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“If a very small warming makes such a difference,”
Alley said, “it raises the question of what happens
when more warming occurs.”
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SCIENCE
VOL 319 11 JANUARY 2008
CLIMATE TIPPING POINTS COME IN FROM THE COLD
Tipping points, once considered too alarmist for proper scientific
circles, have entered the climate change mainstream.
At the meeting, a nearly packed half-day session considered the
prospects for a climate system that is still creeping through change
but might soon cross a threshold into an entirely new way of
operating. The new climate regime may have no sea ice in Arctic
summers, a much smaller ice sheet on West Antarctica and higher sea
levels, or wildly redirected storm tracks. Current understanding of
climate allows that such drastic transitions can happen, the speakers
agreed. Earth may even be in the midst of one now.
As evidence, glaciologist Richard Alley of Pennsylvania State
University in State College cited Earth’s response to the warming
that has occurred over the past 3 decades. Compared with the soaring
global temperatures that the strengthening greenhouse could drive
through the rest of this century, that warming has been rather small,
Alley said. Yet it is having immediate and often unexpected effects,
he observed. Since the 1960s, mountain glaciers around the world have
begun to shrink and are dwindling rapidly. Arctic summer sea ice took
a severe hit last year after decades of slow losses. And the
Greenland ice sheet is now clearly shrinking under some unexpected
attacks. Warming seas are weakening glaciers’ surprisingly fragile
ice tongues, which help slow glaciers’ rush to the sea. And meltwater
on the surfaces of glaciers is plunging down giant cracks to glacier
beds, where it’s lubricating the glaciers’ seaward slip-sliding. ”
If a very small warming makes such a difference,” Alley said, “it
raises the question of what happens when more warming occurs.” Sea
ice specialist Josefino Comiso of NASA’s Goddard Space Flight Center
in Greenbelt, Maryland, said at the meeting that in the Arctic, “the
tipping point for perennial sea ice has likely already been reached.”
Ice persisting from year to year has not been steadily shrinking in
area, he said. The decline of summer sea ice accelerated in the
mid-1990s; since then, summer ice has been disappearing more than
three times faster than before. And the feedback between solar
warming of newly ice-free Arctic waters and the loss of still more
ice has become more and more obvious.
Only colder summers and colder winters can save summer sea ice from
oblivion, he said, an unlikely development at this point. Other
speakers presented possible tipping points that have gotten less
attention: an eventual sudden shift in jet streams that would bring
rapid climate change to North America and Europe and the abrupt
collapse of the Amazon tropical forest, among others. Clearly, the
possibilities are proliferating faster than researchers can confirm
or deny them.   -R.A.K.
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The meeting discussed above was the December, 2007 annual Fall
Meeting of the American Geophysical Union, an association of
scientific societies including specialists in the atmospheric,
biological, chemical, geologic and other sciences. L.O.
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