From driving cars to cattle ranching, a lot of things people do result in the emission of gases into the atmosphere that trap solar heat the way a glass roof traps warm air in a greenhouse.

Among the "greenhouse gases" are carbon dioxide, methane, nitrous oxide and chlorofluorocarbons (CFCs), the manmade gases also responsible for depleting the atmosphere's ozone layer. Ozone itself, when close to the Earth's surface as air pollution from automobiles and industry, acts as a greenhouse gas.Even water vapor is a powerful greenhouse gas, but scientists don't know whether human activities have changed its concentration in the air. Human impact on other greenhouse gases is better understood.

Burning fossil fuels - coal, oil, natural gas - causes the carbon in them to unite with oxygen in the air, sending carbon dioxide into the atmosphere. The same thing happens when fallen trees and other vegetation decompose or burn.

Cattle produce methane as a byproduct of their digestion. Rice paddies and municipal landfills also send methane billowing up. Certain power plants produce nitrous oxide, while CFCs come from a host of processes, including refrigeration and foam blowing.

Like most gases, these are transparent to the sun's visible light. But unlike other gases, they absorb invisible infrared wavelengths that radiate heat.

Sunlight warms the Earth's surface unimpeded by the greenhouse gases. Then, when the surface reflects infrared rays back toward space, the greenhouse gases in the lowest seven miles or so of the atmosphere intercept the radiation and are themselves warmed in the process. Their warmth is then reflected back to Earth.

The atmosphere has always worked this way. Without the greenhouse effect, Earth would be incapable of retaining heat.

What's changed over the last 150 years is that humans are tilting the natural balance of greenhouse gases by pouring more of them into the atmosphere. The growing greenhouse load is trapping more heat and thus nudging the planet toward higher temperatures.