Scientists, public differ sharply on cause of global warming
- Published Monday, October 30, 2006, by the San Jose Mercury News
Scientists, American public disagree sharply over global warming
By Robert S. Boyd
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
Last Wednesday, 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
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
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
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
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