Global Climate Change

The global climate has changed throughout geologic time on timescales that vary from a few years to millions of years. Some short-term global changes occur during volcanic eruptions. In 1991 Mount Pinatubo in the Philippines erupted, producing huge quantities of gas and ash reaching the stratosphere, where it blocked some of the solar radiation from reaching the earth's surface, resulting in a worldwide cooling. It is estimated that 20,000,000 tons of SO2 gas erupted into the stratosphere, and the resulting H2SO4 (sulfuric acid) reflected 2 to 4 percent of the incoming solar radiation back into the space, reducing the mean global temperature at the surface about 0.5°C. In some parts of the Northern Hemisphere the drop in temperature was as much as 1°C. Some older eruptions (for example, the famous Krakatoa, Indonesia, eruption of 1883) reduced worldwide temperatures about 0.5°C, about the same as the change with Mount Pinatubo, cooling the earth for almost ten years. In 1815 Tambora

(Indonesia) produced an even larger eruption, causing the Northern Hemisphere to experience the "year without a summer" in 1816. In northern New England snow fell June 5 and 6, and western Europe experienced many crop failures.

That volcanic eruptions cause climatic change was recognized by Benjamin Franklin, who in 1784 suggested that the cold weather in Europe during 1783 and 1784 was due to massive lava eruptions in Iceland. Single eruptions like these appear to have short-term effects on global climate with no long-term consequences. However, scientists have wondered what would happen if a number of volcanoes erupted continuously over a long period of time. Would the earth's climate cool to such a drastic degree that many animals and plants would become extinct because of the loss of habitat? One of the proposals that explains the extinction of the dinosaurs involves such a scenario, that is, when huge volumes of basalt erupted in India over a 500,000-year interval. The effect of eruptions such as Mount Pinatubo are one of the reasons that people became concerned about one possible outcome of nuclear war—a nuclear winter. Megaton exploding weapons would create huge volumes of dust that would be thrown up into the atmosphere like the dust of volcanic eruptions. Combined with the ash of the resulting fires, the dust would cause a dramatic cooling of the earth, a "winter" that could cause famine and other forms of deprivation that would kill most of the world's population. This explanation may be part of the most commonly accepted scenario of the dinosaur demise that occurred at the end of the Cretaceous period. It is believed that a meteorite struck the earth in the Yucatan Peninsula area of Mexico, causing a voluminous amount of debris to be ejected from the surface into the atmosphere. The debris resulted in a meteoritic winter that killed the dinosaurs and many other animals.

Another type of short-term climate change is the so-called El Niño effect, which occurs every 2 to 7 years around Christmastime, when the typical weather pattern in the Pacific Ocean breaks down. The trade winds weaken, and low pressure establishes itself in the central Pacific, causing winds to blow into the Pacific Basin from the west. This change causes warm surface water to move toward South and Central America, reversing the directions of some currents and placing warm water along the coasts. The warm water produces heavy rain over the coastal deserts, subjecting them to disastrous flooding and erosion. The western United States also experiences major flooding events as a result of El Niño. This temporary influx of rain on arid and semiarid regions creates the conditions for disastrous downslope movements in the form of mud slides, destroying entire villages and taking its toll of human life. Shifts in warm water in the southern Pacific and Indian Oceans cause cooler water to move along the coast of Australia, resulting in decreased rainfall.

One result of this temporary climate change is the occurrence of major brush fires, which destroy not only vegetation but also wild and domestic animals. The warm water El Niño brings to the coasts of North and South America substantially reduces the upwelling of cold, deep water, adversely affecting the cold water fisheries off the coast of Peru and Ecuador and creating economic hardships.

The most dramatic recent global climate change occurred during the Pleistocene epoch when the earth cooled sufficiently to allow glaciers to form, enlarge, and ultimately cover about 30 percent of the land surface (see Glaciation and Ice Caps and Glaciers). The glacial periods alternated many times with warm intervals called interglacials. Presently, we are in a warm period, and if all of the factors are still in place that caused the previous cooling, the earth should experience another glacial episode about 10,000 years from now. Glaciation causes changes in ocean circulation and weather patterns and a drop in sea level; all of these changes combined with other factors produce shifts in the climatic belts and cause the redistribution of plants and animals. The earth began to cool about 40 million years ago, eventually bringing about a dramatic cooling. Although it is still not entirely clear why glaciation occurs, it involves changing positions and configurations of the ocean basins and continents, uplifts of mountains, and changes in the amount of solar radiation striking the earth, as described by the Milankovitch cycles. Once ice and snow accumulate, there is a feedback process that enhances solar radiation loss as the white surface reflects solar radiation back into space. The glaciers underwent a series of advances and retreats with corresponding shifts in climate.

About 60 million years ago the climatic parameters were very different than they are today. The polar latitudes were warmer, and the ocean surface temperatures near Antarctica were 10° to 15° warmer than today. The data suggesting these temperature differences are derived from analysis of the composition of the shells of one-celled animals, which are used as geologic thermometers. At this time there was less of a temperature difference between the tropics and the poles than there is today. Ocean circulation may have been slower because there were no large volumes of cold polar water to sink and cause vertical circulation, and the vertical changes in temperature in the oceans were thus not as pronounced. Most of the United States was located in tropical and subtropical climates. In the arctic, nicely preserved fossils, including crocodiles and palm trees, have been discovered. The area of tundra and deserts were greatly reduced as compared to today.

Why were climates so different at that time? Perhaps it is the result of the continents being low and partly covered by oceans. Europe, North America, and Greenland were beginning to separate, and as the North Atlantic began forming, large amounts of greenhouse gases were released from volcanic eruptions, causing global warming.

A major ice age that lasted tens of millions of years occurred during the Carboniferous period. For this length of time the conditions that created glaciation had to persist and were probably related to changes in size, shape, and orientation of the continents and oceans. For one thing, continents had to have been located at the poles (as glaciers cannot be built on the ocean surface), and indeed, part of the super continent Pangaea was located in the southern polar area. Pangaea also blocked oceanic circulation. Many scientists believe that the possibility of glaciation increases when there is more north-south oceanic circulation than east-west flow. The north-south movement of warm water evaporates more cold water and provides the moisture for snow accumulation. Glaciation ended because Pangaea broke up, changing oceanic circulation and moving away from the poles.

In the central United States there are thick deposits of sediments that contain many layers of coal formed during the Carboniferous period. In general, the layer sequence includes coal, clay, sand, clay, and coal and is repeated many times. The coal derived from plants that lived in swamps shows that the climate was warm at the time. The sequence of layers is interpreted to be the result of flooding, the gradual lowering of sea level, and the rising of sea level and flooding again, repeated over and over. The best explanation for this oscil lating sequence is the rise and fall of the oceans during glaciation.

Fossil coral reefs, ancient soils like laterite, the formation of the aluminum ore bauxite, and others features are the tools geologists use to determine the climates of the past. With such evidence and techniques like paleomag-netic data, the latitudinal position of continents can be determined, giving us a picture of the global climatic changes that the earth has undergone through geologic time. In the references cited at the end of this entry, in the books on historical geology, the reader will find numerous examples of the past climates of the earth and graphs showing, for example, qualitative mean global temperature and precipitation through geologic time reconstructed from the data derived from rocks.

Today, as in the past, gases in the atmosphere play a key role in global climate. Gases in the atmosphere allow solar radiation to pass through to the surface where it is converted to heat when it strikes the surface. It is the same process that causes automobile interiors to get very hot. Shortwave radiation (ultraviolet) passes through the glass (the atmosphere) and is converted to infrared radiation when it strikes the surfaces within the car (the earth's surface). The glass prevents the heat from escaping, and the interior builds to surprisingly high temperatures. This retention of the heat is known as the greenhouse effect. In addition to the natural greenhouse phenomena, people are adding industrially produced gases to the atmosphere and changing the parameters, which many scientists say is causing the earth to warm up. With the production of CO2, the primary greenhouse gas, from burning trees, coal, oil, and natural gas, estimates of the amount of future CO2 production and the future increase in industrialization indicate that temperatures could rise by 2° to 10° degrees by 2100. Keep in mind that there are many other estimates based on different parameters that give more weight to one factor than another. But the results are the same: temperatures will rise, the polar caps will continue to melt causing sea level to rise, and overall global climate will change. Using the estimate of 5°C for the total global warming in the past 11,000 years, this increase indicates that a substantial further change in climate will take place. Intuition and computer modeling suggest that many changes will occur in respect to temperature ranges, locations of increased and decreased precipitation, cloud cover changes, and increases in the severity of adverse weather. One model suggests that the interior of the United States will experience severe summer drought in the grain-producing farmlands, and that wet and cool climates will be replaced by hot and dry climates and vice versa.

In addition to melting of glaciers, other effects have been recognized as resulting from increased heating of the earth. High mountain zones where the temperature is always below freezing have risen 500 feet since 1970. Butterflies, mosquitoes, and plants are now found at higher elevations. The spread of infectious disease is occurring; for example malaria, which has returned to the Korean Peninsula, was also reported during a hot spell in Toronto, Canada. One future result of global warming will be increased health hazards related to the increase of heat waves and the spread of infectious diseases. Many pathogens mature more quickly as temperatures increase, and mosquitoes that carry malaria can survive hot weather in pools of water that are hidden from the scorching sun, although their predators, such as lacewings and lady bugs, cannot.

Historically there are many examples of how changing climate causes social, political, and geopolitical changes. During the Little Ice Age between 1430 and 1890, canals in Venice froze several times, and during the Thirty War

(1618-1648) the Swedish army was able to cross over the frozen Baltic Sea during the winter. In 1972 a severe drought in the Soviet Union and the lack of a snow cover in the Ukraine caused major crop shortages. As a result the Soviet government bought vast quantities of wheat and other grains from the United States, resulting in the rise of American food prices, an important factor in the egregious inflation that followed. Current estimates for 2100 suggest that melting of glaciers will raise sea level some 21 feet and submerge many coastal areas, including parts of many major cities. Most of the world's cities with the largest populations are located along coasts. And if all of the glaciers melt, sea level will rise about 240 feet. In the United States, one-half of the population lives within 100 miles of the coastline and would likely be affected.

Geologists use a phrase that summarizes many aspects of geology: "the present is the key to the past;" we can also say that the "present is the key to the future."

—Sidney Horenstein

See also: Climatology; Glaciation; Habitat Tracking; Pleistocene Epoch

Bibliography

Abbott, Patrick L. 1996. Natural Disasters. Dubuque, IA: Wm. C. Brown Publishers; Akin, Wallace. 1990. Global Patterns: Climate, Vegetation, and Soils. Norman: University of Oklahoma Press; Glaritz, Michael H. 1996. Currents of Change: El Nino's Impact on Climate and Society. Cambridge: Cambridge University Press; Houghton, J. 1994. Global Warming: A Complete Briefing. Oxford: Lion Publishing; Lamb, H. H. 1995. Climate, History and the Modern World, London: Methuen; Lutgens, Frederick K., and Edward J. Tar-buck. 2001. The Atmosphere: An Introduction to Meteorology, 8th ed. Upper Saddle River, NJ: Prentice Hall; Stanley, Steven. 1999. Earth System History. New York: W. H. Freeman and Company.

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