Greenhouse effect

is a warming of the lower atmosphere and surface of a planet by a complex process involving sunlight, gases, and particles in the atmosphere. On the earth, the greenhouse effect began long before human beings existed. However, recent human activity may have added to the effect. The amounts of heat-trapping atmospheric gases, called greenhouse gases, have greatly increased since the mid-1800's, when modern industry became widespread. Since the late 1800's, the temperature of the earth's surface has also risen. The greenhouse effect is so named because the atmosphere acts much like the glass roof and walls of a greenhouse, trapping heat from the sun.

Causes of climate change
Impact Global Warming
Limited Global Warming
Agreement on global warming
Analyzing global warming
Kyoto Protocol
Greenhouse effect
Scientific research
Why climates vary
Ocean problems
Southern Ocean
Pacific Ocean
Ozone hole
Environmental problems by petroleum
Changes in the atmosphere
Increasing Temperatures
Can Earth Explode ?
NASA Study
El Nino
The Procedure Of Implementation Afforestation And Reforestation Project Under The Clean Development Mechanism (CDM) In Indonesia
Reducing emissions from deforestation and forest degradation (REDD) in developing countries



Climate is the weather of a place averaged over a length of time. Earth's climate varies from place to place, creating a variety of environments. Thus, in various parts of Earth, we find deserts; tropical rain forests; tundras (frozen, treeless plains); conifer forests, which consist of cone-bearing trees and bushes; prairies; and coverings of glacial ice.

Climate also changes with time. For example, a thousand years ago, northern latitudes were milder than they are today. The warmer climate enabled Vikings from Iceland to settle on the southern coast of Greenland. But the colder climate that developed over the following centuries wiped out the settlements.

One major environmental concern is that human activity may be changing the global climate. The burning of fossil fuels-coal, oil, and natural gas-to power motor vehicles, heat buildings, generate electric energy, and perform various industrial tasks is increasing the amount of carbon dioxide (CO2) gas released into the atmosphere. Fossil fuels contain carbon, and burning them produces CO2. This gas slows the escape of heat released by Earth into space. Thus, an increase in atmospheric CO2 may cause global warming-a rise in the temperature of the air next to Earth's surface.

Global warming could change rainfall patterns, leading to shifts in plant and animal populations. It could also melt enough polar ice to raise the sea level, and it could increase the frequency and severity of tropical storms.

Why climates vary

Climates vary from place to place because of five main factors: (1) latitude (distance from the equator), (2) altitude (height above sea level), (3) topography (surface features), (4) distance from oceans and large lakes, and (5) the circulation of the atmosphere.

The role of latitude. The sun continually sends electromagnetic radiation into space. Most of the radiation is visible light, and it also includes infrared (heat) rays and ultraviolet rays. About 30 percent of the radiation that reaches Earth's atmosphere is reflected back into space, mostly by clouds. The remaining 70 percent is absorbed by the atmosphere and Earth's surface, heating them.

The intensity of the solar radiation reaching the atmosphere decreases with increasing latitude. The intensity depends on how high in the sky the sun climbs. The closer a place is to the equator, the higher the climb.

At latitudes between 23 1/2° north and 23 1/2° south, the sun is directly overhead at noon twice a year. In these cases, the sun's rays shine directly down toward the surface. The radiation that reaches the atmosphere is therefore at its most intense.

In all other cases, the rays arrive at an angle to the surface and are therefore less intense. The closer a place is to the poles, the smaller the angle and therefore the less intense the radiation. Due to decreases in the intensity of radiation, average temperatures decline from the equator to the poles. Seasonal changes in solar radiation and the number of hours of sunlight also vary with latitude.

In tropical latitudes (those near the equator), there is little difference in the amount of solar heating between summer and winter. Average monthly temperatures therefore do not change much during the year.

In middle latitudes, from the Tropic of Cancer to the Arctic Circle and from the Tropic of Capricorn to the Antarctic Circle, solar heating is considerably greater in summer than in winter. In these latitudes, summers are therefore warmer than winters.

In high latitudes, north of the Arctic Circle and south of the Antarctic Circle, the sun never rises during large portions of the year. Therefore, the contrast in solar heating between summer and winter is extreme. Summers are cool to mild, and winters are bitterly cold.

The role of altitude. The higher a place is, the colder it is. Air temperature drops an average of about 3.5 Fahrenheit degrees per 1,000 feet of altitude (6.5 Celsius degrees per 1,000 meters). The temperature of the air determines how much precipitation falls as snow, rather than rain. Even in the tropics, it is not unusual for mountaintops to be snow-covered.

The role of topography. The surface features of Earth influence the development of clouds and precipitation. As humid air sweeps up the slopes of a mountain range, the air cools, and so clouds form. Eventually, rain or snow falls from the clouds. Some of the rainiest places on earth are on windward slopes, those facing the wind.

As winds blow down the opposite slopes, known as the leeward slopes, the air warms, and clouds thin out or vanish. Leeward slopes of mountain ranges are therefore dry. In addition, a rain shadow (dry area) may stretch hundreds of kilometers downwind of a mountain range.

Oceans and large lakes make the air temperature less extreme in places downwind of them. An ocean or lake surface warms up and cools down more slowly than a land surface. Thus, between summer and winter, the temperature of the water varies less than the temperature of the land. The temperature of the water strongly influences the temperature of the air above it. Therefore, air temperatures over the ocean or a large lake also vary less than air temperatures over land. As a result, places that are immediately downwind of the water have milder winters and cooler summers than places at the same latitude but well inland.

San Francisco and St. Louis, for example, are at about the same latitude and therefore receive about the same amount of solar radiation during the year. But San Francisco is immediately downwind of the Pacific Ocean, and St. Louis is well inland. Consequently, San Francisco has milder winters and cooler summers.

Atmospheric circulation influences climate by producing winds that distribute heat and moisture. Six belts of wind encircle Earth: (1) trade winds that blow between 30° north latitude and the equator, (2) trade winds that blow between the equator and 30° south latitude, (3) westerlies (winds from the west) that blow between 30° and 60° north of the equator, (4) westerlies blowing between 30° and 60° south of the equator, (5) polar winds north of 60° north latitude, and (6) polar winds south of 60° south latitude.

Trade winds north of the equator blow from the northeast. South of the equator, they blow from the southeast. The trade winds of the two hemispheres meet near the equator, causing air to rise. As the rising air cools, clouds and rain develop. The resulting band of cloudy and rainy weather near the equator is called the doldrums.

Westerlies blow from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere. Westerlies steer storms from west to east across middle latitudes.

Westerlies and trade winds blow away from the 30° latitude belt. Over broad regions centered at 30° latitude, surface winds are light or calm. Air slowly descends to replace the air that blows away. Descending air warms and is dry. The tropical deserts, such as the Sahara of Africa and the Sonoran of Mexico, occur under these regions of descending air.

Polar winds blow from the northeast in the Arctic and from the southeast in the Antarctic. In the Northern Hemisphere, the boundary between the cold polar easterly winds and the mild westerly winds is known as the polar front. A front is a narrow zone of transition, usually between a mass of cold air and a mass of warm air. Where the air masses overlap, storms can develop and move along the polar front, bringing cloudy weather, rain, or snow.

As the seasons change, the global wind belts shift north and south. In the spring, they move toward the poles. In the fall, they shift toward the equator. These shifts help explain why some areas have distinct rainy seasons and dry seasons. Parts of Central America, North Africa, India, and Southeast Asia have wet summers and dry winters. Southern California and the Mediterranean coast have dry summers and wet winters.

Contributor: Joseph M. Moran, Ph.D., Professor Emeritus, Department of Earth Science, University of Wisconsin, Green Bay; Associate Director, Education Program, American Meteorological Society.

Source : World Book 2005.