WASHINGTON -- Studies that predict dangerous greenhouse warming slightly overestimate the effect of carbon dioxide on climate while giving too little weight to some other chemicals, European researchers report.
The actual impact of carbon dioxide on climate is about 15 percent less than estimated by the U.N.-sponsored Intergovernmental Panel on Climate Change, according to a report in the July 15 edition of Geophysical Research Letters.
But the study concluded that the impact of other chemicals, including methane and nitrous oxide, has been underestimated.
Jerry Mahlman, director of the Geophysical Fluid Dynamics Laboratory at Princeton, said the finding "isn't a stunner, but it's an important contribution if it's true."
Many climate experts are concerned that increasing amounts of industrial gases, primarily carbon dioxide, being added to the atmosphere will trap heat somewhat like a greenhouse, causing climate to warm up.
They say that this so-called radiative forcing could add 1.5 degrees to 4.5 degrees to the planet's average temperature over time, raising concern that could lead to droughts, melting of glaciers and ice caps and various other weather changes.
The greenhouse warming theory holds that these gasses being added to the atmosphere trap some of the heat that would normally be radiated out into space. Carbon dioxide is released into the atmosphere industrially through the burning of coal and other fossil fuels.
The IPCC report estimated that effect as adding 2.45 watts of energy per square meter at the Earth's surface. The new study reduces that net estimate to 2.25 watts per square meter.
Mahlman, who was not part of the team that prepared the research, said the change "does not substantially reduce the uncertainly in net greenhouse response." Instead of a range of uncertainly between 1.5 degrees and 4.5, it might mean a range of 1.35 and 4.3, he suggested.
"Nevertheless, it's important," he said, to improve the accuracy of climate studies.
Gunnar Myhre of the University of Oslo, Norway, doesn't expect the findings to reduce concerns about warming.
"Our results will only change the IPCC estimate of radiative forcing, not the IPCC estimate of temperature change," he said in an interview via e-mail.
The greenhouse warming issue has become politically charged, especially in the United States with Congress indicating reluctance to approve a global climate agreement signed last year in Kyoto, Japan.
The Kyoto agreement calls on the United States and other industrial countries to curb greenhouse gases, mainly by slowing down the growing amount of carbon dioxide released into the atmosphere from burning fossil fuels such as coal.
The differences found in the new study "ought not have a great impact on policy," Keith P. Shine of the University of Reading, England, said in an interview via e-mail. "They are part of a process of continual refinement as our understanding improves."
The study was done by Shine, Myhre, Eleanor J. Highwood the University of Reading and Frode Stordal of the Norwegian Institute for Air Research.
Geophysical Research Letters is published by the American Geophysical Union, an organization of earth science specialists.