Environmental science


· Botkin, D. B., & Keller, E. A. (2014). Environmental science: Earth as a living planet (9th ed.). Hoboken, NJ: John Wiley & Sons, Inc

· Chapter 4: The Big Picture: Systems of Change

4.1 Basic Systems Concepts

A system is a set of components, or parts, that func- tion together as a whole. A single organism, such as your body, is a system, as are a sewage-treatment plant, a city, and a river. On a much different scale, the entire Earth is a system. In a broader sense, a system is any part of the universe you can isolate in thought (in your brain or on your computer) or, indeed, physically, for the purpose of study. Key systems concepts that we will explain are:

• how a system is connected to the rest of the environment;

• how matter and energy flow between parts of a system;

• whether a system is static or dynamic—whether it changes over time;

• average residence time—how long something stays within a system or part of a system;

• feedback—how the output from a system can affect its inputs; and

• linear and nonlinear flows.

In its relation to the rest of the environment, a sys- tem can be open or closed. In an open system, some en- ergy or material (solid, liquid, or gas) moves into or out of the system. The ocean is an open system with regard to water because water moves into the ocean from the atmosphere and out of the ocean into the atmosphere. In a closed system, no such transfers take place. For our purposes, a materially closed system is one in which no matter moves in and out of the system, although energy and information can move across the system’s boundar- ies. Earth is a materially closed system (for all practical purposes).

Systems respond to inputs and have outputs. For example, think of your body as a complex system and imagine you are hiking in Yellowstone National Park and see a grizzly bear. The sight of the bear is an input. Your body reacts to that input: The adrenaline level in your blood goes up, your heart rate increases, and the hair on your head and arms may rise. Your response—perhaps to move slowly away from the bear—is an output.

Static and Dynamic Systems

A static system has a fixed condition and tends to remain in that exact condition. A dynamic system changes, often continually, over time. A birthday balloon attached to a pole is a static system in terms of space—it stays in one place. A hot-air balloon is a simple dynamic system in terms of space—it moves in response to the winds, air density, and controls exerted by a pilot (Figure 4.4a and 4.4b).

An important kind of static system is one with classical stability. Such a system has a constant condition, and if it is disturbed from that condition, it returns to it once the disturbing factor is removed. The pendulum of an old-fashioned grandfather clock is an example of classical stability. If you push it, the pendulum moves back and forth for a while, but then friction gradually dissipates the energy you just gave it and the pendulum comes to rest exactly where it began. This rest point is known as the equilibrium (Figure 4.4c).

We will see that the classic interpretation of popula- tions, species, ecosystems, and Earth’s entire biosphere has been to assume that each is a stable, static system. But the more these ecological systems are studied scientifically, the clearer it becomes that these are dynamic systems, always changing and always requiring change. An important prac- tical question that keeps arising in many environmental controversies is whether we want to, and should, force ecological systems to be static if and when they are natu- rally dynamic. You will find this question arising in many of the chapters in this book.

Open Systems

With few exceptions, all real systems that we deal with in the environment are open to the flow of matter, en- ergy, and information. (For all practical purposes, as we noted earlier, Earth as a planet is a materially closed system.) An important distinction for open systems is whether they are steady-state or nonsteady-state. In a steady-state system, the inputs (of anything of interest) are equal to the outputs, so the amount stored within the system is constant. An idealized example of a steady- state system is a dam and lake into which water enters from a river and out of which water flows. If the water input equals the water output and evaporation is not considered, the water level in the lake does not change, and so, in regard to water, the lake is in a steady state. (Additional characteristics of systems are discussed in A Closer Look 4.1.)

Two of the river’s great floods were in 1927 and 1993 (Figure 4.6). After the 1927 flood, the federal government commissioned the Army Corps of Engineers to build six major dams on the river (Figure 4.7). (The attempt to control the river’s flow also included many other alterations of the river, such as straightening the channel and building levees.) Of the six dams, the three largest were built upstream, and each of their reservoirs was supposed to hold the equivalent of an entire year’s average flow. The three smaller, downstream dams were meant to serve as safety valves to control the flow more precisely.

The underlying idea was to view the Missouri as a large plumbing system that needed management. When rain- fall was sparse in the huge watershed of the river, the three upstream dams were supposed to be able to augment the flow for up to three years, ensuring a constant and adequate supply of water for irrigation and personal use. In flood years, the six dams were supposed to be able to store the danger- ous flow so that the water could be released slowly, the floods controlled, and the flow once again constant. In addition, levees—narrow ridges of higher ground—were built along the river and into it to protect the settled land along the river from floodwaters not otherwise contained. But these ideal- istic plans did not stop the Missouri from flooding in 1993 (Figures 4.6 and 4.8).

Taking the large view, standing way back from the river, led to a perception of the Missouri River as one huge lake (Figure 4.9) into which water flowed, then drained down- stream and out at its mouth at St. Louis, Missouri, into the Mississippi, which carried the waters to New Orleans, Louisiana, and out into the Gulf of Mexico. The Army Corps of Engineers’ hope was that the Missouri River could be managed the way we manage our bathwater—keeping it at a constant level by always matching the outflow down the drain with inflow from the spigot. This is a perception of the river as a system held in steady state, a term we defined earlier.

An environmental water engineer could use this kind of systems diagram (Figure 4.10) to plan the size of the various dams to be built on the Missouri River, taking into account the desired total storage among the dams and the role of each dam in managing the river’s flow. In Figure 4.10, the amount stored in a dam’s reservoir is listed as Xn, where X is the amount of water stored and n is the number of the compart- ment. (In this case the dams are numbered in order from up- stream to downstream.) Water flows from the environment— tributaries, watersheds, and direct rainfall—into each of the reservoirs, and each is connected to the adjacent reservoirs by the river. Finally, all of the Missouri’s water flows into the Mississippi, which carries it to the Gulf of Mexico.

Looking at Figure 4.10, can you think of problems as- sociated with the input–output of the river and managing water reservoirs in such a large system? Can you name some consequences likely to arise when attempting to keep a river in a steady state?

The average residence time (ART) is the ratio of the size of a reservoir of some material—say, the amount of water in a reservoir—to the rate of its transfer through the res- ervoir. The equation is


where S is the size of the reservoir and F is the rate of transfer.

For example, we can calculate the average residence time for water in the Gavins Point Dam (see Figure 4.10), the farthest downstream of all the dams on the Missouri River, by realizing that the average flow into and out of the dam is about 25 million acre-feet (31 km3) a year, and that the dam stores about 492,000 acre-feet (0.6 km3). This suggests that the average residence time in the dam is only about seven days:

33 ART 5 S/F 5 0.6 km ﰀ31 km per year

S/F 5 0.019 year (about 7 days)

If the total flow were to go through Garrison Dam, the largest of the dams, the residence time would be 347 days, almost a year.

The ART for a chemical element or compound is im- portant in evaluating many environmental problems. For example, knowing the ART of a pollutant in the air, water, or soil gives us a more quantitative understanding of that pollutant, allows us to evaluate the extent to which the pollutant acts in time and space, and helps us to develop strategies to reduce or eliminate the pollutant.

Figure 4.11 shows a map of Big Lake, a hypotheti- cal reservoir impounded by a dam. Three rivers feed a combined 10 m3/sec (2,640 gal/sec) of water into the lake, and the outlet structure releases an equal 10 m3/sec. In this simplified example, we will assume that evaporation of water from the lake is negligible. A water pollutant, MTBE (methyl tertiary—butyl ether), is also present in the lake. MTBE is added to gasoline to help reduce emissions of carbon monoxide. MTBE readily dissolves in water and so travels with it. It is toxic; in small concentrations of 20–40 mg/l (milliononths of grams per liter) in water, it smells like turpentine and is nauseating to some people. Concern over MTBE in California led to a decision to stop add- ing it to gasoline. The sources of MTBE in “Big Lake” are urban runoff from Bear City gasoline stations, gas- oline spills on land or in the lake, and gasoline engines used by boats on the lake.

68 Chapter 4 The Big Picture: Systems of Change ART 5 S 5 ART 5 1,000,000,000 m3

10 m3ﰀsec

ART 5 100,000,000 sec or 108 sec Convert 108 sec to years:

seconds 5 60 sec 3 60 minute 3 24 hours 3 365 days year 1 minute 1 hour 1 day 1 year

Canceling units and multiplying, there are 31,536,000 secﰀyear, which is

3.1536 3 107 secﰀyear Then the ART for Big Lake is

100,000,000sec or 108sec 31,536,000 secﰀyr 3.1536 3 107 secﰀyr

Therefore the ART for water in Big Lake is 3.17 years.

ART of MTBE in Big Lake

The concentration of MTBE in water near the dam is measured as 10 mgﰀl. Then the total amount of MTBE in the lake (size of reservoir or pool of MTBE) is the product of volume of water in the lake and concentration of MTBE:

109 m3 3 103l 3 10mg51013mgor107 g m3 l

which is 104 kg, or 10 metric tons, of MTBE. 3 The output of water from Big Lake is 10 m ﰀsec, and this contains 10 mgﰀl of MTBE; the transfer rate of

water F water 109m3

or 10m3ﰀsec The units m3 cancel out and

MTBE (gﰀsec) is MTBEﰀsec5 sec 3 m3 3 l 3 mg

5 0.1 gﰀsec Because we assume that input and output of MTBE are

equal, the input is also 0.1 gﰀsec. ARTMTBE 5 S 5 107 g 5 108 sec, or 3.17 years

10 m3 103 l 10 mg 1026 g

F 0.1 gﰀsec

Thus, as we suspected, the ARTs of the water and MTBE are the same. This is because MTBE is dissolved in the water. If it attached to the sediment in the lake, the ART of the MTBE would be much longer. Chemicals with large reservoirs or small rates of transfer tend to have long ARTs. In this exercise we have calculated the ART of water in Big Lake as well as the input, total amount, and ART of MTBE.

Often we want real systems in the environment to be in a steady state, and we try to manage many of them so they will be. Attempts to force natural ecological and environmental systems into a steady state often fail. In fact, such attempts commonly make things worse in- stead of better, as we will see in many chapters in this book.

The Balance of Nature: Is a Steady State Natural?

An idea frequently used and defended in the study of our natural environment is that natural systems, left un- disturbed by people, tend toward some sort of steady state. The technical term for this is dynamic equi- librium, but it is more familiarly referred to as the balance of nature (see Figure 4.12). Certainly, negative feedback operates in many natural systems and may tend to hold a system at equilibrium. Nevertheless, we need to ask how often the equilibrium model really applies.4

If we examine natural ecological systems or eco- systems (simply defined here as communities of or- ganisms and their nonliving environment in which nutrients and other chemicals cycle and energy flows) in detail and over a variety of time frames, it is evident that a steady state is seldom attained or maintained for very long. Rather, systems are characterized not only by human-induced disturbances but also by natural disturbances (sometimes large-scale ones called natural disasters, such as floods and wildfires). Thus, changes over time can be expected. In fact, studies of such di- verse systems as forests, rivers, and coral reefs suggest that disturbances due to natural events, such as storms, floods, and fires, are necessary for the maintenance of those systems, as we will see in later chapters. The envi- ronmental lesson is that systems change naturally. If we are going to manage systems for the betterment of the environment, we need to gain a better understanding of how they change.4, 5

Residence Time

By using rates of change or input–output analysis of sys- tems, we can derive an average residence time—how long, on average, a unit of something of interest to us will remain in a reservoir. This is obviously important, as in the case of how much water can be stored for how long in a reservoir. To compute the average residence time (assuming input is equal to output), we divide the total volume of stored water in the reservoir by the av- erage rate of transfer through the system. For example, suppose a university has 10,000 students, and each year 2,500 freshmen start and 2,500 seniors graduate. The av- erage residence time for students is 10,000 divided by 2,500, or four years.

Average residence time has important implications for environmental systems. A system such as a small lake with an inlet and an outlet and a high transfer rate of water has a short residence time for water. On the one hand, from our point of view, that makes the lake espe- cially vulnerable to change because change can happen quickly. On the other hand, any pollutants soon leave the lake.

In large systems with a slow rate of transfer of water, such as oceans, water has a long residence time, and such systems are thus much less vulnerable to quick change. However, once they are polluted, large systems with slow transfer rates are difficult to clean up. (See Working It Out 4.1.)


Feedback occurs when the output of a system (or a com- partment in a system) affects its input. Changes in the output “feed back” on the input. There are two kinds of feedback: negative and positive. A good example of feedback is human temperature regulation. If you go out in the sun and get hot, the increase in temperature affects your sensory perceptions (input). If you stay in the sun, your body responds physiologically: Your pores open, and you are cooled by evaporating water (you sweat). The cooling is output, and it is also input to your sen- sory perceptions. You may respond behaviorally as well: Because you feel hot (input), you walk into the shade (output) and your temperature returns to normal. In this example, an increase in temperature is followed by a re- sponse that leads to a decrease in temperature. This is an example of negative feedback, in which an increase in output leads to a further decrease in output. Negative feedback is self-regulating or stabilizing. It is the way that steady-state systems can remain in a constant condition.

Positive feedback occurs when an increase in output leads to a further increase in output. A fire starting in a forest provides an example of positive feedback. The wood may be slightly damp at the beginning and so may not burn readily. Once a fire starts, wood near the flame dries out and begins to burn, which in turn dries out a greater quantity of wood and leads to a larger fire. The larger the fire, the faster more wood becomes dry and the more rap- idly the fire grows. Positive feedback, sometimes called a “vicious cycle,” is destabilizing.

Environmental damage can be especially serious when people’s use of the environment leads to positive feedback. For example, off-road vehicles—including bicycles—may cause positive feedback to soil erosion (Figure 4.13). The vehicles’ churning tires are designed to grip the earth, but they also erode the soil and up- root plants. Without vegetation, the soil erodes faster, exposing even more soil (positive feedback). As more soil is exposed, rainwater more easily carves out ruts and gullies (more positive feedback). Drivers of off-road vehicles then avoid the ruts and gullies by driving on adjacent sections that are not as eroded, thus widen- ing paths and further increasing erosion (more posi- tive feedback). The gullies themselves increase erosion because they concentrate runoff and have steep side slopes. Once formed, gullies tend to get longer, wider, and deeper, causing additional erosion (even more posi- tive feedback). Eventually, an area of intensive off-road vehicle use may become a wasteland of eroded paths and gullies. Positive feedback has made the situation increasingly worse.

Some systems have both positive and negative feed- backs, as can occur, for example, for the human popu- lation in large cities (Figure 4.14). Positive feedback on the population size may occur when people perceive greater opportunities in cities and move there, hoping for a higher standard of living. As more people move to cit- ies, opportunities may increase, leading to even more mi- gration to cities. Negative feedback can then occur when crowding increases air and water pollution, disease, crime, and discomfort. These negatives encourage some people to migrate from the cities to rural areas, reducing the city’s population. Positive


Air pollution, disease, crime, discomfort, traffic

Jobs, health care, social services, higher standard of living

People leave city

People move into the city

Practicing your critical thinking skills, you may ask, “Is negative feedback generally desirable, and is positive feedback generally undesirable?” Reflecting on this ques- tion, we can see that, although negative feedback is self- regulating, it may in some instances not be desirable. The period over which the positive or negative feedback occurs is the important factor. For example, suppose we are inter- ested in restoring wolves to Yellowstone National Park. We will expect positive feedback in the wolf population for a time as the number of wolves grows. (The more wolves, the greater their population growth, through exponential growth.) Positive feedback, for a time, is desirable because it produces a change we want.

We can see that whether we view positive or negative feedback as desirable depends on the system and potential changes. Nevertheless, some of the major environmen- tal problems we face today result from positive feedback mechanisms. These include resource use and growth of the human population.

4.2 System Responses: Some Important Kinds of Flows5

Within systems, there are certain kinds of flows that we come across over and over in environmental science. (Note that flow is an amount transferred; we also refer to the flux, which is the rate of transfer per unit time.) Because these are so common, we will explain a few of them here.

Linear and Nonlinear Flows

An important distinction among environmental and eco- logical systems is whether they are characterized by linear or nonlinear processes. Put most simply, in a linear process, if you add the same amount of anything to a compartment in a system, the change will always be the same, no mat- ter how much you have added before and no matter what else has changed about the system and its environment. If you harvest one apple and weigh it, then you can estimate how much 10 or 100 or 1,000 or more of the apples will weigh—adding another apple to a scale does not change the amount by which the scale shows an increase. One apple’s effect on a scale is the same, no matter how many apples were on the scale before. This is a linear effect.

Many important processes are nonlinear, which means that the effect of adding a specific amount of some- thing changes, depending on how much has been added before. If you are very thirsty, one glass of water makes you feel good and is good for your health. Two glasses may also be helpful. But what about 100 glasses? Drinking more and more glasses of water leads quickly to diminish- ing returns and eventually to water becoming a poison.

Lag Time

Many responses to environmental inputs (includ- ing human population change; pollution of land, water, and air; and use of resources) are nonlinear and may involve delays, which we need to recog- nize if we are to understand and solve environmen- tal problems. For example, when you add fertilizer to help a tree grow, it takes time for it to enter the soil and be used by the tree.

Exponential Growth Linear Growth

Lag time is the delay between a cause and the appearance of its effect. (This is also referred to as the time between a stimulus and the ap- pearance of a response.) If the lag time is long, especially compared to human lifetimes (or at- tention spans or our ability to continue measur- ing and monitoring), we can fail to recognize the change and know what is the cause and what is the effect. We can also come to believe that a possible cause is not hav- ing a detrimental effect, when in reality the effect is only delayed. For example, logging on steep slopes can increase the likelihood and rate of erosion, but in comparatively dry environments this may not become apparent until there is heavy rain, which might not occur until a num- ber of years afterward. If the lag time is short, cause and effect are easier to identify. For example, highly toxic gas released from a chemical plant will likely have rapid effects on the health of people living nearby.

With an understanding of input and output, positive and negative feedback, stable and unstable systems, and systems at steady state, we have a framework for interpret- ing some of the changes that may affect systems.

Selected Examples of System Responses

Although environmental science deals with very com- plex phenomena, there are recurring relationships that we can represent with a small number of graphs that show how one part of a system responds to inputs from another part. These graphs include responses of indi- vidual organisms, responses of populations and species, responses of entire ecosystems and then large units of the biosphere, the planetary system that includes and sustains life, such as how the atmosphere responds to the burning of fossil fuels. Each of these graphs has a math- ematical equation that can explain the curve, but it is the shape of the graph and what that shape represents that are the keys to understanding environmental systems. These curves represent, in one manifestation or another, the fundamental dynamics found in these systems. The graphs show (1) a straight line (linear), (2) the positive exponential, (3) the negative exponential, and (4) the saturation (Michaelis-Menton) curve. An example of each is shown in Figures 4.15 to 4.17.

Exponential growth is a particularly important kind of positive feedback. Change is exponential when it increases or decreases at a constant rate per time pe- riod, rather than by a constant amount. For instance, suppose you have $1,000 in the bank and it grows at 10% per year. The first year, $100 in interest is added to your account. The second year, you earn more, $110, because you earn 10% on a higher total amount of $1,100. The greater the amount, the greater the inter- est earned, so the money increases by larger and larger amounts. When we plot data in which exponential growth is occurring, the curve we obtain is J-shaped. It looks like a skateboard ramp, starting out nearly flat and then rising steeply.

Two important qualities of exponential growth are (1) the rate of growth measured as a percentage and (2) the doubling time in years. The doubling time is the time necessary for the quantity being measured to double. A useful rule is that the doubling time is approximately equal to 70 divided by the annual percentage growth rate. Working It Out 4.2 describes exponential growth calculations and explains why 70 divided by the annual growth rate is the doubling time.

Figure 4.16 shows two examples of negative exponen- tial relations. The saturation (Michaelis-Menton) curve (Figure 4.17) shows initial fast change, followed by a leveling off at saturation. At the point of saturation, the net CO2 fixed (for soybean) is at a light-intensity value of about 3,000 (Figure 4.17a). As light intensity increases above about 3,000, net fixed CO2 is nearly constant (that is, fixed CO2 saturates at light intensity of 3,000 and does not change if intensity increases).

4.3 Overshoot and Collapse

Figure 4.18 shows the relationship between carrying ca- pacity (maximum population possible without degrading the environment necessary to support the population) and the human population. The carrying capacity starts out being much higher than the human population, but if a population grows exponentially (see Working It Out4.2), it eventually exceeds—overshoots—the carrying ca- pacity. This ultimately results in the collapse of a popula- tion to some lower level, and the carrying capacity may be reduced as well. In this case, the lag time is the period of exponential growth of a population before it exceeds the carrying capacity. A similar scenario may be posited for harvesting species of fish or trees.

4.4 Irreversible Consequences

The adverse consequences of environmental change do not necessarily lead to irreversible consequences. Some do, however, and these lead to particular problems. When we talk about irreversible consequences, we mean conse- quences that may not be easily rectified on a human scale of decades or a few hundred years.

Good examples of this are soil erosion and the har- vesting of old-growth forest (Figure 4.19). With soil ero- sion, there may be a long lag time until the soil erodes to the point where crops no longer have their roots in active soil that has the nutrients necessary to produce a success- ful crop. But once the soil is eroded, it may take hundreds or thousands of years for new soil to form, and so the consequences are irreversible in terms of human plan- ning. Similarly, when old-growth forests are harvested, it may take hundreds of years for them to be restored. Lag times may be even longer if the soils have been damaged or eroded by timber harvesting.

4.5 Environmental Unity

Our discussion of positive and negative feedback sets the stage for another fundamental concept in environmen- tal science: environmental unity—the idea that it is impossible to change only one thing; everything affects everything else. Of course, this is something of an over- statement; the extinction of a species of snails in North America, for instance, is hardly likely to change the flow characteristics of the Amazon River. However, many aspects of the natural environment are in fact closely linked, and thus changes in one part of a system often have secondary and tertiary effects within the system and on adjacent systems as well. Earth and its ecosystems are complex entities in which any action may have many effects.

We will find many examples of environmental unity throughout this book. Urbanization illustrates it. When cities, such as Chicago and Indianapolis, were developed in the eastern and midwestern United States, the clearing of forests and prairies and the construction of buildings and paved streets increased surface-water runoff and soil erosion, which in turn affected the shape of river chan- nels—some eroded soil was deposited on the bottom of the channel, reducing channel depth and increasing flood hazard. Increased fine sediment made the water muddy, and chemicals from street and yard runoff pol- luted streams.6, 7 These changes affected fish and other life in the river, as well as terrestrial wildlife that depended on the river. The point here is that land-use conversion can set off a series of changes in the environment, and each change is likely to trigger additional changes.

4.6 Uniformitarianism

Uniformitarianism is the idea that geological and biolog- ical processes that occur today are the same kinds of pro- cesses that occurred in the past and vice versa. Thus, the present is the key to the past, and the past the key to the future. For example, we use measurements of the current rate of erosion of soils and bedrock by rivers and streams to calculate the rate at which this happened in the past and to estimate how long it took for certain kinds of deposits to develop. If a deposit of gravel and sand found at the top of a mountain is similar to stream gravels found today in an adjacent valley, we may infer by uniformitarianism that a stream once flowed in a valley where the mountaintop is now. The concept of uniformitarianism helps explain the geologic and evolutionary history of Earth.

Uniformitarianism was first suggested in 1785 by the Scottish scientist James Hutton, known as the father of geology. Charles Darwin was impressed by the concept, and it pervades his ideas on biological evolution. Today, uniformitarianism is considered one of the fundamental principles of the biological and Earth sciences.

Uniformitarianism does not demand or even sug- gest that the magnitude and frequency of natural pro- cesses remain constant, only that the processes themselves continue. For the past several billion years, the continents, oceans, and atmosphere have been similar to those of to- day. We assume that the physical and biological processes that form and modify the Earth’s surface have not changed significantly over this period. To be useful from an envi- ronmental standpoint, the principle of uniformitarianism has to be more than a key to the past; we must turn it around and say that a study of past and present process- es is the key to the future. That is, we can assume that in the future the same physical and biological processes will operate, although the rates will vary as the environ- ment is influenced by natural change and human activ- ity. Geologically short-lived landforms, such as beaches (Figure 4.20) and lakes, will continue to appear and dis- appear in response to storms, fires, volcanic eruptions, and earthquakes. Extinctions of animals and plants will continue, in spite of, as well as because of, human activity.

Obviously, some processes do not extend back through all of geologic time. For example, the early Earth atmosphere did not contain free oxygen. Early photo- synthetic bacteria converted carbon dioxide in the atmo- sphere to hydrocarbons and released free oxygen; before life, this process did not occur. But the process began a long time ago—3.5 billion years ago—and as long as there are photosynthetic organisms, this process of carbon dioxide uptake and oxygen release will continue.

Knowledge of uniformitarianism is one way that we can decide what is “natural” and ascertain the characteris- tics of nature undisturbed by people. One of the environmental questions we ask repeatedly, in many contexts, is whether human actions are consistent with the processes of the past. If not, we are often concerned that these ac- tions will be harmful. We want to improve our ability to predict what the future may bring, and uniformitarianism can assist in this task.

4.7 Earth as a System

The discussion in this chapter sets the stage for a rela- tively new way of looking at life and the environment—a global perspective, or thinking about our entire planet’s life-supporting and life-containing system. This is known as Earth Systems Science, and it has become especially important in recent years, with concerns about climate change (see Chapter 20).

Our discussion of Earth as a system—life in its en- vironment, the biosphere, and ecosystems—leads us to the question of how much life on Earth has affected our planet. In recent years, the Gaia hypothesis—named for Gaia, the Greek goddess Mother Earth—has become a hotly debated subject.8 The hypothesis states that life ma- nipulates the environment for the maintenance of life. For example, scientists have evidence that algae floating near the surface of the ocean influence rainfall at sea and the carbon dioxide content of the atmosphere, thereby signifi- cantly affecting the global climate. It follows, then, that the planet Earth is capable of physiological self-regulation.

The idea of a living Earth can be traced back at least to Roman times in the writing of Lucretius.4 James Hutton, whose theory of uniformitarianism was discussed earlier, stated in 1785 that he believed Earth to be a superorgan- ism, and he compared the cycling of nutrients from soils and rocks in streams and rivers to the circulation of blood in an animal.8 In this metaphor, the rivers are the arteries

and veins, the forests are the lungs, and the oceans are the heart of Earth.

The Gaia hypothesis is really a series of hypotheses. The first is that life, since its inception, has greatly affected the planetary environment. Few scientists would disagree. The second hypothesis asserts that life has altered Earth’s environment in ways that have allowed life to persist. Cer- tainly, there is some evidence that life has had such an ef- fect on Earth’s climate. A popularized extension of the Gaia hypothesis is that life deliberately (consciously) controls the global environment. Few scientists accept this idea.

Since the Gaia hypothesis was introduced 40 years ago, there has been heated debate over it.9-11 The hypoth- esis, like previous paradigm shifts on how we view Earth history, is moving through a path that is as yet incom- plete. When new ideas are suggested, they often are not believed because they threaten our previous ideas. After a while, scientific evidence is gathered to test the hypothe- sis, and attempts are made to negate or support it in scien- tific journals. Generally, the original hypothesis is refuted, modified, or accepted. If after years (sometimes decades or longer) a modified hypothesis is accepted, scientists often proclaim that they knew it was basically right all along.

Scientific evaluation of the Gaia hypothesis generally accepts that:9-11

• Through biogeochemical cycles, life has and is playing a significant role in producing Earth’s physical and chem- ical environment.

• Theremaybemechanismsthroughwhichlifeplaysarole of particular importance in modulating Earth’s climate.

• It produced an important metaphor to be used in scien- tific exploration.

• Biologic evolution is an important factor in the chang- ing thoughts about the original Gaia hypothesis.

• The hypothesis has encouraged the study of Earth as a single, unified system, rather than a set of components.

Criticism of the Gaia hypothesis includes the following:9-11

• Gaia is limited by its broad generality in defining a description of the role of life in Earth history.

• Attempts to test a metaphor will likely be difficult and perhaps futile.

• Global change (the temperature of the atmosphere, land, and ocean is accelerating) is making a stable envi- ronment controlled by life less likely.

• Gaia has metaphorical and religious connotations.

• The Gaia hypothesis predicts that biological by-products in the atmosphere should act to regulate Earth’s climate. However, available evidence suggests that biological by-products such as carbon dioxide and methane make the Earth warmer when it is warm and colder when it is cold. The Gaia hypothesis implies that negative feed- backs linked to life should regulate Earth’s climate over geologic time. However, over the past 300 million years of Earth history peaks in past temperature correspond to peaks in past levels of carbon dioxide.

• Feedback between life and the environment does not necessarily enhance the environment, although this may appear to an observer to be the case. Biologic evo- lution through natural selection will favor organisms that do well in their environment at the time they are living. Gaia feedbacks can evolve by natural selection, but so can anti-Gaia feedback. Natural selection will favor a trait that gives a particular life-form a reproduc- tive advantage, whether or not that trait improves the environment.

Countering all this criticism are the following:9

• Several interpretations of the original Gaia hypoth- esis have been discarded. For example, referring to Gaia as a “superorganism” has been largely abandoned in favor of an interpretation that involves a tightly coupled system of life with its nonliving environment.

• Gaia thinking has evolved over recent decades and will continue to evolve.

• The Gaia hypothesis assumes that regulatory feedbacks that control the environment are a probable outcome of planets with abundant life. If this assumption is proven wrong, the assumption will have served to encourage an important research agenda. It seems unlikely (a value judgment) that our Earth system has evolved by ran- dom processes resulting in the good luck of abundant, sustained life on Earth.

One positive note associated with the Gaia hypothesis is that it may have made us more conscious of our effects on the planet, leading us to understand that we can make a difference in the future of our planet. The future status of the human environment may depend in part on actions we take now and in coming years. This aspect of the Gaia hypothesis exemplifies the key theme of thinking globally, which was introduced in Chapter 1.

4.8 types of Change

Change comes in several forms. Some changes brought on by human activities involve rather slow processes—at least from our point of view—with cumulative effects. For example, in the middle of the 19th century, people began to clear-cut patches of the Michigan forests. It was com- monly believed that the forests were so large that it would be impossible to cut them all down before they grew back just as they were. But with many people logging in differ- ent, often isolated areas, it took less than 100 years for all but about 100 hectares to be clear-cut.

Another example: With the beginning of the Indus- trial Revolution, people in many regions began to burn fossil fuels, but only since the second half of the 20th century have the possible global effects become widely evident. Many fisheries appear capable of high harvests for many years. But then suddenly, at least from our perspective—sometimes within a year or a few years— an entire species of fish suffers a drastic decline. In such cases, long-term damage can be done. It has been difficult to recognize when harvesting fisheries is overharvesting and, once it has started, figuring out what can be done to enable a fishery to recover in time for fishermen to continue making a living. A famous example of this was the harvesting of anchovies off the coast of Peru. Once the largest fish catch in the world, within a few years thefish numbers declined so greatly that commercial harvest was threatened. The same thing has happened with the fisheries of Georges Banks and the Grand Banks in the Atlantic Ocean.

You can see from these few examples that environ- mental problems are often complex, involving a variety of linkages among the major components and within each component, as well as linear and exponential change, lag times, and the possibility of irreversible consequences.

As stated, one of our goals in understanding the role of human processes in environmental change is to help manage our global environment. To accomplish this goal, we need to be able to predict changes, but as the examples above demonstrate, prediction poses great challenges. Al- though some changes are anticipated, others come as a surprise. As we learn to apply the principles of environ- mental unity and uniformitarianism more skillfully, we will be better able to anticipate changes that would other- wise have been surprises.


• A system is a set of components or parts that func- tion together as a whole. Environmental studies deal with complex systems, and solutions to environmental problems often involve understanding systems and their rates of change.

• Systems respond to inputs and have outputs. Feed- back is a special kind of system response, where the output affects the input. Positive feedback, in which increases in output lead to increases in input, is destabilizing, whereas negative feedback, in which increases in output lead to decreases in input, tends to stabilize or encourage more constant conditions in a system.

• Relationships between the input (cause) and output (effect) of systems may be linear, exponential, or repre- sented by a logistic curve or a saturation curve.

• The principle of environmental unity, simply stated, holds that everything affects everything else. It empha- sizes linkages among parts of systems.

• The principle of uniformitarianism can help predict future environmental conditions on the basis of the past and the present.

• Although environmental and ecological systems are complex, much of what happens with them can be characterized by just a few response curves or equations: the straight line and the exponential, the logistic, and the saturation curves.

• Exponential growth, long lag times, and the possibil- ity of irreversible change can combine to make solving environmental problems difficult.

• Change may be slow, fast, expected, unexpected, or cha- otic. One of our goals is to learn to better recognize change and its consequences in order to better manage the environment


Botkin, D.B., M. Caswell, J.E. Estes, and A. Orio, eds., Chang- ing the Global Environment: Perspectives on Human In- volvement (New York: Academic Press, 1989). One of the first books to summarize the effects of people on nature; it includes global aspects and uses satellite remote sensing and advanced computer technologies.

Bunyard, P., ed., Gaia in Action: Science of the Living Earth (Edinburgh: Floris Books, 1996). This book presents investiga- tions into implications of the Gaia hypothesis.

Lovelock, J., The Ages of Gaia: A Biography of Our Living Earth (New York: Norton, 1995). This small book explains the Gaia hypothesis, presenting the case that life very much affects our planet and in fact may regulate it for the benefit of life.

Botkin, D. B., & Keller, E. A. (2014). Environmental science: Earth as a living planet (9th ed.). Hoboken, NJ: John Wiley & Sons, Inc

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