5-5 A rapidly growing population is altering our planetary habitat

One of the distinguishing characteristics of whether or not something is “alive” is whether or not it can change its environment. Indeed, Earth’s original atmosphere has been changed from predominantly carbon dioxide–based to nitrogen-oxygen–based because of the biological processes of living things. We can observe the roots of giant trees causing cracks in sidewalks, ant colonies creating giant mounds of soil, and turtles eating away at coral. What makes Earth truly unique among all of the planets is that it is teeming with environment-altering life, from the floors of the oceans to the tops of mountains and from frigid polar caps to blistering deserts. Clearly our planet and its inhabitants interact, but what if things get out of balance?

The Biosphere and Natural Climate Variation

All life on Earth subsists in a relatively thin layer called the biosphere, which includes the oceans, the lowest few kilometers of the troposphere, and the crust to a depth of almost 2 mi (3 km). Figure 5-22 is a portrait of Earth’s biosphere based on NASA satellite data. The biosphere, which has taken billions of years to evolve to its present state, is a delicate, highly complex system in which plants and animals depend on each other for their mutual survival.

Figure 5-22: Earth’s Biosphere This image, based on data from the SeaWIFS spacecraft, shows the distribution of plant life over Earth’s surface. The ocean colors show where free-floating microscopic plants called phytoplankton are found.

The state of the biosphere depends crucially on the temperatures of the oceans and atmosphere. Even small temperature changes can have dramatic consequences. An example that recurs every three to seven years is the El Niño phenomenon, in which temperatures at the surface of the equatorial Pacific Ocean rise by 2°C to 3°C. Ordinarily, water from the cold depths of the ocean is able to well upward, bringing with it nutrients that are used by microscopic marine organisms called phytoplankton that live near the surface (see Figure 5-22). But during an El Niño, the warm surface water suppresses this upwelling, and the phytoplankton starve. This wreaks havoc on organisms such as mollusks that feed on phytoplankton, on the fish that feed on the mollusks, and on the birds and mammals that eat the fish. During one particularly severe El Niño, one-quarter of the adult sea lions off the Peruvian coast starved, along with all of their pups.

Many different factors can change the surface temperature of our planet. One is that the amount of energy radiated by the Sun can vary up or down by a few tenths of a percent. Reduced solar brightness may explain the period from 1450 to 1850, when European winters were substantially colder than they are today. Another is when volcanoes spew ash into the atmosphere, slightly blocking the Sun’s energy for a time.

Other factors are the gravitational influences of the Moon and the other planets. Thanks to these influences, the shape of Earth’s orbit varies within a period of 90,000 to 100,000 years, the tilt of its rotation axis varies between 22.1° and 24.5° with a 40,000-year period, and the orientation of its rotation axis changes due to precession with a 26,000-year period. These variations can affect climate by altering the amount of solar energy that heats Earth during different parts of the year. They help explain why Earth periodically undergoes an extended period of low temperatures called an ice age, the last of which ended about 11,000 years ago.

One of the most important factors affecting global temperatures is the abundance of greenhouse gases, such as CO₂. Geologic processes can alter this abundance, either by removing CO₂ from the atmosphere (as happens when fresh rock is uplifted and exposed to the air, where it can absorb atmospheric CO₂) or by supplying new CO₂. From time to time in our planet’s history, natural events have caused dramatic increases of the amount of greenhouse gases in the atmosphere. One such event may have taken place 251 million (2.51 × 10⁸) years ago, when Siberia went through a period of intense volcanic activity. The tremendous amounts of CO₂ released in this event would have elevated the global temperature by several degrees. Remarkably, the fossil record reveals that 95% of all species on Earth became extinct at this same time. The coincidence of these two events suggests that greenhouse-induced warming can have catastrophic effects on life.

Human Effects on the Biosphere: Deforestation

Our species is having an increasing effect on the biosphere because our population is skyrocketing. Figure 5-23 shows the sharp rise in the human population that began in the late 1700s with the Industrial Revolution and the spread of modern ideas about hygiene. This rise accelerated in the twentieth century thanks to medical and technological advances ranging from antibiotics to high-yield grains. In 1960 there were 3 billion people on Earth; in 1975, 4 billion; and in 1999, 6 billion. We reached 7 billion in 2011. Projections by the United Nations Population Division show that there will be more than 8 billion people on Earth by the year 2030 and more than 9 billion by 2050.

Figure 5-23: The Human Population Data and estimates from the U.S. Bureau of the Census, the Population Reference Bureau, and the United Nations Population Fund were combined to produce this graph showing the human population from 500 b.c.e. to 2000 c.e. The population began to rise in the eighteenth century and has been increasing at an astonishing rate since 1900.

Every human being has basic requirements: food, clothing, and housing. We all need fuel for cooking and heating. To meet these demands, we cut down forests, cultivate grasslands, and build sprawling cities. A striking example of this activity is occurring in the Amazon rain forest of Brazil. Tropical rain forests are vital to our planet’s ecology because they absorb significant amounts of CO₂ and release O₂ through photosynthesis. Although rain forests occupy only 7% of the world’s land areas, they are home to at least 50% of all plant and animal species on Earth. Nevertheless, to make way for farms and grazing land, people simply cut down the trees and set them on fire—a process called slash-and-burn. This burning process releases CO₂ once held within the plants back into the atmosphere. Such deforestation, along with extensive lumbering operations, is occurring in Malaysia, Indonesia, and Papua New Guinea. The rain forests that once thrived in Central America, India, and the western coast of Africa are almost gone (Figure 5-24).

Figure 5-24: RIVUXG The Deforestation of Amazonia The Amazon, the world’s largest rain forest, is being destroyed at a rate of 20,000 km² per year in order to provide land for grazing and farming and as a source for lumber. About 80% of the logging is being carried out illegally.

Human Effects on the Biosphere: The Ozone Layer

Human activity is also having potentially disastrous effects on the upper atmosphere. Certain chemicals released into the air—in particular chlorofluorocarbons (CFCs), which have been used in refrigeration and electronics, and methyl bromide, which is used in fumigation—are destroying the ozone in the stratosphere. Without a high-altitude ozone layer to absorb ultraviolet light from the Sun, solar ultraviolet radiation would beat down on Earth’s surface with greatly increased intensity. Such radiation breaks apart most of the delicate molecules that form living tissue. Hence, a complete loss of the ozone layer would lead to a catastrophic ecological disaster.

In 1985 scientists discovered a region with an abnormally low concentration of ozone over Antarctica. Since then this ozone hole has generally expanded from one year to the next (Figure 5-25). Smaller but still serious effects have been observed in the stratosphere above other parts of Earth. As a result, there has been a worldwide increase in the number of deaths due to skin cancer caused by solar ultraviolet radiation. By international agreement, CFCs are being replaced by compounds that do not deplete stratospheric ozone, and sunlight naturally produces more ozone in the stratosphere. In recent years, we have observed a slight slowing of ozone layer damage, but it is not expected to heal for many decades.

Figure 5-25: The Antarctic Ozone Hole These two false-color images show that there was a net decrease of 50% in stratospheric ozone over Antarctica between October 1979 and September 2003. The amount of ozone at midlatitudes, where most of the human population lives, decreased by 10% to 20% over the same period.

Human Effects on the Biosphere: Global Warming and Climate Change

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The most troubling influence of human affairs on the biosphere is a consequence of burning fossil fuels (petroleum and coal) in automobiles, airplanes, and power plants as well as burning forests and brushland for agriculture and cooking. This burning can cause atmospheric pollution, but that is not what concerns scientists the most. Burning fossil fuels rapidly releases carbon dioxide into Earth’s atmosphere—extra CO₂ that is predicted to interfere with the normal balancing process of thermal energy being emitted out into space—resulting in an overall increase in average global temperatures. This has been predicted by scientists for some time and we are now starting to see the dramatic effects of excess CO₂ humans have released into our atmosphere. It is not that CO₂ in the atmosphere is unwanted, it is that we are adding CO₂ to the atmosphere much faster than plants and geological processes can extract it, causing Earth’s temperature to rapidly rise. Figure 5-26 shows how the carbon dioxide content of the atmosphere has increased since 1958, when scientists began to measure this quantity on an ongoing basis.

Figure 5-26: Atmospheric Carbon Dioxide is increasing This graph shows measurements of atmospheric carbon dioxide in parts per million (ppm). The sawtooth pattern results from plants absorbing more carbon dioxide during spring and summer. The CO₂ concentration in the atmosphere has increased by 21% since continuous observations started in 1958.

You might wonder what the relationship is between CO₂ and Earth’s global temperature. To put the values shown in Figure 5-26 into perspective, we need to know the atmospheric CO₂ concentration in earlier eras. Scientists have learned this by analyzing air bubbles trapped at various depths in the ice that blankets the Antarctic and Greenland. Each winter a new ice layer is deposited, so the depth of the bubble indicates the year in which it was trapped. Figure 5-27 uses data obtained in this way to show how the atmospheric CO₂ concentration has varied since 1000 C.E. While there has been some natural variation in the concentration, its value has skyrocketed since the beginning of the Industrial Revolution around 1800. Data from older, deeper bubbles of trapped air show that in the 650,000 years before the Industrial Revolution, the CO₂ concentration was never greater than 300 parts per million. The present-day CO₂ concentration is greater than this by 25% and has grown to its present elevated level in just over half a century. If there are no changes in our energy consumption habits, by 2050 the atmospheric CO₂ concentration will be greater than 600 parts per million.

Figure 5-27: Atmospheric CO₂ and Changes in Global Temperature This figure shows how the carbon dioxide concentration in our atmosphere (upper curve) and Earth’s average surface temperature (lower curve) have changed since 1000 C.E. The increase in CO₂ since 1800 due to burning fossil fuels has strengthened the greenhouse effect and caused a dramatic temperature increase.

Increasing atmospheric CO₂ is of concern because this strengthens a greenhouse effect and raises Earth’s average surface temperature. Figure 5-27 also shows how this average temperature has varied since 1000 C.E. Like the CO₂ data, the temperature data from past centuries come from analyzing trapped air bubbles. The recent dramatic increase in atmospheric CO₂ concentration has produced an equally dramatic increase in the average surface temperature. This temperature increase is called global warming. Other explanations for global warming have been proposed, such as changes in the Sun’s brightness, but only greenhouse gases produced by human activity can explain the steep temperature increase shown in Figure 5-27. These gases include methane (CH₄) and nitrous oxide (N₂O), which are released in relatively small amounts by agriculture and industry but which are far more effective greenhouse gases than CO₂.

Impact of Global Warming

The effects of global warming can be seen around the world. Each of the last 12 years was one of the warmest on record since 1997, and each of the years since 2000 has seen increasing numbers of droughts, water shortages, and unprecedented heat waves. Glaciers worldwide are receding; the size of the ice cap around the north pole has decreased by nearly 40% since 1979, and portions of the Antarctic ice shelf have broken off (Figure 5-28). Earth’s most dramatic changes due to global warming are evident in the Arctic Circle. It is generally accepted that the quickly thinning ice at the north pole will thaw and become open sea instead of solid ice within the next decades.

Figure 5-28: RIVUXG A Melting Antarctic Ice Shelf Global warming caused the Larsen B ice shelf to break up in early 2002. This ice shelf, which was about the size of Rhode Island, is thought to have been part of the Antarctic coast for the past 12,000 years.

Few scientists disagree that global warming is occurring. Unfortunately, global warming is predicted to intensify in the decades to come. The degree to which this will occur and what might be done about it is under grand debate. The UN Intergovernmental Panel on Climate Change predicts that if nothing is done to decrease the rate at which we add greenhouse gases to our atmosphere, the average surface temperature will continue to rise by an additional 1.4°C to 5.8°C during the twenty-first century. What is worse, the temperature increase is predicted to be greater at the poles than at the equator. The global pattern of atmospheric circulation (Figure 5-7) depends on the temperature difference between the warm equator and cold poles, so this entire pattern will be affected. The same is true for the circulation patterns in the oceans. As a result, temperatures will rise in some regions and decline in others and the patterns of rainfall will be substantially altered. Agriculture depends on rainfall, so these changes in rainfall patterns can cause major disruptions in the world food supply. Studies suggest that the climate changes caused by a 3°C increase in the average surface temperature would cause a worldwide drop in cereal crops of 20 million to 400 million tons, putting 400 million more people at risk of hunger.

The melting of polar ice due to global warming poses an additional risk to our civilization. When floating ice such as that found near the north pole melts, the ocean level remains the same. (You can see this by examining a glass of water with an ice cube floating in it. The water level does not change when the ice melts.) But the ocean level rises when ice on land melts and runs off into the sea. The Greenland ice cap has been melting at an accelerating rate since 2000 and has the potential to raise sea levels by half a meter or more. Low-lying coastal communities such as Boston and New Orleans, as well as river cities such as London, will have a greater risk of catastrophic flooding. Some island nations of the Pacific will disappear completely beneath the waves. Enormous glaciers are melting at an alarming rate (Figure 5-29). These observations closely match—if not exceed—the scientific predictions of what happens when atmospheric CO₂ is increased.

Figure 5-29: RIVUXG Disappearing Glacier At an altitude of about 7000 ft (2100 m), Grinnell Glacier is a centerpiece in Montana’s Glacier National Park. It is evident in this series of pictures that this glacier has lost about 40% of its size just since 1966 and, at its current melting rate, will disappear by the year 2030, along with all of the other glaciers in the park.

While global warming is an unintended consequence of human activity, the solution to global warming will require concerted and thoughtful action. Global warming cannot be stopped completely: Even if we were to immediately halt all production of greenhouse gases, the average surface temperature would increase an additional 2°C by 2100, thanks to the natural inertia of Earth’s climate system. Instead, our goal is to minimize the effects of global warming by changing the ways in which we produce energy, making choices about how to decrease our requirements for energy, and finding ways to remove CO₂ from the atmosphere and trap it in the oceans or beneath our planet’s surface. Confronting global warming is perhaps the greatest challenge to face our civilization in the twenty-first century.