (for a similar number of vehicle miles traveled, year-on-year). Correspondingly, total emissions have generally declined since 2007. 

Source: EIA (2014)

U.S. CO2 Emissions from the Electric Power Sector

This figure shows the emissions of carbon dioxide (CO2) from the burning of fossil fuels for electric power generation. The electricity-generating fuels shown here are coal, natural gas, petroleum and non-biomass waste. Natural gas, which emits about half the amount of CO2 as coal, is being used more extensively due to its lower price and displacing coal-fired generation, while petroleum-fired electricity generation continues to be retired.

Source: EIA (2015)

U.S. Trends in Greenhouse Gas Emissions

This figure shows the trend in U.S. greenhouse gas emissions between 1990 and 2014. Emissions increased by 7.7 percent between 1990 and 2014.

Greenhouse gas emissions have been declining since 2007 for a few reasons:

  1. A greater share of electricity is being generated with natural gas and renewable energy. This has offset coal-fired electricity generation, which emits about two times the amount of carbon dioxide (a greenhouse gas) as natural gas-fired electricity generation per unit of electric energy. Energy efficiency has also contributed by keeping electricity demand growth very low.
  2. Economic activity decreased during the Great Recession, which ran from December 2007 until June 2009. Additionally, the structure of the U.S. economy continues its long-term shift from a manufacturing-based to a service-based economy, which is less energy-intensive.
  3. Consumption of fossil fuels in the transportation sector has decreased due to lower economic activity, more fuel-efficient vehicles on the road, greater use of biofuels and other social shifts that reduce total vehicle miles traveled, including an aging population, technology (telework), growing cities and greater use of public transportation.

Emissions have decreased 8 percent from 2005 to 2014.

Source: Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2014 (EPA 2016)


Greenhouse Gas Emissions by Sector

In 2014, the United States emitted 6.9 billion metric tons of greenhouse gases (CO2e). Greenhouse gases are emitted by all sectors of the economy, including electric power (30% of total), transportation (27%), industry (20%), residential & commercial (13%), and agriculture (10%).  

Source: Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2014 (EPA 2016)

U.S. Greenhouse Gas Emissions by Gas

In 2014, the United States emitted 6.9 billion metric tons of greenhouse gases (CO2e). Carbon dioxide accounted for the largest percentage of greenhouse gases (81%), followed by methane (10%), nitrous oxide (6%), and other greenhouse gases (3%). Total U.S. emissions for 2014 totaled 6,873 million metric tons of CO2e and net emissions, taking sinks into account, totaled 6,187 tons CO2e.

Source: Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2014 (EPA 2016)

Congressional Briefing Series on Science and Impacts: Sea Level Rise

Promoted in Energy Efficiency section: 
Sea level rise is one of the most widespread climate impacts expected to result from human-induced global warming. New evidence from modern satellite observations on the one hand, and from the study of how large polar ice sheets responded to ancient global warming events on the other, suggests that global warming is already causing sea level to rise and that it could rise faster and to a greater extent this century—and beyond—than previously estimated. This briefing will help congressional staff understand recent scientific progress and current scientific thought on sea level rise.

Friday February 9, 2007
10:00-11:30 AM
2325 Rayburn House Office Building


Sea level rise is one of the most widespread climate impacts expected to result from human-induced global warming. New evidence from modern satellite observations on the one hand, and from the study of how large polar ice sheets responded to ancient global warming events on the other, suggests that global warming is already causing sea level to rise and that it could rise faster and to a greater extent this century—and beyond—than previously estimated. This briefing will help congressional staff understand recent scientific progress and current scientific thought on sea level rise.

Following a brief introduction to global climate change by Dr. Jay Gulledge, two leading sea level experts, Dr. Steve Nerem and Dr. Jonathan Overpeck, will describe the present state of the science on global sea level rise, with emphasis on state-of-the-art satellite measurements of contemporary sea level change, the various climate processes that contribute to sea level rise, and lessons learned from studying ancient climate–sea level relationships. Following short scientific presentations from each scientist, there will be ample time for the audience to interact directly with these internationally recognized experts.


R. Steven Nerem, Ph.D.
University of Colorado
Dr. Steve Nerem is Professor of Aerospace Engineering Sciences at the University of Colorado at Boulder and a fellow of the Cooperative Institute for Research in Environmental Sciences. Prior to joining the CU faculty in 2000, he was Assistant Professor and then Associate Professor of Aerospace Engineering for four years at the University of Texas at Austin. Prior to that he was a geophysicist with NASA/Goddard Space Flight Center for six years. He earned his Ph.D. in Aerospace Engineering from The University of Texas at Austin. Dr. Nerem has authored approximately 60 peer-reviewed journal publications covering a variety of topics related to his specialty, which involves satellite orbit determination, remote sensing, and measuring the Earth's shape, gravity field, and sea level from space. He is a Contributing Author for the 2007 Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Dr. Nerem has received more than a dozen awards for his work, including NASA's Exceptional Scientific Achievement Medal for his research in the area of gravity field determination.

Jonathan T. Overpeck, Ph.D.
University of Arizona
Dr. Overpeck is Director of the Institute for the Study of Planet Earth and professor of Geosciences at the University of Arizona, Tucson. Prior to joining the faculty in 1999 he was head of the NOAA Paleoclimatology Program at the National Geophysical Data Center in Boulder, Colorado for nine years. He earned a Ph.D. in geological sciences from Brown University. Dr. Overpeck has authored over 100 papers that focus on global change dynamics, with a major focus on how and why climate systems vary on timescales of decades and longer. Current work focuses on the Asian and West African Monsoon systems, tropical Atlantic variability, El Niño-Southern Oscillation dynamics, Arctic environmental change, and reconstruction of ancient environments. He is a Coordinating Lead Author for the 2007 Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Dr. Overpeck has received numerous awards recognizing his climate research, including the U.S. Department of Commerce Gold Medal and the American Meteorological Society Walter Orr Roberts Award.

Jay Gulledge, Ph.D.
Pew Center on Global Climate Change
Dr. Gulledge is Senior Research Fellow for Science and Impacts at the Pew Center on Global Climate Change. He serves as the Center’s in-house scientist and coordinates its work to communicate the state of knowledge on the science and environmental impacts of global climate change to policy-makers and the public. He is also an adjunct Associate Professor at the University of Wyoming, home to his academic research on biological cycling of atmospheric greenhouse gases, which he publishes regularly in peer-reviewed journals. Prior to joining the Pew Center, he served on the faculties of Tulane University and University of Louisville. Dr. Gulledge earned a PhD in ecosystem sciences from the University of Alaska Fairbanks. He currently serves as an associate editor of Ecological Applications, a peer-reviewed journal published by the Ecological Society of America.

Main Greenhouse Gases

Learn more about climate science on our basics page

The tables below present characteristics of major greenhouse gases. The Global Warming Potential (GWP) indicates the warming effect of a greenhouse gas, while the atmospheric lifetime expresses the total effect of a specific greenhouse gas after taking into account global sink availability. The lifetime indicates how long the gas remains in the atmosphere and increased radiative forcing quantifies the contribution to additional heating over an area. The vast majority of emissions  are carbon dioxide followed by methane and nitrous oxide. Lesser amounts of CFC-12, HCFC-22, Perflouroethane and Sulfur Hexaflouride are also emitted and their contribution to global warming is magnified by their high GWP, although their total contribution is still small compared to the other gases.



Chemical Formula

 Anthropogenic Sources

Atmospheric Lifetime1(years)

 GWP2 (100 Year Time Horizon)



Fossil-fuel combustion, Land-use conversion, Cement Production





Fossil fuels,
Rice paddies,
Waste dumps





Industrial processes, Combustion



Tropospheric Ozone O3Fossil fuel combustion, Industrial emissions, Chemical solventshours-daysN.A.



Liquid coolants,








Sulfur Hexaflouride


Dielectric fluid




Pre-1750 Tropospheric
(parts per billion)

Current Tropospheric
(parts per billion) 






1,870 / 1,7488



 323 / 3228

Tropospheric Ozone 2534



.534 / .5328



.218 / .19410

Sulfur Hexaflouride


.00712 /.006738, 10

Source of graphical information and notes:
Blasing, T.J. ad K. Smith 2011.  "Recent Greenhouse Gas Concentrations."  In Trends: A Compendium of Data on Global Change.  Carbon Dioxide Information Analysis Cetner, Oak Ridge National Laboratory, US Department of Energy, Oak Ridge, TN, USA.


  1. The atmospheric lifetime is used to characterize the decay of an instanenous pulse input to the atmosphere, and can be likened to the time it takes that pulse input to decay to 0.368 (l/e) of its original value. The analogy would be strictly correct if every gas decayed according to a simple exponential curve, which is seldom the case. For example, CH4 is removed from the atmosphere by a single process, oxidation by the hydroxyl radical (OH), but the effect of an increase in atmospheric concentration of CH4 is to reduce the OH concentration, which, in turn, reduces destruction of the additional methane, effectively lengthening its atmospheric lifetime. An opposite kind of feedback may shorten the atmospheric lifetime of N2O (IPCC 2007, Section 2.10.3).
  2. The Global Warming Potential (GWP) provides a simple measure of the radiative effects of emissions of various greenhouse gases, integrated over a specified time horizon, relative to an equal mass of CO2 emissions.
  3. Pre-1750 concentrations of CH4,N2O and current concentrations of O3, are taken from Table 4.1 (a) of the IPCC Intergovernmental Panel on Climate Change), 2001. Following the convention of IPCC (2001), inferred global-scale trace-gas concentrations from prior to 1750 are assumed to be practically uninfluenced by human activities such as increasingly specialized agriculture, land clearing, and combustion of fossil fuels. Preindustrial concentrations of industrially manufactured compounds are given as zero. The short atmospheric lifetime of ozone (hours-days) together with the spatial variability of its sources precludes a globally or vertically homogeneous distribution, so that a fractional unit such as parts per billion would not apply over a range of altitudes or geographical locations. Therefore a different unit is used to integrate the varying concentrations of ozone in the vertical dimension over a unit area, and the results can then be averaged globally. This unit is called a Dobson Unit (D.U.), after G. M. B. Dobson, one of the first investigators of atmospheric ozone. A Dobson unit is the amount of ozone in a column which, unmixed with the rest of the atmosphere, would be 10 micrometers thick at standard temperature and pressure.
  4. Because atmospheric concentrations of most gases tend to vary systematically over the course of a year, figures given represent averages over a 12-month period for all gases except ozone (O3), for which a current global value has been estimated (IPCC, 2001, Table 4.1a).
  5. The value given by IPCC 2001, page 185, is 280 ± 10 ppm. This is supported by measurements of CO2 in old, confined, and reasonably well-dated air. Such air is found in bubbles trapped in annual layers of ice in Antarctica, in sealed brass buttons on old uniforms, airtight bottles of wine of known vintage, etc. Additional support comes from well-dated carbon-isotope signatures, for example, in annual tree rings. Estimates of "pre-industrial" CO2 can also be obtained by first calculating the ratio of the recent atmospheric CO2 increases to recent fossil-fuel use, and using past records of fossil-fuel use to extrapolate past atmospheric CO2 concentrations on an annual basis. Estimates of "pre-industrial" CO2 concentrations obtained in this way are higher than those obtained by more direct measurements; this is believed to be because the effects of widespread land clearing are not accounted for. Ice-core data provide records of earlier concentrations. For concentrations back to about 1775, see A. Neftel et al.
  6. Recent CO2 concentration (388.5 ppm) is the 2010 average taken from globally averaged marine surface data given by the National Oceanic and Atmospheric Administration Earth System Research Laboratory, web site:
  7. Pre-industrial concentrations of CH4 are evident in the "1000-year" ice-core records in CDIAC's Trends Online However, those values need to be multiplied by a scaling factor of 1.0119 to make them compatible with the AGAGE measurements of current methane concentrations, which have already been adjusted to the Tohoku University scale. Ten thousand-year records of CH4, CO2 and N2O, from ice-core data, are also presented graphically in IPCC 2007, (Figure SPM.1).
  8. The first value in a cell represents Mace Head, Ireland, a mid-latitude Northern-Hemisphere site, and the second value represents Cape Grim, Tasmania, a mid-latitude Southern-Hemisphere site. "Current" values given for these gases are annual arithmetic averages based on monthly background concentrations for October 2009 through September 2010. The SF6 values are from the AGAGE gas chromatography - mass spectrometer (gc-ms) Medusa measuring system.
  9. Source: IPCC (2007). The pre-1750 value for N2O is consistent with ice-core records from 10,000 B.C.E. through 1750 C.E. shown graphically in figure SPM.1 on page 3.
  10. For SF6 data from January 2004 onward see For data from 1995 through 2004, see the National Oceanic and Atmospheric Administration (NOAA), Halogenated and other Atmospheric Trace Species (HATS) site at:

United States Emissions

In this section, you can find information about the main sources of greenhouse gases emitted in the United States.

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



U.S. Greenhouse Gas Emissions by Gas

Greenhouse Gas Emissions by Sector

U.S. Trends in Greenhouse Gas Emissions

Trends in CO2 Emissions

U.S. CO2 Emissions from the Electric Power Sector


Explore All the Facts & Figures Sections:

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


Adaptation to Climate Change: International Policy Options

Adaptation to Climate Change cover

Adaptation to Climate Change: International Policy Options

Prepared for the Pew Center on Global Climate Change
November 2006

Ian Burton, University of Toronto
Elliot Diringer, Pew Center on Global Climate Change
Joel Smith, Stratus Consulting Inc.

This report examines options for future international efforts to help vulnerable countries adapt to the impacts of climate change both within and outside the climate framework. Options outlined in the report include stronger funding and action under the UN Framework Convention on Climate Change, mandatory climate risk assessments for multilateral development finance, and donor country support for climate "insurance" in vulnerable countries.

Press release

Download entire report (pdf)



From its inception, the international climate effort has focused predominantly on mitigation—reducing greenhouse gas (GHG) emissions to prevent dangerous climate change. The next stage of the international effort must deal squarely with adaptation—coping with those impacts that cannot be avoided. This is both a matter of need, as climate change is now underway, and a matter of equity, as its impacts fall disproportionately on those least able to bear them. It also may be a condition for further progress on mitigation. Indeed, substantial new mitigation commitments post-2012 may be politically feasible only if accompanied by stronger support for adaptation.

Ambitious mitigation efforts can lessen, but not prevent, future climate change. While steep reductions in emissions could stabilize atmospheric GHG concentrations at lower levels than under “business as usual,” they likely would be well above current, let alone pre-industrial, levels.2 With higher concentrations will come further rises in temperatures and sea level, changes in precipitation, and more extreme weather. The early impacts of climate change already are being felt worldwide.3 Future impacts will affect a broad array of human and natural systems, with consequences for human health, food and fiber production, water supplies, and many other areas vital to economic and social well being. While certain impacts may in the nearer term prove beneficial to some, in the long term, the effects will be largely detrimental.4

Anticipating and adapting to these impacts in order to minimize their human and environmental toll is a significant challenge for all nations. Meeting it requires action at multiple levels, from the local to the international, within both public and private spheres. This paper explores one critical dimension of this multifaceted challenge—how adaptation can be best promoted and facilitated through future multilateral efforts.

Among the many issues confronting governments, two are especially daunting. The first is equity and its relation to cost. Difficult questions of fairness suffuse the climate debate but are particularly stark in the case of adaptation: those most vulnerable to climate change are the ones least responsible for it. Stronger international adaptation efforts—whatever form they might take, and whether understood as assistance or as compensation—will be possible, let alone effective, only insofar as affluent countries are prepared to commit resources. This is a question not of policy design but, rather, of negotiation and political will. Second, reliable information and relevant experience are in short supply. Relative to mitigation, the adaptation challenge is much less well understood—needs as well as solutions. A high priority in the near term is strengthening the knowledge base with better data and modeling to refine projections of future impacts, and with early insights from the field on the most effective responses.

It is at the same time essential to begin considering how future international efforts can best be structured. This paper examines underlying issues and lays out an array of possibilities. To set the issue in context, it looks first at the history and evolving nature of human adaptation to climate. It then highlights key issues in the design of adaptation policy, and summarizes and assesses international adaptation efforts to date. Finally, the paper outlines three broad and potentially complementary approaches to future international efforts:

  • Adaptation Under the UNFCCC—Initiating new steps under the UN Framework Convention on Climate Change (UNFCCC) to facilitate comprehensive national adaptation strategies and to provide reliable assistance for high-priority implementation projects.
  • Integration with Development—Integrating adaptation across the full range of development-related assistance through measures such as mandatory climate risk assessments for projects financed with bilateral or multilateral support.
  • Climate “Insurance”—Committing stable funding for an international response fund or to support insurance-type approaches covering climate-related losses and promoting proactive adaptation in vulnerable countries.

1. This report was prepared initially as input to the Climate Dialogue at Pocantico convened by the PewCenterin 2004-5, and in its final form reflects contributions from the dialogue. The Pocantico dialogue brought together 25 senior policymakers and stakeholders from 15 countries to recommend options for advancing the international climate change effort beyond 2012. The group’s report is available at: /global-warming-in-depth/all_reports/climate_dialogue_at_ pocantico/index.cfm.

2. Metz et al. (2001).

3. Parmesan, C. and G. Yohe (2003); Root, T. L. et al. (2003); Stott et al. (2004).

4. McCarthy et al. (2001).


Elliot Diringer
Ian Burton
Joel Smith
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