The big-bang advocates argue that government should take a much longer view than private investors. Stern, in particular, argues that policy makers should give the same weight to future generations’ welfare as we give to those now living. Moreover, the proponents of fast action hold that the damage from emissions may be much larger than the policy-ramp analyses suggest, either because global temperatures are more sensitive to greenhouse-gas emissions than previously thought or because the economic damage from a large rise in temperatures is much greater than the guesstimates in the climate-ramp models.

As a professional economist, I find this debate painful. There are smart, well-intentioned people on both sides — some of them, as it happens, old friends and mentors of mine — and each side has scored some major points. Unfortunately, we can’t just declare it an honorable draw, because there’s a decision to be made.

Personally, I lean toward the big-bang view. Stern’s moral argument for loving unborn generations as we love ourselves may be too strong, but there’s a compelling case to be made that public policy should take a much longer view than private markets. Even more important, the policy-ramp prescriptions seem far too much like conducting a very risky experiment with the whole planet. Nordhaus’s preferred policy, for example, would stabilize the concentration of carbon dioxide in the atmosphere at a level about twice its preindustrial average. In his model, this would have only modest effects on global welfare; but how confident can we be of that? How sure are we that this kind of change in the environment would not lead to catastrophe? Not sure enough, I’d say, particularly because, as noted above, climate modelers have sharply raised their estimates of future warming in just the last couple of years.

So what I end up with is basically Martin Weitzman’s argument: it’s the nonnegligible probability of utter disaster that should dominate our policy analysis. And that argues for aggressive moves to curb emissions, soon.

The Political Atmosphere
As I’ve mentioned, the House has already passed Waxman-Markey, a fairly strong bill aimed at reducing greenhouse-gas emissions. It’s not as strong as what the big-bang advocates propose, but it appears to move faster than the policy-ramp proposals. But the vote on Waxman-Markey, which was taken last June, revealed a starkly divided Congress. Only 8 Republicans voted in favor of it, while 44 Democrats voted against. And the odds are that it would not pass if it were brought up for a vote today.

Prospects in the Senate, where it takes 60 votes to get most legislation through, are even worse. A number of Democratic senators, representing energy-producing and agricultural states, have come out against cap and trade (modern American agriculture is strongly energy-intensive). In the past, some Republican senators have supported cap and trade. But with partisanship on the rise, most of them have been changing their tune. The most striking about-face has come from John McCain, who played a leading role in promoting cap and trade, introducing a bill broadly similar to Waxman-Markey in 2003. Today McCain lambastes the whole idea as “cap and tax,” to the dismay of former aides.

Oh, and a snowy winter on the East Coast of the U.S. has given climate skeptics a field day, even though globally this has been one of the warmest winters on record.

So the immediate prospects for climate action do not look promising, despite an ongoing effort by three senators — John Kerry, Joseph Lieberman and Lindsey Graham — to come up with a compromise proposal. (They plan to introduce legislation later this month.) Yet the issue isn’t going away. There’s a pretty good chance that the record temperatures the world outside Washington has seen so far this year will continue, depriving climate skeptics of one of their main talking points. And in a more general sense, given the twists and turns of American politics in recent years — since 2005 the conventional wisdom has gone from permanent Republican domination to permanent Democratic domination to God knows what — there has to be a real chance that political support for action on climate change will revive.

If it does, the economic analysis will be ready. We know how to limit greenhouse-gas emissions. We have a good sense of the costs — and they’re manageable. All we need now is the political will.

Paul Krugman is a Times columnist and winner of the 2008 Nobel Memorial Prize in Economic Science. His latest book is “The Return of Depression Economics and the Crisis of 2008.”