Animals, fungi, and most bacteria can’t convert solar energy directly into the molecules they need to sustain themselves, so they seize their energy by eating other life-forms. Plants provide the energy source for the primary consumer level of the food chain, which includes grazing animals, caterpillars, and vegetarians, as well as a host of species from bacteria to termites that eat decayed plant matter. Most animals occupy this second trophic level.
Higher up the food chain are animals that feed on other animals in one form or another. Primary carnivores (like wolves) eat herbivores (like rabbits); secondary carnivores (like killer whales) eat primary carnivores (like fish). Other feeding strategies include organisms (like many bacteria, termites, and vultures) that scavenge dead bodies and waste products, and omnivores (like human beings and raccoons) that get their food from many sources, both plant and animal. All of these animal consumers, however, are ultimately dependent on the photosynthetic producers at the base of the food chain.
The food chain is a very inefficient system. As energy is transferred up from one level to another, much of it is lost at each step. Plants use only a few percent of the energy in the sunlight that falls on them. Grazing animals typically recover only about 10 percent of the energy stored in the grass they eat. The lost 90 percent escapes through animal metabolism as heat or is locked in molecules that are not easily digested.
Energy is obtained by living things through the food chain, which includes several trophic levels. Energy-producing photosynthetic plants provide energy for animal consumers and decomposers.
Roughly another 90 percent of the available energy in the food chain is lost in the move from herbivores to carnivores. This ever-diminishing flow of energy from lower to higher trophic levels means that each level supports fewer individuals. Vast schools of small fish will feed relatively few big fish. On the African plains there are few lions compared to the large herds of grazing animals. This fact also explains why beef (from the second trophic level) is approximately ten times more expensive than grain, and why there are no lion steaks available in your supermarket.
Each ecosystem is maintained by the energy that flows through it, but no matter how the energy moves through the biosphere, its ultimate fate is always the same. Sooner or later it is converted into heat and radiated back into space, just one more part of the great energy balance that keeps our planet going.
NUTRIENTS AND THE CARBON CYCLE
Unlike energy, which is constantly lost and must be constantly replenished in any ecosystem, the atoms and molecules that make up the structure and nutrients of organisms are recycled. Atoms do not disappear, but move from one organism to another and from one chemical form to another, continuously shifting back and forth between living and nonliving parts of the system. We describe the history of atoms in terms of the so-called chemical cycles. Chemical cycles essential for life include the water cycle, as well as cycles for the elements carbon, nitrogen, oxygen, phosphorus, sulfur, and others.
Each part of the cycle followed by any atom or molecule is complex, with many alternative pathways. Consider the movements of just one carbon atom, entering the cycle from the atmosphere as a molecule of the gas carbon dioxide (CO2). A blade of grass combines that molecule of carbon dioxide with water through photosynthesis to create part of a glucose molecule. Shortly thereafter, the glucose is processed in cellular chemical factories to form part of the cellulose fibers that support each grass blade. The carbon atom has become an integral part of the structure of grass.
A hungry mouse nibbles at the grass, chewing and swallowing the carbon atom, which is added to the mouse’s chemical stockpile. The unfortunate mouse is spotted and eaten by an owl, who adds the atom to its own energy reserve. As the owl burns its carbon-rich fuel by respiration, the carbon atom returns to the atmosphere in another molecule of carbon dioxide.
Carbon might follow other pathways. Some carbon atoms end up in the soil as animal droppings or by death and decay. There, bacteria, worms or other scavengers obtain raw materials directly from the carbon-rich earth. Layers of dead plant matter can pile up, become deeply buried, and transform by the Earth’s temperature and pressure to form fossil fuel deposits such as coal, oil, and natural gas. Snails and beetles convert other carbon atoms into the chemicals that make their hard outer shells. In the ocean, corals and shellfish use a similar process to manufacture durable carbonate reefs and shells, which can accumulate to form thick limestone formations. In the past century humans have altered the natural carbon cycle by burning hundreds of billions of tons of fossil fuel, thus increasing the concentration of carbon dioxide in the atmosphere.
Every other atom essential to life—oxygen, hydrogen, nitrogen, and so on—follows a similar cycle through the biosphere. The details differ from element to element, of course, but the main principle is the same: materials cycle through the biosphere and never leave.
HUMANS AND THE ENVIRONMENT
Humans are an integral part of the ecosystem. Like all other living things, we depend ultimately on the energy in sunlight and the photosynthetic reactions in the first level of the food chain. But humans, unlike any other species, have learned to shape and alter their environment in remarkable ways. By developing agriculture, building cities, and, more recently, building manufacturing plants, we have, for better or worse, profoundly altered the biosphere. Many of the most important questions now on the national and international political agendas have to do with this fact. None of these issues is a purely scientific one; many economic and social factors impinge on one another. Nevertheless, each has a strong scientific component, and it is impossible to discuss any of them intelligently without some basic understanding of the underlying science. Below are three of the many environmental problems about which we all will have to make intelligent decisions as citizens: ozone depletion, acid rain and urban air pollution, and the greenhouse effect.
Ozone Depletion
The ozone molecule consists of three (as opposed to the usual two) oxygen atoms. Only about one molecule per million in the atmosphere is ozone, but these molecules play a crucial role in our environment in two ways. Ozone near Earth’s surface (“bad ozone”) is a noxious pollutant, irritating to eyes and lungs. Ozone 50,000 feet up, on the other hand (“good ozone”), absorbs the sun’s harmful ultraviolet radiation and thus provides an effective sunscreen for those of us living on the ground. Without the ozone layer, humans and other terrestrial life would be constantly bombarded with high-energy radiation and consequently put at higher risk of medical problems such as skin cancer and eye damage.
The ozone layer was put at risk because of the widespread use of a class of chemicals known as chlorofluorocarbons (CFCs for short). For decades CFCs were used extensively as the working fluids in refrigerators and air conditioners, as cleaners during the manufacture of microchips, and in the manufacture of foam products. When they were widely introduced in the 1960s, the molecules’ stability was considered an asset since they wouldn’t break down and add to pollution. But that very stability led to problems, because CFCs last long enough to filter through to the upper atmosphere. There the molecules’ chlorine atoms act as catalysts in a complex set of reactions that convert two molecules of ozone to three molecules of ordinary oxygen, depleting the ozone layer faster than it can be recharged by natural processes—another example of the law of unintended consequences.
In 1984, scientists working in the Antarctic made a startling discovery that focused world attention on the ozone layer. During the months of September and October (the Antarctic spring), the concentrations of ozone above the pole dropped by 50 percent. This celebrated “ozone hole” has reappeared, with varying degrees of intensity, every year since. It appears that the massive ozone depletion associated with the hole is the result of the special conditions in Antarctica—the isolation of the air during the Antarctic winter and the presence of ice clouds that form during the period when the sun doesn’t shine.
The public was justifiably concerned
about the progressive destruction of the ozone layer, and a sense of urgency prompted measures to reduce the use of CFCs. Chemical industries were quick to respond to the evident danger—DuPont, for example, quickly phased out all production of the chemicals, and all industrial nations soon followed suit.
As environmental problems go, ozone depletion has been a relatively simple one to deal with. The solution was obvious, its cost was relatively low, and it required no real change in behavior or lifestyle to reverse the present trend.
Acid Rain and Urban Air Pollution
Burning always introduces carbon dioxide and water vapor into the atmosphere, but combustion also produces three other significant sources of pollution:
Nitrogen oxides. Whenever the temperature of the air is raised above about 500°C, nitrogen in the air combines with oxygen to form what are called NOx compounds (pronounced nox): nitrogen monoxide (NO), nitrogen dioxide (NO2), and others.
Sulfur compounds. Petroleum-and coal-based fossil fuels usually contain small amounts of sulfur, either as a contaminant or as an integral part of their structure. The result is that sulfur dioxide (SO2) is released into the atmosphere as well.
Hydrocarbons. The long-chain molecules that make up hydro carbons seldom burn perfectly. As a result, a third class of pollutants, bits and pieces of unreacted hydrocarbon molecular chains, enters the atmosphere.
Emission of these pollutants can cause serious environmental problems. Urban air pollution occurs when sunlight hitting nitrogen compounds and hydrocarbons in the air triggers chemical reactions that in the end produce ozone. And whereas ozone in the stratosphere is essential to life on Earth, ozone at ground level is a caustic gas that stings and can damage the human respiratory system. This “bad ozone” is a major product of modern urban air pollution associated with smog—the brownish stuff that you often see over major cities during the summer.
Urban air pollution can be a serious problem, but it is also often a transitory one. If the air quality in a city declines, then people can be alerted about it immediately as part of the weather forecast. And just as with the weather, the intensity of air pollution varies on a daily basis and can change swiftly with the arrival of a thunderstorm or stiff winds.
Acid rain is a much longer-term problem associated with nitrogen and sulfur compounds, which interact with other atmospheric chemicals to form tiny droplets of nitric and sulfuric acid. When it rains, these droplets of acid wash out and they become, in effect, a rain of dilute acid rather than water. You can see dramatic effects of acid rain in many European cities, where great historical monuments made of limestone are particularly susceptible to the effect of acid. Over the years, the acid rain simply dissolves the fabric of the building.
In the mid-twentieth century, local adverse effects of pollution were often dealt with by the construction of tall smokestacks, particularly in the industrial parts of the Midwest. The effect was to put the pollutants high enough in the atmosphere to be taken away by prevailing winds. But that approach didn’t really solve the problem; it merely displaced the acid rain to the forests of New England.
Governments have responded to problems of urban air pollution by regulating tailpipe emission from vehicles and requiring large industrial plants to use available technology to scrub gases before they are released into the atmosphere. It is not unusual for the equipment needed to clean emissions from a coal-burning electrical plant to cost more than the equipment used to generate the electricity itself. In the future, if plug-in hybrid and (perhaps) all electric cars come into widespread use, the electricity needed to run those cars will come from plants with this sort of equipment. It is, after all, easier to clean one smokestack than tens of thousands of individual tailpipes.
Acid rain and air pollution are moderate environmental problems. We understand the sources and consequences of the pollution, and we know what has to be done to prevent the pollution. The costs of dealing with these problems, however, are much greater than those of reversing the ozone layer depletion. Political and economic questions become very important. How much are we willing to pay for clean air? How many midwestern jobs are we willing to lose to save New England forests? These are not easy questions, nor can they be answered by science alone.
The Greenhouse Effect and Global Climate Change
Although it is possible to talk about removing materials that cause acid rain from smokestacks or exhaust pipes, or even converting to fuels that do not produce those materials, one product must inevitably be produced whenever we burn a fossil fuel: carbon dioxide. Whenever you drive a car, cook food, or use an electric light, chances are that you are adding carbon dioxide to the atmosphere.
This addition of carbon dioxide to the atmosphere gives rise to what scientists call the greenhouse effect. In a greenhouse (or even a car left in the open with the windows rolled up), sunlight passes through the glass and is absorbed by materials on the inside. The heated material then gives the energy back in the form of infrared radiation, but the glass is opaque at infrared wavelengths, so the energy remains trapped, warming the interior of the greenhouse or car. Like glass, carbon dioxide transmits visible light coming in from the sun, but absorbs infrared radiation that rises from the ground and holds this heat in the atmosphere instead of reflecting it back into space. The term “greenhouse effect” as applied to planet Earth refers to the possibility of global warming due to the accumulation of carbon dioxide from the massive burning of fossil fuels that has taken place since the beginning of the industrial revolution.
Several points should be made about the greenhouse effect. First, there has always been carbon dioxide in the atmosphere, so we are not introducing a totally new substance to the environment. In fact, without the greenhouse effect that arises from naturally occurring carbon dioxide and other gases (notably water vapor and methane), the temperature of Earth’s surface would be about 20 degrees below zero! Nevertheless, the dramatic increase in atmospheric carbon dioxide produced by human activities over the past century, and associated changes in global climate, are the main greenhouse concerns today.
Scientists agree on four key points:
Carbon dioxide absorbs infrared radiation and acts as a greenhouse gas, as do other molecules such as methane, water, and ozone.
Burning fossil fuels by human beings has increased the amount of carbon dioxide in Earth’s atmosphere, and recent studies suggest that the rate of increase is accelerating.
The climate of our planet is changeable. Today we are living in a period of warming that began in about 1850, following a cool period called the Little Ice Age, which in turn followed a centuries-long era known as the Medieval Warm Period. Since 1850, the average global temperature has increased by approximately 0.5°C, and evidence suggests that at least part of this warming is due to human activities.
Average global temperatures increased significantly during the past several decades, with the 1990s being the warmest decade on record, and twenty of the twenty-five warmest years in recorded history occurring since 1980. Furthermore, 2004 and 2005 were among the four warmest years on record—at least a full degree warmer than the thirty-year average. Several individual months during that period also set historic records, so there can be little doubt that Earth has become warmer in recent years. On the other hand, satellite data shows little increase in warming since the late 1990s.
In spite of these stark facts, uncertainties persist about the rate and extent of global climate change. The primary scientific tools for predicting future climate are enormously complex computer codes called global circulation models (GCM). These models work by splitting the atmosphere and the oceans into boxes a few hundred kilometers on a side, calculating changes in each box, and then adding up the effect of all of the boxes. On a short time scale, these types of programs are used to generate daily weather reports. When they are applied to climate pre dictions (which involve weather a hundred years in the future rather than the weather a few days from now), they require an understanding of li
terally hundreds of processes on Earth, from the formation of icebergs to the details of ocean chemistry, and can take months to run on the fastest computers available. This is not a criticism; after all, it is what you would expect from a computer model designed to represent a complex and multifaceted system like Earth’s climate. It is important to realize, however, that the complexity of the GCM and our current imperfect understanding of many of the effects we have to put into them give rise to uncertainties in the predictions that come from the models. Just because something comes from a computer doesn’t mean it’s true, any more than seeing words in print guarantees their veracity.
In particular, we can identify a number of areas that contribute to the uncertainty of GCM predictions.
Clouds. Clouds play an important but complex role in determining climate. Low-lying clouds reflect sunlight, thereby cooling the surface, while high clouds trap heat, which contributes to warming. Thus, it is not only the amount of cloud cover that we have to be able to predict, but the type of cloud as well. At present, we simply do not understand the physics and chemistry of cloud formation well enough to predict these sorts of things from first principles.
Oceans. Far more carbon dioxide is dissolved in the world’s oceans and their sediments than in the atmosphere, and there exists a complex and imperfectly understood interaction between these two reservoirs. It is thought that even small changes in ocean currents can have an effect on atmospheric carbon dioxide.
Science Matters Page 30