Greenhouse gases -- carbon dioxide, methane, nitrous oxide, and others -- are transparent to certain wavelengths of the Sun's radiant energy, allowing them to penetrate deep into the atmosphere or all the way to Earth's surface. Clouds, ice caps, and particles in the air reflect about 30 percent of this radiation, but oceans and land masses absorb the rest, then release it back toward space as infrared radiation. The greenhouse gases and clouds effectively prevent some of the infrared radiation from escaping; they trap the heat near Earth's surface where it warms the lower atmosphere. If this natural barrier of atmospheric gases were not present, the heat would escape into space, and Earth's mean global temperatures could be as much as 33 degrees Celsius cooler [about -18 degrees Celsius as opposed to 15 degrees Celsius].
The greenhouse effect is important to life on Earth, without it the Earth would be far too cold for us. But some scientists are concerned that humans are producing too many greenhouse gases, and that we may be warming the Earth too much.