Most cost estimates clustered between 1 and 2 percent of the world’s gross domestic product (estimated at about $85 trillion today), if temperatures were to increase 2.5 degrees Celsius (4.5 degrees Fahrenheit) above the preindustrial era. A more recent estimate by William D. Nordhaus of Yale, the foremost American economist studying climate change, concluded that allowing uncontrolled carbon emissions would raise temperatures above the preindustrial era by 3.4 degrees Celsius (6.1 degrees Fahrenheit) by the end of the century and cost the world 2.8 percent of G.D.P. in 2095.

If damages were higher than expected, the government said the social cost of carbon could rise sharply — to a whopping $123 per ton of CO 2 .

The discrepancy between the estimates of the value of climate damage stem from radically different views on how much weight the people of the present should give to damages caused by the climate in the distant future.

The estimate of $65 a ton is inspired by a moral stance: if warming will impose a cost of 1 percent of the world’s income in the future, we should spend about 1 percent of our income to prevent it — or perhaps somewhat less to account for the trend that people 100 years from now are likely to be much richer than people today.

By contrast, $13.50 a ton comes from the business world. Essentially, it requires that spending to prevent climate change should yield at least the same rate of return, in terms of reduced damages from warming, as any other capital investment.

The two outlooks lead to entirely different decisions. The government’s rendition of the moral approach implies that it is worth making every investment to reduce carbon emissions that has a rate of return of at least 2.5 percent, in terms of avoided damages. Businesss logic suggests that no investment should be made if the return — after taxes — is less than 5 percent.

The moralist would try to keep the atmosphere from warming more than 2 degrees above its temperature in the preindustrial era, the agreed-upon target at the United Nations climate summit in Copenhagen four years ago. The executive would not, noting that aiming for this goal would cost trillions more than it saved.