Southern AERA Quarterly Activity Bulletin of The South Carolina Department of Natural Resources-Southeast Regional Climate Center
Volume 5, No. 3
What is the Greenhouse Effect??
The greenhouse effect is a natural phenomenon that is caused by certain "greenhouse" gases in the Earth's atmosphere. These greenhouse gases are transparent to the short wavelength radiation (mostly visible "light") that comes from the sun, but opaque to the long wavelength radiation (infared radiation) that leaves the Earth. These gases then reemit the trapped energy, partly toward the Earth's surface. The result of the greenhouse effect is a global temperature 33 degrees Celsius, or 59 degrees Fahrenheit warmer than it would otherwise be. Some of the most important greenhouse gases include carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and water vapor. Within an acceptable range, these gases are advantageous in would be too cold on earth to live here without them.
Carbon dioxide is the best-known greenhouse gas because of the large amount of it present in the atmosphere. Carbon dioxide is present naturally, but the amount of this greenhouse gas in the atmosphere has been growing since the Industrial Revolution. The combustion of fossil fuels (coal, natural gas, and petroleum) characteristic of industrial societies has raised the amount of carbon dioxide in the atmosphere at a great rate.
Another factor that has contributed to the large amount of carbon dioxide in the air is deforestation. Carbon dioxide is released when organic material is burned or decays. In addition, forests are a sink for Carbon dioxide, meaning that Carbon dioxide is absorbed by forests. Large amounts of tropical rainforests have been cleared for agriculture and ranching. These tropical areas have also been the focus of inefficient logging operations. The United Nations estimates that during the 1980s, rainforests were destroyed at a rate of 38 million acres per year.
Deforestation and the combustion of fossil fuels has raised the amount of carbon dioxide in the atmosphere to the highest levels ever recorded. In fact, from the mid-nineteenth century to 1994, there was an increase of more than 25% of the carbon dioxide in the air. This increase closely matches the growth in carbon dioxide emissions. This increase in carbon dioxide in the atmosphere is believed by many climatologists to be a cause of an enhanced greenhouse effect. In addition to carbon dioxide, there are several other gases that may contribute to global warming. Although they are present in smaller quantities, they are also important.
Methane is an extremely effective greenhouse gas. It is actually a more effective absorber of long wave radiation than carbon dioxide. Methane occurs naturally in the atmosphere. It is produced in places where oxygen is scarce such as swamps, bogs and rice paddies. Methane is also produced in the intestinal tracts of cattle and sheep. The concentrations of methane in the atmosphere has doubled since 1800 because of the increase in human population; as population has risen, so have the number of rice paddies and cattle.
Another greenhouse gas whose increase is closely tied with agriculture is nitrous oxide. When fertilizers are used to increase crop yields, some of the nitrogen goes into the air as nitrous oxide. This gas is also results from high-temperature combustion of fossil fuels.
Chlorofluorocarbons, (CFCs) are manufactured chemicals with many uses. They have been used to make foam, cleaners, aerosol sprays, and coolants for refrigeration and air conditioning equipment. CFCs have become notorious for their ozone destroying capabilities, but are also very effective greenhouse gases. CFCs were not invented until the 1920s, but they already contribute to the greenhouse effect as much as methane. Atmospheric concentrations of CFCs are already diminishing, however, as a result of laws that have been put in place to limit their availability.
Water vapor is also a greenhouse gas, as it absorbs long waves that would otherwise escape to space. It exists naturally in our atmosphere and the amount of it has not risen considerably because of human effects on the environment.
Carbon dioxide is the most important gas causing the greenhouse effect, but trace gases (methane, nitrous oxide, and CFCs) could collectively double the impact of carbon dioxide in the future. Although it has been proven that these gases do trap the Earth's long wave radiation causing a warming effect and that the emissions of these gases have been increasing over time, there is still speculation surrounding the issue of global warming.
Speculations and Certainties
There has been speculation as to whether or not an enhanced greenhouse effect is influencing the Earth and whether or not it ever will. Since the late 19th century, however, the mean surface temperature has risen 0.3 - 0.6 degrees Celsius. Thirteen of the hottest years in over a century were between 1980 and 1992. Satellites show a shrinking snow cover in the Northern Hemisphere. If emissions continue to grow at projected rates, computer models predict that there will be an increase in temperature 1.5 - 5.5 degrees Celsius between 1990 and 2100. A temperature increase this large would come close to equaling the amount of warming that has taken place since the last glacial stage 18,000 years ago. According to most of the studies done by the Inter-governmental Panel on Climate Change, "the observed warming trend is unlikely to be entirely natural in origin". The IPCC points out that their ability to understand the extent of human influence on climate change is limited due to the "noise of natural variability", but also states that "the balance of evidence suggests that there is a discernible human influence on global climate". Even with reports such as this, speculation lingers due to the fact that most scientists agree that many more observations need to be conducted on this issue.
The issue of global warming is extremely complex, and involves many relationships between greenhouse gases and processes such as carbon and energy flows on Earth. Although studies show that the recent warming trend is likely to be tied with human intervention, there is still much more research to be done. The future of the climate may very well depend on the decisions made by humans about greenhouse emissions.
Internet Resources on the Greenhouse Effect: