The 'Greenhouse Earth' is surrounded by a shield of atmospheric gases, rather than a glass or plastic canopy. The air that makes up our atmosphere consists primarily of nitrogen and oxygen molecules (N2 at 78% and O2 at 21%). A large number of 'trace gases' make up the remainder of air's composition. Many of these - including carbon dioxide (CO2) and methane (CH4) are the so called 'greenhouse' gases.
If you have ever felt the piercing cold of the clear winter night sky and wondered why you feel warmer on a cloudy winter night, you have experienced the atmospheric greenhouse effect firsthand. The radiative laws of physics tell us that any object warmer than absolute zero will radiate energy. Cooler objects emit longer waves (in the infrared region) while hotter ones radiate shorter wavelengths. Our sun, powered by its hot, nuclear fusion reaction, produces radiant energy in the visible and ultraviolet regions with relatively short wavelengths. Of the sunlight that strikes the earth, about 70% is absorbed by the planet and its atmosphere, while the other 30% is immediately reflected. If the earth did not reradiate most of this newly absorbed energy back to space (at longer infrared wavelengths) the world would continue to grow hotter. Instead, an energy balance is maintained, with the earth at a certain temperature, and its atmosphere at a somewhat cooler temperature, but warmer than space itself.
The earth is about 60 degrees F (33 deg.C) warmer than it would be if it did not have the atmospheric blanket of greenhouse gases and clouds around it. Clouds are simply water vapor (gaseous H2O molecules). Water vapor and greenhouse gases keep the earth warm because their molecules have the ability to absorb some of the infrared radiation, IR, being emitted from the earth. Once warmed, these molecules then radiate a portion of this heat energy back to earth, creating more warming on the surface of our planet. It is this radiation of IR energy by the atmospheric gases back to earth that scientists call the 'greenhouse effect'. The greenhouse theory is one of the most well-established theories of atmospheric science, and has been verified by many measurements on our planet, as well as on Mars and Venus, which have very different atmospheres and temperatures than our Earth.
The graphs at the right show the CO2 buildup and temperature increases that Earth has experienced in the last 100 years. CO2, produced when oil, gas, coal or wood are burned, is the primary greenhouse gas warming the earth. While the 1.3 degree F global rise (0.7 deg C) does not seem to be very much, it represents a large change in a single century, especially when compared with climate changes that resulted from the historic glacial ages. ***
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