WASHINGTON, DC, June 7 -- In a report requested by the Bush administration, a National Research Council committee confirmed global warming is occurring but said it is unknown how much can be blamed on fossil fuel burning and other human activity.
"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."
The committee -- made up of 11 of the nation's top climate scientists, including seven members of the National Academy of Sciences -- also emphasized that much more systematic research is needed to reduce current uncertainties in climate-change science.
A White House spokesman cautioned it is too soon to blame human activity for the temperature increases.
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.
President George W. Bush will meet with European leaders next week to discuss climate change policy. Instead of supporting the Kyoto treaty, he is widely expected to endorse a cap and trade carbon dioxide emission system that would have the US and other developed nations cap carbon dioxide emissions by 2012 and by 2025 eliminate most man-made CO2 emissions through carbon sequestration and new technology. Developing nations would cap CO2 by 2035 and seek to end most emissions by 2050.
NAS said computer models suggest that average global surface temperatures will rise between 2.5 and 10.4° F. (1.4 and 5.8° C.) 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° F. (about 0.6° C.) 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 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.