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The Greenhouse Effect

Within the atmosphere are small amounts of a number of important gases, popularly called greenhouse gases, because they alter the flow of life and heat-energy through the atmosphere, much as does the glass shell of a greenhouse. Their effect on incoming solar energy is minimal, but collectively they act as an insulating blanket around the planet. They do this by absorbing and returning to the earth’s surface much of its outgoing heat, trapping it within the lower atmosphere. A greenhouse effect is natural and essential to a livable climate on Earth.

Greenhouse gas concentrations, however, are being drastically affected by human activities. One of the most important gases, carbon dioxide, is an important nutrient for plants, but it is potentially dangerous to our climate if enormously augmented. Its concentration has increased from about 280 parts per million in 1850 to about 350 today, mainly by a large increase in fossil fuel burning, forest removal, and agriculture. Other gases, such as nitrous oxide, methane, and surface ozone, although they are less abundant, are also increasing rapidly and are potentially dangerous. Man-made chloroflourocarbons (CFCs) are used as, among other things, coolants in refrigerators and air conditioners. The most common industrially produced CFCs, although measured in parts per trillion, are among the most potent and the most rapidly increasing greenhouse gases in existence. One free chlorine atom, produced in the stratosphere by the effects of ultraviolet light on CFCs, can eliminate 100,000 molecules of ozone.

The result? Increased concentration of greenhouse gases enhances the global greenhouse effect, trapping more heat near the Earth’s surface. A warmer atmosphere can hold more water vapor, which is itself a powerful greenhouse gas, and amplify the warming. On the other hand, the increase in airborne moisture may mean more clouds, which would cut off sunlight and limit or modulate warming.

On the basis of climate models, some scientists predict a potential increase in global surface temperature between 1.5° C and 4.5° C in the next fifty years. This may not seem like much, but 4.5° C equals the total temperature rise since the peak of the last ice age 18,000 years ago, and the increase will be even higher in some regions. The average could be slightly lower in the tropics, but at least doubled at high latitude-mainly because of the disappearance of ice and snow. Snow-free land surfaces absorb more of the Sun’s rays than snow surfaces, so that warming by the Sun will increase as the duration of snow-cover diminishes. Moreover, the increases in temperature can be quite large where there is relatively low energy from the Sun because of the very shallow, strong temperature inversions typical of the Arctic cold season. Reduced ice-cover on the polar seas will also increase the heat transfer from water to the overlying air.

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