The rising of the Earth's average global temperature because of greenhouse gases accumulating in the atmosphere. The scientific emphasis has swung from whether global warming exists to how to minimize the damage it will cause. One example of many: according to the British Antarctic Survey and U. S. Geological Survey as of 5005, 87% of 244 glaciers studied have retreated over the last fifty years, and average retreat rates are accelerating. If the Greenland ice sheet melts, sea levels worldwide will rise twenty feet. See Greenhouse Effect.