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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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