The prices don’t have to be high to show effects. A 5-cent surcharge per disposable bag in Washington, D.C., cut bag use 80 percent within a year by some estimates.

Of course, wishing we had a strong cap on carbon won’t make it so. That’s where decisive, immediate action from President Obama’s administration comes in: everything from applying the Clean Air Act to global warming pollution, as instructed by the Supreme Court, to reining in methane leakage from our natural gas system.

Put together, these administrative actions alone could get us a long way toward where we need to be, until Congress finally gets ready to follow the will of the people to tackle global warming pollution.

GERNOT WAGNER

Economist

Environmental Defense Fund

New York, April 3, 2013

The perfect can be the enemy of the good, but here moderation or “probity” might turn out to be the enemy of life. In the midst of all the conditionals and uncertainties, one thing that can be said about mainstream climate science is that it has always been behind the curve, always catching up with the moving reality on the ground. The facts have repeatedly aligned with radical predictions, like those of the scientist James Hansen, who is now talking about the oceans boiling off and leaving the earth lifeless.

ROBERT LEVIN

Capitola, Calif., April 3, 2013

In any scientific endeavor, the burden is on those who propose a theory to provide the proof. In the case of man-made global warming, the Intergovernmental Panel on Climate Change’s models have come up short. This is clearly a case where consensus and facts diverge, showing that the former is the realm of politics, not science.

The issue shouldn’t be what to do about climate change, but whether we should institute monumental policies based on less than robust theories. Cheap energy is perhaps one of the greatest factors in lifting a society out of poverty. What is the humanitarian and moral price of raising the cost of energy for developing nations based on speculative science?

TIMOTHY NAYLOR

Brooklyn, April 3, 2013

Mr. Fri makes two excellent points: We must tackle the climate problem now, and “many existing government rules and institutions aren’t up to the job of managing climate policy.”