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THE PHYSICAL ENVIRONMENT

We have seen how living things depend on the physical environment to supply essential materials for biogeochemical cycles. Physical factors such as climate and soil also affect living things. (See Focus On: The Gaia Hypothesis for an intriguing view of living organisms and their abiotic environment.) Climate comprises the average weather condi­tions that occur in a place over a period of years. Factors that determine an area's climate include temperature, precipitation, wind, humidity, fog, and cloud cover. Day-to-day variations, day-to­night variations, and seasonal variations in these factors are also important aspects of climate which we discuss in the remainder of this chapter, (Chap­ter 14 discusses soil, the surface layer of Earth that supports plants and is home to countless numbers of bacteria, fungi, protists, and animals.)

The sun's energy is the product of a massive nuclear fusion reaction (see Chapter 11) and is emitted into space in the form of electromagnetic radiation—especially visible light and infrared and ultraviolet radiation (which are not visible to the human eye) An infinitesimal portion of this en­ergy—one-billionth of the sun's total production strikes the Earth's atmosphere, and of this tiny-trickle of energy a minute part operates the eco-sphere.

Solar Energy at the Equator and the Poles

The most significant local variation in Earth's tempera­ture is produced because the sun's energy doesn't reach all places on Earth uniformly. A combination of the Earth's roughly spherical shape and the tilt of its axis produce a great deal of variation in the exposure of the Earth's surface to the energy deliv­ered by sunlight.

The principal effect of the tilt is on the angles at which the sun's rays strike different areas of the Earth at any one time. On the aver­age, the sun's rays hit the Earth vertically near the equator, making the energy more concentrated and producing higher temperatures. Near the poles the sun's rays hit more obliquely, and as a result their energy is spread over a larger surface area. Also, rays of light entering the atmosphere obliquely near the poles must pass through a deeper envelope of air than those entering near the equator. This causes more of the sun's energy to be scattered and re­flected back to space, which in turn further lowers temperatures near the poles. Thus, because the solar energy that reaches Polar Regions is less con­centrated, temperatures are lower.

Seasonal Variations in Solar Energy

Seasons are determined by two main factors: the inclination of the Earth's axis (the more important factor) and the distance of the Earth from the sun, which varies during the year. Since the Earth's inclination on its axis is always the same (23.5°), during half of the year (March 21 to September 22) the Northern Hemisphere tilts toward the sun, and during Hit-other half (September 22 to March 21) it tilts away from the sun. (The orientation of the Southern Hemisphere is just the opposite at these times.)

Atmospheric Circulation

In large measure, differences in temperature caused by variations in the amount of solar energy reaching the Earth at different locations drive the circulation of the atmosphere. The very warm surface of the Earth near the equator heats the air that is in contact with it, causing this air to expand and rise. As the warm air rises it cools, and then it sinks again. Much of it recirculates almost immediately to the same areas it has left, but the remainder of the heated air flows toward the poles, where eventually it is chilled. Similar upward movements of warm air and its subsequent flow toward the poles occur at higher latitudes (farther from the equator) as well. As air cools by contact with the polar ground and ocean, it sinks and flows to­ward the equator, generally beneath the sheets of warm air that simultaneously flow toward the poles. The constant motion of air transfers heat from the equator toward the poles, and as the air returns, n cools the land over which it passes. This continu­ous turnover does not equalize temperatures over the surface of the Earth, but it does moderate them.

Surface Winds

In addition to global circulation patterns, the Earth's atmosphere exhibits complex horizontal movements that are commonly referred to as winds. The nature of wind, with its turbulent gusts, eddies, and lulls, is difficult to understand or predict. It results in part from differences in at­mospheric pressure and from the rotation of the Earth.

The gases that constitute the atmosphere have weight and exert a pressure that is, at sea level, about 1,013 millibars (14-7 pounds per square inch). Air pressure is variable, however, changing with altitude, temperature, and humidity. Winds tend to blow from areas of high atmospheric pres­sure to areas of low pressure, and the greater the difference between the high- and low-pressure areas, the stronger the wind.

The Earth's rotation also influences the direc­tion of wind. The Earth's rotation from west to cast causes moving air to he deflected from its path and swerve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This tendency is known as the Coriolis effect.

The Coriolis effect can be visualized by imagin­ing that you and a friend are standing about 10 feet apart on a merry-go-round that is turning clock­wise. Suppose you throw a ball directly at your friend. By the time the ball reaches the place where your friend was, he or she is no longer in that spot. The ball will have swerved far to the left of your friend. This is how the Coriolis effect works in the Southern Hemisphere.

    To visualize how the Coriolis effect works in the Northern Hemisphere, imagine you and your friend arc standing on the same merry-go-round, only this time it is moving counterclockwise. Now when you throw the ball, it will swerve far to the right of your friend.

The Earth's atmosphere has three prevailing winds—major surface winds that blow more or less continually. Prevailing winds that blow from the northeast near the North Pole or from the southeast near the South Pole ate called polar easterlies. Winds that blow in the mid-lati­tudes from the southwest {in the Northern Hemisphere or from the northwest (in the Southern Hemisphere) are called westerlies. Tropical winds that blows from the northeast (Northern Hemi­sphere) or the southeast (Southern Hemisphere) are called trade winds.

Patterns of Circulation in the Oceans

The persistent prevailing winds blowing over the ocean produce mass movements of surface ocean water known as currents. The prevailing winds generate circular ocean currents called gyres. For example, in the North Atlantic, the tropical trade winds tend to blow toward the west, whereas the westerlies in the mid-latitudes blow toward the east. This helps establish a clockwise gyre in the North Atlantic. Thus, surface ocean currents and winds tend to move in the same direction, although there are many variations on this general rule. Other fac­tors that contribute to ocean currents include the Coriolis effect, the varying density of water, and the positions of land masses.

The paths traveled by surface ocean currents are partly caused by the Coriolis effect . The Earth's rotation from west to east causes surface ocean currents to swerve to the right in the Northern Hemisphere, creating a circular, clockwise pattern of water currents. In the South­ern Hemisphere, ocean currents swerve to the left, thereby moving in a circular, counterclockwise pat­tern.

The varying density (mass per unit volume) of seawater affects deep ocean currents. Water that is colder is denser than warmer water.' Thus, colder ocean water sinks and flows under warmer water, creating currents far below the surface. Deep ocean currents often travel in different directions and at different speeds than do surface currents, in part because the Coriolis effect is more pronounced at greater depths.

The positions of land masses also affect oceanic circulation. As you can observe that the oceans are not distributed uniformly over the globe: there is clearly more water in the Southern Hemi­sphere than in the Northern Hemisphere. There­fore, the circumpolar (around the pole) flow of water in the Southern Hemisphere is almost unimpeded by land masses.

What Causes Climate?

Average temperature, temperature extremes, pre­cipitation, the seasonal distribution of precipita­tion, day length, and season length are the most important dimensions of climate that affect living organisms. Variations in these climatic factors pro­duce the Earth's major ecosystems, including tun­dra, desert, rain forest, and grassland.

Latitude (distance north or south of the equa­tor) and the inclination of the Earth on its axis determine day length, season length, and, to a large degree, temperature.

Differences in precipitation depend upon sev­eral factors. The heavy rainfall of some areas of the tropics results mainly from the equatorial upwelling of moisture-laden air. High surface-water tempera­tures cause the evaporation of vast quantities of water from tropical oceans, and prevailing winds blow the resulting moist air over land masses. Heat­ing of the air by land surface that has been warmed by the sun causes moist air to rise. As it rises, the air cools, and moisture condenses from water vapor to a liquid, then falls as precipitation. The air eventually returns to Earth on both sides of the equator between the Tropics of Cancer and Capricorn (lati­tudes 23.5° north and 23.5° south). By then most of its moisture has precipitated, and the dry air returns to the equator. This air makes little biological dif­ference over the ocean, but its lack of moisture pro­duces some of the great tropical deserts, such as the Sahara Desert.

Air is also dried by long journeys over land masses. Near the windward (the side from which the wind blows) coasts of continents, rainfall may be heavy. However, in the temperate zones—the areas between the tropics and the polar zones— continental interiors are usually dry, because they are far from oceans that replenish water in the air passing over them.

Moisture is also removed from air by mountains when they cause humid air masses to rise and thus

The El Nino-Southern Oscillation Event and the World's Climate

Seasonal weather forecasting requires an un­derstanding of not only how the atmosphere operates but also how the oceans interact with the atmosphere. Consider the El Nino-Southern Oscillation event, a periodic warm­ing of surface waters of the tropical East Pa­cific that alters both oceanic and atmos­pheric circulation patterns and results in unusual weather in remote areas of the Earth. Every three to seven years, a warm mass of water that is normally restricted to the west­ern Pacific (near Australia) expands eastward,

Increasing surface temperatures in the east Pacific to 3 to 4 degrees over normal. Ocean currents, which normally flow westward in this area, slow down, stop altogether, or even

Reserve and go eastward. The phenomenon is called El Nino (Spanish for "the child") because the warming usually reaches the fishing grounds off Peru just before Christmas.

The El Nino-Southern Oscillation has a devastating effect on the fisheries off South

America. The higher temperatures and accompanying changes in ocean circulation patterns prevent nutrient-laden deeper waters from upwelling {coming to the surface). This severely decreases the populations of anchovies and other marine organisms. During the 1972 El Nino, for example, the anchovy pop­ulation decreased by 90 percent.

The El Nino-Southern Oscillation also alters air currents, directing unusual weather to areas far from the tropical pacific. The 1991 El Nino, for example, resulted in a much warmer-than-usual winter across much of Alaska, western Canada, and the northern United States. El Nino was responsible for the torrential rains that hit Texas and south­ern California during the 1991-1992 winter season. In addition, the effects of El Nino have been linked to droughts in Africa, Aus­tralia, and Hawaii.

release their water as precipitation. If prevailing winds blow onto a mountain range, precipitation occurs primarily on the windward slopes of the mountains. This situation exists on the west coast of North America where precipitation falls on the western slopes of the mountains. Downwind (in this case, east of the mountain range), a low-precipitation rain shadow develops, often creating a desert. Thus, some of the regional differ­ences in worldwide precipitation re­sult from the drying of air as it is returned to more equatorial areas; some result from long travel over

continents; and some result from cooling produced by mountainous regions.