Some Experts on Global Warming Foresee ‘Tipping Point’ When It Is Too Late to Act

There are three specific events that these scientists describe as especially worrisome and potentially imminent, although the time frames are a matter of dispute: widespread coral bleaching that could damage the world’s fisheries within three decades; dramatic sea level rise by the end of the century that would take tens of thousands of years to reverse; and, within 200 years, a shutdown of the ocean current that moderates temperatures in northern Europe.

The debate has been intensifying because Earth is warming much faster than some researchers had predicted. James E. Hansen, who directs NASA’s Goddard Institute of Space Studies, last week confirmed that 2005 was the warmest year on record, surpassing 1998. Earth’s average temperature has risen nearly 1 degree Fahrenheit over the past 30 years, he noted, and another increase of about 4 degrees over the next century would “imply changes that constitute practically a different planet.”

“It’s not something you can adapt to,” Hansen said in an interview. “We can’t let it go on another 10 years like this. We’ve got to do something.”
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While both the Greenland and the Antarctic ice sheets as a whole are gaining some mass in their cold interiors because of increasing snowfall, they are losing ice along their peripheries. That indicates that scientists may have underestimated the rate of disintegration they face in the future, Oppenheimer said. Greenland’s current net ice loss is 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’s really no putting it back for thousands of years, if ever.”
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This tipping point debate has stirred controversy within the administration; Hansen said senior political appointees are trying to block him from sharing his views publicly.

When Hansen posted data on the Internet in the fall suggesting that 2005 could be the warmest year on record, NASA officials ordered Hansen to withdraw the information because he had not had it screened by the administration in advance, according to a Goddard scientist who spoke on the condition of anonymity. More recently, NASA officials tried to discourage a reporter from interviewing Hansen for this article and later insisted he could speak on the record only if an agency spokeswoman listened in on the conversation.

“They’re trying to control what’s getting out to the public,” Hansen said, adding that many of his colleagues are afraid to talk about the issue. “They’re not willing to say much, because they’ve been pressured and they’re afraid they’ll get into trouble.”

But Mary L. Cleave, deputy associate administrator for NASA’s Office of Earth Science, said the agency insists on monitoring interviews with scientists to ensure they are not misquoted.

“People could see it as a constraint,” Cleave said. “As a manager, I might see it as protection.”

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