Like medieval priests, today’s carbon brokers will sell you an indulgence that forgives your carbon sins. It will run you about $500 for 5 tons of forgiveness—about how much the typical American needs every year. Or about $2,000 a year for a typical four-person household. Your broker will spend the money on such things as reducing methane emissions from hog farms in Brazil. But if you really want to make a difference, you must send a check large enough to forgive the carbon emitted by four poor Brazilian households, too—because they’re not going to do it themselves. To cover all...