June 13, 2001 WASHINGTON -- In a report requested by the Bush administration, a committee of the National Academies' National Research Council summed up science's current understanding of global climate change by characterizing the global warming trend over the last 100 years, and examining what may be in store for the 21st century and the extent to which warming may be attributable to human activity. The committee -- made up of 11 of the nation's top climate scientists, including seven members of the National Academy of Sciences, one of whom is a Nobel-Prize winner -- also emphasized that much more systematic research is needed to reduce current uncertainties in climate-change science.
"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.
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