Some scientists worry about making too many doomsday predictions.


By ANDREW C. REVKIN


The language was apocalyptic. A leading climate scientist warned that Earth’s rising temperatures were poised to set off irreversible disasters if steps were not taken quickly to stop global warming.

“The climate is nearing tipping points,”the NASA climate scientist James E. Hansen wrote in February in The Observer newspaper of London.“If we do not change course, we’ll hand our children a situation that is out of their control.”

The resulting calamities, Dr. Hansen and other like-minded scientists have warned, could be widespread and overwhelming: the loss of untold species as ocean reefs and forests are disrupted; the transformation of the Amazon into parched savanna; a dangerous rise in sea levels resulting from the melting of the kilometer-high ice sheets in West Antarctica and Greenland; and the thawing of the Arctic tundra, which would release torrents of the greenhouse gas methane into the atmosphere.

But the idea that the planet is nearing tipping points - thresholds at which change suddenly becomes unstoppable - has driven a wedge between scientists who otherwise share deep concerns about the implications of a humanwarmed climate.

Environmentalists and some climate experts are increasingly warning of impending tipping points in their efforts to stir public concern. The term confers a sense of immediacy and menace to potential threats from a warming climate - dangers that otherwise might seem too distant for people to worry about.

But other scientists say there is little hard evidence to back up specific predictions of catastrophe. They worry that the use of the term“tipping point”can be misleading and could backfire, fueling criticism of alarmism and threatening public support for reducing greenhouse gas emissions.

“I think a lot of this threshold and tipping point talk is dangerous,”said Kenneth Caldeira, an earth scientist at Stanford University in California and the Carnegie Institution, an organization for scientific discovery based in Washington, D.C., and an advocate of swift action to reduce carbon dioxide emissions.“If we say we passed thresholds and tipping points today, this will be an excuse for inaction tomorrow.”

While studies of climate patterns in the distant past show the potential for drastic shifts, these scientists say, there is enormous uncertainty in making specific predictions about the future.

In some cases, there are big questions about whether climate-driven disasters are even plausible. And even in cases where most scientists agree that rising temperatures could lead to unstoppable change, no one knows where the thresholds lie that would set off such shifts.

Dr. Hansen defends the use of the term tipping point and said that it accurately depicts some probable consequences of unchecked global warming. There is abundant evidence, he says, that rising temperatures can have an abrupt, calamitous and“nonlinear”effect on glaciers and ecosystems.

But other scientists, who study the response to climate change of polar ice and tropical forests, said that they saw scant evidence of runaway disruption.

For example, the idea that recent sharp retreat of summer sea ice around the North Pole has now taken on its own momentum has been challenged recently in papers by the earth scientists John S. Wettlaufer of Yale University and Ian Eisenman of the California Institute of Technology. They contend that thin ice floes have the capacity to regrow quickly as summer ends, balancing out the melting that occurs as sunlight hits and heats dark open water.

More generally, Dr. Wettlaufer has stressed the importance of being“caustically honest about what we know and don’t know.”