2. Global Warming

Heating by the greenhouse effect

Our planetís surface is now kept at a comfortable temperature because the atmosphere traps some of the radiant heat from the Sun and keeps it near the surface, warming the planet and sustaining living creatures. Jean Baptise Joseph Fourier (1768-1830) first conceived the mechanism in the 1820s, while wondering how the Sunís heat could be retained to keep the Earth hot. Fourierís idea, still accepted today, is that the atmosphere lets some of the Sunís radiation in, but it doesnít let all of the radiation back out. Visible sunlight passes through our transparent atmosphere to warm the Earthís land and oceans, and some of this heat is reradiated in infrared form. The longer infrared rays are less energetic than visible ones and do not slice through the atmosphere as easily as visible light.

So our atmosphere absorbs some of the infrared heat radiation, and some of the trapped heat is reradiated downward to warm the planetís surface and the air immediately above it. Fourier likened the thin atmospheric blanket to a huge glass bell jar, made out of clouds and gases, that holds the Earthís heat close to its surface.

The warming by heat-trapping gases in the air is now known as the ďgreenhouse effectĒ, but this is a misnomer. The air inside a garden greenhouse is heated because it is enclosed, preventing the circulation of air currents that would carry away heat and cool the interior. Nevertheless, the term is now so common that we will also sometimes designate the heat-trapping gases as greenhouse gases, and let greenhouse effect designate the process by which an atmosphere traps heat near a planetís surface.

Right now, the warming influence is literally a matter of life and death. It keeps the average surface temperature of the planet at 288 degrees kelvin (15 degrees Celsius or 59 degrees Fahrenheit). Without this greenhouse effect, the average surface temperature would be 255 degrees kelvin (-18 degrees Celsius or 0 degrees Fahrenheit); a temperature so low that all water on Earth would freeze, the oceans would turn into ice and life, as we know it, would not exist.

The gases that absorb the most infrared heat radiation are minor ingredients of our atmosphere. They are water vapor and carbon dioxide, with water vapor absorbing the most. Sixty to seventy percent of the Earthís greenhouse warming is now caused by water vapor and carbon dioxide provides just a few degrees.

The main constituents of the atmosphere, nitrogen (77 percent) and oxygen (21 percent) play no part in the greenhouse effect. The two atoms in these diatomic molecules are bound tightly together and are therefore incapable of absorbing significant infrared radiation. In contrast, water vapor and carbon dioxide molecules consist of three atoms that are less constrained in their motion, so they absorb the heat radiation.

Why doesnít the atmosphere just keep heating up until it explodes? The greenhouse warming rises to a fixed temperature that balances the heat input from sunlight and the heat radiated into space. The level of water in a pond similarly remains much the same even though water is running in one end and out the other.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University