Every ton of carbon dioxide that is emitted into the atmosphere by burning coal, oil, or gas adds to long-term global warming, and therefore to the long-term costs that society will incur through droughts, floods, heat waves, extreme storms, and rising sea levels. While these future costs cannot be predicted with precision, recent estimates put the current social cost of each added ton of atmospheric CO2 at $10-100, with the US government using a middle-range estimate of about $40 per ton to guide energy regulation.