In the United States, this reflects more efficient vehicles, a shift from coal to natural gas for electricity generation (natural gas has about half of coal's emissions), more renewables and an overall less-energy-intensive economy.
In Europe and Japan, sluggish economic growth and stagnant — or declining — populations also depress demand.
Still, climate policy is mired in paralysis. It is not that people disbelieve warming. In a Pew poll, 84 percent of Democrats and 46 percent of Republicans agreed there is "solid evidence the earth is warming." But concrete action founders on three political obstacles.
First, it requires collective agreement. All major countries, including China, must join; otherwise, the declining emissions of some will be swamped by the gains of others. Understandably, poorer countries are reluctant to renounce the cheap energy they need to reduce poverty.
Second, it requires that governments inflict pain (higher energy costs) on today's citizens for hypothetical gains (less global warming) for tomorrow's citizens. The politics are ugly.
Third, environmentalists have described the outlook for global warming in such dire terms that it's doubtful that any set of plausible policies would avert calamity. If failure is fated, why bother?
There's no obvious way around these obstacles. Why not try something different? The crux of the matter is putting a price on carbon — through a tax on oil, coal and natural gas — that reflects global warming's costs.
This would promote energy efficiency and favor renewables. The trouble, as Pindyck says, is that we don't know what that tax should be, because we don't know global warming's full effects.
But we do know the size of the budget deficit; and we do know that revenues from a carbon tax might help finance a simplification of the income tax. By addressing multiple problems, an admittedly unpopular carbon tax might command broader support.
Who knows? It might even pass.
Samuelson writes for The Washington Post Writers Group.