Weeks after President George Bush took office, the National Academy of Sciences warned him: "We believe that global environmental change may well be the most pressing international issue of the next century." Of the environmental threats surfacing, global warming is perhaps the most imposing. Scientists believe man-made atmospheric changes may result in a global mean temperature rise of from 1.5 to 5.5 degrees C. by the middle of the next century. As glaciers melt, sea level could rise as much as five feet, transforming heavily populated coastal areas into ocean. The climate shift could disrupt agriculture, defeat present water supply and flood control efforts, and doom plant and animal species that are unable to adapt.