An analysis of admittedly spotty temperature records indicates that the world's average temperature has gone up about 0.5°C (1°F) in the past century, with the '90s being the hottest decade in recent history. This fact is quoted widely in the scientific community, although there are nagging doubts even among researchers. Recent satellite records, using different kinds of instrumentation, fail to show a warming trend.

If we accept that there has been moderate warming, we turn to computer models to see if humans are to blame and what will happen to the earth's climate in the future. These models are complex because climate depends on thousands of things, from Antarctic sea ice to sub-Saharan soil conditions. While the electronic simulations are monuments to the ingenuity and perseverance of their creators, they provide us with, at best, a fuzzy view of the future. They have difficulty handling factors like clouds and ocean currents (two major influences on climate), and if you fed the climate of 1900 into any of them, they couldn't predict the climatic history of the 20th century. Like everything else in this frustrating field, the models' limitations force us to make important decisions in the face of imperfect knowledge.

The most authoritative predictions about future warming come from the Intergovernmental Panel on Climate Change, a worldwide consortium of more than 2,000 climate scientists. The current forecast is that by 2100 the earth's temperature will go up 1° to 3.5°C (2° to 7°F), with the best guess being an increase of 2°C (4°F). MORE>>

PAGE 1 | 2 | 3