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| Extreme heat waves such in Texas are due to global warming |
By Justin Gillis, The New York Times, July 10, 2012
Some of the weather
extremes bedeviling people around the world have become far more likely because
of human-induced global warming, researchers reported on
Tuesday. Yet they ruled it out as a cause of last year’s devastating floods in
Thailand, one of the most striking weather events of recent years.
A new study found that global warming made the
severe heat wave that afflicted Texas last year 20 times as likely as it would
have been in the 1960s. The extremely warm temperatures in Britain last
November were 62 times as likely because of global warming, it said.
The findings, especially the specific numbers attached to some extreme
events, represent an increased effort by scientists to respond to a public
clamor for information about what is happening to the earth’s climate. Studies
seeking to discern any human influence on weather extremes have usually taken
years, but in this case, researchers around the world managed to study six
events from 2011 and publish the results in six months.
Some of the researchers acknowledged that given the haste of the work,
the conclusions must be regarded as tentative.
“This is hot new science,” said Philip W. Mote, director of the Climate Change
Research Institute at Oregon State University, who led the research on the
Texas heat wave and drought. “It’s controversial. People are trying different
methods of figuring out how much the odds may have shifted because of what we
have put into the atmosphere.”
The general conclusion of the new research is that many of the
extremes being witnessed worldwide are consistent with what scientists expect
on a warming planet. Heat waves, in particular, are probably being worsened by
global warming, the scientists said. They also cited an intensification of the
water cycle, reflected in an increase in both droughts and heavy downpours.
The study on extreme weather was released along with a broader report
on the state of the world’s climate. Both are to be published soon in the Bulletin
of the American Meteorological Society. The broad report found no surcease of the climate trends
that have led to widespread concern about the future.
The Arctic continued to warm more rapidly than the planet as a whole
in 2011, scientists reported, and sea ice in the Arctic was at its
second-lowest level in the historical record. In 2010, rains were so heavy that
the sea level actually dropped as storms moved billions of gallons of water
onto land, they said, but by late 2011 the water had returned to the sea, which
resumed a relentless long-term rise.
So far this year in the United States, fewer weather disasters seem to
be unfolding than in 2011. But it is still turning out to be a remarkable year,
with wildfires, floods, storms that knocked out electrical power for millions
and sizzling heat waves in March and June.
Globally, the new research makes clear that some of the recent weather
damage resulted not from an increased likelihood of extremes, but from changes
in human exposure and vulnerability. The 2011 floods in Thailand are a prime
example.
An analysis by Dutch and British scientists found that the amount of
rain falling in Thailand last year, while heavy, was not particularly unusual
by historical standards, and that “climate change cannot be shown to have
played any role in this event.”
More important, the researchers said, was rapid development in parts
of Thailand. Farm fields have given way to factories in the floodplains of
major rivers, helping to set the stage for the disaster.
In the new report, researchers in Oregon and Britain found that
natural climate variability played a big role in setting the stage for the heat
wave in Texas. The weather in 2011 was heavily influenced by a weather pattern
called La Niña, which has effects worldwide, including making
drought in the American Southwest more likely.
But even taking that into account, the researchers found, the overall
warming of the planet since the 1960s made it about 20 times as likely that
such a heat wave would occur in Texas in a La Niña year.
Martin P. Hoerling, a meteorologist with the
National Oceanic and Atmospheric Administration who was not involved in the new
study but is conducting his own research on the Texas disaster, agreed that
human-induced global warming had probably made the odds of record-setting heat
somewhat more likely. But he said his research showed that the rainfall deficits
were unrelated to global warming.
He said he was skeptical about several aspects of the new paper,
including the claim of a 20-fold increase in likelihood.
More broadly, he said he was worried that the newly published studies
had been done so hastily that the conclusions may not stand the test of time.
“We need to think carefully about what kind of questions we can credibly pursue
with this sort of rapid turnaround,” Dr. Hoerling said.
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