Over the next 100 years, global warming will raise temperatures by two to six degrees on average on the earth's surface, melt a third of all glaciers, and cause a rise in the level of the oceans by three feet. Vast lands may flood, people will starve, and deserts could expand and become more extreme. That was the grim outlook foreseen by scientists from more than 30 countries in a report issued in late October by the White House Office of Science and Technology Policy in the United States. The estimates are only slightly lower than those made last year by another working group.