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发帖数: 29846 | 1 December 8, 2014 - 4:06 PM
By SETH BORENSTEIN, Associated Press
WASHINGTON (AP) — Don't blame man-made global warming for the devastating
California drought, a new federal report says.
A report issued Monday by the National Oceanic and Atmospheric
Administration said natural variations — mostly a La Nina weather
oscillation — were the primary drivers behind the drought that has now
stretched to three years.
Study lead author Richard Seager of Columbia University said the paper has
not yet been published in a peer-reviewed scientific journal. He and NOAA's
Martin Hoerling said 160 runs of computer models show heat-trapping gases
should slightly increase winter rain in parts of California, not decrease.
"The conditions of the last three winters are not the conditions that
climate change models say would happen," Hoerling said. But he said the La
Nina, which is the cooler flip side of the warming of central Pacific ocean,
can only be blamed for about one-third of the drought. The rest of the
causes can be from just random variation, he said.
Some outside climate scientists criticized the report, saying it didn't take
into effect how record warmth worsened the drought. California is having
its hottest year on record, based on the first 11 months of the year and is
4.1 degrees warmer than 20th-century average, according to the National
Climatic Data Center.
"This study completely fails to consider what climate change is doing to
water in California," wrote Kevin Trenberth, head of climate analysis at the
National Center for Atmospheric Research. He said the work "completely
misses" how hotter air increases drying by evaporating more it from the
ground.
In droughts, extra heat from global warming enhances the drying in a
feedback effect, Trenberth and others said. But Hoerling said that is less
of a factor in California because it is so near the ocean and its rain comes
in storms coming off the Pacific.
Peer-reviewed studies are divided on whether the drought can be blamed on
climate change. Others published earlier this year point more directly to
changes in pressure of the Pacific that blocked rain from coming into
California, but Hoerling and Seager dismissed them as not adequate.
Hoerling, who specializes in the complicated field of studying the cause of
climate extremes, in the past has downplayed other scientists' claims that
regional droughts are caused by man-made warming. However, Hoerling
acknowledges that climate change is happening, will worsen weather in the
future and has produced past studies attributing strange weather — such as
more frequent Mediterranean droughts — to heat-trapping gases from the
burning of fossil fuels.
Scientists can't even agree on how bad the drought is. Hoerling said the
drought isn't even in the top five worst for California. But a new peer-
reviewed study in the journal Geophysical Research Letters by researchers at
the University of Minnesota and Woods Hole Oceanographic calls this "the
most severe drought in the last 1,200 years."
Deke Arndt, climate monitoring chief for NOAA's National Climatic Data
Center, said by some drought measures, the current California drought "is
slightly more intense than, but still comparable to, the late 1970s episode.
I'd put them at 1a and 1b on the list of historical multi-year drought
episodes affecting California in modern times." |
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