A Hotter Earth?

Greenhouse effect is a warming of the lower atmosphere and surface of a planet by a complex process involving sunlight, gases, and particles in the atmosphere. The greenhouse effect is so named because the atmosphere acts much like the glass roof and walls of a greenhouse, trapping heat from the sun.

On the earth, the greenhouse effect began long before human beings existed. However, recent human activity may have added to the effect. Since the mid-1800's, human activities--chiefly the burning of fossil fuels (coal, oil, and natural gas) and the clearing of land--have increased the amounts of heat-trapping atmospheric gases, called greenhouse gases. The chief greenhouse gases are water vapour, carbon dioxide, methane, and ozone. The greenhouse particles include cloud droplets, soot, and dust.

The burning of fossil fuels produces the greenhouse gas carbon dioxide. Most of this burning takes place in automobiles, electric power plants, and industrial facilities. The clearing of land reduces the amount of carbon dioxide that trees and other plants remove from the atmosphere in a process called photosynthesis.

The atmosphere reflects toward space about 30 percent of the energy in incoming sunlight. The atmosphere absorbs about another 30 percent, and the remaining 40 percent or so reaches the earth's surface.

The earth's surface reflects about 15 percent of the solar energy that reaches it back toward space. The remaining energy heats the lands and seas. The warmed lands and seas then send most of the heat back into the atmosphere, chiefly as infrared rays and in evaporated water. Infrared rays are much like light waves but are invisible to the human eye.

When the rays from the lands and seas strike certain substances in the atmosphere, such as greenhouse gases and particles, those substances absorb the rays. As a result, the gases and particles are heated. They then are cooled by sending out infrared rays of their own. Some of the rays go into space. The remainder radiate back toward the earth's surface, adding to the warming of the surface layer of air. Without the natural greenhouse effect, the average temperature of the earth's surface would be about 59 Fahrenheit degrees (33 Celsius degrees) colder than it is now.

Since the late 1800's, the temperature of the earth's surface has also risen.

The increase in surface temperature, which is called global warming, could alter the ecology of many parts of the earth. For example, global warming could change rainfall patterns, melt enough polar ice to raise the sea level, increase the severity of tropical storms, and lead to shifts in plant and animal populations. Ocean currents and wind patterns could change, making some areas cooler than they are now. One remote possibility is that a warming of northern regions will result in more winter snowfall, causing some ice sheets to advance. Continued global warming could have a beneficial impact in some areas and a harmful impact in others. For example, people could begin to farm in regions where it is currently too cold. At the same time, global warming could cause sea levels to rise and thereby increase the threat of flooding in low-lying coastal areas, many of which are densely populated. A rapid and large-scale climate change could severely harm the earth's ecosystems (the living organisms and physical environment in particular areas). For example, such a change could make it difficult for many species to survive in the regions they now inhabit. Some could be forced to migrate, while others could become extinct.

Because global warming might do much harm, many scientists recommend a reduction in the emissions of greenhouse gases. The least controversial way to reduce such emissions would be to use gains in efficiency to decrease the use of energy. For example, manufacturers would adopt more efficient processes to produce goods. Makers of electrical equipment would introduce more efficient motors, light bulbs, and other devices. New manufacturing equipment and the new devices might be more costly than those they replaced.

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