Global warming debate heats up

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

This “tipping point” scenario has begun to consume many prominent researchers in the United States and abroad, because the answer could determine how drastically countries need to reduce their greenhouse gas emissions in the coming years. While scientists remain uncertain when such a point might occur, many say it is urgent that policymakers cut global carbon dioxide emissions in half over the next 50 years or risk the triggering of changes that would be irreversible.

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 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.”

Princeton University geosciences and international affairs professor Michael Oppenheimer, who also advises the advocacy group Environmental Defense, said one of the greatest dangers lies in the disintegration of the Greenland or West Antarctic ice sheets, which together hold about 20 percent of the fresh water on the planet. If either of the two sheets disintegrates, sea level could rise nearly 20 feet in the course of a couple of centuries, swamping the southern third of Florida and Manhattan up to the middle of Greenwich Village.

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.”

Last year the British government sponsored a scientific symposium on “Avoiding Dangerous Climate Change,” which examined a number of possible tipping points. A book based on that conference, due to be published Tuesday, suggests that disintegration of the two ice sheets becomes more likely if average temperatures rise by more than 5 degrees Fahrenheit, a prospect “well within the range of climate change projections for this century.”

The report concludes that a temperature rise of just 1.8 degrees Fahrenheit “is likely to lead to extensive coral bleaching,” destroying critical fish nurseries in the Caribbean and Southeast Asia. Too-warm sea temperatures stress corals, causing them to expel symbiotic micro-algae that live in their tissues and provide them with food, and thus making the reefs appear bleached. Bleaching that lasts longer than a week can kill corals. This fall there was widespread bleaching from Texas to Trinidad that killed broad swaths of corals, in part because ocean temperatures were 2 degrees Fahrenheit above average monthly highs.

Many scientists are also worried about a possible collapse of the Atlantic thermohaline circulation, a current that brings warm surface water to northern Europe and returns cold, deep-ocean water south. Hans Joachim Schellnhuber, who directs Germany’s Potsdam Institute for Climate Impact Research, has run multiple computer models to determine when 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 the current will collapse within 200 years.

Some scientists, including President Bush’s chief science adviser, John Marburger III, emphasize that much uncertainty remains about when abrupt global warming might occur.

“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; NASA’s Hansen said senior political appointees are trying to block him from sharing his views publicly.

When Hansen posted data on the Internet last fall suggesting that 2005 could be the warmest on record, according to a Goddard scientist who did not want to be identified, NASA officials ordered Hansen to withdraw the information because he had not screened it with the administration in advance. 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.”