Greenhouse Effect

B

Carbon Dioxide

Carbon dioxide constantly circulates in the environment through a variety of natural processes known as the carbon cycle. Volcanic eruptions and the decay of plant and animal matter both release carbon dioxide into the atmosphere. In respiration, animals break down food to release the energy required to build and maintain cellular activity. A byproduct of respiration is the formation of carbon dioxide, which is exhaled from animals into the environment. Oceans, lakes, and rivers absorb carbon dioxide from the atmosphere. Through photosynthesis, plants collect carbon dioxide and use it to make their own food, in the process incorporating carbon into new plant tissue and releasing oxygen to the environment as a byproduct.

In order to provide energy to heat buildings, power automobiles, and fuel electricity-producing power plants, humans burn objects that contain carbon, such as the fossil fuels oil, coal, and natural gas; wood or wood products; and some solid wastes. When these products are burned, they release carbon dioxide into the air. In addition, humans cut down huge tracts of trees for lumber or to clear land for farming or building. This process, known as deforestation, can both release the carbon stored in trees and significantly reduce the number of trees available to absorb carbon dioxide.

As a result of these human activities, carbon dioxide in the atmosphere is accumulating faster than Earth’s natural processes can absorb the gas. By analyzing air bubbles trapped in glacier ice that is many centuries old, scientists have determined that carbon dioxide levels in the atmosphere have risen by 35 percent since 1750. And since carbon dioxide increases can remain in the atmosphere for centuries, scientists expect these concentrations to double or triple in the next century if current trends continue.

C

Methane

Many natural processes produce methane, also known as natural gas. Decomposition of carbon-containing substances found in oxygen-free environments, such as wastes in landfills, release methane. Ruminating animals such as cattle and sheep belch methane into the air as a byproduct of digestion. Microorganisms that live in damp soils, such as rice fields, produce methane when they break down organic matter. Methane is also emitted during coal mining and the production and transport of other fossil fuels.

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Methane has more than doubled in the atmosphere since 1750, and could double again in the next century. Atmospheric concentrations of methane are far less than carbon dioxide, and methane only stays in the atmosphere for a decade or so. But methane is an extremely effective heat-trapping gas—one molecule of methane is nearly 30 times more efficient at trapping infrared radiation radiated from the Earth’s surface than a molecule of carbon dioxide.

D

Nitrous Oxide

Nitrous oxide is released by the burning of fossil fuels, and automobile exhaust is a large source of this gas. In addition, many farmers use nitrogen-containing fertilizers to provide nutrients to their crops. When these fertilizers break down in the soil, they emit nitrous oxide into the air. Plowing fields also releases nitrous oxide.

Since 1750 nitrous oxide has risen by 18 percent in the atmosphere. Although this increase is smaller than for the other greenhouse gases, nitrous oxide traps heat about 300 times more effectively than carbon dioxide and can stay in the atmosphere for a century.

E

Ozone

Ozone is both a natural and human-made greenhouse gas. Ozone in the upper atmosphere is known as the ozone layer and shields life on Earth from the Sun’s harmful ultraviolet radiation, which can cause cancer and other damage to plants and animals. However, ozone in the lower atmosphere is a component of smog (a severe type of air pollution) and is considered a greenhouse gas. Unlike other greenhouse gases, which are well-mixed throughout the atmosphere, ozone in the lower atmosphere tends to be limited to industrialized regions.

F

Fluorinated Compounds

Some of the most potent greenhouse gases emitted are produced solely by human activities. Fluorinated compounds, including CFCs and HCFCs, are used in a variety of manufacturing processes. For each of these synthetic compounds, one molecule is several thousand times more effective in trapping heat than a single molecule of carbon dioxide.

CFCs, first synthesized in 1928, were widely used in the manufacture of aerosol sprays, blowing agents for foams and packing materials, as solvents, and as refrigerants. Nontoxic and safe to use in most applications, CFCs are harmless in the lower atmosphere. However, in the upper atmosphere, ultraviolet radiation breaks down CFCs, releasing chlorine into the atmosphere. In the mid-1970s, scientists began observing that higher concentrations of chlorine were destroying the ozone layer in the upper atmosphere. Beginning in 1987 with the Montréal Protocol on Substances that Deplete the Ozone Layer, representatives from 47 countries established control measures that limited the consumption of CFCs. By 1992 the Montréal Protocol was amended to completely ban the manufacture and use of CFCs worldwide, except in certain developing countries and for use in special medical processes such as asthma inhalers.

Scientists devised substitutes for CFCs, including HCFCs, HFCs, and PFCs. Since HCFCs still release ozone-destroying chlorine in the atmosphere, production of this chemical will be completely phased out by the year 2030. Although HFCs and PFCs contain no chlorine and are not harmful to the ozone layer, they are nevertheless powerful greenhouse gases.

Another synthetic chemical, sulfur hexafluoride, is one of the most potent greenhouse gases ever produced. This synthetic gas compound has nearly 24,000 times the warming effect as an equal amount of carbon dioxide. However, it is released into the atmosphere in relatively small quantities.