A team of scientists from Columbia University has shown that
warm winters in the northern hemisphere likely can be
explained by the action of upper-atmosphere winds that are
closely linked to global warming.
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Global mean surface temperatures have increased in the range
of 0.6 to 1.2°F since the late 19th century. But far
more severe warming has taken place over wide regions of
northern Eurasia, Canada and Alaska, with temperatures
averaging 7 to 10°F warmer in the last 35 years,
according to data previously compiled by the NASA Goddard
Institute for Space Studies in New York City.
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The research, which appears in the June 3 issue of the
British journal Nature, offers no predictions on what
temperatures future winters will bring, but suggests a
continuation of the current trend for three to four more
decades. If warming trends continue, said Drew Shindell,
associate research scientist at Columbia's Center for
Climate Systems Research and lead author of the report,
northern regions of Europe and Asia and, to a lesser extent,
North America, can expect winters that are both warmer and
wetter, with increased rain and snow.
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"Based on this research, it's quite likely that the warmer
winters over the continents are indeed a result of the
increasing amount of greenhouse gases in the atmosphere,"
Dr. Shindell said. "This research offers both a plausible
physical mechanism for how this takes place, and reproduces
the observed trends both qualitatively and even
quantitatively."
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Other authors of the Nature paper were Gavin A. Schmidt,
associate research scientist at Columbia's Center for
Climate Systems Research; Ron L. Miller, associate research
scientist in the Department of Applied Physics and Applied
Mathematics at Columbia, and Lionel Pandolfo, assistant
professor in the Department of Earth and Ocean Sciences at
the University of British Columbia. Drs. Shindell, Schmidt
and Miller also maintain an affiliation with the NASA
Goddard Institute.
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The physical mechanism the authors suggest is a
redistribution of heat closely related to recent changes in
atmospheric wind patterns, an indirect consequence of
greenhouse warming. Greenhouse gases trap heat at the
Earth's surface, while cooling the stratosphere, a region of
the atmosphere that extends from about seven to about 30
miles above the planet's surface. This cooling has increased
the speed of the stratospheric jet stream and has
strengthened a lower atmosphere vortex of west-to-east,
counterclockwise winds that naturally forms over the polar
region each winter.
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During the winter, the ocean retains heat better than the
land. So when the dominant west-to-east winds increase, they
carry warmer air from the oceans to the continents, and
colder continental air to the oceans. In North America, the
Rockies intercept the warmer winds, so the effect is
stronger west of the mountains and is mitigated in central
and eastern portions.
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The Columbia team used several versions of the NASA Goddard
Institute's general circulation model, a computer construct
that predicts the Earth's climate when certain inputs are
varied. Model simulations suggest that much of the increase
in surface winds and in continental surface temperatures
during the winter months is induced by the buildup of
greenhouse gases in the atmosphere. In the model, increasing
greenhouse gas emissions lead to a warmer surface and, at
the same time, a colder stratosphere. The large wintertime
continental temperature increases produced in the model
correspond quite well with what scientists actually observe.
But when the researchers used a version of the climate model
that did not adequately represent the stratosphere, the
results did not jibe as well with reality.
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Colder polar temperatures in winter, and warmer temperatures
in the middle latitudes, are actually part of a natural
cycle of climate variability, which made the warming trend
more difficult for the scientists to isolate. The
temperature differences are reflected in sea-level pressure,
which decreases in the Arctic region and increases at the
middle latitudes; this cycle is called the Arctic
Oscillation and is second only to El Niño in its
effects on global weather. In the NASA Goddard Institute
simulations, increasing greenhouse gases caused a preference
for one phase of this cycle over another, with stronger
west-to-east surface winds at the Northern Hemisphere middle
latitudes, leading to the increased surface temperatures
over land.
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"Despite appearing as part of a natural climate oscillation,
the large increases in wintertime surface temperatures over
the continents may therefore be attributable in large part
to human activities," Dr. Shindell said. "The impact of
greenhouse gases on climate through surface wind changes may
be as large as, or in some areas larger than, the more
direct impact of global warming."
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The research was supported by the National Oceanic and
Atmospheric Administration and by the National Aeronautics
and Space Administration.
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