During the 20th century, we raised the average temperature of the Earth by about 1°F (0.6°C). That doesn't sound like a lot, yet many effects are already evident, including: record-setting storms and heat waves, species extinctions, the melting of polar ice caps, and long-term droughts that have undermined entire regions. To put this one degree of warming in perspective, consider that only about 10°F (5.6°C) separates us from the coldest ice age, when Canada and the northern U.S. were under a mile thick sheet of ice and sea level was about 400 feet lower than today.

Because global warming doesn't happen instantly—the Earth takes time to respond—even if we stop all global warming pollution tomorrow, the Earth will continue to warm for decades (about another 1°F). But, of course, we can't stop the pollution overnight. So the key question is, how much additional warming are we going to allow, and what will be its impacts?

Polar bears on melted ice in Beaufort Sea, Aug. 2004 (Dan Crosbie, Canadian Ice Service)

The climate history of the Earth shows what a planet that is a few degrees warmer would be like. We have to go back about 3 million years to find a time when the Earth was several degrees warmer than at present. At that time, sea level was about 80 feet higher. With a sea level rise of 80 feet, coastal cities such as New York, Tokyo, and Calcutta would be submerged.

Along with sea level rise, there is little doubt that rapid warming will cause water shortages, disease epidemics, and a mass extinction of species. What's harder to predict is the overall path that the climate system will follow. Earth's climate history shows that the stability we've enjoyed over the past 8,000 years is exceptional. Earth's climate is prone to large, abrupt mode shifts in response to relatively small influences. We are carrying out an uncontrolled experiment on a system that we don't yet understand.

The Earth climate system has proven beyond any doubt that it is capable of jumping abruptly from one state of operation to another. ... My lifetime study of Earth's climate system has humbled me. I'm convinced that we have greatly underestimated the complexity of this system. (1997)

—Dr. Wallace Broecker, Newberry Professor of Geology at Columbia University, Lamont-Doherty Earth Observatory