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Debate on Climate Shifts to Issue of Irreparable Change

by: Juliet Eilperin    29 January 2006

Some Experts on Global Warming Foresee 'Tipping Point' When It Is TooLate to Act

Now that most scientists agree human activity is causing Earth to warm,the central debate has shifted to whether climate change is progressingso rapidly that, within decades, humans may be helpless to slow orreverse the trend.

This "tipping point" scenario has begun to consume many prominentresearchers in the United States and abroad, because the answer coulddetermine how drastically countries need to reduce their greenhouse gasemissions in the coming years. While scientists remain uncertain whensuch a point might occur, many say it is urgent that policymakers cutglobal carbon dioxide emissions in half over the next 50 years or riskthe triggering of changes that would be irreversible.

There are three specific events that these scientists describe asespecially worrisome and potentially imminent, although the time framesare a matter of dispute: widespread coral bleaching that could damagethe world's fisheries within three decades; dramatic sea level rise bythe end of the century that would take tens of thousands of years toreverse; and, within 200 years, a shutdown of the ocean current thatmoderates temperatures in northern Europe.

The debate has been intensifying because Earth is warming much fasterthan some researchers had predicted. James E. Hansen, who directsNASA's Goddard Institute of Space Studies, last week confirmed that2005 was the warmest year on record, surpassing 1998. Earth's averagetemperature has risen nearly 1 degree Fahrenheit over the past 30years, he noted, and another increase of about 4 degrees over the nextcentury would "imply changes that constitute practically a differentplanet."

"It's not something you can adapt to," Hansen said in an interview. "Wecan't let it go on another 10 years like this. We've got to dosomething."

Princeton University geosciences and international affairs professorMichael Oppenheimer, who also advises the advocacy group EnvironmentalDefense, said one of the greatest dangers lies in the disintegration ofthe Greenland or West Antarctic ice sheets, which together hold about20 percent of the fresh water on the planet. If either of the twosheets disintegrates, sea level could rise nearly 20 feet in the courseof a couple of centuries, swamping the southern third of Florida andManhattan up to the middle of Greenwich Village.

While both the Greenland and the Antarctic ice sheets as a whole aregaining some mass in their cold interiors because of increasingsnowfall, they are losing ice along their peripheries. That indicatesthat scientists may have underestimated the rate of disintegration theyface in the future, Oppenheimer said. Greenland's current net ice lossis equivalent to an annual 0.008 inch sea level rise.

The effects of the collapse of either ice sheet would be "huge,"Oppenheimer said. "Once you lost one of these ice sheets, there'sreally no putting it back for thousands of years, if ever."

Last year, the British government sponsored a scientific symposium on"Avoiding Dangerous Climate Change," which examined a number ofpossible tipping points. A book based on that conference, due to bepublished Tuesday, suggests that disintegration of the two ice sheetsbecomes more likely if average temperatures rise by more than 5 degreesFahrenheit, a prospect "well within the range of climate changeprojections for this century."

The report concludes that a temperature rise of just 1.8 degreesFahrenheit "is likely to lead to extensive coral bleaching," destroyingcritical fish nurseries in the Caribbean and Southeast Asia. Too-warmsea temperatures stress corals, causing them to expel symbioticmicro-algae that live in their tissues and provide them with food, andthus making the reefs appear bleached. Bleaching that lasts longer thana week can kill corals. This fall there was widespread bleaching fromTexas to Trinidad that killed broad swaths of corals, in part becauseocean temperatures were 2 degrees Fahrenheit above average monthlymaximums.

Many scientists are also worried about a possible collapse of theAtlantic thermohaline circulation, a current that brings warm surfacewater to northern Europe and returns cold, deep-ocean water south. HansJoachim Schellnhuber, who directs Germany's Potsdam Institute forClimate Impact Research, has run multiple computer models to determinewhen climate change could disrupt this "conveyor belt," which,according to one study, is already slower than it was 30 years ago.According to these simulations, there is a 50 percent chance thecurrent will collapse within 200 years.

Some scientists, including President Bush's chief science adviser, JohnH. Marburger III, emphasize there is still much uncertainty about whenabrupt global warming might occur.

"There's no agreement on what it is that constitutes a dangerousclimate change," said Marburger, adding that the U.S. government spends$2 billion a year on researching this and other climate changequestions. "We know things like this are possible, but we don't haveenough information to quantify the level of risk."

This tipping point debate has stirred controversy within theadministration; Hansen said senior political appointees are trying toblock him from sharing his views publicly.

When Hansen posted data on the Internet in the fall suggesting that2005 could be the warmest year on record, NASA officials ordered Hansento withdraw the information because he had not had it screened by theadministration in advance, according to a Goddard scientist who spokeon the condition of anonymity. More recently, NASA officials tried todiscourage a reporter from interviewing Hansen for this article andlater insisted he could speak on the record only if an agencyspokeswoman listened in on the conversation.

"They're trying to control what's getting out to the public," Hansensaid, adding that many of his colleagues are afraid to talk about theissue. "They're not willing to say much, because they've been pressuredand they're afraid they'll get into trouble."

But Mary L. Cleave, deputy associate administrator for NASA's Office ofEarth Science, said the agency insists on monitoring interviews withscientists to ensure they are not misquoted.

"People could see it as a constraint," Cleave said. "As a manager, Imight see it as protection."

John R. Christy, director of the Earth Science System Center at theUniversity of Alabama in Huntsville, said it is possible increasedwarming will be offset by other factors, such as increased cloudinessthat would reflect more sunlight. "Whatever happens, we will adapt toit," Christy said.

Scientists who read the history of Earth's climate in ancientsediments, ice cores and fossils find clear signs that it has shiftedabruptly in the past on a scale that could prove disastrous for modernsociety. Peter B. deMenocal, an associate professor at theLamont-Doherty Earth Observatory of Columbia University, said thatabout 8,200 years ago, a very sudden cooling shut down the Atlanticconveyor belt. As a result, the land temperature in Greenland droppedmore than 9 degrees Fahrenheit within a decade or two.

"It's not this abstract notion that happens over millions of years,"deMenocal said. "The magnitude of what we're talking about greatly,greatly exceeds anything we've withstood in human history."

These kinds of concerns have spurred some governments to make majorcuts in the carbon dioxide emissions linked to global warming. Britainhas slashed its emissions by 14 percent, compared with 1990 levels, andaims to reduce them by 60 percent by 2050. Some European countries,however, are lagging well behind their targets under the internationalKyoto climate treaty.

David Warrilow, who heads science policy on climate change forBritain's Department of Environment, Food and Rural Affairs, said thatwhile the science remains unsettled, his government has decided to takea precautionary approach. He compared consuming massive amounts offossil fuels to the strategy of the Titanic's crew, who were unable toavoid an iceberg because they were speeding across the Atlantic inhopes of breaking a record.

"We know there are icebergs out there, but at the moment we'reaccelerating toward the tipping point," Warrilow said in an interview."This is silly. We should be doing the opposite, slowing down whilst webuild up our knowledge base."

The Bush administration espouses a different approach. Marburger saidthat though everyone agrees carbon dioxide emissions should decline,the United States prefers to promote cleaner technology rather thanimpose mandatory greenhouse gas limits. "The U.S. is the world leaderin doing something on climate change because of its actions on changingtechnology," he said.

Stanford University climatologist Stephen H. Schneider, who is helpingoversee a major international assessment of how climate change couldexpose humans and the environment to new vulnerabilities, saidcountries respond differently to the global warming issue in partbecause they are affected differently by it. The small island nation of Kiribati is made up of 33 small atolls, none of which is more than 6.5feet above the South Pacific, and it is only a matter of time beforethe entire country is submerged by the rising sea.

"For Kiribati, the tipping point has already occurred," Schneider said."As far as they're concerned, it's tipped, but they have no economicclout in the world."