Economists: For those who believe that climate change is a serious problem, the decisions that the U.S. makes now are of momentous importance. In Copenhagen, Denmark, in December, the world will decide whether to reinvigorate or abandon its effort to avert serious climate change, and what the U.S. does between now and then will in large part determine the outcome. So the fact that President Barack Obama clearly intends to turn the U.S. from being a laggard into a leader in this task is therefore …