Science

IPCC Fifth Assessment Report

What is the IPCC and why is it important?

The Intergovernmental Panel on Climate Change (IPCC) is the leading international body for the assessment of climate change. It was established by the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO) in 1988 to provide the world with a clear scientific review of the current state of knowledge on climate change.

Thousands of scientists from all over the world volunteer and are selected to review and assess the latest relevant scientific, technical and socio-economic data to understand climate change, its potential impacts, and options for adaptation and mitigation. The IPCC aims to reflect a range of views and expertise in order to publish comprehensive and objective assessments.

The IPCC does not conduct independent research, rather it convenes climate experts from around the world every five to seven years to synthesize the latest climate research findings in peer-reviewed and published scientific/technical literature. The IPCC issued comprehensive assessments in 1990, 1995, 2001, and 2007.

IPCC reports are never policy prescriptive but the conclusions are relevant to nations, states, and businesses interested in enacting policies to limit future warming and reduce the costs of climate change.

What is the Fifth Assessment report?

The Fifth Assessment Report (AR5) is the latest in a series of reports from the IPCC assessing scientific, technical, and socio-economic information regarding climate change. It will be released in three installments over the course of 2013 and 2014, and an additional synthesis report will be published in October 2014.

More than 830 scientists are involved in writing the reports and hundreds more will review and edit the draft reports.

AR5 will comprise three sections, or working groups:

  • Working Group I provides a comprehensive assessment of the physical science basis of climate change: Released Sept. 27, 2013.
  • Working Group II assesses the scientific, technical, environmental, economic and social aspects of vulnerability to climate change as well as consequences for ecological systems, socio-economic sectors and human health: Released March 31, 2014.
  • Working Group III assesses all relevant options for mitigating climate change through limiting or preventing greenhouse gas emissions and taking actions to remove them from the atmosphere: Released April 11, 2014.

The syntheses report is scheduled to be released Oct. 31, 2014

Each working group report has a Summary for Policymakers that distills the key points from the hundreds of pages found in the respective full report. The Summary for Policymakers tend to be of most interest to the media and non-scientists.

What are the key points in the IPCC Working Group I Summary for Policymakers?

The Summary for Policymakers includes the key conclusions from the longer report in a format suitable for a broader audience. The SPM will include observations of changes in the atmosphere, ocean, and cryosphere, including changes in sea level. It will also discuss our confidence in attributing climate change to human activities. The SPM will also include projections of global and regional climate change for the 21st century, including projected changes in the water cycle, extreme weather, sea level, sea ice, and the carbon cycle.

It answers such questions as:

  • What is the state of the science in understanding and attributing climate changes?
  • What are the primary drivers of climate change?
  • How do recent changes compare to paleoclimatic records?
  • In what ways is climate change is already occurring around the globe and how fast are these systems changing?
  • What are models projecting for the 21st century climate and how accurate are they?
  • Which types of climate changes might be irreversible?

What are the key points in the IPCC Working Group II Summary for Policymakers?

The Summary for Policymakers includes important statements and conclusions from the WGII report and is aimed at a broader audience. The WGII SPM is divided into three sections: 1. Observed Impacts, Vulnerability, and Adaptation in a Complex and Changing World; 2. Future Risks and Opportunities for Adaptation; and 3. Managing Future Risks and Building Resilience.

The WGII SPM answers such questions as:

  • What climate-related impacts are already being observed today?
  • What are the key future risks in areas such as water resources, coastal and marine systems, food security, human health, security, and economic growth?
  • Which groups of people are the most vulnerable?
  • What kind of adaptation actions have already been undertaken and which actions could be taken to reduce these future risks?
  • How can adaptation decisions be made when the future is uncertain?
  • What constitutes effective adaptation action and climate-resilient development?

What are the key points in the IPCC Working Group III Summary for Policymakers?

The Summary for Policymakers includes key statements and conclusions from the WGIII report and is aimed at a broader audience. It assesses all relevant options for mitigating climate change through limiting or preventing greenhouse gas emissions, as well as activities that remove them from the atmosphere. It lays out a number of baseline scenarios that, without mitigation efforts, would lead to substantial warming by the end of the 21st century.

It also describes a number of potential mitigation scenarios:

  • To avoid 2 degrees C (3.6 degrees F) of warming relative to pre-industrial time, the report indicates that atmospheric concentrations of greenhouse gases need to be stabilized around 450 ppm CO2-eq or lower. Given that we are currently around 430 CO2-eq, this is a tall order, requiring large-scale changes in energy systems and land use. For example, achieving this level of stabilization will require more rapid improvements in energy efficiency and a tripling to nearly a quadrupling of the share of zero- and low-carbon energy supply from renewables, nuclear energy, and fossil energy with carbon capture and storage, or bioenergy with carbon capture and storage, by the year 2050.
  • The aggregate economic cost of mitigation varies widely, but generally increases based on the stringency of the level of mitigation. In general, the costs of mitigation only offset a relatively small fraction of global projected economic growth for the 21st century.
  • The 2020 individual country-pledged goals (under the Cancún Agreements) are unlikely to put us on a path to avoid 2 degrees C (3.6 degrees F) of warming; further substantial reductions beyond 2020 would need to be made. Continuing on the pathways consistent with the Cancún pledges is more consistent with scenarios likely to keep temperature change below 3 degrees C relative to pre-industrial levels.
  • If we do not strengthen mitigation efforts between now and 2030, it will be more difficult and more expensive to achieve warming targets, such as avoiding 2 degrees of warming relative to pre-industrial levels.

Additional resources:

The IPCC's growing certainty of human involvement in global warming

IPCC homepage

WGI homepage: The Physical Science Basis

WGII homepage: Impacts, Adaptation, and Vulnerability

WGIII homepage: Mitigation of Climate Change

C2ES summaries of the Fourth Assessment Report

IPCC Fourth Assessment Report, Working Group II

IPCC AR4 WGII: "Impacts, Adaptation & Vulnerability"

The second installment to the IPCC Fourth Assessment Report was released April 6, 2007. The Working Group II installment to the report addresses "Impacts, Adaptation and Vulnerability". The WGII report provides a detailed analysis of observed changes in natural and human systems and the relationship between those observed changes and climate change, as well as a detailed assessment of projected future vulnerability, impacts, and response measures to adapt to climatic changes for main sectors and regions.

According to the IPCC, the report from Working Group II on the impacts of climate change answers the following questions:

  • What is the current state of knowledge on impacts of climate change?
  • What is the state of knowledge on impacts under different levels of adaptation?
  • What are the impacts under different levels of mitigation?
  • What is the state of knowledge concerning observed effects?

For the full report, visit the IPCC website. For a summary of the report, click here (pdf).

Back to coverage of the IPCC's Fourth Assessment Report

IPCC Fourth Assessment Report, Working Group I

IPCC AR4 WGI: "The Physical Science of Climate Change"

The first installment to the IPCC Fourth Assessment Report was released February 2, 2007. According to the IPCC, the Working Group I Report, "The Physical Science Basis", assesses the current state of knowledge about the natural and human drivers of climate change, reflecting the progress of the climate change science in the observation of the atmosphere, the Earth's surface and oceans. It provides a paleoclimatic perspective and evaluates the Earth's surface and oceans. Main topics include changes in atmospheric composition, observation of various climate parameters, coupling between changes in climate and biogeochemistry, evaluation of models and attribution of climate change.

Working Group I Report, "The physical science basis", assesses the current state of knowledge about the natural and human drivers of climate change, reflecting the progress of the climate change science in the observation of the atmosphere, the Earth's surface and oceans. It provides a paleoclimatic perspective and evaluates future projections of climate change. Main topics include changes in atmospheric composition, observation of various climate parameters, coupling between changes in climate and biogeochemistry, evaluation of models and attribution of climate change..

According to the IPCC, the report from Working Group I on the science of climate change answers the following questions:

  • What progress has been made in understanding and attributing climate change?
  • What do observations of the atmosphere, oceans, sea level, snow and ice tell us?
  • How has climate been behaving in the last hundreds of thousands of years?
  • Which are the projections of future changes?

For the full report, visit the IPCC website. For a summary of the report, click here (pdf). Relevant materials, including a statement from C2ES can be linked to from the "Related Content" box above.

Back to coverage of the IPCC's Fourth Assessment Report

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 sector, 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 declined since 2007. 

Source: EIA (2012)

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 (2012)

U.S. Trends in Greenhouse Gas Emissions

This figure shows the trend in U.S. greenhouse gas emissions between 1990 and 2010. Emissions increased by 10.5 percent between 1990 and 2010.

Greenhouse gas emissions declined in 2008 and 2009 for two main reasons:

  1. A larger share of electricity was generated with natural gas (and to a much lesser extent renewable energy). This 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.
  2. Economic activity decreased during the Great Recession, which ran from December 2007 until June 2009.

Emissions increased 3.1 percent from 2009 to 2010, as economic growth returned to the United States.


 
Source: Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2010 (EPA 2012)


 

Greenhouse Gas Emissions by Sector

In 2010, the United States emitted over 6.8 billion metric tons of greenhouse gases (CO2e). Greenhouse gases are emitted by all sectors of the economy, including electric power (34% of total), transportation (27%), industry (21%), residential & commercial (11%), and agriculture (7%).  


 
Source: Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2010 (EPA 2012)

U.S. Greenhouse Gas Emissions by Gas

In 2010, the United States emitted over 6.8 billion metric tons of greenhouse gases (CO2e). Carbon dioxide accounted for the largest percentage of greenhouse gases (84%), followed by methane (10%), nitrous oxide (4%), and other greenhouse gases (2%). Total U.S. emissions for 2010 totaled 6,821 million metric tons of CO2e and net emissions, taking sinks into account, totaled 5,747 tons CO2e.


Source: Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2010 (EPA 2012)

Congressional Briefing Series on Science and Impacts: Sea Level Rise

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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.

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