Lizette Kabré. Climate congress, Copenhagen 2009.

[UPDATE, 1:45 p.m.: A roundup of economists’ and scientists’ views at the Copenhagen climate meeting and a reaction from Mike Hulme, a participating scientist.]

COPENHAGEN — A scientist known for his aggressive stance on climate policy made an apocalyptic prediction on Thursay.

Hans Joachim Schellnhuber, the director of the Potsdam Institute for Climate Impact Research in Germany, said that if the buildup of greenhouse gases and its consequences pushed global temperatures 9 degrees Fahrenheit higher than today — well below the upper temperature range that scientists project could occur from global warming — Earth’s population would be devastated. [ UPDATED, 6:10 p.m: The preceding line was adjusted to reflect that Dr. Schellnhuber was not describing a worst-case warming projection. h/t to Joe Romm.]

“In a very cynical way, it’s a triumph for science because at last we have stabilized something –- namely the estimates for the carrying capacity of the planet, namely below 1 billion people,” said Dr. Schellnhuber, who has advised German Chancellor Angela Merkel on climate policy and is a visiting professor at Oxford.

At that temperature, there would be “no fluctuations anymore, we can be fairly sure,” said Dr. Schellnhuber, exercising his characteristically dark sense of humor at the morning plenary session on the closing day of an international climate change conference in Copenhagen, Denmark. [Earlier post: The conference organizers have sought to jog policymakers with a stronger assessment of global warming’s risks, but some scientists warned the approach could backfire.]

“What a triumph,” Dr. Schellnhuber said. “On the other hand do we want this alternative? I think we can do much, much better,” he told the conference.

Dr. Schellnhuber, citing his own research, said that at certain “tipping points,” higher temperatures could cause areas of the ocean to become deoxygenated, resulting in what he calls “oxygen holes” between 600 and 2,400 feet deep. These are areas so depleted of the gas that they would badly disrupt the food chain.

Unabated warming would also lead to “disruption of the monsoon, collapse of the Amazon rain forest and the Greenland ice sheet will meltdown,” he said.

But on the bright side, he noted, in a joking reference to the meeting’s Danish hosts, the retreat of the sheath of ice covering Greenland, which is Danish-controlled territory, “would increase your usable land by, I don’t know, 10,000 percent.”

“But I’m not sure whether you want to do this,” he said.