Artic thaw & global warming

Publié le par Scorpio

Many scientists see no escape for dramatic Arctic thaw



In 1969, Roy Koerner, a Canadian government glaciologist, was one of four men (and 36 dogs) who completed the first surface crossing of the Arctic Ocean, from Alaska through the North Pole to Norway.

Today, he said, such a trek would be impossible: There is just not enough ice. In September, the area covered by sea ice reached a record low. "I recently reviewed a proposal by one guy to go across by kayak," Koerner said.

Many scientists say it has taken a long time for them to accept that global warming, partly the result of carbon dioxide and other heat-trapping gases in the atmosphere, could shrink the Arctic 's summer cloak of ice.

But many of those same scientists have concluded that the momentum behind human-caused warming, combined with the region's tendency to amplify change, has put the familiar Arctic past
the point of no return.

The particularly sharp warming and melting in the last few decades is thought by many experts
to result from a mix of human and natural causes. But a number of recent computer simulations of global climate run by half a dozen research centers around the world show that in the future human influence will dominate.

Even with just modest growth in emissions of the greenhouse gases, almost all of the summer sea ice is likely to disappear by late in the century. Some of the simulations, including an advanced model at the National Center for Atmospheric Research in Boulder, Colo. , show much of the summer ice disappearing by 2050, said Marika Holland, a scientist there.

Of the various simulations, all done for an international scientific report on climate trends to be issued in 2007, the only ones that retain much summer sea ice in the Arctic by 2100 are those that assume global greenhouse-gas emissions are held constant at rates measured in 2000 — something that only five years later is already impossible.

The other models all produce an Arctic Ocean in summer akin to the "open polar sea" that was sought by oceanographers and explorers in the mid-1800s.

The models are, of course, impressionistic views of a far more complicated Arctic reality, so their projections are uncertain. But what worries field scientists, who form their opinions based on empirical clues embedded in ice or recorded by thermometers, is that observations of change and evidence pointing to past patterns are agreeing with the models.

"Even if you would stop every engine right now, there is no escape unless you physically take the CO2 out of the air again," said Henk Brinkhuis, an expert on past Arctic ecosystems at Utrecht University in the Netherlands. "You may argue for a long time whether this process will take 20, 50 or 100 years, but it doesn't change the fact that it will happen."

Field work suggests that past Arctic warm spells, like a stretch through the 1920s and 1930s, were limited to certain regions, while the recent warming has largely progressed in concert with rising temperatures around the Northern Hemisphere — a sign of large forces at work, climate scientists say, not regional variability.

The inevitability of summer ice retreats, Arctic experts say, is a result of the nature of the climate system, which is something like a heavy flywheel. Once started, flywheels tend to keep going. Within a few decades, say many scientists focused on the region, the insulating power of greenhouse gases will dominate natural climate fluctuations, possibly for centuries.

The flywheel in the Arctic moves faster than in other areas because the region amplifies change.
The most obvious mechanism is the difference in how bright white sea ice and the dark sea act under sunlight. Ice reflects most of the solar energy striking it back into space. Water absorbs most of it.

A result is that each area of ocean exposed by melting ice soaks up heat, melting more ice, exposing more sea, soaking up even more heat — and so on, until the annual marathons held each spring on the floating ice near the North Pole are replaced by boat races. "You might call it the temperatization of the Arctic; we haven't really invented a word for it yet," said Charles Voeroesmarty, director of the Complex Systems Research Center at the University of New Hampshire and one of 21 co-authors of a recent article in Eos, a journal of the American Geophysical Union, about the changes.

Titled simply "Arctic System on Trajectory to New, Seasonally Ice-Free State," it says, "There seem to be few, if any, processes or feedbacks within the Arctic system that are capable of altering the trajectory."

Climatologists say the effects eventually could extend far beyond the sparsely populated north, contributing to climate and ocean shifts that could dry the American West and possibly slow north-flowing warm currents in the Atlantic Ocean that keep northern Europe milder than it would otherwise be.

The most that can be expected, some climate scientists say, is to limit the human contribution to warming enough to forestall the one truly calamitous, if slow motion, threat in the far north: the melting of Greenland's ice cap.


Rising two miles high and spreading over an area twice the size of California, this vast reservoir — essentially the Gulf of Mexico frozen and flipped onto land — contains enough water to raise sea levels worldwide more than 20 feet.

In recent years, the ice sheets of Greenland have been building in the middle through added snowfall but melting even more around the edges in summer. Many Greenland experts say the melting is already winning out.

James E. Hansen, a scientist for the National Aeronautics and Space Administration who has been designing simulations of earth's climate for nearly four decades, is among those who say prompt cuts in emissions can avert a Greenland meltdown.

"It is physically and technologically possible, but there has to be a will to achieve it," Hansen said.

Other scientists are not as optimistic.

Fresh studies of ancient glacial ice and sea-floor sediments show that, if anything, the computer simulations projecting strong warming and ice retreats in the region over the long run may be substantial underestimates, Brinkhuis said.

"Everything we are seeing shows things can move more and faster than we think," he added, referring to geologic and glacial records of past Arctic changes.

The current increase in greenhouse gases, he continued, is similar to past natural changes that profoundly altered the world.

"We have not seen such fast carbon dioxide rises as we have now other than in extreme cases in the past," Brinkhuis said, including periods like one about 50 million years ago that turned the Arctic Ocean into a warm, weed-covered lake.

Brinkhuis and many other veteran Arctic researchers caution that there is something of
a paradox in Arctic trends: While the long-term fate of the region may be mostly sealed, no one should presume that the recent sharp warming and seasonal ice retreats that have caught the world's attention will continue smoothly into the future.

In the short run, the natural fluctuations will most likely sustain those on both sides of the debate over how to respond to global warming, with cool years embraced by skeptics and hotter ones by proponents of cutting the heat-trapping gases, said Richard B. Alley, a geoscientist at Pennsylvania State University .

But he and other scientists say it is clear
that in the long run, the Arctic will get warmer, a conclusion at the heart of the Arctic Climate Impact Assessment, a report commissioned by the eight Arctic nations and released last year.

Holland, the Colorado modeler, has never seen the real Arctic . She said a colleague who spends most summers slogging through ponds of meltwater on Arctic Ocean floes recently proposed creating a program called "Take a Modeler to the Arctic ." The proposal was only half in jest, she said.

"I'd like to see what it's like before it actually disappears," Holland said.


Publié dans climate change

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