Earth science and the environment

An understanding of the systems of the earth, how they interact, and how they are affected by anthropogenic disturbances is essential to any study of the environment. As the model of the earth is refined and improved, it will be possible to predict the planet’s response to anticipated perturbations with greater precision and confidence and adopt better public policy.

Global and Solid Earth Systems

The planet Earth has a large number of interconnected and interacting systems. Although the rates with which these systems respond to perturbations vary greatly, over time they all influence one another. Some of these systems are global in scale, while others are local. Many provide feedback that can either amplify or suppress a change.

The systems that have the slowest response times involve the solid earth. The surface of the earth is divided into about one dozen tectonic plates. These plates show little interior deformation over hundreds of millions of years, and yet their boundaries are constantly changing. They move across the earth at rates on the order of centimeters per year and have been doing so for at least several hundred million years. The environmental issues most closely related to this motion are the hazards of earthquakes and volcanoes; however, less obvious connections exist.

Large areas of uplift, such as the Himalayan Mountains and the Tibetan Plateau, dramatically change atmospheric circulation patterns, which in turn affect precipitation and evaporation. Rates at which tectonic plates are formed along midocean ridges vary, with far-reaching effects. The new material is hot and expanded. The time it takes to cool and contract does not depend on the rate at which it is forming, so faster growth results in larger regions underlain by expanded rock. The sea level rises as a result of this process. Some scientists have proposed a connection between volcanic processes acting along the East Pacific Rise and the occurrence of El Niño events.

Although the plate tectonic system interacts with other Earth systems and has long-term effects, it operates at such a slow rate that it is unlikely to cause significant changes to the environment during the course of a few human generations. In one hundred years, a plate may move a few meters. This is more than enough to cause tremendous earthquakes, but it is not likely to produce noticeable climate change.

Continental glaciation causes vertical movements of the earth’s crust, which occur much more rapidly than plate motions. Approximately 20,000 years ago, a mass of ice about three kilometers thick covered much of Canada, the northern United States, and Scandinavia. The weight of this ice forced the crust to sink into the mantle, a process called isostasy; when the ice melted away, the crust began to rise again. As a result, some Viking fishing villages are now well above sea level.

Ocean Systems

Much faster than solid rock systems, the circulation of the oceans is one of the most significant processes affecting the environment. It is convenient to divide this circulation into surface current circulation and thermohaline circulation (involving the effects of both temperature and salinity).

Heat from the sun warms the surface water, which then expands and floats on the cooler water beneath it. This warm body of water has a lens shape, with the center a few meters higher than the edges. As water tries to flow down the slope of the surface, Coriolis force—a result of the earth’s rotation—forces it to move sideways instead. In the Northern Hemisphere, this produces a clockwise movement; in the Southern Hemisphere, it produces a counterclockwise movement. This ocean-scale circular current is called a gyre. The Gulf Stream is the northwest segment of the North Atlantic gyre.

The Gulf Stream moves 35 million cubic meters of water past Chesapeake Bay every second. The edge of the Gulf Stream forms an abrupt boundary between cool, nutrient-rich waters near the continents and the warm, nearly lifeless waters of the Sargasso Sea. Although interconnected with weather and climate in a complex way, the operation of the Gulf Stream is not the principal source of heat responsible for the mild climate of Europe. The principal source of warmth for Europe is heat transported northward by atmospheric circulation, plus heat stored in Atlantic surface waters during the summer.

This heat transfer drives the thermohaline circulation. Cold, salty Antarctic water sinks to the bottom and moves northward, eventually rising to join the surface circulation. The largest amount of bottom water is in the Pacific, warming as it circulates southward and westward. Warm water from the Indian Ocean rounds Africa into the Atlantic, moves north as the Gulf Stream, and cools and sinks in the Arctic. This circulation transports heat from regions around the equator to those around the pole and keeps the oceans mixed uniformly.

Atmospheric Systems

Atmospheric systems constitute the other major heat redistribution scheme on Earth. Because of Earth’s spherical shape, the equatorial regions receive and absorb far more solar energy than upper latitudes.

As solar radiation reaches Earth, much of it is absorbed, raising the temperature of the surface. This heats the air above, which expands and rises into the atmosphere, just like a hot air balloon. The air cools as it rises, and the moisture therein condenses and precipitates, producing most of the earth’s rain forests. The cooler, drier air moves to the north and south until it eventually descends over belts of latitudes between 20 degrees north and 30 degrees north, and between 20 degrees south and 30 degrees south. Descending, this air warms up and gains moisture. At the surface, this air evaporates water effectively. Most of Earth’s major deserts are in these latitudinal belts. The convection cell is completed by a return flow across the surface toward the equator. Because of the Coriolis force produced by the earth’s rotation, this flow is deflected to come out of the northeast in the Northern Hemisphere and the southeast in the Southern Hemisphere. Regardless of direction, these winds are referred to as trade winds.

Another convection cell forms at somewhat higher latitudes, causing the surface winds to come out of the southwest in the Northern Hemisphere and out of the northwest in the Southern Hemisphere. These winds, called the westerlies, blow across most of the United States. As they travel over the mountains in the western states, they are forced up into higher elevations, where the air loses its water. Deserts lie to the east of these mountains, in what is called the mountains’ “rain shadow.”

The trade winds of both hemispheres meet along the equator and result in surface winds that tend to push the surface water of the oceans to the west. This surface water is warmer and less dense than the water beneath it, and the boundary between the two is a region of rapidly changing temperature called the thermocline. Because only warm water is pushed to the west, it piles up there, pushing the thermocline lower and lower. Under these conditions, the air just above the surface of the ocean is heated, and moisture is added to it as it travels from east to west. When it reaches the western edge of the Pacific, it rises and its water condenses. Every few years, however, the situation reverses itself. Surface winds weaken, the accumulated warm surface water flows eastward, rain comes to the deserts of Peru, and weather patterns over the entire planet are severely disrupted. The atmospheric effects of such a reversal are called the Southern Oscillation, whereas the oceanic effects are called El Niño. As they are linked and occur together, it has become common practice to refer to the phenomenon as the El Niño Southern Oscillation (ENSO).

Greenhouse Gases

Most of the energy that reaches the earth from the sun is electromagnetic in the visible light portion of the spectrum. A cloudless atmosphere lets this pass through easily. Upon reaching the earth’s surface, some of this energy is absorbed by surface material, which then radiates the energy back, but at a much lower frequency in the infrared region, which humans detect as heat. On its way back to outer space, this energy may be absorbed by a greenhouse gas molecule. When this molecule reradiates the energy, the chances are about equal that it will be directed toward or away from the earth. In this way, a significant fraction of the infrared radiation is redirected back toward the surface of the earth.

The most important greenhouse gas is water vapor. The distribution of water vapor is uneven and subject to wide variations. It is generally assumed that the average relative humidity of the atmosphere remains constant. Because warm air contains more moisture at the same level of relative humidity than cooler air, the amount of water in the atmosphere will increase if global surface temperatures rise. Thus, water vapor is expected to enhance any effects produced by other greenhouse gases.

The greenhouse gas of greatest concern is carbon dioxide. It is well known that the trace amounts of carbon dioxide present in the atmosphere increased by 12 percent between 1958 and 1998. The amount of coal, oil, and natural gas burned in that time period is also known, and the 12 percent increase represents only about one-half of the carbon dioxide produced by this burning. Where the other half has gone remains a subject of research. Scientists are trying to determine if the observed increases in atmospheric carbon dioxide have led to changes in global surface temperatures. It appears that such temperatures rose approximately 0.5 degree Celsius between 1880 and 1980, a change well within historical rates of temperature fluctuations, leaving the significance of the increase in atmospheric carbon dioxide open to question.

Earth scientists studying gas bubbles trapped in the ice of Greenland and Antarctica have observed that carbon dioxide concentrations in the atmosphere and global temperatures changed together over the past 200,000 years. A causal relationship has not been proved, however, and the situation is complicated by biologic activity, the solubility of carbon dioxide in seawater, and many other factors that vary with temperature.

Water and the Surface of the Earth

Water evaporates from the oceans, is transported by weather systems, and falls on the land as rain and snow. Some of this water soaks into the soil, where it contributes to the chemical and physical breakdown of rocks, a process called weathering. Some water can soak deep into the ground, where it is stored as groundwater. Much of it runs across the surface, transporting loose grains of sand and silt and eroding the surface. It takes only a few tens of millions of years to erode a mountain range flat.

In addition to being transported by water, loosened surface materials can move downhill by gravity—a process called mass wasting. Slow mass wasting is called soil creep. One common sign of soil creep is trees with bent bases; young trees are tipped by creep and keep trying to grow upright. Eventually, their roots become firmly enough established to resist creep, and the trees grow straight from then on. Rapid forms of mass wasting can be extremely destructive to life and property. Slump is a slope failure where a large block of material drops but remains largely intact. Mudflows and avalanches are fast-moving quantities of debris that can cause great losses of life and property.

Local Systems

The environment of Earth is maintained by the interactions of global systems. Local areas—such as a flood plain, a watershed, or an earthquake-prone region—also have interconnected systems of environmental significance. Here, too, Earth science provides essential background for policymakers.

Flooding occurs when too much water tries to move through a channel at once. Earth scientists can determine how much water a channel can hold and how long it will take various volumes of water to move down tributaries. Therefore they also can predict how high a particular flood will be and when the crest will occur. By studying geological evidence of previous floods, scientists can establish the historical frequency of floods of various sizes. Taking into account the effects of agriculture and development, they can then estimate how frequently floods of different magnitudes are likely to occur in the future. This information can be used to determine how much money to invest in flood control or how restrictive to make zoning within the flood plain.

During floods there is an overabundance of water, but at other times water can be in short supply. In many places, underground aquifers are tapped for drinking water. Earth scientists can study a watershed and evaluate the quantity, quality, and flow patterns of its underground water. They can determine how much water can be safely withdrawn without long-term detrimental effects. If pollution occurs, scientists can determine the likely paths of the pollutants, trace them using remote sensing techniques, and suggest strategies to contain or eliminate their threat.

Threats posed by earthquakes are more difficult to perceive than those posed by pollution or flooding. Large earthquakes are unlikely to strike a region twice within the span of one person’s life. They may leave a record in the soils and structures of a region, and Earth scientists have been able to interpret these to extend knowledge of major earthquakes back several centuries. To detect small earthquakes, which recur frequently, Earth scientists have developed instruments called seismographs. The behavior of geologic units subjected to prolonged periods of violent shaking can be very different from their behavior under normal conditions. Earth scientists have developed theories to explain such behavior, as well as criteria that can be used to identify those materials that pose the greatest risk. By combining results from these and other areas of research, researchers have been able to estimate the risks posed by earthquakes at various locations. This information can be used to develop building codes and zoning laws to minimize the risk of death and destruction posed by major earthquakes in the future.

Significance

The environmentalists’ slogan “Think globally; act locally” is an acknowledgment of the interconnectedness of many planetary processes and the importance of making local decisions based on some understanding of potential global consequences. Earth science endeavors to develop that understanding.

Earth scientists, by quantifying the many feedback loops with which systems interact with themselves and one another, can identify those natural processes that human influences are most likely to disturb. By studying the past behavior of these systems, scientists can make reasonable estimates of how they might respond to future disturbances. By informing public policy, legislation and law enforcement, and civic leaders in general, Earth scientists can use their knowledge and insight to influence the course taken by the governments of the world.

Principal Terms

El Niño Southern Oscillation: a reversal in precipitation patterns, ocean upwelling, and thermocline geometry that is accompanied by a weakening of the trade winds; the phenomenon typically recurs every three to seven years

greenhouse gas: an atmospheric gas capable of absorbing electromagnetic radiation in the infrared part of the spectrum

gyre: an ocean-scale surface current that moves in a circular pattern

hydrologic cycle: the cycle of water movement on the earth from ocean to land and back

mass wasting: downhill movement of material by gravity, as opposed to transport by water or wind.

plate tectonics: a theory that holds that the surface of the earth is divided into roughly one dozen rigid plates that move relative to one another, producing earthquakes, volcanoes, mountain belts, trenches, and many other large-scale features of the planet

thermohaline conveyor belt: a system of oceanic circulation driven by the cooling and sinking of salty surface waters in the Nordic seas

Bibliography

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Cutter, Susan L., and William H. Renwick. Exploitation Conservation Preservation: A Geographic Perspective on Natural Resource Use. 4th ed. Hoboken, N.J.: John Wiley & Sons, 2003. Discusses the social, economic, political, and environmental effects of resource use. Covers detailed content useful to the graduate student. Presents many varying views of unresolved issues.

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Duffield, Wendell, and John Sas. Geothermal Energy: Clean Power from the Earth’s Heat. USGS Circular 1249. 2003. Describes historical and current geothermal energy use. Covers global geothermal applications, mining of geothermal energy, hydrothermal systems, and dry geothermal systems, and the environmental impact of geothermal energy use. Includes numerous color diagrams and images.

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Sinding-Larsen, Richard, and Friedrich-W. Wellmer, eds. Non-renewable Resource Issues: Geoscientific and Societal Challenges. Hoboken, N.J.: Springer, 2012. Discusses use, overuse, and future needs of natural resources. Considers possible resources needs of the future. Discusses concepts of land use, sustainability, and cultural needs.

U.S. Department of the Interior, Minerals Management Service (USDOI MMS).Programmatic Environmental Assessment: Arctic Ocean Outer Continental Shelf Seismic Surveys. U.S. Department of the Interior Minerals Management Service, Alaska OCS Region. 2006. Provides an overview of seismic surveys and the exploration of the Alaskan continental shelf. Content includes alternative scenarios for surveys and their evaluation. Addresses the environmental impact of such surveys.

Zektser, Igor S., et al., eds. Geology and Ecosystems. New York: Springer, 2010. Covers various geomorphologies and geological processes. Evaluates environmental impacts of geological research, industries, and natural dynamics. Appropriate for environmental scientists, geologists, ecologists, and similar professionals.