Energy flows within and among systems. Plants and other photosynthetic organisms such as the algae in Mono Lake absorb solar energy and use it in photosynthesis to convert carbon dioxide and water into sugars they need to survive, grow, and reproduce. Animals such as the brine shrimp in Mono Lake eat those plants and the energy is transferred. When migrating gulls use Mono Lake as a stopover, they consume the brine shrimp and transfer the energy and nutrients elsewhere. Some transfers occur more effectively than others. Some transfers occur within a given system while others, like those of migrating gulls, result in transfers of material and energy to another system.

Learning Objectives

After reading this module you should be able to

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Earth’s systems cannot function, and organisms cannot survive, without energy. Energy is the ability to do work, or transfer heat. Water flowing into a lake has energy because it moves and can move other objects in its path.

All living systems absorb energy from their surroundings and use it to organize and reorganize molecules within their cells and to power movement. The sugars in plants are also an important energy source for many animals. Humans, like other animals, absorb the energy they need for cellular respiration from food. This provides the energy for our daily activities, from waking to sleeping to walking, and everything in between. Constructed human systems also utilize energy. If you took mass transportation or an automobile to get to school, you most likely utilized fossil fuel energy that was converted to the energy of motion of your vehicle.

The basic unit of energy in the metric system is the joule ( J). A joule is the amount of energy used when a 1-watt light bulb is turned on for 1 second—a very small amount. Although the joule is the preferred energy unit in scientific study, many other energy units are commonly used. Conversions between these units and joules are given in TABLE 5.1.

Although we often use the words “energy” and “power” interchangeably, they are not the same thing. We have seen that energy is the ability to do work. Power is the rate at which work is done:

energy = power × time

power = energy ÷ time

When we talk about generating electricity, we often hear about kilowatts and kilowatt-hours. The kilowatt (kW) is a unit of power while the kilowatt-hour (kWh) is a unit of energy. Therefore, the capacity of a turbine is given in kW because that measurement refers to the turbine’s power. Your monthly home electricity bill reports energy use—the amount of energy from electricity that you have used in your home—in kWh. “Do the Math: Calculating Energy Use and Converting Units” (see page 46) gives you an opportunity to practice working with these units.

Forms of Energy

Energy exists in different forms and can be converted from one form to another. Potential energy, kinetic energy, light energy, chemical energy, and sound energy are all important energy forms in the environmental sciences.

Electromagnetic Radiation

Ultimately, most energy on Earth derives from the Sun. The Sun emits electromagnetic radiation, a form of energy that includes, but is not limited to, visible light, ultraviolet light, and infrared energy, which we perceive as heat. The scale at the top of FIGURE 5.1 shows these and other types of electromagnetic radiation.

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Electromagnetic radiation is carried by photons, massless packets of energy that travel at the speed of light and can move even through the vacuum of space. The amount of energy contained in a photon depends on its wavelength—the distance between two peaks or troughs in a wave, as shown in the inset in FIGURE 5.1. Photons with long wavelengths, such as radio waves, have very low energy, while those with short wavelengths, such as X-rays, have high energy. Photons of different wavelengths are used by humans for different purposes. For example, high-energy, short-wavelength X-rays are used for diagnostic medical purposes while lower-energy, long-wavelength infrared rays are used to identify heat loss from buildings during an environmental energy audit.

Potential Energy

Many stationary objects possess a large amount of potential energy—energy that is stored but has not yet been released. For example, water impounded behind a dam contains a great deal of potential energy. Potential energy stored in chemical bonds is known as chemical energy. The energy in food is a familiar example. By breaking down the high-energy bonds in the salad you had for lunch, your body obtains energy to power its activities and functions. Likewise, an automobile engine combusts gasoline and releases its chemical energy to propel the car.

Kinetic Energy

We noted that water impounded behind a dam contains a great deal of potential energy. When the water is released and flows downstream, that potential energy becomes kinetic energy, the energy of motion (FIGURE 5.2). The kinetic energy of moving water can be captured at a dam and transferred to a turbine and generator, and ultimately to the energy in electricity. Can you think of other common examples of kinetic energy? A car moving down the street, a flying honeybee, and a football traveling through the air all have kinetic energy. Sound also has kinetic energy because it travels in waves through the coordinated motion of atoms. Systems can contain potential energy, kinetic energy, or some of each.

All matter, even the frozen water in the world’s ice caps, contains some energy. When we say that energy moves matter, we mean that it is moving the molecules within a substance. The measure of the average kinetic energy of a substance is its temperature.

Changes in temperature—and, therefore, in energy—can convert matter from one state to another such as liquid water freezing and becoming ice. At a certain temperature, the molecules in a solid substance start moving so fast that they begin to flow, and the substance melts into a liquid. At an even higher temperature, the molecules in the liquid move still faster, with increasing amounts of energy. Finally the molecules move with such speed and energy that they overcome the forces holding them together and become gases.

Energy Conversions

Individual organisms rely on a continuous input of energy in order to survive, grow, and reproduce. But interactions beyond the organism can also be seen as a process of converting energy into organized structures such as leaves and branches. Consider a forest ecosystem. Trees absorb water through their roots and carbon dioxide through their leaves. By combining these compounds in the presence of sunlight, they convert water and carbon dioxide into sugars that will provide them with the energy they need. But then a deer grazes on tree leaves, and later a mountain lion eats the deer. At each step, energy is converted by organisms into work.

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The form and amount of energy available in an environment determines what kinds of organisms can live there. Plants thrive in tropical rainforests where there is plenty of sunlight and water. Many food crops, not surprisingly, can be planted and grown in temperate climates that have a moderate amount of sunlight. Life is much more sparse at high latitudes, toward the North and South Poles, where less solar energy is available to organisms. These landscapes are populated mainly by small plants and shrubs, insects, and migrating animals. Plants cannot live at all on the deep ocean floor, where no solar energy penetrates. The animals that live there, such as eels, anglerfish, and squid, get their energy by feeding on dead organisms that sink from above. Chemical energy, in the form of sulfides emitted from deep-ocean vents, supports a plantless ecosystem that includes sea spiders, 2.4-m (8-foot) tube worms, and bacteria (FIGURE 5.3).

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In Chapter 1 we saw that some theories have no known exceptions. Two theories concerning energy fall in this category. The laws of thermodynamics are among the most significant principles in all of science.

First Law of Thermodynamics

Impounded water behind a dam is quiet, still, and unmoving. It doesn’t seem like it contains a lot of energy. But in fact that water contains a great deal of potential energy. If you release that water by opening a gate in the dam, the water rushes out. The potential energy of the impounded water becomes the kinetic energy of the water rushing through the gates of the dam. This is an illustration of the first law of thermodynamics, which states that energy is neither created nor destroyed but it can change from one form to another.

The first law of thermodynamics dictates that you can’t get something from nothing. When an organism needs biologically usable energy, it must convert it from an energy source such as the Sun or food. The potential energy contained in firewood never goes away but is transformed into heat energy permeating a room when the wood is burned in a fireplace. Sometimes it may be difficult to identify where the energy is going, but it is always conserved.

Look at FIGURE 5.4, which uses a car to show the first law in action through a series of energy conversions. Think of the car, including its fuel tank, as a system. The potential energy of the fuel (gasoline) is converted into kinetic energy when the battery supplies a spark in the presence of gasoline and air. The gasoline combusts, and the resulting gases expand, pushing the pistons in the engine—converting the chemical energy in the gasoline into the kinetic energy of the moving pistons. Energy is transferred from the pistons to the drivetrain, and from there to the wheels, which propel the car. The combustion of gasoline also produces heat, which dissipates into the environment outside the system. The kinetic energy of the moving car is converted into heat and sound energy as the tires create friction with the road and the body of the automobile moves through the air. When the brakes are applied to stop the car, friction between brake parts releases heat energy. No energy is ever destroyed in this example, but chemical energy is converted into motion, heat, and sound. Notice that some of the energy stays within the system and some energy, for example the heat from burning gasoline, leaves the system.

Second Law of Thermodynamics

We have seen how the potential energy of gasoline is transformed into the kinetic energy of moving pistons in a car engine. But as FIGURE 5.4 shows, some of that energy is converted into a less usable form—in this case, heat. The heat that is created is called waste heat, meaning that it is not used to do any useful work. And it is inevitable—it is a natural law—that any time there is a conversion of energy from one form to another, some of that energy will be lost as heat. This is one of the implications of another very important law: The second law of thermodynamics tells us that when energy is transformed, the quantity of energy remains the same, but its ability to do work diminishes.

Energy Efficiency

To quantify the second law of thermodynamics, we use the concept of energy efficiency. Energy efficiency is the ratio of the amount of energy expended in the desired form to the total amount of energy that is introduced into the system. Two machines or engines that perform the same amount of work, but use different amounts of energy to do that work, have different energy efficiencies. Consider the difference between modern woodstoves and traditional open fireplaces. A woodstove that is 70 percent efficient might use 2 kg of wood to heat a room to a comfortable 20°C (68°F), whereas a fireplace that is 10 percent efficient would require 14 kg of wood to achieve the same temperature—a sevenfold greater energy input (FIGURE 5.5).

We can also calculate the energy efficiency of transforming one form of energy into other forms of energy. Let’s consider what happens when we convert the chemical energy of coal into the electricity that provides light from a reading lamp and the heat that the lamp releases. FIGURE 5.6 shows the process.

A modern coal-burning power plant can convert 1 metric ton of coal, containing 24,000 megajoules (MJ; 1 MJ = 1 million joules) of chemical energy into about 8,400 MJ of electricity. Since 8,400 is 35 percent of 24,000, this means that the process of turning coal into electricity is about 35 percent efficient. The rest of the energy from the coal—65 percent—is lost as waste heat.

In the electrical transmission lines between the power plant and the house, 10 percent of the electrical energy from the plant is lost as heat and sound, so the transport of energy away from the plant is about 90 percent efficient. We know that the conversion of electrical energy into light in an incandescent bulb is 5 percent efficient; again, the rest of the energy is lost as heat. From beginning to end, we can calculate the energy efficiency of converting coal into incandescent lighting by multiplying all the individual efficiencies:

Calculating energy efficiency:

Energy Quality

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Most of us have an intuitive sense about the relative effectiveness of various energy sources. For example, we realize that gasoline is a more useful source of energy than paper. This difference is a function of each material’s energy quality, the ease with which an energy source can be used for work. A high-quality energy source has a convenient, concentrated form so that it does not take too much energy to move it from one place to another.

Gasoline, for example, is a high-quality energy source because its chemical energy is concentrated (about 44 MJ/kg), and because we have technology that can conveniently transport it from one location to another. In addition, it is relatively easy to convert gasoline energy into work and heat. Wood, on the other hand, is a lower-quality energy source. It has less than half the energy concentration of gasoline (about 20 MJ/kg) and is more difficult to use to do work. Imagine using wood to power an automobile. Clearly, gasoline is a higher-quality energy source than wood. Energy quality is one important factor humans must consider when they make energy choices. Considering that wood has less than half the energy content of gasoline, could you imagine driving a car powered by wood? It would not be practical for many reasons, including its lower energy quality.

Entropy

The second law of thermodynamics also tells us that all systems move toward randomness rather than toward order. This randomness in a system, called entropy, is always increasing unless new energy from outside the system is added to create order.

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Think of your bedroom as a system. At the start of the week, your books may be in the bookcase, your clothes may be in the dresser, and your shoes may be lined up in a row in the closet. But what happens if, as the week goes on, you don’t expend energy to put your things away (FIGURE 5.7)? Unfortunately, your books will not spontaneously line up in the bookcase, your clothes will not fall folded into the dresser, and your shoes will not pair up and arrange themselves in the closet. Unless you bring energy into the system to put things in order, your room will slowly become more and more disorganized.

The energy you use to pick up your room comes from the energy stored in food. Food is a relatively high-quality energy source because the human body easily converts it into usable energy. The molecules of food are ordered rather than random. In other words, food is a low-entropy energy source. Only a small portion of the energy in your digested food is converted into work, however; the rest becomes body heat, which may or may not be needed. This waste heat has a high degree of entropy because heat is the random movement of molecules. Thus, in using food energy to power your body to organize your room, you are decreasing the entropy of the room, but increasing the entropy in the universe by producing waste body heat.

Another example of the second law can be found in the observation that energy always flows from hot to cold. A pot of water will never boil without an input of energy, but hot water left alone will gradually cool as its energy dissipates into the surrounding air. This application of the second law is important in many of the global circulation patterns that are powered by the energy of the Sun.

Why do environmental scientists study whole systems rather than focusing on the individual plants, animals, or substances within a system? Imagine taking apart your cell phone and trying to understand how it works simply by focusing on the microphone. You wouldn’t get very far. Similarly, it is important for environmental scientists to look at the whole picture and not just the individual parts of a system in order to understand how that system works. With a working knowledge of how a system functions, we can predict how changes to any part of the system—for example, changes in the water level at Mono Lake—will change the entire system.

Studying systems allows scientists to think about how matter and energy flow in the environment. In this way, researchers can learn about the complex relationships between organisms and the environment. In this section we will explore system dynamics and changes in systems across space and over time. In each case we will focus on how energy and matter flow in the environment.

System Dynamics

As we suggested at the beginning of this chapter, the Mono Lake ecosystem changes over time. Some years there is more algae growing in the lake and that feeds more brine shrimp. Some years fewer migrating gulls stop over and feed, removing less matter and energy from the system, while in other years, more gulls stop over. These changing parameters describe the system dynamics of the Mono Lake ecosystem. There are a number of terms used to describe systems and we will present some of them in this section.

Open and Closed Systems

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Systems can be either open or closed. In an open system, exchanges of matter or energy occur across system boundaries. Most systems are open. Even at remote Mono Lake, water flows in and birds fly to and from the lake. The ocean is also an open system. Energy from the Sun enters the ocean, warming the waters and providing energy to plants and algae. Energy and matter are transferred from the ocean to the atmosphere as energy from the Sun evaporates the water, giving rise to meteorological events such as tropical storms in which clouds form and send rain back to the ocean surface. Matter, such as sediment and nutrients, enters the ocean from rivers and streams and leaves it through geologic cycles and other processes.

In a closed system, matter and energy exchanges do not occur across system boundaries. Closed systems are less common than open systems. Some underground cave systems are almost completely closed systems.

As FIGURE 5.8 shows, Earth is an open system with respect to energy. Solar radiation enters Earth’s atmosphere, and heat and reflected light leave it. But because of its gravitational field, Earth is essentially a closed system with respect to matter. Only an insignificant amount of material enters or leaves the Earth system. All important material exchanges occur within the system.

Inputs and Outputs

By now you have seen numerous examples of both inputs, which are additions to a given system, and outputs, which are losses from the system. People who study systems often conduct a systems analysis, in which they determine inputs, outputs, and changes in the system under various conditions. For instance, researchers studying Mono Lake might quantify the inputs to that system—such as water and salts—and the outputs—such as water that evaporates from the lake and brine shrimp removed by migratory birds. Because no water flows out of the lake, salts are not removed, and even without the aqueduct, Mono Lake, like other terminal lakes, would slowly become saltier.

Steady States

In any given period at Mono Lake, the same amount of water that enters the lake eventually evaporates. In many cases, the most important aspect of conducting a systems analysis is determining whether your system is in steady state—that is, whether inputs equal outputs, so that the system is not changing over time. This information is particularly useful in the study of environmental science. For example, it allows us to know whether the amount of a valuable resource or a harmful pollutant is increasing, decreasing, or staying the same.

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The first step in determining whether a system is in steady state is to measure the amount of matter and energy within it. If the scale of the system allows, we can perform these measurements directly. Consider the leaky bucket shown in FIGURE 5.9. We can measure the amount of water going into the bucket and the amount of water flowing out through the holes in the bottom. However, some properties of systems, such as the volume of a lake or the size of an insect population, are difficult to measure directly, so we must calculate or estimate the amount of energy or matter stored in the system. We can then use this information to determine the inputs to and outputs from the system to determine whether it is in steady state.

Many aspects of natural systems, such as the water vapor in the global atmosphere, have been in steady state for at least as long as we have been studying them. The amount of water that enters the atmosphere by evaporation from oceans, rivers, and lakes is roughly equal to the amount that falls from the atmosphere as precipitation. Until recently, the oceans have also been in steady state: The amount of water that enters from rivers and streams has been roughly equal to the amount that evaporates into the air. One concern about the effects of global climate change is that some global systems, such as the system that includes water balance in the oceans and atmosphere, may no longer be in steady state.

It’s interesting to note that one part of a system can be in steady state while another part is not. Before the Los Angeles Aqueduct was built, the Mono Lake system was in steady state with respect to water but not with respect to salt. The inflow of water equaled the rate of water evaporation but salt was slowly accumulating, as it does in all terminal lakes.

Feedbacks

Most natural systems are in steady state. Why? A natural system can respond to changes in its inputs and outputs. For example, during a period of drought, evaporation from a lake will be greater than combined precipitation and stream water flowing into the lake. Therefore, the lake will begin to dry up. Soon there will be less surface water available for evaporation, so the evaporation rate will continue to fall until it matches the new, lower precipitation rate. When this happens, the system returns to steady state, and the lake stops shrinking.

Of course, the opposite is also true. In very wet periods, the size of the lake will grow, and evaporation from the expanded surface area will continue to increase until the system returns to a steady state at which inputs and outputs are equal.

Adjustments in input or output rates caused by changes to a system are called feedbacks; the results of a process feed back into the system to change the rate of that process. Feedbacks, which can be diagrammed as loops or cycles, are found throughout the environment.

Feedback can be either negative or positive. In natural systems, scientists most often observe negative feedback loops, in which a system responds to a change by returning to its original state, or by decreasing the rate at which the change is occurring. FIGURE 5.10a shows how the negative feedback loop for Mono Lake works: When water levels drop, there is less lake surface area, so evaporation decreases. With less evaporation, the water in the lake slowly returns to its original volume.

Positive feedbacks also occur in the natural world. FIGURE 5.10b shows an example of how births in a population can give rise to a positive feedback loop in which change in a system is amplified. The more members of a species that can reproduce, the more births there will be, creating even more of the species to give birth, and so on.

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It’s important to note that positive and negative here do not mean good and bad; instead, positive feedback amplifies changes, whereas negative feedback resists changes. People often talk about the balance of nature. That balance is the logical result of systems reaching a state at which negative feedbacks predominate—although positive feedback loops play important roles in environmental systems as well.

Environmental scientists are especially concerned with the extent to which Earth’s temperature is regulated by feedback loops. Understanding the role of feedback loops in temperature regulation, as well as the types of feedbacks and their scale, can help us make better predictions about climatic changes in the coming decades.

In general, warmer temperatures at Earth’s surface increase the evaporation of water. The additional water vapor that enters the atmosphere by evaporation causes two kinds of clouds to form. Low-altitude clouds reflect sunlight back into space. The result is less heating of Earth’s surface, less evaporation, and less warming—a negative feedback loop. High-altitude clouds, on the other hand, absorb terrestrial energy that might otherwise have escaped the atmosphere, leading to higher temperatures near Earth’s surface, more evaporation of water, and more warming—a positive feedback loop. In the absence of other factors that compensate for or balance the warming, this positive feedback loop will continue making temperatures warmer, driving the system further away from its starting point. This and other potential positive feedback loops may play critical roles in climate change.

The health of many environmental systems depends on the proper operation of feedback loops. Sometimes, natural or anthropogenic factors lead to a breakdown in a negative feedback loop and drive an environmental system away from its steady state. As you study the exploitation of natural resources, try to determine what factors may be disrupting the negative feedback loops of the systems that provide those resources.

Change Across Space and over Time

Differences in environmental conditions affect what grows or lives in an area, which creates geographic variation among natural systems. Variations in temperature, precipitation, or soil composition across a landscape can lead to vastly different numbers and types of organisms. In Texas, for example, sycamore trees grow in river valleys where there is plenty of water available, whereas pine trees dominate mountain slopes because they can tolerate the cold, dry conditions there. Paying close attention to these natural variations may help us predict the effect of any change in an environment. We know that if the rivers that support the sycamores in Texas dry up, then the trees will probably die.

Natural systems are also affected by the passage of time. Thousands of years ago, when the climate of the Sahara was much wetter than it is today, it supported large populations of Nubian farmers and herders. Small changes in Earth’s orbit relative to the Sun, along with a series of other factors, led to the disappearance of monsoon rains in northern Africa. As a result of these changes, the Sahara—now a desert nearly the size of the continental United States—became one of Earth’s driest regions (FIGURE 5.11). Other, more dramatic changes have occurred on the planet. In the last few million years, Earth has moved in and out of several ice ages; 70 million years ago, central North America was covered by a sea; 240 million years ago, Antarctica was warm enough for 2-meter-long (6.6-foot) salamander-like amphibians to roam its swamps. Natural systems respond to such changes in the global environment with migrations and extinctions of species as well as the evolution of new species.

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Throughout Earth’s history, small natural changes have had large effects on complex systems, but human activities have increased both the pace and the intensity of these natural environmental changes, as they did at Mono Lake. Studying variations in natural systems over space and time can help scientists learn more about what to expect from the alterations humans are making to the world today.