By ROBERT LEE HOTZ, Times Science Writer
Global warming
may boost world temperatures by up to 11 degrees Fahrenheit by
the end of the 21st century, a figure substantially higher than
previous estimates, according to a confidential draft report prepared
by an influential group of climate scientists sponsored by the
United Nations.
Moreover, "there is now stronger evidence for human influence
on global climate," the scientists concluded in their preliminary
report, which was distributed to more than 100 governments this
week for review.
Several scientists
familiar with the new report, prepared by an international group
known as the Intergovernment Panel on Climate Change, said its
findings significantly strengthen the case for a human role in
climate change. Although there is general agreement that the climate
is warming, the question of how much of the change is caused by
human action has been a major topic of scientific inquiry.
The issue
has also figured in the presidential campaign. Vice President
Al Gore has frequently asserted that global warming is a major
problem on which the government must begin taking action.
Texas Gov. George W. Bush has been more skeptical. "I don't
think we've got all the facts," he said in the second campaign
debate earlier this month. "I think it's an issue that we
need to take very seriously, but I don't think we know the solution
to global warming yet."
In the new draft report, the scientists conclude that it is "likely"
that human actions "have contributed substantially"
to the observed warming. The major human contribution is the release
of so-called greenhouse gases into the atmosphere through the
burning of coal, oil, natural gas and other fossil fuels.
That "is
a stronger conclusion" than was offered by earlier assessments,
said Kevin Trenberth, head of the climate analysis section at
the National Center for Atmospheric Research in Boulder, Colo.
"This is not the work of one individual scientist. This is
a consensus reached across scientists in the international community.
It has gone through extensive reviews."
The report
is not likely to quiet all debate on the issue, however. Some
longtime critics of projections about global warming said that
they remain skeptical.
Paradoxically,
the report also suggests that some pollution control efforts may
unintentionally be making the planet a hotter place. Greenhouse
gases can contribute to global warming by trapping solar heat
and preventing it from being reflected back into space.
Mounting evidence
suggests that the Earth's atmosphere has been steadily warming
for nearly 150 years as a result of carbon gas produced by burning
oil, gas and coal, with the warmest years on record occurring
in the last decade.
Overall, the
panel's draft predicts that temperatures worldwide may increase
2.1 to 11 degrees Fahrenheit (1 to 5.8 degrees Celsius). Earlier
assessments projected an increase of 2.1 to 7.2 degrees Fahrenheit
(1 to 3.5 degrees Celsius). An 11-degree shift in average temperatures
would be a major change in climate. This week, for example, 11
degrees was the difference in average temperatures between Los
Angeles and Seattle.
The greater
increase is projected in large measure because of efforts to control
pollution from industrial facilities and power plants. Pollution-control
measures have greatly reduced the amount of sulfate particles
that cause acid rain and a variety of health problems. But those
particles also have a cooling effect in the atmosphere because
they deflect the sun's heat. As sulfate levels drop, the temperature
will effectively rebound.
"These
sulfate particulates have had a masking effect," said one
atmosphere expert who has seen the report but who asked not to
be identified. "We are cleaning up this air pollution, and
that is making global warming worse." The new report, which
will not be made final until it has been approved early next year,
is the first formal update in five years of an assessment prepared
by the climate change panel. The panel is a technical group sponsored
by the U.N. and the World Meteorological Organization and comprising
hundreds of scientists who assess scientific, social and economic
aspects of global climate change. The panel does no original research
of its own but attempts to arrive at a measured technical assessment
of often-conflicting studies on climate change.
The contents
of the report were first disclosed Wednesday by the Associated
Press. Several scientists familiar with it agreed to discuss the
findings with The Times, on condition they not be identified.
With international negotiations now underway to limit the amount
of such greenhouse gases that nations may release, almost any
attempt to reach a scientific consensus on climate change is controversial.
So far, no major industrial nation has ratified an agreement negotiated
in Kyoto, Japan, in 1997 to stave off global warming by reducing
greenhouse emissions.
The U.N. group's
first report on climate change, released in 1995, generated considerable
criticism over allegations that political bias had colored its
assessments.
Scientists
who worked on the report were at pains to rebut such charges in
advance this time. "This is a cold-eyed, objective rendition
of the science," said one such climate scientist. "We
could not do it any better."
To ensure
that the new draft is based fairly on the scientific data, "there
have been skeptics involved, as authors and reviewers," Trenberth
said.
The report
will be the subject of an international meeting next year in China,
where the dozens of participating governments will all have the
opportunity to review and perhaps temper its conclusions.
"Until that point, the whole report is not considered final,"
Trenberth said. The final report, he said, "involves a negotiation
between the scientists who determine what can be said and the
governments who determine how it can be said."
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