. Shortwave solar radiation can pass through the clear atmosphere relatively unimpeded, but longwave infrared radiation emitted by the warm surface of the Earth is absorbed partially and then re-emitted by a number of trace gases--particularly water vapor and carbon dioxide--in the cooler atmosphere above. Because, on average, the outgoing infrared radiation balances the incoming solar radiation, both the atmosphere and the surface will be warmer than they would be without the greenhouse gases. One should distinguish between the "natural" and a possible "enhanced" greenhouse effect. The natural greenhouse effect causes the mean temperature of the Earth's surface to be about 33 degrees C warmer than it would be if natural greenhouse gases were not present. This is fortunate for the natural greenhouse effect creates a climate in which life can thrive and man can live under relatively benign conditions. Otherwise, the Earth would be a very frigid and inhospitable place. On the other hand, an enhanced greenhouse effect refers to the possible raising of the mean temperature of the Earth's surface above that occurring due to the natural greenhouse effect because of an increase in the concentrations of greenhouse gases due to human activities. Such a global warming would probably bring other, sometimes deleterious, changes in climate; for example, changes in precipitation, storm patterns, and the level of the oceans.