WASHINGTON—When it comes to global warming, scientists and the American public aren't talking on the same wavelength.
Most scientists believe that humans and their machines are mainly responsible for the 1.4 degree Fahrenheit rise in the world's average temperature in the last 100 years. Most Americans think otherwise.
Last month, a group of 18 climate scientists, including two Nobel Prize winners, submitted an affidavit to the Supreme Court declaring that they're 99 percent certain that "greenhouse gas emissions from human activities cause global climate change, endangering human health and welfare."
Only 41 percent of those polled last summer by the Washington-based Pew Research Center, however, accepted the argument that climate change is due primarily to human activities, such as burning fossil fuels in cars, trucks and factories.
The rest of the 1,501 adults in the survey either said there's no solid evidence that the Earth is warming, or that if there is, the extra heat is the result of natural climate patterns, such as fluctuations in the sun's radiation.
This public skepticism flies in the face of the most widely accepted scientific assessment of the cause of global warming, which lays the blame primarily on "greenhouse gases" generated by humans.
The leading greenhouse gas is carbon dioxide (CO2), which is emitted by cars, trucks and factories and traps the sun's heat in the atmosphere.
The official scientific consensus is contained in a massive report issued in 2001 by the Intergovernmental Panel on Climate Change (IPCC), an organization that the United Nations created to collect and assess the work of climate scientists.
The IPCC report concluded that "most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations." The authors defined "most" to mean more than half and "likely" to mean that they're 66 percent to 90 percent sure that their statement is true.
Hundreds of scientists from around the globe are now working on an updated IPCC report to be completed next year. The report will reflect the results of the last five years of research and more accurately define the human and natural roles in global warming.
"It will be much better quantified," said Kevin Trenberth, a climate analyst at the National Center for Atmospheric Research in Boulder, Colo. "But there are no major new revelations."
Disentangling the causes of warming is a complicated detective story.
"Nature is a perverse old girl, and she doesn't tell us how much (warming) is due to her and how much is due to us," said Stephen Schneider, a climate scientist at Stanford University in Palo Alto, Calif. "So we look for fingerprints that you'd expect if the cause was humans rather than nature."
One "fingerprint," for example, is the observation that the lower atmosphere is warming while the upper atmosphere is cooling. Schneider said that's one piece of evidence that the phenomenon is caused by people, not the sun, because the sun would warm all levels of the atmosphere equally.
Despite the uncertainties and controversies, continued research has strengthened confidence in the IPCC's conclusion.
"You're never going to say you're 100 percent sure," Schneider said. "You have to lay out the odds."
Thomas Karl, the director of the National Climatic Data Center in Asheville, N.C., told the House Government Reform committee in July that "the odds are better than two to one" that humans have caused most of the warming.
"The chances are two out of three that this is right and only one out of three that it's wrong," Gabriele Hegerl, a climate scientist at Duke University in Durham, N.C., and one of the IPCC authors, said in an e-mail.
When skeptics recently raised objections to one line of evidence of CO2's role in global warming, the House Government Reform committee asked the National Academies to clarify the issue.
In June, a special committee of the Academies, headed by Gerald R. North, an atmospheric scientist at Texas A&M University in College Station, declared that there are "multiple lines of evidence for the conclusion that climatic warming is occurring in response to human activities."
The North committee found that volcanic activity and natural variations in the sun's radiation can explain most of the ups and downs in the Earth's temperature before 1750, the beginning of the Industrial Revolution.
But since 1750, the committee declared, only a combination of natural and human causes can explain the rising temperatures. And human activity "dominates the warming" since 1950, another report from the National Academies said last year.
"Human activities are almost definitely required to explain the observed climate changes since the mid-20th century," said Peter Thorne, a climate expert at the Hadley Centre for Climate Prediction and Research in Bracknell, England. "Natural causes and natural climate variability alone are an inadequate explanation."
Since 1750, more than 300 billion tons of carbon have been added to the atmosphere, according to the Energy Department. This raised the amount of carbon dioxide in the air by 35 percent and accounts for about half of the warming effect so far.
"The predominant cause of this increase in carbon dioxide is the combustion of fossil fuels and the burning of forests," Karl said in his congressional testimony.
Another important greenhouse gas is methane, a type of natural gas produced by cattle raising, rice cultivation and cement manufacture. Ozone and nitrous oxide from agricultural and industrial sources also contribute to warming.
Variations in the amount of heat radiated by the sun, such as during its 11-year sunspot cycle, make little difference in the long-range rise in Earth's temperature.
"The energy reaching the Earth from the sun has been measured precisely enough to conclude that Earth's warming was not due to changes in the sun," the National Academies committee reported.
Some factors, both human and natural, help cool the planet rather than warm it.
Volcanoes, for example, throw up tons of sulfur and ash that reflect the sun's heat back into space. Aerosols from refrigerators, tailpipes and other human sources also reflect rather than absorb heat. Some clouds trap heat in the atmosphere; others bounce it back into space.
On balance, however, the warming outweighs the cooling and is expected to accelerate for the rest of the century.
For more information, go to www.nap.edu/books/0309095069/html/
(c) 2006, McClatchy-Tribune Information Services.
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