Greenhouse Effect | Research & Encyclopedia Articles

The greenhouse effect is a natural phenomenon that traps radiation within the earth's atmosphere. Natural greenhouse gases include water vapor, carbon dioxide, nitrous oxide, methane, and ozone, all essential to support life. The enhanced greenhouse effect, the direct result of human activities, increases concentrations of these gases in the atmosphere, and leads to pollution of the lower atmosphere and contributes to global warming. These gases let in sunlight but tend to insulate Earth against the loss of heat, as do the glass walls of a greenhouse. A higher concentration of the greenhouse gases means a warmer climate. For example, the twentieth century was been 1° warmer on worldwide average than the nineteenth century—warming at a rate 20 times faster than average.

Carbon dioxide (CO₂) is considered the predominant greenhouse gas and has the greatest impact on global heat. From April 1958, when monthly measurements of CO₂ from atop the Mauna Loa volcano began, through June 1991, the CO₂ concentration in parts per million went from 316 ppm to almost 360 ppm. The peak concentration is due to the destruction of tropical rain forests and the burning of fossil fuels, which accounts for half of the greenhouse gases added to the atmosphere. CO₂ is dumped into the atmosphere at a much faster rate than it can be withdrawn or absorbed by the oceans or living things in the biosphere; since 1765, its presence in the atmosphere has increased by over 27%. CO₂ buildup in the next few decades to centuries could be one of the principal controlling factors of the near-future climate.

Methane, another greenhouse gas, is produced when oxygen is not freely available and bacteria have access to organic matter, such as in swamps, bogs, rice paddies and moist soils. Methane also is produced in the guts of termites and cows, in garbage dumps, landfills, emissions from coal mining, natural gas production and distribution, and changing land use. Methane concentrations have increased over 100% since 1765.

Nitrous oxide concentrations have increased in recent years due to fertilizer use and chemical production, such as in the manufacture of nylon. Nitrous oxide is also dispersed during fossil fuel combustion, biomass burning and changing land use.

CFCs (chlorofluorocarbons), also implicated in ozone layer depletion, act as greenhouse gases. While useful and widely used as refrigerants, their total effect is significant because compared to a molecule of carbon dioxide, each molecule of CFC absorbs much more radiation, thereby trapping heat in the atmosphere.

Other greenhouse gases are ground-level ozone (sunlight reacting with automobile emissions). and water vapor. Water vapor represents about two% of total atmospheric composition, and is the most abundant greenhouse gas. With methane and carbon dioxide, it plays an important role in regulating the temperature of the planet through the production of clouds.

Rain forest destruction also contributes to global warming. When the canopy of leaves is removed through clear-cutting or burning, the sudden warming of the forest floor releases methane and CO₂, in a kind of biochemical burning. The massive increase in the number of dead tree trunks and branches leads to a population explosion of termites, which themselves produce methane. Dead trees can no longer store CO₂ or convert it to oxygen.

Two factors which appear to mitigate the effect of enhanced greenhouse gases are aerosols and dust. Aerosols, minute solid particles, are finely dispersed in the atmosphere and have become an influence on the greenhouse effect. Aerosols are produced by combustion, but they also come from natural sources, primarily volcanoes. By blocking light, aerosols and dust can offset warming from greenhouse gasses. For example, a significant cooling trend in the spring and summer of 1992 seemed to correlate with the eruption of Mount Pinatubo in the Philippines. The fall and winter of 1992 were fairly mild on worldwide average. As all the particulate matter from the Mt. Pinatubo eruption settled out of the atmosphere, the surface cooling effects abated and the global warming trend resumed.

Anthropogenic (human-caused) greenhouse gases now appear responsible for increasing the global average temperature. According to current projections, global temperatures may rise as much as 35.6–37.4° F (2–3° C) above the pre-industrial temperatures by the year 2100. To place this change in perspective, the temperature rise that brought the planet out of the most recent ice age was only about 37.4–39.2° F (3–4° C).

The top 10 warmest years of average global recorded temperatures were in the last 15 years of the twentieth centry and saw devastating fires in Yellowstone National Park, flooding in Bangladesh, record number of hurricanes and tornados, and a deadly heat wave and drought in the southeastern United States. It is probable, based on computer models, that a resumption in warming will accompany changes in regional weather. A 40-year trend of increased precipitation in Europe and decreased precipitation in the African Sahel (Ethiopia, the Sudan, Somalia) may be an early consequence of global warming due to the greenhouse effect. Longer and more frequent heat waves would result in public health threats as well as inconveniences such as road buckling, electrical brownouts, or blackouts.

Precipitation is likely to increase regionally because as the temperature increases, more evaporation takes place, leading to more precipitation. The average precipitation event is likely to be heavier: wetter monsoons in coastal subtropics; more frequent and heavier winter snows at high altitudes and high latitudes; an earlier snowmelt, and a wetter spring.

Increases in rain- or snowfall are not expected to offset the effects of higher temperatures on soil, however. Higher temperatures are expected to dry the soil in North America and southern Europe, among other places, by boosting the rates of evaporation and transpiration through plants. More favorable agricultural conditions in high latitudes could move the center of agriculture farther north into Canada and Siberia and out of the United States.

Other consequences of global warming from the enhanced greenhouse effect include the reduction of sea ice, coastal sea level rises of several feet per century, more frequent and powerful hurricanes, and more frequent and severe forest fires. In the United States, the frequency of tornadoes is near or above record levels for the years 1990–1994.

The rise of sea level is the most easily predicted consequence. The one-degree increase in temperature over the past century contributed to a 4–-in (10–20-cm) rise in mean sea level. This could lead to severe and frequent storm damage, flooding and disappearance of wetlands and lowlands, coastal erosion, loss of beaches and low islands, wildlife extinctions, and increased salinity of rivers, bays and aquifers.

However, because the global atmosphere operates as a complex system, it is difficult, even with today's sophisticated computer models, to predict the exact nature of the changes we are likely to cause with increased greenhouse gases. Scientists have predicted that low-lying areas and islands, including the Seychelles, the Maldives, the Marshall Islands, and large areas of Bangladesh, Egypt, Florida, Louisiana, and North Carolina will disappear over the next few decades.

The earth's natural atmospheric cleanser — rain — may wash excess greenhouse gases out of the atmosphere. But until rates of greenhouse gases slow their rapid increases or actually begin to decrease, the planet will get warmer. In response to climate projections, the United Nations Framework Convention on Climate Change (UNFCC), adopted and signed by 162 countries in 1992 at the Rio Earth Summit, sets country-by-country standards to reduce the emissions of greenhouse gases, particularly carbon dioxide.

Policy-makers in the United States, including Vice President Albert Gore Jr., propose stricter requirements for more fuel-efficient cars, "environment taxes" that penalize heavy polluters and help pay for cleansing the atmosphere, and trading technological advances for rain forest protection in Third World countries. However, because global warming often is made a political ping-pong ball, changes in political administrations worldwide can extend to policy makers and climate researchers, who depend on government assistance for research.

The greatest controversy over slowing the rate of greenhouse gases injected into the atmosphere seems to be how to do it. Some scientists advocate increased use of nuclear power to reduce dependence on fossil fuels, but that carries its own controversies. Nuclear power plants are so energy-intensive just to build, the trade-off is negligible. Conservation and a switch from a dependence on fossil fuels to dependence on renewable resources such as wind and solar energy, slows the rate of increase of carbon dioxide emissions into the atmosphere.

Environmental Economics; Environmental Policy; The Global 2000 Report

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Changing by Begrees: Steps to Reduce Greenhouse Gases. U.S. Congress, Office of Technology Asessment, 1991.

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