The Greenhouse Effect

One of the most remarkable socioscientific issues the world faces today concerns possible changes in the earth's climate. In one sense, the idea that humans could have any effect on our planet's climate seems absurd. Look out the nearest window. Try to imagine the huge reaches of the earth's atmosphere as it extends thousands of miles around the earth and dozens of miles into space. How could the tiny contributions of human activities seriously change the nature of that enormous space?

Historically, most scientists have accepted this attitude. Humans might be able to control many forces of nature. But the idea that they could significantly alter the planet's most fundamental properties has always seemed highly unlikely.

Those attitudes--among both scientists and laypersons--have recently begun to change. Evidence appears to be accumulating that human activities may, in fact, be altering our atmosphere in some fundamental and, perhaps, troublesome ways. The term sometimes used to describe these changes is the greenhouse effect.

The term greenhouse effect refers to the accumulation of carbon dioxide in the earth's atmosphere, as a result of human activities, possibly producing climatic changes. Greenhouse effects are also described as global warming. Here is how the greenhouse effect occurs.

Radiation from the sun is a mixture of many different forms of electromagnetic energy: ultraviolet light, infared radiation, Xrays, and visible light, for example. Most forms of solar radiation are absorbed by molecules in the earth's upper atmosphere. Ozone in the stratosphere, for example, absorbs ultraviolet radiation. The only form of radiation that penetrates the atmosphere and reaches the earth's surface in large amounts is visible light.

About a third of the light that strikes the earth's surface is recycled back into the atmosphere. The other two-thirds is absorbed by rocks, soil, water, and other materials on the earth's surface. The visible solar radiation warms these materials and is then returned to the atmosphere in the form of heat. The process described here, then, is one in which visible light from the sun passes through the atmosphere, strikes the earth, is transformed into heat, and then reradiates into the atmosphere.

The heat radiation returned to the atmosphere is absorbed by molecules of carbon dioxide. The average temperature of the earth's atmosphere is, therefore, partially determined by the amount of reflected heat captured and held by carbon dioxide molecules.

This process is similar to the one that occurs in a greenhouse. Glass is transparent to visible light. Sunlight passes easily through greenhouse windows, strikes the plants and the floor of the building, is changed to heat, and reflects back into the air inside the greenhouse. But glass is not transparent to heat. So the heat produced by reflection of light off soil is held within the glass walls and ceiling of the building. The greenhouse continues to get warmer and warmer. The term "greenhouse effect" comes from the similarity of atmospheric processes to energy changes that take place within a greenhouse.

The amount of carbon dioxide in the atmosphere from natural processes remains roughly constant. It reaches the atmosphere when complex organic compounds are broken down through digestion, combustion, or decay. Those processes can be represented in a general way in the following chemical equation:

Carbon dioxide is also removed from the atmosphere by a number of natural processes. For example, green plants use carbon dioxide to make organic materials by the process known as photosynthesis. Oxygen is given off as a waste product in this reaction. The process is essentially the reverse of the one described above:

Humans affect the carbon dioxide balance in the atmosphere in many ways. When trees are cut down, the rate at which equation 2 occurs is reduced. Less carbon dioxide is removed from the atmosphere and concentrations in the atmosphere increase. This fact helps explain why we are concerned about the destruction of the earth's tropical rain forests.

By far the most important anthropogenic (human) influence on carbon dioxide in the atmosphere is the combustion of fossil fuels. The burning of coal, oil, and natural gas always produces carbon dioxide as one product. The more fossil fuels humans use to heat homes and buildings, to drive cars and other motor vehicles, to operate factories, and to generate electricity, the more carbon dioxide will be added to the air. Between 1950 and 1990, carbon dioxide emissions from all anthropogenic sources more than tripled, from 1,639 to 5,400 million metric tons of carbon equivalents.

Scientific experts largely agree about everything concerning the greenhouse effect said thus far. What they may not agree on is the importance of this information. Many authorities believe that increased levels of carbon dioxide will lead to a warming of the atmosphere. As the annual average temperature of the atmosphere increases, changes will begin to occur on the earth, they say. Polar ice caps will begin to melt, the earth's oceans will start to rise, and dramatic climatic changes will soon appear. Deserts may become fertile farmland, and heavily inhabited areas may become too arid for human survival.

Other scientists are less certain about these predictions. They think the earth's atmosphere is capable of adjusting to huge new additions of carbon dioxide without serious harm. They think that warnings of global disaster are not based on sound scientific evidence and should not be taken too seriously.

The problem is that no way exists for deciding who is right in this matter ... at least at the present moment. Atmospheric and climatic changes take place very slowly over decades and centuries. Even an unusually severe drought that lasts ten years means little or nothing in the grand scale of the history of the planet Earth.

We probably will not know for a very long time whether global warming is truly a matter of human concern or not. Under those circumstances, critics say, it would be foolish to spend the money needed and to make the sacrifices necessary to reduce our release of carbon dioxide into the atmosphere.

But can we afford to wait until we are certain about what is really happening in the earth's atmosphere, others ask. If we take no action until we have positive proof about global warming, it will be too late. The effect will already have caused its most severe changes on the earth's climate and on the nature of human civilization. We must act now, they insist, to make sure that such conditions can never occur.

Adapted from
Science and Social Issues by David E. Newton, pages 116-118, Walch Publisher, Maine, 1992.

created by: Debbie Anholt, anholt@lclark.edu
updated: May 9, 2000