A system is any set of interactions set apart from the rest of the universe for the purposes of study, observation, and measurement. Theoretically, a system is isolated from its environment, but this is an artificial construct, since nothing is ever fully isolated. Earth is largely a closed system, meaning that it exchanges very little matter with its external environment in space, but the same is not true of the systems within the planet—geosphere, hydrosphere, biosphere, and atmosphere—which interact to such a degree that they are virtually inseparable. Together these systems constitute an intricate balance, a complex series of interrelations in which events in one sector exert a profound impact on conditions in another.
HOW IT WORKS
An isolated system is one so completely sealed off from its environment that neither matter nor energy passes through its boundaries. This is an imaginary construct, however, an idea rather than a reality, because it is impossible to create a situation in which no energy is exchanged between the system and the environment. Under the right conditions it is perhaps conceivable that matter could be sealed out so completely that not even an atom could pass through a barrier, but some transfer of energy is inevitable. The reason is that electromagnetic energy, such as that emitted by the Sun, requires no material medium in which to travel.
In contrast to an isolated system is a closed system, of which Earth is an approximation. Despite its name, a closed system permits the exchange of energy with the environment but does not allow matter to pass back and forth between the external environment and the system. Thus, Earth absorbs electromagnetic energy, radiated from the Sun, yet very little matter enters or departs Earth's system. Note that Earth is an approximation of a closed system: actually, some matter does pass from space into the atmosphere and vice versa. The planet loses traces of hydrogen in the extremities of its upper atmosphere, while meteorites and other forms of matter from space may reach Earth's surface.
Earth more closely resembles a closed system than it does an open one—that is, a system that allows the full and free exchange of both matter and energy with its environment. The human circulatory system is an example of an open system, as are the various "spheres" of Earth (geosphere, hydrosphere, biosphere, and atmosphere) discussed later. Whereas an isolated system is imaginary in the sense that it does not exist, sometimes a different feat of imagination is required to visualize an open system. It is intricately tied to its environment, and therefore the concept of an open system as a separate entity sometimes requires some imagination.
Using Systems in Science
To gain perspective on the use of systems in science as well as the necessity of mentally separating an open system from its environment, consider how these ideas are used in formulating problems and illustrating scientific principles. For example, to illustrate the principle of potential and kinetic energy in physics, teachers often use the example of a baseball dropping from a great height (say, the top of a building) to the ground.
At the top of the building, the ball's potential energy, or the energy it possesses by virtue of its position, is at a maximum, while its kinetic energy (the energy it possesses by virtue of its motion) is equal to zero. Once it is dropped, its potential energy begins to decrease, and its kinetic energy to increase. Halfway through the ball's descent to the ground, its potential and kinetic energy will be equal. As it continues to fall, the potential energy keeps decreasing while the kinetic energy increases until, in the instant it strikes the ground, kinetic energy is at a maximum and potential energy equals zero.
KEEPING OUT IRRELEVANT DETAILS.
What has been described here is a system. The ball itself has neither potential nor kinetic energy; rather, energy is in the system, which involves the ball, the height through which it is dropped, and the point at which it comes to a stop. Furthermore, because this system is concerned with potential and kinetic energy only in very simple terms, we have mentally separated it from its environment, treating it as though it were closed or even isolated, though in reality it would more likely be an open system.
In the real world, a baseball dropping off the top of a building and hitting the ground could be affected by such conditions as prevailing winds. These possibilities, however, are not important for the purposes of illustrating potential and kinetic energy, and even if they were, they could be incorporated into the larger energy system.
THE "MAGIC" OF A SYSTEM.
Since kinetic energy and potential energy are inversely related, the potential energy at the top of the building will always equal the kinetic energy at the point of maximum speed, just before impact. This is true whether the ball is dropped from 10 ft. (3 m) or 1,000 ft. (305 m). It may seem almost magical that the sum of potential and kinetic energy is always the same or that the two values are perfectly inverse. In fact, there is nothing magical here: the system has a certain total energy, and this does not change, though the distribution of that energy can and does vary.
Suppose one had a money jar known to contain $20. If one reaches in and grasps a five-dollar bill, two one-dollar bills, three quarters, a dime, and two nickels ($7.95), there must be $12.05 left in the jar. There is nothing magical in this; rather, what has been illustrated is the physical principle of conservation. In physics and other sciences, "to conserve" something means "to result in no net loss of" that particular component. It is possible that within a given system, the component may change form or position, but as long as the net value of the component remains the same, it has been conserved. Thus, the total energy is conserved in the situation involving the baseball, and the total amount of money is conserved in the money-jar.
Applying the System Principle to Earth
In the baseball illustration, the distribution between types of energy varies, but the total amount is always the same. Likewise in the money-jar illustration, the total amount of money remains fixed even though the distribution according to various denominations may vary. The same is true of Earth, though here it is the total amount of matter. This includes valuable resources, among them materials that can be mined to produce energy—for instance, fossil fuels such as coal or petroleum—as well as waste products. Because Earth is a closed system, there are no additional resources, nor is there any dumping ground other than the one beneath our feet. Thus, the situation calls for prudence both in the use of the planet's material wealth and in the processing of materials that will leave a byproduct of waste.
The fact that a closed system is by definition finite leads to the principle that the relationships between its constituent parts are likewise finite, and therefore changes in one part of the system are liable to produce effects in another part. Conditions in the baseball or money-jar illustrations are so simple that it is easy to predict the effect of a change. For instance, if we substitute a basketball for a baseball, this will change the total energy, because the latter is a function of the ball's mass. If the denominations making up the $20 in the money jar are replaced with a collection of two-dollar bills and dimes, this will make it impossible to reach in and pull out an odd-numbered value in dollars or cents.
What about the changes that result when one aspect of Earth's system is altered? In some cases, it is easy to guess; in others, the interactions are so complex that prediction requires sophisticated mathematical models. It is perhaps no accident that chaos theory was developed by a meteorologist, the American Edward Lorenz (1917-). Chaos theory, the study of complex systems that appear to follow no orderly laws, involves the analysis of phenomena that appear connected by something than an ordinary cause and effect relationship. The classic example of this is the "butterfly effect, " the idea that a butterfly beating its wings in China can change the weather in New York City. This, of course, is a farfetched scenario, but sometimes changes in one sector of Earth's system can yield amazing consequences in an entirely different part.
The Four "Spheres"
The systems approach is relatively new to the earth sciences, themselves a group of disciplines whose diversity reflects the breadth of possible approaches to studying Earth (see Studying Earth). At one time, earth scientists tended to investigate specific aspects of Earth without recognizing the ways in which these aspects connect with one another; today, by contrast, the paradigm of the earth sciences favors an approach that incorporates the larger background.
Given the complexities of Earth itself, as well as the earth sciences, it is helpful to apply a schema (that is, an organizational system) for dividing larger concepts and entities into smaller ones. For this reason, earth scientists tend to view Earth in terms of four interconnected "spheres. " One of these terms, atmosphere, is a familiar one, while the other three (geosphere, hydrosphere, and biosphere) may sound at first like mere scientific jargon.
UNDERSTANDING THE SPHERES.
In fact, each sphere represents a sector of existence on the planet that is at once clearly defined and virtually inseparable from the others. Each is an open system within the closed system of Earth, and overlap is inevitable. For example, the seeds of a plant (biosphere) are placed in the ground (geosphere), from which they receive nutrients for growth. In order to sustain life, they receive water (hydrosphere) and carbon dioxide (atmosphere). Nor are they merely receiving: they also give back oxygen to the atmosphere, and by providing nutrition to an animal, they contribute to the biosphere.
Each of the spheres, or Earth systems, is treated in various essays within this book. These essays examine these subsystems of the larger Earth system in much greater depth; what follows, by contrast, is the most cursory of introductions. It should be noted also that while these four subsystems constitute the entirety of Earth as humans know and experience it, they are only a small part of the planet's entire mass. The majority of that mass lies below the geosphere, in the region of the mantle and core.
A CURIOUS AND INSTRUCTIVE POINT.
As a passing curiosity, it is interesting to note that modern scientists have identified four subsystems and given them the name spheres. As discussed in the essay Earth, Science, and Nonscience, the ancient Greeks were inclined to divide natural phenomena into fours, a practice that reached its fullest expression in the model of the universe developed by the Greek philosopher Aristotle (384-322 b.c.) He even depicted the physical world as a set of spheres and suggested that the heaviest material would sink to the interior of Earth while the lightest would rise to the highest points.
These points of continuity with ancient science are notable because almost everything about Aristotle's system was wrong, and, indeed, the differences between his model of the physical world and the modern one are instructive. There are four spheres in the modern earth sciences because these four happen to be useful ways of discussing the larger Earth system—not, as in the case of the Greeks, because the number four represents spiritual perfection. Furthermore, scientists understand these spheres to be artificial constructs, at least to some extent, rather than a key to some deeper objective reality about existence, as the ancients would have supposed.
Nor are the spheres of the modern earth sciences literally spheres, as Aristotle's concentric orbits of the planets around Earth were. If anything, the use of the term sphere represents a holdover from the Greek way of viewing the material world. Finally, unlike such ancient notions as the concept of the four elements, four qualities, or four humors, the idea of the four spheres is not simply the result of pure conjecture. Instead, the concept of these four interrelated systems came about by application of the scientific method and entered the vocabulary of earth scientists because the ideas involved clearly reflected and illustrated the realities of Earth processes.
The Spheres in Brief
The geosphere itself may be defined as the upper part of the planet's continental crust, the portion of the solid earth on which human beings live, which provides them with most of their food and natural resources. Even with the exclusion of the mantle and core, the solid earth portion of Earth's system is still by far the most massive. It is estimated that the continental and oceanic crust to a depth of about 1.24 mi. (2 km) weighs 6 × 1021 kg—about 13,300 billion billion pounds. The mass of the biosphere, by contrast, is about one millionth that figure. If the mass of all four spheres were combined, the geosphere would account for 81.57%, the hydrosphere 18.35%, the atmosphere 0.08%, and the biosphere a measly 0.00008%. (Of that last figure, incidentally, animal life—of which humans are, of course, a very small part—accounts for less than 2%.)
Not only is the geosphere the largest, it is also by far the oldest of the spheres. Its formation dates back about four billion years, or within about 0.5 billion years of the planet's formation. As Earth cooled after being formed from the gases surrounding the newborn Sun, its components began to separate according to density. The heaviest elements, such as iron and nickel, drifted toward the core, while silicon rose to the surface to form the geosphere.
ATMOSPHERE, HYDROSPHERE, AND BIOSPHERE.
In that distant time Earth had an atmosphere in the sense that there was a blanket of gases surrounding the planet, but the atmospheric composition was quite different from today's mixture of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%. The atmosphere then consisted largely of carbon dioxide from Earth's interior as well as gases brought to Earth by comets. Elemental hydrogen and helium escaped the planet, and much of the carbon was deposited in what became known as carbonate rocks. What remained was a combination of hydrogen compounds, including methane, ammonia, nitrogen-and sulfur-rich compounds expelled by volcanoes, and (most important of all) H2O, or water.
Simultaneous with these developments, the gases of Earth's atmosphere cooled and condensed, taking the form of rains that, over millions of years, collected in deep depressions on the planet's surface. This was the beginning of the oceans, the largest but far from the only component of Earth's hydrosphere, which consists of all the planet's water except for water vapor in the atmosphere. Thus, the hydrosphere includes not only saltwater but also lakes, streams, groundwater, snow, and ice.
Water, of course, is necessary to life, and it was only after its widespread appearance that the first life-forms appeared. This was the beginning of the biosphere, which consists of all living organisms as well as any formerly living material that has not yet decomposed. (Typically, following decomposition an organism becomes part of the geosphere.) Over millions of years, plants formed, and these plants gradually began producing oxygen, helping to create the atmosphere as it is known today—an example of interaction between the open systems that make up the larger Earth system.
Earth As an Organism
Clearly, a great deal of interaction occurs between spheres and has continued to take place for a long time. Earth often is described as a living organism, a concept formalized in the 1970s by the English meteorologist James Lovelock (1919-) and the American biologist Lynn Margulis (1938-), who developed the Gaia hypothesis. Sometimes called the Gaian hypothesis, this principle is named after the Greek earth goddess, a prototype for "Mother Earth," and is based on the idea that Earth possesses homeostatic or self-regulating mechanisms that preserve life. (Lovelock's neighbor William Golding [1911-1993], author of Lord of the Flies, suggested the name to him.)
Though the Gaia hypothesis seems very modern and even a bit "New Age" (that is, relating to a late twentieth-century movement that incorporates such themes as concern for nature and spirituality), it has roots in the ideas of the great Scottish geologist James Hutton (1726-1797), who described Earth as a "superorganism." A forward-thinking person, Hutton maintained that physiology provides the model for the study of Earth systems. Out of Hutton's and, later, Lovelock's ideas ultimately grew the earth science specialty of geophysiology, an interdisciplinary approach incorporating aspects of geo-chemistry, biology, and other areas.
The Gaia hypothesis is far from universally accepted, however, and remains controversial. One reason is that it seems to contain a teleologic, or goal-oriented, explanation of physical behaviors that does not fully comport with the findings of science. An animal responds to external conditions in such a way as to preserve life, but this is because it has instinctive responses "hardwired" into its brain. Clearly, if the Earth is an "organism, " it is an organism in quite a different sense than an animal, since it does not make sense to describe Earth as having a "brain."
Homeostasis and Cycles
Nonetheless, Lovelock, Margulis, and other supporters of the Gaia hypothesis have pointed to a number of anomalies that have yet to be explained fully and for which the Gaia hypothesis offers one possible solution. For example, it would have taken only about 80 million years for the present levels of salt in Earth's oceans to have been deposited there from the geosphere; why, then, is the sea not many, many times more salty than it is? Could it be that Earth has somehow regulated the salinity levels in its own seas?
Earth's systems unquestionably display a homeostatic and cyclical behavior typical of living organisms. Just as the human body tends to correct any stresses imposed on it, Earth likewise seeks equilibrium. And just as blood, for instance, cycles through the body's circulatory system, so matter and energy move between various spheres in the course of completing certain cycles of the Earth system. These include the energy and hydrologic cycles; a number of biogeochemical cycles, such as the carbon and nitrogen cycles; and a rock cycle of erosion, weathering, and buildup. (Each of these systems is discussed in a separate essay, or as part of a separate essay, in this book.)
Though particulars of the Gaia hypothesis remain a matter of question, it is clear that Earth regulates these cycles and does so through a process of feedback and corrections. To appreciate the idea of feedback, consider a financial example. In the early 1990s, the U.S. Congress placed a steep tax on luxury boats, presumably with the aim of getting more money from wealthy taxpayers. The result, however, was exactly the opposite: boat owners sold their crafts, and many of those considering purchases cancelled their plans. Rather than redistributing wealth from the rich to those less fortunate, the tax resulted in the government's actually getting less money from rich yacht owners.
Whereas Congress expected the rich to provide positive feedback by giving up more tax money, instead the yacht owners responded by acting against the tax—a phenomenon known as negative feedback. Feedback itself is the return of output to a system, such that it becomes input which then produces further output. Feedback that causes the system to move in a direction opposite that of the input is negative feedback, whereas positive feedback is that which causes the system to move in the same direction as the input. The luxury tax would have made perfect sense if the purpose had been to halt the production and purchase of expensive boats, in which case the output would have been deemed positive.
In the luxury-tax illustration, negative feedback is truly "negative" in the more common sense of the word, but this is not typically the case where nature in general or Earth systems in particular are concerned. In natural systems negative feedback serves as a healthy corrective and tends to stabilize a system. To use an example from physiology, if a person goes into a cold environment, the body responds by raising the internal temperature. Likewise, in chemical reactions the system tends to respond to any stress placed on it by reducing the impact of the stress, a concept known as Le Châtelier's principle after the French chemist Henry Le Châtelier (1850-36).
Positive feedback, on the other hand, is often far from "positive" and is sometimes described as a "vicious cycle." Suppose rainwater erodes a portion of a hillside, creating a gully. Assuming the rains continue, the opening of this channel for the water facilitates the introduction of more water and therefore further erosion of the hillside. Given enough time, the rain can wash a deep gash into the hill or even wash away the hill entirely.
Given the interconnectedness of systems on Earth, it is easy to see how changes in one part of the larger Earth system can have far-reaching impacts on another sector. For example, the devastating Alaska earthquake of March 1964 produced tsunamis felt as far away as Hawaii, while the Exxon Valdez oil spill that afflicted Alaska exactly 25 years later had an effect on the biosphere and hydrosphere over an enormous area.
El Niño is a familiar example of far-reaching consequences produced by changes in Earth systems. Spanish for "child" (because it typically occurs around Christmastime), El Niño begins on the western coast of South America. There, every few years, trade winds slacken, allowing the wind from the west to push warm surface water eastward. Lacking vital nutrients, this warm water brings about a decline in the local marine life. It also causes heavy rains and storms.
IMPACT OF EL NIÑO AROUND THE WORLD.
To the extent described, El Niño is largely a local phenomenon. But it can affect the jet streams, or high-level winds, that push storms across the Western Hemisphere. This can result in milder weather for western Canada or the northern United States, as the winds push more severe storms into Alaska, but it also can bring about heavy rains in the Gulf of Mexico region. Nor are its effects limited to the Western Hemisphere. El Niño has been known to alter the pattern of monsoons, or rainy seasons, in India, Southeast Asia, and parts of Africa, thus producing crop failures that affect millions of people.
Aside from the indirect effects, such as the famines in the Eastern Hemisphere, the direct effects of the El Niño phenomenon can be devastating. The El Niño of 1982-83, which affected the United States, the Caribbean, western South America, Africa, and Australia, claimed some 2,000 lives and cost about $13 billion in property damage. It returned with a vengeance 15 years later, in 1997-98, killing more than 2,100 people and destroying $33 billion worth of property.
Years Without Summer
Whereas El Niño is an example of a disturbance in the hydrosphere that affects the atmosphere and ultimately the biosphere, an even more terrifying phenomenon can begin with an eruption in the geosphere, which spreads to the atmosphere and then the hydrosphere and biosphere. This phenomenon might be called "years without summer"; an example occurred in 1815-16.
In June of 1816 snow fell in New England, and throughout July and August temperatures hovered close to freezing. Frosts hit in September, and New Englanders braced themselves for an uncommonly cold winter, as that of 1816-17 turned out to be. It must have seemed as though the world were coming to an end, yet the summer of 1817 proved to be a normal one. The cause behind this year without summer in 1816 lay in what is now Indonesia, and it began a year earlier.
In 1815, Mount Tambora to the east of Java had erupted, pouring so much volcanic ash into the sky that it served as a curtain against the Sun's rays, causing a brutally cold summer in New England the following year. An eruption of Mount Katmai in Alaska in 1912 produced far-reaching effects, including some lowering of temperatures, but its impact was nothing like that of Tambora. Nor did the 1980 Mount Saint Helens eruption in Washington State prove nearly as potent in the long run as the eruption of Tambora did (though it produced a devastating immediate impact).
THE CATACLYSM OF a.d. 535.
Even the eruption of Mount Tambora may have been overshadowed by another, similar event, known simply as the catastrophe, or cataclysm, of a.d. 535. In the late twentieth century, the British dendrochronologist Mike Baillie discovered a pattern of severely curtailed growth in tree rings dating to the period a.d. 535-541. More or less simultaneous with Baillie's work was that of the amateur archaeologist David Keys, who found a number of historical texts by Byzantine, Chinese, and Anglo-Saxon scholars of the era, all suggesting that something cataclysmic had happened in a.d. 535. For example, the Byzantine historian Procopius (d. 565) wrote, "The sun gave forth its light without brightness … for the whole year."
Some geologists have maintained that the cataclysm resulted from the eruption of another Indonesian volcano, the infamous Krakatau, which had a devastating eruption in 1883 and which could have produced enough dust to cause an artificial winter. Whatever the cause, the cataclysm had an enormous impact that redounds from that time perhaps up to the present. The temperature drop may have sparked a chain of events, beginning in southern Africa, that ultimately brought a plague to the Byzantine Empire, forcing Justinian I (r. a.d. 483-565) to halt his attempted reconquest of western Europe. At the same time, the cataclysm may have been responsible for food shortages in central Asia, which spawned a new wave of European invasions, this time led by the Avars.
The result was that the fate of Europe was sealed. For a few years it had seemed that Justinian could reconquer Italy, thus reuniting the Roman Empire, whose western portion had ceased to exist in a.d. 476. Forced to give up their reconquest, with the Avars and others overrunning Europe while the plague swept through Greece, the Byzantines turned their attention to affairs at home and increasingly shut themselves off from western Europe. Thus the Dark Ages, the split between Catholicism and Eastern Orthodoxy, the Crusades—even the Cold War, which reflected the old east-west split in Europe—may have been the results of a volcano on the other side of the world.
WHERE TO LEARN MORE
Cox, Reg, and Neil Morris. The Natural World. Philadelphia: Chelsea House, 2000.
Earth's Energy Budget (Web site). <http://radar.metr.ou.edu/OK1/meteorology/EnergyBudget.html.>.
Farndon, John. Dictionary of the Earth. New York: Dorling Kindersley, 1994.
The Gaia Hypothesis (Web site). <http://www.magna.com/au/~prfbrown/gaia_jim.html>.
Geophysiology Online (Web site). <http://www.webtorre.com/geo/GeoPhysOnline.asp>.
Knapp, Brian J. Earth Science: Discovering the Secrets of the Earth. Illus. David Woodroffe and Julian Baker. Danbury, CT: Grolier Educational, 2000.
Kump, Lee R., James F. Kasting, and Robert G. Crane. The Earth System. Upper Saddle River, NJ: Prentice Hall, 2000.
Lovelock, J.E. Gaia: A New Look at Life on Earth. New York: Oxford University Press, 2000.
Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 2nd. ed. New York: John Wiley and Sons, 1999.
A blanket of gases surrounding Earth and consisting of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include watervapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%.
A combination of all living things on Earth—plants, mammals, birds, reptiles, amphibians, aquatic life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed. Typically, after decomposing, a formerly living organism becomes part of the geosphere.
A system that permits the exchange of energy with its external environment but does not allow matter to pass between the environment and the system. Compare with isolated system, on the one hand, and open system, on the other.
In physics and other sciences, "to conserve" something means "to result in no net loss of" that particular component. It is possible that within a given system, the component may change form or position, but as long as the net value of the component remains the same, it has been conserved.
A form of energy with electric and magnetic components, which travels in waves and, depending on the frequency and energy level, can take the form of long-wave and short-wave radio; microwaves; infrared, visible, and ultraviolet light; x rays; and gamma rays.
In discussing systems, the term environment refers to the surroundings—everything external to and separate from the system.
The return of output to a system, such that the output becomes input that produces further output. Feedback that causes the system to move in adirection opposite to that of the input is negative feedback, whereas positive feedback is that which causes the system to move in the same direction as the input.
The concept, introduced in the 1970s, that Earth behaves much like a living organism, possessing self-regulating mechanisms that preserve life. Sometimes called the Gaian hypothesis, it is named after Gaia, the Greek goddess of the earth.
The upper part of Earth's continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources.
A tendency toward equilibrium.
The quality of beingself-regulating.
The entirety of Earth's water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice.
A system that is so fully separated from the rest of the universe that it exchanges neither matter nor energy with its environment. This is an imaginary construct, since full isolation isimpossible.
A system that allowscomplete, or near-complete, exchange of matter and energy with its environment.
A set of principles and procedures for systematic study that includes observation; the formation of hypotheses, theories, and laws; and continual testing and reexamination.
Any set of interactions that can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement.
A tidal wave produced by an earthquake or volcanic eruption. The term comes from the Japanese words for "harbor" and "wave."