Greenhouse effect, phenomenon whereby the earth's surface becomes hotter. Sunlight radiated at visible and near-ultraviolet wavelengths provides most of the earth's incoming energy. Some of it is absorbed, some reradiated. Although the atmosphere is transparent to incoming solar radiation, reradiated heat from the earth's surface is absorbed by atmospheric water vapor and carbon dioxide and again reradiated toward the surface. Over time, the process causes the surface temperature of the earth to rise. A relatively small increase in the amount of carbon dioxide in the atmosphere might have the effect of causing a long-term warming of the earth that could threaten life.