When President Obama goes to the Copenhagen climate change summit next week, he is expected to once again declare that the U.S. will reduce its carbon emissions 83% by 2050. Even though no legally binding agreement is expected, what Mr. Obama says in Denmark will define the U.S. position in subsequent international negotiations. He will not say how the cuts will be accomplished. For Americans, the details are worth knowing. Annual U.S. carbon-dioxide emissions currently average about 5.5 tons of carbon per person. Achieving Mr. Obama's goal would mean reducing this to 0.63 tons per person by midcentury, taking expected...