Greenhouse Effect

In environmental science, the greenhouse effect is a popular term for the role that the variable atmospheric constituents carbon dioxide, water vapor, and trace-gases play in keeping the Earth's surface warmer than it would be without their presence. The atmosphere, when clear, is nearly transparent to the primarily shorwave radiation from the Sun, most of which is absorbed at the Earth's surface. The Earth, being much cooler than the Sun, reemits radiation most strongly at shorwave (infrared) wavelengths. The atmosphere's carbon dioxide, water vapor, and trace-gases then absorb much of this radiation and reemit a large proportion of it back toward the Earth. The term greenhouse effect implies that a comparable effect keeps the interior of a greenhouse warm. Actually, the main role of the glass in a greenhouse, besides that of admitting solar radiation, is to prevent convection currents from mixing cooler air outside with the warm air inside.


Although H₂O is an important atmospheric constituent contributing to the greenhouse effect, it is a major reason why humid regions experience less cooling at night than do dry regions, variations in CO₂ in particular, have played an important role in climatic changes. For this reason, many environment sciences have expressed concern over the global increase in amounts of atmospheric        
CO₂ in recents decades, largely as a result of the burning of fossil fuels. If the many other determinants of the Earth's present global climate remain more or less constant, the CO₂ increase should raise the average temperature at the Earth's surface. Because warm air can contain more H₂O before reaching saturation than can cooler air, the amount of H₂O would probably also increase as the atmosphere warmed. Conceivably, such a process could go on indefinitely. Some natural checks might develop. For example, negative feedbacks such as increased cloud cover and increased sea absorption of CO₂ could lower temperatures. Even a limited rise in average surface temperature, however, might lead to a significant rise in sea level and result in other environmental disruptions.
A great deal remains unknown about the cycling of carbon through the environment, and particulary about the role of oceans in this carbon cycle. Further uncertainty occurs in greenhouse effect studies because the historical temperature records being used tend to represent warmer urban areas rather than the environment as a whole. In addition, the effects of trace gases such as methane are only beginning to be understood. Despite such problems, a number of scientists maintain that the rise in global temperatures in the 1980_s is indeed a result of the greenhouse effect. 

A report issued in 1988 by three major international organizations called for furher research into the greenhouse effect and at the same time urged immediate governmental action to counteract the apparent global warming trend. By the end of the 1980_s no government had yet adopted any of the radical measures proposed by the report.