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Debate on Climate Shifts to Issue of Irreparable Change
"There's no agreement on what it is that constitutes a dangerous climate change," said Marburger, adding that the U.S. government spends $2 billion a year on researching this and other climate change questions. "We know things like this are possible, but we don't have enough information to quantify the level of risk."
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."
John R. Christy, director of the Earth Science System Center at the University of Alabama in Huntsville, said it is possible increased warming will be offset by other factors, such as increased cloudiness that would reflect more sunlight. "Whatever happens, we will adapt to it," Christy said.
Scientists who read the history of Earth's climate in ancient sediments, ice cores and fossils find clear signs that it has shifted abruptly in the past on a scale that could prove disastrous for modern society. Peter B. deMenocal, an associate professor at the Lamont-Doherty Earth Observatory of Columbia University, said that about 8,200 years ago, a very sudden cooling shut down the Atlantic conveyor belt. As a result, the land temperature in Greenland dropped more 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 major cuts in the carbon dioxide emissions linked to global warming. Britain has slashed its emissions by 14 percent, compared with 1990 levels, and aims to reduce them by 60 percent by 2050. Some European countries, however, are lagging well behind their targets under the international Kyoto climate treaty.
David Warrilow, who heads science policy on climate change for Britain's Department of Environment, Food and Rural Affairs, said that while the science remains unsettled, his government has decided to take a precautionary approach. He compared consuming massive amounts of fossil fuels to the strategy of the Titanic's crew, who were unable to avoid an iceberg because they were speeding across the Atlantic in hopes of breaking a record.
"We know there are icebergs out there, but at the moment we're accelerating toward the tipping point," Warrilow said in an interview. "This is silly. We should be doing the opposite, slowing down whilst we build up our knowledge base."
The Bush administration espouses a different approach. Marburger said that though everyone agrees carbon dioxide emissions should decline, the United States prefers to promote cleaner technology rather than impose mandatory greenhouse gas limits. "The U.S. is the world leader in doing something on climate change because of its actions on changing technology," he said.
Stanford University climatologist Stephen H. Schneider, who is helping oversee a major international assessment of how climate change could expose humans and the environment to new vulnerabilities, said countries respond differently to the global warming issue in part because 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 6.5 feet above the South Pacific, and it is only a matter of time before the 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 economic clout in the world."