Human activity -- particularly the burning of fossil fuels -- has made the blanket of greenhouse gases around the earth "thicker." The resulting increase in global temperatures is alterating the complex web of systems that allow life to thrive on earth, such as cloud cover, rainfall, wind patterns, ocean currents, and the distribution of plant and animal species.

The greenhouse effect and the carbon cycle. More of the sun's energy is being trapped in the atmosphere, and much more of the world's carbon (in the form of carbon dioxide) is resting in the air rather than in trees, soil, and subterranean deposits.

Current evidence of climate change. Some consequences of global warming are already apparent.

Future effects. The complexity of the climate system means predictions vary widely, but even the minimum changes forecast could mean frequently flooded coastlines, disruptions to food and water supplies, and the extinction of many species.

The Intergovermental Panel on Climate Change. An international group of experts formed in 1988 reviews scientific research and offers assessments of climate change and its effects.