ESL 91 ON THE WEB
The Greenhouse Effect
from Interdisciplinary English
by Loretta F. Kasper, Ph.D.
The Greenhouse Effect is the term used to describe the role the atmosphere plays in helping to warm the earth's surface. Since the late 1940s, concentrations of carbon dioxide in the earth's atmosphere have been steadily increasing. The amount of carbon dioxide in the atmosphere has increased by 0.4 percent a year because of the use of fossil fuels such as oil, gas, and coal, as well as the destruction of tropical forests, called deforestation. Other gases that contribute to the greenhouse effect, such as methane and chlorofluorocarbons, are increasing even faster.
How Do Greenhouse Gases Cause Global Warming?
Normally, rays from the sun that reach the surface of the earth warm the ground and are then re-emitted, or sent, as heat back into space. But carbon dioxide and gases such as methane, nitrous oxide, halocarbons, and ozone in the atmosphere create a blanket that traps some of the heat. This blanket acts like glass in a greenhouse: it lets warming rays through, but blocks the heat trying to get out. This heating effect is at the root of the theories concerning global warming.
The result of the greenhouse effect could be a worldwide rise in temperature, estimated at 2° to 6° C (4° to 11° F) over the next 100 years. Such a global warming trend might affect the world's climate and lead to a partial melting of the polar ice caps. This means that the increase in global temperature caused by the greenhouse effect could start melting the ice on the polar ice caps. This melting is something that has happened before in the earth's history between the various ice ages. If the polar caps melted, sea levels would rise. Because many of the world's major cities are built on the coasts, a general rise in sea level would cause massive flooding and subsequent destruction of these cities.
Possibly an increase in cloud cover or absorption of excess carbon dioxide by the oceans would halt the greenhouse effect before it reached the stage of polar melting. Nevertheless, research reports released in the U.S. in the 1980s indicate that the greenhouse effect is definitely under way and that the nations of the world should be taking immediate steps to deal with it.
The Ozone Layer
The earth has an umbrella of gas to product it from the dangerous radiation that the sun produces. This umbrella of gas is called the ozone layer. Ozone is a form of oxygen, created when ultraviolet radiation from the sun meets oxygen in the atmosphere. The ozone layer absorbs most of the dangerous ultraviolet radiation reaching the earth from the sun.
Under normal circumstances the atmospheric concentrations and interactions of ultraviolet radiation, ozone, oxygen, and other chemicals is in balance. Ozone is being made and broken down all the time, so that the amount of ozone in the atmosphere stays more or less the same. Unfortunately, pollutants and chemicals can destroy the ozone, upsetting the balance in the atmosphere. When this happens, the ozone layer is depleted, and the amount of dangerous ultraviolet radiation reaching the earth may increase.
The Chemical Threat To The Ozone Layer: CFCs
As a result of human activity, the composition of the atmosphere is changing, and the ozone layer is under threat from chemicals that we use on earth. In the 1970s, scientists discovered that certain long-used chemicals posed a possible threat to the ozone layer. The guilty chemicals are chlorofluorocarbons, called CFCs for short. CFCs are used in aerosol cans, refrigerators, some air conditioning systems, and some packaging materials.
When CFCs are released into the atmosphere, these chlorine-containing chemicals rise up to the ozone layer. When the CFC reaches the ozone layer, the ultraviolet radiation from the sun strikes it and releases chlorine from it. The chlorine then reacts with and destroys ozone molecules. The chlorine itself remains unchanged, so it continues to destroy ozone over and over again. CFCs can last for more than 100 years in the atmosphere, slowly moving up through the atmosphere before breaking down to produce the chemicals that destroy the ozone layer
The Hole In The Ozone
Recent satellite pictures have shown quite clearly that there is a hole appearing in the ozone over the Antarctic pole. In other areas of the world, the ozone layer has thinned out. Scientists have collected samples from these parts of the atmosphere and have found high levels of ozone-depleting chemicals in these areas.
Consequences of Ozone Depletion
The ozone layer is necessary for life as we know it. The ozone layer absorbs most of the dangerous ultraviolet radiation. If more of this radiation got through, it would cause an increase in skin cancer and in eye diseases such as cataracts. It has been estimated that a one percent depletion of the ozone layer would result in an extra 70,000 cases of skin cancer every year worldwide.
Increased ultraviolet radiation is not just harmful to human beings, it is harmful to all life on earth. Ultraviolet radiation damages crops, plants, and trees which form the basis of the food chains that support life on earth. The result of this damage would be a threat to the world's food supply. Plankton is the basis of the food chain in the sea. Plankton consists of tiny plants and animals and is eaten by larger marine creatures. If the plankton were killed as a result of increased levels of ultraviolet radiation, the fish would starve, the seas would die, and a major source of human food would be lost.
What Can Be Done To Save The Ozone Layer?
There are alternatives for virtually all the uses of CFCs. For example, aerosol sprays can be replaced by pump-action sprays, which cause no damage to the environment. The CFCs in refrigerators can be recycled; that is, taken out of an old refrigerator and put into a new one to be used again.
In 1987, a treaty for the protection of the ozone layer was signed and later ratified by 36 nations including the United States. A total ban on the use of CFCs was proposed by the European Union in 1989. In order to monitor ozone depletion on a global level, in 1991 NASA launched an Upper Atmosphere Research Satellite, which measures variations in the ozone level at different altitudes.
In addition, governments are working to reduce air pollution which produces greenhouse gases which can lead to the destruction of the ozone layer. The United States passed the Clean Air Act of 1967, with amendments in 1970, 1977, and 1990. The Clean Air Act is the legal basis for air-pollution control throughout the U.S. The Environmental Protection Agency (EPA) has primary responsibility for carrying out the requirements of the Clean Air Act, which specifies air-quality standards for hazardous substances. These standards are in the form of concentration levels that are believed to be low enough to protect public health.
On the international scene, in March, 1985, 49 countries signed an agreement designed to protect the ozone layer. This "Montréal Protocol," which was renegotiated in 1990, calls for the phaseout of certain chlorocarbons and fluorocarbons by the year 2000 and provides aid to developing countries in making this transition.