A red sun pulsates over a field of withered crops. Torrential rains erode farmlands and warm weather melts ice caps. Rising sea levels flood coastal cities. Do such images accurately convey what global warming will bring? One of the difficulties in answering that question is the fact that many outcomes are possible. The IPCC says that global temperatures one hundred years from now could rise by as little as 1. 4° C (or 2. 5° F) or as much as 5. 8° C (or 10. 4° F). If the results are somewhere at the low end or in the middle, they would, of course, be easier to deal with than if the highest predictions occur.

The high-end prediction does have frightening potential. There is a theoretical possibility that the giant West Antarctic ice sheet could collapse. This is a giant sheet of ice that...