Such figures typically come from a model that combines all sorts of engineering and marketplace estimates. These will include, for instance, engineers’ best calculations of how much it costs to generate electricity in various ways, from coal, gas and nuclear and solar power at given resource prices. Then estimates will be made, based on historical experience, of how much consumers would cut back their electricity consumption if its price rises. The same process is followed for other kinds of energy, like motor fuel. And the model assumes that everyone makes the best choice given the economic environment — that power generators choose the least expensive means of producing electricity, while consumers conserve energy as long as the money saved by buying less electricity exceeds the cost of using less power in the form either of other spending or loss of convenience. After all this analysis, it’s possible to predict how producers and consumers of energy will react to policies that put a price on emissions and how much those reactions will end up costing the economy as a whole.

There are, of course, a number of ways this kind of modeling could be wrong. Many of the underlying estimates are necessarily somewhat speculative; nobody really knows, for instance, what solar power will cost once it finally becomes a large-scale proposition. There is also reason to doubt the assumption that people actually make the right choices: many studies have found that consumers fail to take measures to conserve energy, like improving insulation, even when they could save money by doing so.

But while it’s unlikely that these models get everything right, it’s a good bet that they overstate rather than understate the economic costs of climate-change action. That is what the experience from the cap-and-trade program for acid rain suggests: costs came in well below initial predictions. And in general, what the models do not and cannot take into account is creativity; surely, faced with an economy in which there are big monetary payoffs for reducing greenhouse-gas emissions, the private sector will come up with ways to limit emissions that are not yet in any model.

What you hear from conservative opponents of a climate-change policy, however, is that any attempt to limit emissions would be economically devastating. The Heritage Foundation, for one, responded to Budget Office estimates on Waxman-Markey with a broadside titled, “C.B.O. Grossly Underestimates Costs of Cap and Trade.” The real effects, the foundation said, would be ruinous for families and job creation.

This reaction — this extreme pessimism about the economy’s ability to live with cap and trade — is very much at odds with typical conservative rhetoric. After all, modern conservatives express a deep, almost mystical confidence in the effectiveness of market incentives — Ronald Reagan liked to talk about the “magic of the marketplace.” They believe that the capitalist system can deal with all kinds of limitations, that technology, say, can easily overcome any constraints on growth posed by limited reserves of oil or other natural resources. And yet now they submit that this same private sector is utterly incapable of coping with a limit on overall emissions, even though such a cap would, from the private sector’s point of view, operate very much like a limited supply of a resource, like land. Why don’t they believe that the dynamism of capitalism will spur it to find ways to make do in a world of reduced carbon emissions? Why do they think the marketplace loses its magic as soon as market incentives are invoked in favor of conservation?

Clearly, conservatives abandon all faith in the ability of markets to cope with climate-change policy because they don’t want government intervention. Their stated pessimism about the cost of climate policy is essentially a political ploy rather than a reasoned economic judgment. The giveaway is the strong tendency of conservative opponents of cap and trade to argue in bad faith. That Heritage Foundation broadside accuses the Congressional Budget Office of making elementary logical errors, but if you actually read the office’s report, it’s clear that the foundation is willfully misreading it. Conservative politicians have been even more shameless. The National Republican Congressional Committee, for example, issued multiple press releases specifically citing a study from M.I.T. as the basis for a claim that cap and trade would cost $3,100 per household, despite repeated attempts by the study’s authors to get out the word that the actual number was only about a quarter as much.

The truth is that there is no credible research suggesting that taking strong action on climate change is beyond the economy’s capacity. Even if you do not fully trust the models — and you shouldn’t — history and logic both suggest that the models are overestimating, not underestimating, the costs of climate action. We can afford to do something about climate change.

But that’s not the same as saying we should. Action will have costs, and these must be compared with the costs of not acting. Before I get to that, however, let me touch on an issue that will become central if we actually do get moving on climate policy: how to get the rest of the world to go along with us.

The China Syndrome
The United States is still the world’s largest economy, which makes the country one of the world’s largest sources of greenhouse gases. But it’s not the largest. China, which burns much more coal per dollar of gross domestic product than the United States does, overtook us by that measure around three years ago. Over all, the advanced countries — the rich man’s club comprising Europe, North America and Japan — account for only about half of greenhouse emissions, and that’s a fraction that will fall over time. In short, there can’t be a solution to climate change unless the rest of the world, emerging economies in particular, participates in a major way.

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.”