The greenhouse effect is a naturally occurring phenomenon in which the atmosphere of the Earth traps heat from the sun (see The Greenhouse Effect Diagram attachment). Typically, our atmosphere absorbs just the right amount of heat so that living things can survive, and reflects back into the atmosphere the rest of the heat. Essentially, the atmosphere acts like the glass in a greenhouse. As a result of this, the Earth's surface is about 53°F (12°C) warmer than it would be without the greenhouse effect.

However, some gases (especially CO₂, methane, nitrous oxide, and CFCs) also contribute to trapping heat within the atmosphere. Methane is a product of natural decay from living (or once-living) things; nitrogen oxides are generally a result of human-made burning, automobiles and similar internal-combustion engines; and CFCs are a class of chemicals once commonly used in air conditioners, refrigerators and as the pressurizing gas in aerosol spray cans. The higher the concentration of these gases in the atmosphere, the warmer the Earth's temperature becomes. CO₂ makes up about 50% of the heat-retaining gases in the atmosphere, and methane makes up about 20%. Each molecule of methane absorbs infrared radiation 20 times more effectively than a molecule of CO₂.

Figure 2. Composition of the Earth's atmosphere, today and in earlier times. click for copyright

Global warming is a scientific theory that says that increased amounts of greenhouse gases (such as carbon dioxide) in the atmosphere trap too much heat and cause the average temperature on Earth to increase. Scientists advise that even a 2-3ºF increase in the average temperature of the Earth could trigger disasters. Some scientists predict that if the amount of CO₂ in the air doubles, the average world temperature would rise about 4.5%.

The amount of CO₂ entering the air increases when fossil fuels are burned and the excess cannot be used by plants (especially since we are eliminating them, too). The excess CO₂ absorbs heat energy from the sun, keeping the heat near the surface of the Earth, which raises the Earth's temperature. The concentration of CO₂ in the atmosphere has doubled in the last 100 years and scientists expect it to double in the next 100 years as well.

Scientists predict that these changes will trigger disaster. For example, a major shift in weather patterns could cause droughts, tropical storms and increase temperatures that would make some currently habitable areas of the Earth become uninhabitable. It is also speculated that melting polar ice caps could cause a rise in sea levels and, in turn, flood low-lying areas, such as coastal cities like New York City and San Francisco. The melting of the icecaps could also dilute marine saline concentrations, threatening marine life.

While most scientists believe that the greenhouse effect will gradually warm the Earth's climate, some have a different view. Some scientists predict that as the temperature rises, more water will evaporate from the oceans, resulting in so many clouds that it could block out sunlight, causing an overall decrease in the Earth's average temperature. This increased atmospheric reflectivity is also called an increase in the Earth's albedo.

Forests have been called the "lungs of the Earth" because animals inhale oxygen and exhale carbon dioxide in the process of breathing, and plants take in carbon dioxide and give off oxygen in the process of photosynthesis. See The Carbon Cycle Diagram attachment.

Every year, more than 28 million acres of tropical forest are cut and burned to clear land for farming. Many scientists agree that about 25% of the CO₂ being released into the air comes from burning the rain forests. For example, it is estimated that in 1989 alone, the burning of Brazilian forests probably added about 350 million tons of CO₂ to our atmosphere.

Environmental engineers are concerned about photosynthesis because plants help clean the air of harmful CO₂ gas, replacing it with oxygen. With the decreasing numbers of trees in the world, the air is not being cleaned as well. Additionally, the CO₂ levels continue to increase due to increasing numbers of automobiles and industrial pollution. Environmental engineers are continually challenged to find methods to reduce CO₂ emissions from industry and cars, and find ways to clean our polluted air.