The Paleocene-Eocene Thermal Maximum, which occurred 56 million years ago, saw semi-tropical temperatures – and alligators – well above the Arctic Circle. This was likely caused by very high concentrations of carbon dioxide in the atmosphere. Through the burning of fossil fuels, present-day people run the risk of recreating these catastrophic conditions. This book draws the implications of climate science for economics and policy. It welcomes the analytical clarity economics brings to the question of how to control carbon emissions, but it rejects the economic impulse to search for their “optimal” level. As for policy, it works backward from the conditions for avoiding a climate catastrophe rather than forward from what appears politically feasible at the current moment. Along the way it critiques several common assumptions: that policies should try to balance their economic costs and benefits, that the costs of stabilizing the climate will be modest and only the intransigence of the fossil fuel companies stands in the way, that climate change is not “really” about carbon but some deeper ideological problem and that the problem can be solved by actions individuals or organizations can take one at a time.