Man-made global warming occurs as a result of burning fossil fuels is a negative externality—a spillover from an economic transaction that harms parties not directly involved in the transaction. In this case, the carbon dioxide released into the atmosphere is thought to be boosting temperatures, raising sea levels, and having other effects on the climate that people must adapt to (by using more air conditioning, switching crops, and so forth). Ideally, once the full costs of man-made global warming are calculated, consumers, businesses, governments, and international agencies can adopt policies that take those burdens into account. But, do we have time to wait to figure out all the details of accounting to calculate this to the nth degree? The goal of both approaches is to make polluters pay for the costs they impose on others. But they work only if those costs can be accurately assessed. In the real world things are never so simple.