We quantify the U.S. corporate sector's carbon externality by computing the sector's “carbon burden”—the present value of social costs of its future carbon emissions. Our baseline estimate of the carbon burden is 131% of total corporate equity value. Among individual firms, 77% have carbon burdens exceeding their market capitalizations, as do 13% of firms even with indirect emissions omitted. The 30 largest emitters account for all the decarbonization of U.S. corporations predicted by 2050. Predicted emission reductions, and even firms' targets, fall short of the Paris Agreement. Firms' emissions are predictable by past emissions, investment, climate score, and book-to-market.