ENERGY AND EARTH

CONCEPT

Earth is a vast flow-through system for the input and output of energy. The overwhelming majority of the input to Earth's energy budget comes from the Sun in the form of solar radiation, with geothermal and tidal energy rounding out the picture. Each form of energy is converted into heat and re-radiated to space, but the radiation that leaves Earth travels in longer wavelengths than that which entered the planet. This is in accordance with the second law of thermodynamics, which shows that energy output will always be smaller than energy input and that energy which flows through a system will return to the environment in a degraded form. Yet what the Earth system does in processing that energy, particularly the portion that passes through the biosphere, is amazing. Some biological matter decays and, over the course of several hundred million years, produces fossil fuels that have given Earth a slight energy surplus. Human use of fossil fuels is rapidly depleting those sources, however, while posing new environmental problems, and this has encouraged the search for alternative forms of energy. Many of those forms, most notably geothermal energy, come from Earth itself.

HOW IT WORKS

ENERGY, WORK, AND POWER

Physicists define energy as the ability of an object (and in some cases a non object, such as a magnetic force field) to accomplish work. "Work" in this context does not have the same meaning as it does in everyday life; along with the closely related concept of power, it is defined very specifically in a scientific context.

Work is the exertion of force over a given distance, and therefore it is measured in units of force multiplied by units of length. In the English system used by most Americans, a pound is the unit of force, and the foot-pound (ft-lb) would be the unit of work. However, scientists worldwide use SI, or the International System, which applies metric units. The metric unit of force is the newton (N), and the metric unit of work is the joule (J), equal to 1 newton-meter (N × m).

Power is the rate at which work is accomplished over time and therefore is measured in units of work divided by units of time. The metric unit of power is the watt (W), named after James Watt (1736-1819), the Scottish inventor who developed the first fully viable steam engine and thus helped inaugurate the Industrial Revolution. A watt is equal to 1 J per second, but this is such a small unit that kilowatts, or units of 1,000 W, are more frequently used. Discussing the vast energy budget of Earth itself, however, requires use of an even larger unit: the terawatt (TW), equal to 1012 (one trillion) W.

Ironically, Watt himself—like most people in the British Isles and America—lived in a world that used the British system, in which the unit of power is the foot-pound per second. The latter unit, too, is very small, so for measuring the power of his steam engine, Watt suggested a unit based on something quite familiar to the people of his time: the power of a horse. One horsepower (hp) is equal to 550 ft-lb per second.

In the present context, we will rely as much as possible on SI units, especially because the watt is widely used in America. Horsepower typically is applied in the United States only for measuring the power of a mechanical device, such as an automobile or even a garbage disposal. For measuring electrical power, particularly in larger quantities, the SI kilowatt (kW) is used. When an electric utility performs a meter reading on a family's power usage, for instance, it measures that usage in terms of electrical "work" performed for the family and thus bills them by the kilowatt-hour (kWh).

VARIETIES OF ENERGY

In the most fundamental sense, there are only three kinds of energy: kinetic, potential, and mass, or rest, energy. These types are, respectively, the energy an object possesses by virtue of its motion, its position (or its ability to perform work), and its mass. The first two are understood in relation to each other: for example, a ball held over the side of a building has a certain gravitational potential energy, but once it is dropped, it begins to lose potential energy and gain kinetic energy. The faster it moves, the greater the kinetic energy; but as it covers more distance, the less its potential energy is. (See Earth Systems for more about the kinetic-potential energy system.)

As Earth moves around the Sun, the gravitational interaction between the two bodies is not unlike that of the baseball and the ground in the illustration just given. Earth makes an elliptical, or oval-shaped, path in its orbit, meaning that the distance between it and the Sun is not uniform. At its furthest distance, Earth's potential energy is maximized, but as it comes closer to the Sun in its orbital path, its kinetic energy increases, with a corresponding decrease of potential energy.

MASS AND ENERGY.

Mass, or rest, energy is identified in the famous formula E = mc2, derived by Albert Einstein (1879-1955). In simple terms Einstein's formula means that every object possesses an amount of energy equal to its mass multiplied by the speed of light squared. Given that light travels at 186,000 mi. (299,339 km) per second, this is an enormous figure, even for a small object. A mere baseball, which weighs about 0.333 lb. (0.15 kg), possesses enough energy to yield about 3.75 billion kWh worth of power—enough to run all the lights and appliances in a typical American home for more than 156,000 years!

To release this energy in significant quantities, it would be necessary to accelerate the baseball to a speed close to that of light. Even in ordinary experience, however, very small amounts of mass are converted to energy. For instance, when a fire burns, the mass of the ashes combined with that of the particles and gases sent into the atmosphere is smaller (by an almost imperceptible fraction) than the mass of the original wood. The "lost" mass is converted to energy. These mass-energy conversions occur on a much larger level in nuclear reactions, such as the nuclear fusion of hydrogen atoms to form helium in the solar core (see Sun, Moon, and Earth).

MANIFESTATIONS OF ENERGY.

In discussing kinetic and potential energy, the example of dropping a baseball from a height illustrates these two types of energy in a gravitational field—that is, the gravitational field of Earth. Yet the concept of potential and kinetic energy translates to a situation involving an electromagnetic field as well. For instance, the positive or negative attraction between two electromagnetically charged particles is analogous to the force of gravity, and a system of two or more charges possesses a certain amount of kinetic and potential electromagnetic energy.

Electromagnetic energy, which is the form in which solar power reaches Earth, is a type of energy that (as its name suggests) combines both electrical and magnetic energy. Another important form of energy in the Earth system is thermal, or heat, energy, which is the kinetic energy of molecules, since heat is simply the result of molecular motion.

Other types of energy include sound, chemical, and nuclear energy. Sound waves, which require a physical medium such as air in which to travel, are simply pressure fluctuations that carry varying levels of energy, depending on the frequency (pitch) and amplitude (volume) of the waves. Chemical energy makes possible the forming and releasing of molecular bonds, and, for this reason, chemical reactions often are accompanied by the production of heat. Whereas chemical energy concerns the bonds between atoms, nuclear energy relates to the bonds within them. Nuclear fission reactions involve the splitting of an atomic nucleus, while nuclear fusion is the joining of nuclei.

HEAT AND THERMODYNAMICS

Thermodynamics is the study of the relationships between heat, work, and energy. As with work, energy, and power, heat and the related concept of temperature are terms that have special definitions in the physical sciences. Heat itself is not to be confused with thermal energy, which, as noted earlier, is the kinetic energy that arises from the motion of particles at the atomic or molecular level. The greater the movement of these particles relative to one another, the greater the thermal energy.

Heat is internal thermal energy that flows from one body of matter to another. It is not the same as the energy contained in a system—that is, the internal thermal energy of the system. Rather than being "energy-in-residence," heat is "energy-in-transit." This may seem a little confusing, but all it means is that heat, in its scientific sense, exists only when internal energy is being transferred. As for temperature, it is not (as is commonly believed) a measure of heat and cold. Instead, temperature indicates the direction of internal energy flow between bodies and the average molecular kinetic energy in transit between those bodies.

In any case, temperature could not be a measure of heat and cold, as though these two were equal and opposing entities, because, scientifically speaking, there is no such thing as cold—only an absence of heat. When we place an ice cube in a cup of coffee, we say that the ice is there to "cool the coffee down," but, in fact, the opposite is happening: the coffee is warming up the ice cube, and in the process of doing so, it loses heat. This may seem like a difference of semantics, but it is not. It is a physical law that the flow of heat is always from a high-temperature reservoir to a low-temperature reservoir. Even air conditioners and refrigerators work by pulling heat out of a compartment rather than by bringing cold in.

MEASURING TEMPERATURE AND HEAT.

Temperature, of course, can be measured by either the Fahrenheit or the Centigrade scales familiar in everyday life. Scientists, however, prefer the Kelvin (K) scale, established by William Thomson, Lord Kelvin (1824-1907). Drawing on the discovery made by the French physicist and chemist J. A. C. Charles (1746-1823) that gas at 0°C (32°F) regularly contracts by about 1/273 of its volume for every Celsius degree drop in temperature, Thomson derived the value of absolute zero (the temperature at which molecular motion virtually ceases) as −273.15°C (−459.67°F). The Kelvin and Celsius scales are thus directly related: Celsius temperatures can be converted to Kelvin units (for which neither the word nor the symbol for "degree" is used) by adding 273.15.

Heat, on the other hand, is measured not by degrees but by the same units as work. Energy is the ability to perform work, so heat or work units are also units of energy. Aside from the joule, heat often is measured by the kilocalorie, or the amount of heat that must be added to or removed from 1 kg of water to change its temperature by 1°C. As its name suggests, a kilocalorie is 1,000 calories, a calorie being the amount of heat required to change the temperature in 1 g of water by 1°C. The dietary calorie with which most people are familiar, however, is the same as a kilocalorie.

THE LAWS OF THERMODYNAMICS

The three laws of thermodynamics collectively show that it is impossible for a system to produce more energy than was put into it or even to produce an equal amount of usable energy. In other words, a perfectly efficient system—whether an engine or the entire Earth—is an impossibility. Derived during a period of about 60 years beginning in the 1840s, the laws of thermodynamics helped scientists and engineers improve the machines that powered the height of the Industrial Age. They also revealed the impossibility of constructing anything approaching a perpetual-motion machine, that great quest of dreamers over the ages, which the laws of thermodynamics proved to be an impossible dream.

THE FIRST LAW OF THERMODYNAMICS.

The first law of thermodynamics is related to the conservation of energy, a physical law whereby the total energy in a system remains the same, though transformations of energy from one form to another take place. Such transformations occur frequently in the Earth system, as when a plant receives electromagnetic energy from the Sun and converts it to chemical potential energy in the form of carbohydrates. Likewise humans, by building dams, can harness the gravitational potential energy of flowing water and convert it into electromagnetic energy.

The conservation of energy, in effect, states that "the glass is half full," meaning that we can obtain as much energy from a system as we put into it. While saying the same thing, the first law of thermodynamics in effect states that "the glass is half empty,": that is, that we can obtain no more energy from a system than we put into it. According to this law, because the amount of energy in a system remains constant, it is impossible to perform work that results in an energy output greater than the energy input.

The term law in the physical sciences is no empty expression; it means that a principle has been shown to be the case always and may be expected to remain the case in all situations. It is possible, of course, for a physical law to be overturned in light of later evidence. It is not likely, however, that any set of circumstances in the universe will ever disprove the core truth behind this law, which may be stated colloquially as "You can't get something for nothing."

THE SECOND LAW OF THERMODYNAMICS.

In a 1959 lecture published as The Two Cultures and the Scientific Revolution, the British writer and scientist C. P. Snow (1905-1980) compared transfers of heat and energy to a game. The laws of thermodynamics are its rules, and, as Snow stated, the first law proves that it is impossible to win at this game, while the second law shows the impossibility of breaking even.

The second law of thermodynamics is more complicated than the first and is stated in a number of ways, though they are all interrelated. According to this law, spontaneous or unaided transfers of energy are irreversible and impossible without an increase of entropy in the universe. Entropy is the tendency of natural systems toward breakdown, specifically, the tendency for the energy in a system to be dissipated or degraded. (Later in this essay, we discuss examples of energy that has been degraded—for instance, wood that has been burned to produce ashes.) The second law means that spontaneous processes are irreversible and that it is impossible, without the additional input of energy, to transfer heat from a colder to a hotter body or to convert heat into an equal amount of work.

Whereas the first law showed engineers the impossibility of building a perpetual-motion machine, the second law proves that it is impossible to build even a perfectly efficient engine. Of all the energy we put into our automobiles in the form of gasoline (which is chemical potential energy in the form of hydrocarbons derived from the fossilized remains of dinosaurs in the earth), only about 30% of it goes into moving the car forward. The rest is dissipated in a number of ways, chiefly through heat and sound. Entropy, as it turns out, is inescapable and as inevitable as death. In fact, death itself is a result of entropy in the systems of all living things.

THE THIRD LAW OF THERMODYNAMICS.

The third law of thermodynamics is not as well known as the other two and has little bearing on the discussion at hand, but it deserves at least brief mention. According to the third law, at the temperature of absolute zero entropy also approaches zero, which might sound like a way out of the restrictions imposed by the first two laws. All it really means is that absolute zero is impossible to reach—or, as Snow put it, the third law shows that "you can't escape the game."

In 1824 the French physicist and engineer Nicolas Léonard Sadi Carnot (1796-1832) had shown that an engine could achieve maximum efficiency if its lowest operating temperature were absolute zero. His work influenced that of Kelvin, who established the absolute-temperature scale mentioned earlier. Additionally, Carnot's discoveries informed the development of the third law. Whereas the second law is not derived from the first (though it is certainly consistent with it), the third law relies on the second: if it is impossible to build a perfectly efficient engine, as the second law states, it is likewise impossible to reach absolute zero.

This, of course, has not stopped scientists from attempting to achieve absolute zero, most properly defined as the temperature at which the motion of the average atom or molecule is zero. Helium atoms, in fact, never fully cease their motion, even at temperatures very close to 0K—and scientists have come very, very close. In 1993 physicists at the Helsinki University of Technology Low Temperature Laboratory in Finland used a nuclear demagnetization device to achieve a temperature of 2.8 × 10−10K, or 0.00000000028K. This amounts to a difference of only 28 parts in 100 billion between that temperature and absolute zero.

EARTH'S ENERGY INPUT

Just as households have financial budgets, a system such as Earth (see Earth Systems) has an energy budget. The latter may be defined as the total amount of energy available to a system or, more specifically, the difference between the

THE POHUTU GEYSER AT THE ROTORUA WHAKAREWAREWA THERMAL AREA IN NORTH ISLAND, NEW ZEALAND. (© A. N. T./Photo Researchers. Reproduced by permission.)
THE POHUTU GEYSER AT THE ROTORUA WHAKAREWAREWA THERMAL AREA IN NORTH ISLAND, NEW ZEALAND. (
© A. N. T./Photo Researchers
. Reproduced by permission.)
energy flowing into the system and the energy lost by it. Having reviewed the laws of thermodynamics, one might suspect that a great deal of energy is dissipated in the operation of the Earth system, and, of course, that is absolutely correct.

Earth receives 174,000 TW of energy, or 174 quadrillion J per second. Human civilization, by contrast, uses only 10 TW, or about 0.00574% as much as Earth's total energy. Of that total, there are three principal sources, though one of these sources—the Sun—dwarfs the other two in importance. The breakdown of Earth's energy input, along with the percentage of the total that each portion constitutes, is as follows:

  • Solar radiation: 99.985%
  • Geothermal energy: 0.013%
  • Tidal energy: 0.002%

SOLAR RADIATION.

The Sun radiates electromagnetic energy, which, as mentioned previously, is a form of energy that produces both electric and magnetic fields. Electromagnetic energy travels in waves, and since waves follow regular patterns, it is possible to know that those waves with shorter wavelengths have a higher frequency and thus higher energy levels.

The electromagnetic spectrum contains a variety of waves, each with progressively higher energy levels, including long-wave and short-wave radio; microwaves (used for TV transmissions); infrared, visible, and ultraviolet light; x rays; and ultra-high-energy gamma rays. Visible light is only a very small portion of this spectrum, and each color has its own narrow wavelength range.

Red has the least energy and purple or violet the most; hence, the names infrared for light with less energy than red, and ultraviolet for light with more energy than violet. The order of these wavelengths of light, along with the colors between, is remembered easily by the mnemonic device ROY G. BIV (standing for red, orange, yellow, green, blue, indigo, and violet). (Actually, there are only six major color ranges, and the name really should be ROY G. BV.)

Although it covers the entire electromagnetic spectrum, energy from the Sun is referred to by earth scientists as short-wavelength radiation. This is because the solar energy that enters the Earth system is shorter in wavelength (and thus higher in energy level) than the energy returned to space by Earth. (We discuss the degradation of energy in the Earth system later in this essay.) Without solar radiation, the life-giving processes of the hydrosphere, biosphere, and atmosphere would be impossible. An example is photosynthesis, the biological conversion of electromagnetic energy to chemical energy in plants. (See the later discussion of photosynthesis and the food web.)

GEOTHERMAL ENERGY.

A much smaller, but still significant component of Earth's energy budget is geothermal energy, the planet's internal heat energy. Much of this heat comes from Earth's core, which has temperatures as high as 8,132°F (4,500°C) and from whence thermal energy circulates throughout the planet's interior. Also significant is the heat from radioactive elements, most notably uranium and thorium, near Earth's surface.

This thermal energy heats groundwater, and thus the principal visible sources of geothermal energy include geysers, hot springs, and fumaroles—fissures, created by volcanoes, from which hot gases pour. There are several types of geothermal energy reserves, among them dry and wet steam fields. The first of these reserves occurs when groundwater boils normally, whereas in the second type of reserve, groundwater is super-heated, or prevented from boiling even though its temperature is above the boiling point. In both cases the waters have a much higher concentration of gases and minerals than ordinary groundwater. Another type of reserve can be found under the ocean floors, where natural gas mixes with very hot water.

Geothermal energy powers seismic activity as well as volcanic eruptions and mountain building, which together have played a significant role in shaping Earth as we know it today. Aside from its obvious impact on the planet's terrain, geothermal energy has had an indirect influence on the transfer of vital elements from beneath Earth's surface, a benefit of volcanic activity. (See the later discussion of the human use of geothermal energy in this essay.)

TIDAL ENERGY.

Whereas the principal form of energy in Earth's budget comes from the Sun and the secondary source from Earth itself, the third type of energy input to the Earth system comes chiefly from the Moon. The Sun also affects tides, but because of its close proximity to Earth, the Moon has more influence over the movements of our planet's ocean waters.

Though the Moon is much smaller than Earth, it is larger, in proportion to the planet it orbits, than any satellite in the solar system (with the possible exception of Pluto's moon Charon). Given this fact, combined with its close proximity to Earth, it is understandable that the Moon would exert a powerful pull on its host planet. The gravitational pull of the Moon (and, to a lesser extent, that of the Sun) on Earth causes the oceans to bulge outward on the side of Earth closest to the Moon. At the same time, the oceans on the opposite side of the planet bulge in response. (See Sun, Moon, and Earth for more about tides and the bulges that result from the Moon's gravitational pull.)

This gravitational pull creates a torque that acts as a brake on Earth's rotation, producing a relatively small amount of energy that is dissipated primarily within the waters of the ocean. Incidentally, the lunar-solar tidal torque, by increasing the amount of time it takes Earth to turn on its axis, is causing a gradual increase in the length of a day. Today, of course, there are 365.25 days in a year, but about 650 million years ago there were 400 days. In other words, Earth made 400 revolutions on its axis in the period of time it took it to revolve around the Sun. The change is a result of the fact that Earth's rotation is being slowed by 24 microseconds a year.

ENERGY PROFIT AND LOSS

Focusing now on solar radiation, since it is by far the greatest source of energy input to the Earth system, let us consider Earth's energy budget in terms of "profit and loss." In other words, how much useful energy output is denied to Earth owing to the laws of thermodynamics and other factors?

First of all, a good 30% of the Sun's energy input is reflected back into space unchanged, without entering Earth's atmosphere. This results from our planet's albedo, or reflective power. Albedo is the proportion of incoming radiation that is reflected by a body (e.g., a planet) or surface such as a cloud: the higher the proportion of incoming radiation that a planet deflects, the higher its albedo. The latter is influenced by such factors as solar angle, amount of cloud cover, particles in the atmosphere, and the character of the planetary surface.

Another 25% of solar radiation is absorbed by the atmosphere, while about 45% is absorbed at the planetary surface by living and nonliving materials. Thus, electromagnetic energy from the Sun enters the atmosphere, biosphere, and hydrosphere, where it is converted to other forms of energy, primarily thermal. Some of this thermal energy, for instance, causes the evaporation of water, which cycles through the atmosphere and then reenters the hydrosphere as precipitation. In other cases, absorbed radiation drives atmospheric and hydrologic distribution mechanisms, including winds, water currents, and waves. A very small, but extremely significant portion of incoming solar radiation goes into plant photosynthesis, discussed later.

ENERGY DEGRADATION.

The energy that enters the Earth system—not only solar radiation but also geothermal and tidal energy—ultimately leaves the system. As shown by the second law of thermodynamics, however, the energy that departs the Earth system will be in a degraded form compared with the energy that entered it.

In a steam engine, water in the form of steam goes to work to power gears or levers. In the process, it cools, and the resulting cool water constitutes a degraded form of energy. Likewise, the ashes that remain after a fire or the fumes that are a by-product of an internal combustion engine's operation contain degraded forms of energy compared with that in the original wood or gasoline, respectively. In the same way, Earth receives short-wavelength energy from the Sun, but the energy it radiates to space is in a long-wave length form.

All physical bodies with a temperature greater than absolute zero emit electromagnetic energy in accordance with their surface temperatures, and the hotter the body, the shorter the wavelength of the radiation. The sunlight that enters Earth's atmosphere is divided between the visible portion of the spectrum and the high-frequency side of the infrared portion. (Note that the Sun emits energy across the entire electromagnetic spectrum, but only a small part gets through Earth's atmospheric covering.) Earth, with an average surface temperature of 59°F (15°C), is much cooler than the Sun, with its average surface temperature of about 10,000°F (5,538°C). The radiation Earth sends back into space, then, is on the low-frequency, long-wave-length side of the infrared spectrum.

NO ENERGY LOSS.

In accordance with the conservation of energy, no energy truly has been lost. In a relatively simple system, such as an automobile, chemical potential energy enters the vehicle in the form of gasoline and, after being processed by the engine, exits in a variety of forms. There is the kinetic energy that turns the wheels; the thermal energy of the engine and exhaust; electromagnetic energy from the battery for the headlights, dashboard lights, radio, air conditioning, and so on; and the sound energy dissipated in the noise of the car. If one could add all those energy components together, one would find that all the energy that entered the system left the system. Note, however, that once again the process is irreversible: one can use gasoline to power a battery and hence a car radio, but the radio or the battery cannot generate gasoline.

The Earth system is much more complex, of course, but the same principle applies: about 174,000 TW of energy enter the system, and about 174,000 TW are used in the form of heat. Of the portion that enters the atmosphere, some goes into warming the planet, some into moving the air and water, and a very small part into the all-important biological processes described later, but all of it is used. It should be noted that Earth has a very small energy surplus, owing to the accumulation of undecomposed biomass that ultimately becomes fossil fuels; however, the amount of energy involved is minor compared with the larger energy budget. (For more information about biomass and fossil fuels, see the following discussion.)

THE GREENHOUSE EFFECT.

Not only is there no net loss of energy in the universe, but Earth itself also possesses a remarkably efficient system for making use of the energy it receives. This is the greenhouse effect, whereby the planet essentially recycles the degraded energy it is in the process or returning to space.

Water vapor and carbon dioxide, as well as methane, nitrous oxide, and ozone, all absorb long-wave length radiated energy as the latter makes its way up through the atmosphere. When heated, these radiatively active gases (as they are called) re-radiate the energy, now at even longer wavelengths. In so doing, they slow the planet's rate of cooling. Without the greenhouse effect, surface temperatures would be about 50°F (10°C) cooler than they are—that is, around 17.6°F (−8°C). This, of course, is well below the freezing temperature of water and much too cold for Earth's biological processes. Thus, the greenhouse effect literally preserves life on the planet.

It may be surprising to learn that the greenhouse effect, a term often heard in the context of dire environmental warnings, is a natural and healthful part of the Earth system. Like many useful things, the greenhouse effect is not necessarily better in larger doses, however, and that is the problem. It is believed that human activities have resulted in an increase of radiatively active

A GERMINATING GARDEN PEA SEEKS SUNLIGHT FOR PHOTOSYNTHESIS. (© J. Burgess/Photo Researchers. Reproduced by permission.)
A GERMINATING GARDEN PEA SEEKS SUNLIGHT FOR PHOTOSYNTHESIS. (
© J. Burgess/Photo Researchers
. Reproduced by permission.)
gases, which could lead to global warming. Among such gases are the chlorofluorocarbons (CFCs) used in aerosol cans, now banned by international treaty. Also of concern are the high levels of carbon dioxide produced by the burning of fossil fuels.

REAL-LIFE APPLICATIONS

ENERGY AND THE LIVING WORLD

One of the most interesting components of the entire energy picture is the relationship of energy input, energy output, and the biosphere. Particularly important is the use of solar radiation for the purposes of photosynthesis, an activity that constitutes a small but vital sector of Earth's energy budget.

Though it accounts for only about 1% of the energy received from solar radiation, photosynthesis is essential to the sustenance of life. In photosynthesis, plants receive solar radiation, carbon dioxide, and water, which chemically react to produce carbohydrates and oxygen. Animals depend not only on the oxygen but also on carbohydrates, such as sugars, and thus photosynthesis makes possible the development of the food chains that constitute the world of living creatures.

FOOD WEBS.

Even though food chain is a well-known expression, modern scientists prefer the term food web, because it more accurately portrays the complicated relationships involved. The term food chain, as it is commonly used, implies a strict hierarchical structure in which (to use another popular phrase) "the big fish eat the little fish." In fact, the relationship between participants is not quite so neatly defined.

It is, however, possible to describe a food web in terms of a few key players, or types of players. There are primary producers, which are green plants, and primary consumers—herbivores, or plant-eating animals. Secondary consumers (and those at further levels, such as tertiary and quaternary) are either carnivores—that is, meat-eating animals who eat the herbivores—or omnivores, which are both plant and animal eaters. For example, omnivorous humans eat herbivorous cows, who have eaten plants.

Carnivores and omnivores, however, are not really at the top of the food chain, so to speak; rather, in line with the non-hierarchical idea of the food web, they represent points in an interlocking set of relationships. Materials from plants, herbivores, carnivores, and omnivores ultimately will all be consumed by the lowliest of creatures, that is, detritivores, or decomposers, including bacteria and worms. At each stage energy is transferred, and, as always, the second law of thermodynamics comes into play. The energy is degraded in transfer; specifically, the further away an organism is from the original plant source, the less a given quantity of fuel contributes to its growth. It is interesting to note the economy of energy use at the detritivore stage.

Worms and other decomposers are exceedingly efficient feeders, working the same food particles over and over and extracting more stored energy each time. They then produce waste products that increase the vitamin content of the soil, thus enabling the growth of plants and the continuation of the biological cycle. Also, detritivores may contribute directly (and, from the larger energy-cycle perspective, less efficiently) to providing fuel for carnivores, as when a bird eats a worm.

BIOENERGY

The food web is the mechanism whereby energy is cycled through the biosphere, as fuel in the form of food. Plant matter and other biological forms also can serve as direct sources of heat, providing fuel that can be either burned without processing or converted into gas or alcohol. In such situations the plant matter is described as bioenergy, or energy derived from biological sources that are used as fuel.

Materials that are burned or processed to produce bioenergy are called biomass. Examples of the latter include wood logs burned on a fire, probably the oldest type of fuel known to humankind and still one of the principal forms of heating available in many developing countries. In fact, some of the least technologically advanced and most technologically advanced nations are alike in their use of another variety of biomass: waste.

Dried animal dung provides heating material in many a third world village where electricity is unknown, while at the other end of the technological spectrum, some Western municipalities extract burnable biomass from processed sewage. Since Western countries have at their disposal plenty of other energy sources, they typically burn off the methane gas and dried waste material from treated sewage simply as a means of removing it, rather than as an energy source. Those materials could provide usable energy, however.

The products of sewage treatment, of course, are the result of processing, which involves the conversion of biomass to either gases (for example, methane, as mentioned previously) or alcohol. Farmers in rural China, for instance, often place agricultural waste and sewage in small closed pits, from whence they extract burnable methane gas. In the United States, Brazil, and other countries with abundant farmland, some of the agricultural output is directed not toward production of food but toward production of fuel in the form of ethanol, a type of alcohol made from sugarcane, corn, or sorghum grain. Ethanol can be mixed with gasoline to run an automobile or burned alone in specially modified engines. In either case, the fuel burns much more cleanly, producing less poisonous carbon monoxide than ordinary gasoline.

FOSSIL FUELS

Biomass is potentially renewable, whereas another source of bioenergy is not. This is the bioenergy from fossil fuels—buried deposits of petroleum, coal, peat, natural gas, and other organic compounds. (Actually, fossil fuels typically are considered separate from other forms of bioenergy, because they are nonrenewable.)

Fossil fuels are the product of plants and animals that lived millions of years ago, died, decomposed, and became part of Earth's interior. Over the ages, as more sediment weighed down on these organic deposits, the weight applied more pressure, generated more heat, and led to the concentration of this decomposed material, which became a valuable source of energy.

USING UP RESOURCES.

Given the vast spans of time that have passed, as well as the almost inconceivable numbers of plants and animals, the supplies of fossil-fuel energy stored under Earth's surface are enormous. But they are not infinite, and, as noted earlier, they are nonrenewable; once they are gone, they might as well be gone forever, because it would take hundreds of millions of years to produce additional deposits. Furthermore, humans are using up these energy sources at an alarming rate.

Up until about 1750, Earth's fossil-fuel deposits were largely intact, but after that time industrialized societies began to extract coal for heating, transportation, and industrial uses. Today it remains a leading means of generating electrical power. During the twentieth century, petroleum increasingly was directed toward transportation, and this led to the extraction of still more of Earth's fossil-fuel deposits. If civilization continues to consume these products at its current rate, reserves will be exhausted long before the end of the twenty-first century.

SEARCHING FOR ENERGY SOURCES.

There are several reasons not to panic over the loss of fossil-fuel resources. First, known reserves are just that—they are the ones that energy companies, and their geologists, know about at the present time. As long as plentiful resources are available, corporations and governments do not feel a pressing need to search for more, but as those resources are used up, such searches become economically necessary. For a time at least, these searches will continue to yield new (though increasingly harder to reach) deposits.

Also, fears about Earth running out of fuel are built on the assumption that no one will develop other sources of energy, a few of which are discussed at the conclusion of this essay. Though most of the known alternative energy sources face their own challenges, it is thoroughly conceivable that scientists of the future will develop means of completely replacing fossil fuels as the source of electrical power, fuel for transport, and other uses. After all, there was once a time (during the second half of the nineteenth century) when Americans became increasingly anxious over dwindling reserves of a vital energy resource essential for powering the nation's lamps: whale oil.

ENVIRONMENTAL CONCERNS

While the loss of energy reserves may not be an immediate cause for panic, the effects of fossil-fuel burning on the environment have alarmed scientists and environmentalists alike. At the most basic level, there is the environmental impact posed by the extraction of fossil fuels. Coal mines, for instance, have been used up and now sit abandoned, their land worthless for any purpose. There is also the environmental danger created by hazards in misuse or transport of fossil fuels—for example, the vast oil spill caused by the grounding of the Exxon Valdez near Alaska's Prince William Sound in 1989.

By far the greatest environmental concern raised by fossil fuels, however, is the effect they produce in the atmosphere when burned. For instance, one of the impurities in coal is sulfur, and when coal burns, the sulfur reacts with oxygen in the combustion process to create sulfur dioxide and sulfur trioxide. Sulfur trioxide reacts with water in the air, creating sulfuric acid and thus acid rain, which can endanger plant and animal life as well as corrode metals and building materials.

THE GREENHOUSE EFFECT REVISITED.

Even greater fears center on the release into the atmosphere of carbon monoxide and carbon dioxide, both of which are by-products in the burning of fossil fuels. The first is a poison, whereas the second is a vital part of the life cycle, yet carbon dioxide, in fact, may pose the greater threat.

AN ARRAY OF SOLAR REFLECTORS NEAR ALBU-QUERQUE, NEW MEXICO, HARNESSES SOLAR POWER. (© J. Mead/Photo Researchers. Reproduced by permission.)
AN ARRAY OF SOLAR REFLECTORS NEAR ALBU-QUERQUE, NEW MEXICO, HARNESSES SOLAR POWER. (
© J. Mead/Photo Researchers
. Reproduced by permission.)

Earlier we discussed the greenhouse effect, which, when it occurs naturally, is important to the preservation of life on the planet. The large number of internal-combustion engines in operation on Earth today produce an inordinate amount of carbon dioxide, which in turn provides the atmosphere with more radiatively active gas than it needs. According to many environmentalists, the result is, or will be, global warming. If it takes place over a long period of time, global warming could bring about serious hazards—in particular, the melting of the polar ice caps. It should be noted, however, that not all scientists are in agreement that global warming is occurring or that humans are the principal culprits inducing these environmental changes.

ALTERNATIVE ENERGY SOURCES

Whatever the merits of the various sides in the debate on global warming or the exhaustion of fuel resources, one need hardly be an environmentalist to agree that the world cannot forever rely only on existing fossil fuels and the technology that uses them. Today, even as scientists in laboratories around the world work to develop viable alternative means of powering industry, utilities, and transportation, many alternative energy sources are already in use.

Some of these energy forms are very old, for instance, burnable biomass, water power, or wind power, all of which date back to ancient times. Others are extremely new in concept, most notably, nuclear energy, which was developed in the twentieth century. Still others are new, hightech versions of old-fashioned energy sources, the best example being solar power. In fact, all three major contributors to Earth's energy budget—solar radiation, tidal energy, and geothermal energy (discussed further later in this essay)—have been harnessed by human societies.

HIGH-TECH ENERGY SOLUTIONS.

Several of the more ambitious ideas for energy creation, while they may capture people's imaginations, have significant drawbacks. There is the proposal, for instance, to extract hydrogen gas from water by means of electrolysis, potentially providing an extremely clean, virtually limitless source of energy. Hydrogen gas, however, is highly flammable, as the 1937 explosion of the airship Hindenburg illustrated, and, in any case, the fuel to provide the electricity necessary for electrolysis would have to come from somewhere, presumably, the burning of fossil fuels.

Nuclear energy, of course, has frightened many people in the wake of such well-known disasters as those that occurred at Three Mile Island, Pennsylvania, in 1979 and Chernobyl, in the former Soviet Union, in 1986. In fact, those two situations illustrate more about governments than they do about technology itself. No one died at Three Mile Island, and with an open society and media access to the site, the public outcry became so great that the plant was closed. By contrast, the Soviets' outmoded technology helped bring about the Chernobyl disaster, and the communist dictatorship's practice of censorship and suppression led to a massive cover-up that greatly increased the death toll. As a result, thousands died at Chernobyl, and thousands more died as the result of the indirect effects of nuclear pollution in the environment.

There is no question, however, that nuclear energy does pose an enormous potential environmental threat from its waste products. Spent fuel rods, if simply buried, eventually leak radioactive waste into the water table and could kill or harm vast populations. This all relates to nuclear fission, the only type of peaceful nuclear power in use today. In building the hydrogen bomb in the mid-twentieth century, physicists and chemists used the much greater power of nuclear fusion, or the bonding of atomic nuclei. If nuclear fusion could be produced in controlled reactions, for peaceful use, it would provide safe, cheap, limitless power to the planet.

GEOTHERMAL ENERGY.

Unless or until nuclear fusion or some more advanced alternative energy source is developed, societies will continue to rely on fossil fuels for the bulk of their energy needs. At the same time, alternative sources will continue to supply energy in certain situations. An excellent example of such a source is geothermal energy, harnessed by peoples in areas as widespread as New Zealand and Iceland centuries ago.

Long before the first Europeans arrived in New Zealand, the native Maori people used geothermal energy from geysers to cook food. Modern applications of geothermal energy began with the creation of the first geothermal well, by workers who accidentally drilled into one in Hungary in 1867. Today Hungary is the world's leading producer and consumer of geothermal energy, followed closely by Italy and Iceland. The word fumarole, used earlier in this essay, is Italian in origin, and it is said that the fumaroles near the town of Lardarello, used for the production of electricity since 1904, once inspired Dante Alighieri's (1265-1321) vision of the Inferno, as captured in his celebrated work by that name. As for Iceland, more than 99% of the buildings in the capital city of Reykjavik use geothermal energy for heat.

Heat is not the primary human application for geothermal energy. As noted earlier, in the context of the first law of thermodynamics, energy can be converted from one form to another. Thus, geothermal energy is applied for the creation of electromagnetic energy: steam or heated water from the ground runs turbines, which produce electricity.

Geothermal energy has enormous advantages, including the fact that its raw materials (heated water and steam) are free and relatively inexpensive to extract. It is also inexpensive environmentally, causing virtually no air pollution. Will geothermal energy ever significantly compete with fossil fuels as a significant source of energy for humans? It is conceivable, but at present a number of barriers exist. Geothermal resources exist only in very specific parts of the world, and the extraction of the raw materials may release noxious gases, such as hydrogen sulfide (the same compound that gives intestinal gas its smell). Also, ironically, there are environmental concerns, not because of true damage but because geothermal mines often pose a threat of sight pollution in the midst of otherwise gorgeous natural settings.

WHERE TO LEARN MORE

Altman, Linda Jacobs. Letting Off Steam: The Story of Geothermal Energy. Minneapolis, MN: Carolrhoda Books, 1989.

Brown, Warren. Alternative Sources of Energy. New York: Chelsea House, 1994.

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>.

Earth's Energy Budget, or Can You Spare a Sun? (Web site). <http://www.strategies.org/LESSON8.html>.

Fowler, Allan. Energy from the Sun. New York: Children's Press, 1997.

Gutnik, Martin J., and Natalie Browne-Gutnik. The Energy Question: Thinking About Tomorrow. Hillside, NJ: Enslow Publishers, 1993.

Hancock, Paul L., and Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000.

Knapp, Brian J. Earth Science: Discovering the Secrets of the Earth. Illus. David Woodroffe and Julian Baker. Danbury, CT: Grolier Educational, 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.

KEY TERMS

ABSOLUTE ZERO:

The temperature at which all molecular motion virtually ceases.

ALBEDO:

The reflective power of a surface or body or, more specifically, the proportion of incoming radiation that the surface or body reflects.

ATMOSPHERE:

In general, an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists 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%.

BIOENERGY:

Energy derived from biological sources that are used directly as fuel (as opposed to food, which becomes fuel).

BIOMASS:

Materials that are burned or processed to produce bioenergy.

BIOSPHERE:

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.

CALORIE:

A measure of heat or energy in the SI, or metric, system, equal to the heat that must be added to or removed from 1 g of water to change its temperature by 1°C. The dietary calorie with which most people are familiar is the same as the kilocalorie, or 1,000 calories.

CONSERVATION OF ENERGY:

A law of physics that holds that within a system isolated from all outside factors, the total amount of energy remains the same, though transformations of energy from one form to another take place. The first law of thermodynamics is the same as the conservation of energy.

ELECTROMAGNETIC ENERGY:

A form of energy with electric and magnetic components, which travels in waves.

ELECTROMAGNETIC SPECTRUM:

The complete range of electromagnetic waves on a continuous distribution from a very low range of frequencies and energylevels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electromagnetic spectrum are long-wave and short-wave radio; microwaves; infrared, visible, and ultraviolet light; x rays; and gamma rays.

ENERGY:

The ability of an object (or in some cases a non object, such as a magnetic force field) to accomplish work.

ENERGY BUDGET:

The total amount of energy available to a system or, More specifically, the difference between the energy flowing into the system and the energy lost by it.

ENTROPY:

The tendency of natural systems toward breakdown and, specifically, the tendency for the energy in a system to be dissipated. Entropy is related closely to the second law of thermodynamics.

ENVIRONMENT:

In discussing systems, the term environment refers to the surroundings—everything external to and separate from the system.

FIRST LAW OF THERMODYNAMICS:

A law of physics stating that the amount of energy in a system remains constant, and therefore it is impossible to perform work that results in an energy output greater than the energy input. This is the same as the conservation of energy.

FOSSIL FUELS:

Nonrenewable forms of bioenergy, including petroleum, coal, peat, natural gas, and other organic compounds usable as fuel.

FREQUENCY:

The number of waves, measured in Hertz, passing through a given point during the interval of one second. The higher the frequency, the shorter the wavelength.

GEOTHERMAL ENERGY:

Heat, or thermal, energy from Earth's interior.

GREENHOUSE EFFECT:

Warming of the lower atmosphere and surface of Earth. This occurs because of the absorption of long-wave length radiation from the planet's surface by certain radiatively activegases, such as carbon dioxide and watervapor, in the atmosphere. These gases are heated and ultimately re-radiate energy at an even longer wavelength to space.

HEAT:

Internal thermal energy that flows from one body of matter to another.

HERTZ:

A unit for measuring frequency, equal to one cycle per second. High frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second), megahertz (MHz; 106 or one million cycles per second), and gigahertz (GHz; 109 or one billion cycles per second.)

HYDROSPHERE:

The entirety of Earth's water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice.

JOULE:

The SI measure of work. One joule (J) is equal to the work required to accelerate 1 kg of mass by 1 m per second squared (1 m/s2) over a distance of 1 m. Owing to the small size of the joule, however, it often is replaced by the kilowatt-hour, equal to 3.6 million (3.6 × 106) J.

KELVIN SCALE:

Established by William Thompson, Lord Kelvin (1824-1907), the Kelvin scale measures temperature in relation to absolute zero, or 0K.(Note that units in the Kelvin system, known as kelvins, do not include the word or symbol for "degree.") The Kelvin scale, which is the system usually favored by scientists, is related directly to the Celsius scale; hence, Celsius temperatures can be converted to kelvins by adding 273.15.

KINETIC ENERGY:

The energy that an object possesses by virtue of its motion.

LAW:

A scientific principle that is shown always to be the case and for which no exceptions are deemed possible.

MASS ENERGY:

The energy an object possesses by virtue of its mass. Sometimes called rest energy.

NUCLEAR FISSION:

A nuclear reaction that involves the splitting of an atomic nucleus.

NUCLEAR FUSION:

A nuclear reaction that involves the joining of atomicnuclei.

NUCLEUS:

The center of an atom, a region where protons and neutrons are located and around which electrons spin.

PHOTOSYNTHESIS:

The biological conversion of light energy (that is, electromagnetic energy) to chemical energy in plants.

POTENTIAL ENERGY:

The energy that an object possesses by virtue of its position or its ability to perform work.

POWER:

The rate at which work is accomplished over time, a figure rendered mathematically as work divided by time. The SI unit of power is the watt, while the British unit is the foot-pound per second.

RADIATION:

The transfer of energy by means of electromagnetic waves, which require no physical medium (for example, water or air) for the transfer. Earth receives the Sun's energy via the electromagnetic spectrum by means of radiation.

RADIOACTIVITY:

A term describing a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles or radiation. Forms of particles or energy include alpha particles (positively charged helium nuclei), beta particles (either electrons or subatomic particles called positrons), or gamma rays, which occupy the highest energy level in the electromagnetic spectrum.

SECOND LAW OF THERMODYNAMICS:

A law of physics stating that spontaneous or unaided transfers of energy are irreversible and impossible without an increase of entropy in the universe. It is therefore impossible, without the additional input of energy, to transfer heat from a colder to a hotter body or to convert heat into an equal amount of work.

SI:

An abbreviation of the French term Système International d'Unités, or International System of Units. Based on the metricsystem, SI is the system of measurement units in use by scientists worldwide.

SYSTEM:

Any set of interactions that can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement.

TEMPERATURE:

The direction of internal energy flow between two systems when heat is being transferred. Temperature measures the average molecular kinetic energy in transit between those systems.

TERAWATT:

See watt.

THERMAL ENERGY:

Heat energy, a form of kinetic energy produced by the motion of atomic or molecular particles in relation to one another. The greater the relative motion of these particles, the greater the thermal energy.

WATT:

The metric unit of power, equal to 1 J per second. Because this is such a small unit, scientists and engineers typically speak in terms of kilowatts, or units of 1,000W. Very large figures, such as those relating to Earth's energy budget, usually are given in terawatts, or 1012 (one trillion) W.

WAVELENGTH:

The distance between a crest and the adjacent crest or the trough and an adjacent trough of a wave. Wavelength is related inversely to frequency, meaning that the shorter the wavelength, the higher the frequency.

WORK:

The exertion of force over a given distance. In the metric, or SI, system, work is measured by the joule (J) and in the British system by the foot-pound (ft-lb).

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