like carbon dioxide and methane - and water vapor and clouds - all act to trap escaping infrared radiation. The net effect is to cause the Earth's surface to warm up to an average of 59 degrees. RICHARD C. J. SOMERVILLE, Scripps Institution of Oceanography: It's as real as gravity, and it keeps the planet habitable. We'd have an average surface temperature on the Earth below the freezing point of water were it not for the greenhouse effect. NARRATOR: But since the industrial revolution, there has been a new and growing source of CO2: fossil fuels like coal, oil and natural gas. When fossil fuels are burned, their carbon atoms combine with oxygen in the air to make CO2. Every time we turn on a light or drive our cars, we are adding CO2 to the atmosphere. [www.pbs.org: What's your carbon diet?] CHARLES D. KEELING: It's hard for people to understand how much carbon dioxide is being put into the atmosphere in absolute terms, but in relative terms we can say as follows: At the end of World War II, about a billion metric tons of carbon, in the form of CO2, was being emitted to the atmosphere. When I started my measurements, it had risen to two and a half. At the present time, it's almost seven. NARRATOR: According to the theory of the greenhouse effect, adding more greenhouse gases like CO2 to the atmosphere should cause the average global temperature to rise for as long as we continue