Almost 10 years ago, California’s legislature passed Assembly Bill 32, the Global Warming Solutions Act of 2006. AB 32 set the most ambitious legally binding climate policy in the United States, requiring that California’s greenhouse gas emissions return to 1990 levels by the year 2020. The centerpiece of the state’s efforts—in rhetorical terms, if not practical ones—is a comprehensive carbon market, which California’s leaders promote as a model policy for controlling carbon pollution. Over the course of the past 18 months, however, California quietly changed its approach to a critical rule affecting the carbon market’s integrity. Under the new rule, utilities are rewarded for swapping contracts on the Western electricity grid, without actually reducing greenhouse gas emissions to the atmosphere. Now that the Environmental Protection Agency is preparing to regulate greenhouse gases from power plants, many are looking to the Golden State for best climate policy practices. On that score, California’s experience offers cautionary insights into the challenges of using carbon markets to reduce greenhouse gas emissions.