Abstract: See also, 41 Envtl. L. Rep. 10695 (2011). The public policy debate on the appropriate American response to climate change is now in full swing. There are no longer significant voices disputing that climate change is real or that it is primarily the result of human activity. The issue today is what the United States should do about climate change given the risks the country faces and the likely economic impacts. The question is whether putting a price on carbon domestically is worth the cost. In this Article we make the case that the United States should act aggressively to mitigate the effects of climate change. In doing so we take on and debunk the “climate change winner” argument, which asserts that the United States is likely to fare well in a warmer world, at least compared to most other states and, therefore, faces no rational incentive to invest in expensive mitigation efforts that will largely benefit other states. In this view, impacts on the United States are best addressed through a strategy of adaptation rather than mitigation – the construction of both literal and figurative sea walls to reduce the effects of global warming. The dominant response to this argument has been an appeal to a perceived moral obligation on the United States based on its wealth and its historical greenhouse gas emissions. Though we are sympathetic to this moral argument, this Article takes a different approach. We demonstrate that even if one accepts that the premises of the climate change winner argument – that impacts on the United States will be less severe than elsewhere and that the United States is not morally obliged to help foreign states – the case for American action on climate change is strong. Considering only the narrow self-interest of the United States, we show that the climate change winner argument is wrong. We explain that existing estimates systematically underestimate the likely economic impact of climate change, and we provide rough estimates of what a more complete accounting would reveal. The sources of downward bias in existing models are numerous and include undue optimism about future warming, overlooked asymmetries around expected increases in temperature, and a failure to account for catastrophic events, non-market costs, cross-sectoral impacts, and impacts on productivity. Also ignored by existing estimates are the ways in which climate change impacts abroad will spillover into the United States through economic effects, national security, migration and disease, creating additional costs. This Article shows that climate change is not simply a problem for the rest of the world. It is far likelier than current models suggest to lead to serious negative consequences for the United States. If this is so, the country should take prompt and aggressive action to address climate change, not out of benevolence or guilt, but out of simple self-interest.