06 June 2001

National Academy of Sciences Issues Report on Global Warming

The National Academy of Sciences reports that the observed warming of the Earth's atmosphere over the last 50 years is at least in part being caused by greenhouse gas emissions such as carbon dioxide from the burning of fossil fuels.

The committee's 24-page report, prepared by 11 top climate scientists, including a Nobel laureate, came in response to a White House request for a review of the state of climate science in preparation for ministerial talks on a climate change treaty to be held in Bonn July 16-27.

A June 6 press release on the conclusions of a report by a committee of the academy's National Research Council adds, however, that uncertainties remain about how much natural variation is contributing to global warming.

The report says the conclusion of the United Nations Intergovernmental Panel on Climate Change (IPCC) that global warming is likely the result of increases in greenhouse gases accurately reflects the current thinking of the scientific community.

But the report cautions that uncertainties about this conclusion remain because of the level of natural variability inherent in the climate system over decades and centuries, and the questionable ability of computer models to accurately simulate changes in nature over long periods.

"We know that greenhouse gases are accumulating in Earth's atmosphere, causing surface temperatures to rise," said committee chair Ralph Cicerone, chancellor at the University of California at Irvine. "We don't know precisely how much of this rise to date is from human activities ..."

To reduce some of the uncertainties inherent in current climate change predictions, the committee called for a strong commitment to improving climate models and building a global climate observing system. "More comprehensive measurements of greenhouse gases and increased computational power also will be needed," it said.

The proposed international treaty on climate change, known as the Kyoto Protocol, calls on industrialized countries to limit their emissions of carbon dioxide and other greenhouse gases linked to global warming.

President Bush announced last March that the United States was withdrawing support for the protocol, saying that the protocol's binding limits on emissions could harm the U.S. economy. The protocol has been signed by the United States but not ratified.

According to reports, the administration is expected to unveil an alternative plan of voluntary emissions targets before his upcoming meeting with European Union leaders in Gothenburg, Sweden. The alternative plan follows a cabinet-level review of U.S. climate change policy.

The full text of the report by the National Research Council, entitled "Climate Change Science: An Analysis of Some Key Questions," can be found at the following Web site: http://www.nap.edu/catalog/10139.html

Following is the text of the National Academy of Sciences press release:

Leading Climate Scientists Advise White House on Global Warming

National Academies of Science Press Release

"We know that greenhouse gases are accumulating in Earth's atmosphere, causing surface temperatures to rise," said committee chair Ralph Cicerone, chancellor, University of California at Irvine. "We don't know precisely how much of this rise to date is from human activities, but based on physical principles and highly sophisticated computer models, we expect the warming to continue because of greenhouse gas emissions."

Based on assumptions that emissions of greenhouse gases will accelerate and conservative assumptions about how the climate will react to that, computer models suggest that average global surface temperatures will rise between 2.5 and 10.4 degrees Fahrenheit (1.4 and 5.8 degrees Celsius) by the end of this century.

With regard to the basic question of whether climate change is occurring, the report notes that measurements show that temperatures at the Earth's surface rose by about 1 degree Fahrenheit (about .6 degrees Celsius) during the 20th century. This warming process has intensified in the past 20 years, accompanied by retreating glaciers, thinning arctic ice, rising sea levels, lengthening of the growing season in many areas, and earlier arrival of migratory birds.

The committee said the conclusion of the Intergovernmental Panel on Climate Change (IPCC) that the global warming that has occurred in the last 50 years is likely the result of increases in greenhouse gases accurately reflects the current thinking of the scientific community. However, it also cautioned that uncertainties about this conclusion remain because of the level of natural variability inherent in the climate on time scales from decades to centuries, the questionable ability of models to simulate natural variability on such long time scales, and the degree of confidence that can be placed on estimates of temperatures going back thousands of years based on evidence from tree rings or ice cores.

The greenhouse gas of most concern is carbon dioxide since the naturally occurring chemical also is generated by the continuing burning of fossil fuels, can last in the atmosphere for centuries, and "forces" more climate change than any other greenhouse gas, the committee said. Other significant greenhouse gases include methane, nitrous oxide, water vapor, tropospheric ozone, and chlorofluorocarbons (CFCs), which together have a "forcing" on climate change approximately equal to that of carbon dioxide. Man-made sources of methane, nitrous oxide, and ozone have resulted in substantially increased concentrations in the atmosphere in the 20th century, although each of these gases also has natural sources. CFCs are entirely synthetic compounds.

The best information about past climate variability comes from ice cores drilled miles deep in Antarctica and Greenland, which reveal that temperatures changed substantially over the past 400,000 years. Although most of these changes occurred over thousands of years, some rapid warmings took place over a period of decades.

The ice cores also trapped carbon dioxide and methane, which shows that the gases were present in the atmosphere at their lowest levels during cold eras and at higher levels during warm eras. Carbon dioxide did not rise much above 280 parts per million by volume (ppmv) until the industrial revolution. By the end of the 20th century, it had reached 370 ppmv, with an average increase in the last two decades of 1.5 ppmv a year. Both carbon dioxide and methane are more abundant in the atmosphere now than at any time during the 400,000-year ice core record.

The committee noted that the IPCC has examined a range of scenarios concerning future greenhouse gas emissions. The committee called such scenarios valuable because they provide a warning of the magnitude of climate change that may occur if emission rates continue to climb at a rate similar to last century, but it also said alternative scenarios are needed to illustrate the sensitivity to underlying assumptions, particularly with regard to future technological development and energy policy.

The committee also was asked by the White House to examine whether there were any substantive differences between the IPCC reports and their abridged technical and policy-maker summaries. The IPCC was established by the United Nations and World Meteorological Organization in 1988 and its reports and summaries have been influential in international negotiations related to the Kyoto protocol.

The full IPCC Working Group 1 report does an admirable job of reflecting research activities in climate science, and is adequately summarized in the technical summary, the committee said. The corresponding summary for policy-makers, it added, placed less emphasis on the scientific uncertainties and caveats. Looking to the future, the committee suggested that improvements to the IPCC process may need to be made to ensure the best scientific representation possible, and to keep the process from being seen as too heavily influenced by governments "which have specific postures with regard to treaties, emissions controls, and other policy instruments."

To reduce some of the uncertainties inherent in current climate change predictions, a strong commitment must be made to basic research as well as to improving climate models and building a global climate observing system, the committee said. More comprehensive measurements of greenhouse gases and increased computational power also will be needed.

Although potential impacts from global warming were looked at in the report, it was not part of the committee's charge to make policy recommendations for dealing with them.

The White House requested this fast-track review of the state of climate science in preparation for international discussions on global warming scheduled to take place in the coming weeks. "In view of the critical nature of this issue, we agreed to undertake this study and to use our own funds to support it," said Bruce Alberts, president of the National Academy of Sciences and chair of the National Research Council. The study took a month.

The National Research Council is the principal operating arm of the National Academy of Sciences and National Academy of Engineering. It is a private, nonprofit institution that provides scientific and technical advice under a congressional charter.

NATIONAL RESEARCH COUNCIL
Division on Earth and Life Studies

Committee on the Science of Climate Change

Ralph J. Cicerone1 (chair)
Chancellor, and
Daniel G. Aldrich Professor
Department of Earth System Science and Department of Chemistry
University of California
Irvine

Eric J. Barron
Director
Earth and Mineral Sciences Environment Institute, and
Distinguished Professor of Geosciences
Pennsylvania State University
University Park

Robert E. Dickinson1
Professor of Dynamics and Climate
School of Earth and Atmospheric Sciences
Georgia Institute of Technology
Atlanta

Inez Y. Fung1
Richard and Rhoda Goldman Distinguished Professor for the Physical Sciences;
Professor
Departments of Earth and Planetary Science and of Environmental
Sciences, Policy, and Management; and
Director
Center for Atmospheric Sciences
University of California
Berkeley

James E. Hansen1
Head
NASA/Goddard Institute for Space Studies
New York City

Thomas R. Karl
Director
National Climatic Data Center
National Oceanic and Atmospheric Administration
Asheville, N.C.

Richard S. Lindzen1
Alfred P. Sloan Professor of Meteorology
Department of Earth, Atmospheric, and Planetary Sciences
Massachusetts Institute of Technology
Cambridge

James C. McWilliams
Slichter Professor of Earth Sciences
Department of Atmospheric Sciences
Institute of Geophysics and Planetary Physics
University of California
Los Angeles

F. Sherwood Rowland1,2
Donald Bren Research Professor of Chemistry and Earth System Science
Department of Chemistry
University of California
Irvine

Edward S. Sarachik
Professor
Department of Atmospheric Sciences;
Adjunct Professor
School of Oceanography; and
Director
S.P. Hayes Center of the Joint Institute for the Study of the Atmosphere and the Ocean
University of Washington
Seattle

John M. Wallace1
Professor of Atmospheric Sciences
Department of Atmospheric Sciences, and
Co-Director
Program on the Environment
University of Washington
Seattle

RESEARCH COUNCIL STAFF

Vaughan C. Turekian
Study Director

1 Member, National Academy of Sciences
2 Member, Institute of Medicine