How fast must we act on global warming?

Over the past month, a broad consensus has been reached among scientists that global warming is real and substantially man-made. A blockbuster UN report released Feb. 2 confirmed that the world is rapidly warming and added that the cause is almost certainly greenhouse gases, especially carbon dioxide, released through human activity. Earlier this week, the World Bank's chief scientist, Robert Watson, urged nations to try to limit temperature rise over this century to 2 degrees Celsius (4 degrees F.), a task many scientists say will be difficult. On Sunday, the nation's top scientific society, the American Association for the Advancement of Science, added: "The evidence is clear: Global climate change caused by human activities is occurring now and is a growing threat to society."

So should the world's fight to curb greenhouse-gas emissions start with modest steps or does humanity need a crash diet to curb climate change?

Economists have looked at the question for years and concluded that a gradual approach inflicts the lowest cost on society. But last October, an economic report prepared for the British government shifted the debate and argued for more aggressive action.

In his report to the British government, economist Sir Nicholas Stern tried to settle the question. His "Stern Review: The Economics of Climate Change" calculated that failing to curb greenhouse-gas emissions would cut the world's economic output (GDP) by 5 to 20 percent per year, risking economic disruption on a scale of the two world wars or the Great Depression. Without mitigation (efforts to slow CO2 emissions), temperatures could rise 5 degrees C (9 F.) by the end of the century.

Even before that temperature is reached, Mr. Stern contends that the lives of millions will be greatly disrupted through intensified droughts, coastal flooding, and stronger storms.

On the other hand, quick action to limit the worst effects of climate change would cost nations only about 1 percent of their GDP each year, the Stern report argues. The case that the cost of immediate action is less than the cost of inaction is "blindly obvious," Stern says. The aim should be to stabilize greenhouse-gas levels (principally CO2) in the atmosphere to 450 to 550 parts per million (ppm). The current level is about 430 ppm.

While economists have praised the report as a milestone, not all are persuaded. So for the first time since the "Stern Review" was issued last fall, the former chief economist for the World Bank ventured across the Atlantic to make his case before Congress, the United Nations, and a panel of a half-dozen fellow economists at Yale's Center for the Study of Globalization.

Reasonable people can disagree with conclusions in the report, but "reasonable people are obligated to have a serious discussion" about its findings, Stern said at Yale. Even if climate change turned out to be the biggest hoax in history, Stern argues, the world will still be better off with all the new technologies it will develop to combat it. The price of waiting for more evidence is too high, he says. "You're playing with a planet here."

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