Causes of climate change

Causes of
Climate Change

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
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Reducing emissions from deforestation and forest degradation (REDD) in developing countries


Climate changes over time. About 18,000 years ago, a sheet of glacial ice up to 10,000 feet (3,000 meters) thick covered much of what is now Greenland, Canada, and the northern United States. A warming trend gradually melted almost all the glaciers, except in Greenland. In Canada and the United States, the last large fields of ice had disappeared by about 11,500 years ago. The warming trend ended after a mild period from 7,000 to 5,000 years ago, when the global average temperature was higher than it is today.

Within the past 1,200 years, the period between about A.D. 950 and 1250 was mild. The years from about 1400 to 1850 were cool. Since then, global average temperatures generally have risen.

Many natural processes influence a region's climate. Some of these processes, such as volcanic eruptions, are short-lived and cause short-term changes. Other processes, such as mountain building, occur over long periods and cause long-term changes in climate. Human activity also may affect climate.

Volcanic eruptions can cause short-term cooling over large portions of the planet, especially if the eruptions throw large amounts of sulfur gases high into the atmosphere. The sulfur gases combine with moisture to produce droplets of sulfuric acid and tiny sulfate particles. The sulfur droplets and particles created by major volcanic eruptions absorb some solar radiation and reflect some back to space. As a result, less solar radiation reaches Earth's surface, and so air temperatures fall.

Because the droplets and particles are so small, they can remain suspended in the atmosphere for months or years. Meanwhile, winds carry them around the globe. Scientists believe that volcanic eruptions can cause a maximum global cooling of about 2 Fahrenheit degrees (1 Celsius degree).

In 1991, the eruption of Mount Pinatubo in the Philippines threw large amounts of sulfur gases high into the atmosphere. This eruption likely caused a drop of 1.1 Fahrenheit degree (0.6 Celsius degree) in the global average temperature during the following few years.

Changes in ocean circulation can alter the climate. For example, changes in ocean currents that occur during El Nino can affect the climate for a year or two.

El Nino is a large-scale interaction between the tropical atmosphere and tropical oceans that happens about every two to seven years. Changes in the air pressure over the tropical Pacific Ocean cause the trade winds there to weaken or even reverse direction. This change enables the warm waters of the ocean surface to drift from the western tropical Pacific to the eastern tropical Pacific. This flow makes sea-surface temperatures lower than usual over the western tropical Pacific and higher than usual over the eastern tropical Pacific.

The sea-surface temperature changes, in turn, alter the circulation of the atmosphere in tropical and middle latitudes. These alterations cause weather extremes in various parts of the world. Heavy rains drench the normally arid coastal plain of western South America, drought is more likely in Hawaii and eastern Australia, and winters are wetter than normal along the Gulf of Mexico coast.

Activity on the sun's surface may affect Earth's climate for short periods. Sunspots are dark, relatively cool blotches that appear on the surface of the sun. Faculae are relatively bright, hot areas on the solar surface. The number of sunspots and faculae increases and decreases over a cycle of about 11 years. The average amount of energy given off by the sun is slightly higher during a sunspot maximum-when the number of sunspots and faculae is high. The average amount is slightly lower during a sunspot minimum-when the number is low.

Climatologists are not sure about the relationship between changes on the sun's surface and variations in Earth's climate. During the period from 1645 to 1715, the number of sunspots was unusually low. This episode corresponds to a portion of the Little Ice Age, a time of relatively cool conditions. Climatologists have not proved, however, that the reductions in the number of sunspots caused the cooling.

Changes in carbon dioxide concentration in the atmosphere may cause short-term and long-term variations in the climate. Atmospheric CO2 slows the flow of heat from Earth to space. This gas absorbs heat that radiates from Earth's surface and radiates heat back to the surface.

Human activity is currently increasing the level of atmospheric CO2, but this level has varied significantly throughout the history of Earth. With past variations in atmospheric CO2, the global climate has warmed or cooled. For example, about 100 million years ago, volcanic activity on the floor of the Pacific Ocean released enough CO2 to cause global warming of perhaps 18 Fahrenheit degrees (10 Celsius degrees).

Changes in Earth's orbit about the sun may cause climate changes over tens of thousands to hundreds of thousands of years. Milutin Milankovitch, a Serbian mathematician, proposed in the 1940's that three orbital variations change how sunlight is distributed seasonally and geographically over the planet: (1) a precession (wobble) of Earth's axis, (2) a variation in the tilt of the axis, and (3) a variation in the path of the orbit.

The precession of the axis varies over a period of 23,000 years. This cycle alters the times of the year when earth is closest to the sun and farthest from the sun.

The tilt of the axis changes from 22.1° to 24.5° over a period of 41,000 years. This cycle affects the contrast between winter and summer temperatures.

The path of the orbit varies over a period of 100,000 years. The orbit is always elliptical-that is, shaped like a flattened circle. But during the 100,000-year cycle, the amount of flatness changes from a maximum to a minimum, then back to the maximum. This variation changes the distance between Earth and sun over the course of a year.

These three orbital cycles probably have altered global temperatures at regular intervals throughout Earth's history. For example, climatologists believe that the cycles may have governed the major fluctuations in the planet's glacial ice cover during the Pleistocene Epoch, the period from about 2 million years ago to 11,500 years ago. The three cycles probably produced temperature changes that caused the ice cover to expand and contract at regular intervals.

Continental drift is an extremely slow process that influences the climate over periods of tens of millions of years. The planet's solid outer shell, which is typically 60 miles (100 kilometers) thick, is divided into about 30 large plates that move slowly over the surface of the globe. The continents are embedded in these plates and slowly drift with them, a process known as continental drift. About 200 million years ago, there was just one huge continent, called Pangaea. Pangaea split into fragments that drifted apart and eventually reached their present locations as the continents we know today.

Continental drift helps explain the presence of coral reef fossils in Wisconsin and of tropical plant fossils in areas north of the Arctic Circle. When the coral and plants were alive, Wisconsin and the arctic regions were at much lower latitudes than they are today.

Mountain building, another extremely slow process, likely helped set the stage for the ice ages during the Pleistocene Epoch. About 30 million to 40 million years ago, the Himalaya and the adjacent Tibetan plateau began to rise in southern Asia. At about the same time, mountain building began in the western part of the United States. Half the total uplift of the Himalaya and the Colorado Plateau may have occurred within the past 10 million years. The rise of these massive landforms likely altered the wind belts that encircle the planet. As a result, Earth's climate became more diverse, wetter, and colder.

Human activity also affects the climate. The building of cities, the clearing of forests, and the burning of oil, coal, and natural gas can all cause climatic changes. Climatologists disagree, however, about the impact that human activity has had on climate, particularly on the recent global warming trend.

The construction of cities creates areas that are warmer and drier than the surrounding countryside. Cities are drier because they have storm sewer systems that quickly carry off rainwater and snowmelt.

Cities are warmer for several reasons. The use of storm drainage systems means that less solar radiation is used to evaporate water and more is used to heat the city surfaces and air. The brick, asphalt, and concrete surfaces readily radiate the heat they absorb and so raise urban air temperatures even more. In addition, cities themselves generate heat from a number of sources, including motor vehicles and heating and air conditioning systems.

Large urban areas also affect the climate in the areas downwind of them. Smokestacks and automobile tailpipes in cities emit water vapor and tiny particles that stimulate the formation of clouds. Heat from a city also spurs the growth of clouds. Thus, the climate downwind from many large urban areas is cloudier and wetter than the climate upwind from those same areas.

The burning of fossil fuels has contributed to recent increases in the amount of CO2 in the atmosphere. Since the mid-1800's, the level of atmospheric CO2 has risen about 25 percent, mainly because of an increased use of fossil fuels for transportation, space heating, and generation of electric energy. The clearing of forests also contributes to the buildup of atmospheric CO2 by reducing the rate at which the gas is removed from the air. Trees and other green plants remove CO2 from the air during photosynthesis-the process they use to produce food.

Climatologists disagree about the impact that the burning of fossil fuels and the clearing of forests have had on climate. Global temperature records indicate that a warming trend began in the late 1800's. It was interrupted by an episode of cooling from about 1940 to the late 1970's. The cooling effect of the Mount Pinatubo eruption also interrupted the trend in the early 1990's. Some scientists argue that the buildup of atmospheric CO2 due to human activities has caused the warming trend. Others argue that recent warming is merely a natural fluctuation of the climate.

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.


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