Scientists know that the earth is warming and most of them acknowledge that it has warmed about one degree in the past hundred years. What many of them find alarming is how quickly that change has occurred. Throughout the planet's history there have been periods of warming and cooling, but the average rate of change has been about one degree per thousand years—so the current global warming appears to be happening ten times faster than ever before. Many scientists believe that this is the result of anthropogenic actions, which are caused by human beings rather than nature. Dr. Stephen H. Schneider, biological sciences professor at Stanford University, is one scientist who believes that people are to blame for the current global warming, as he explains:
Humans are not simply passengers holding a temporary ticket on planet Earth's ride through the galaxy. We are actively altering the surface of the land and the composition of the atmosphere. These factors affect the natural flows of energy and materials around the planet and in turn are altering the climate. And while it usually takes nature thousands of years to create several degrees of temperature change on a globally sustained basis, human beings can do so in a century or less. 14
Schneider and other like-minded scientists insist that this accelerated warming is caused by greenhouse gases that humans are adding to the atmosphere. That is why scientists often use the term greenhouse warming to describe the current warming of the earth. Richard Somerville shares his views on the cause of this increase in temperature: "The concern is that we human beings are modifying that greenhouse effect by adding to the atmosphere gases that increase the natural abundance of these so-called gases. . . . We're adding them through lots of processes, the most important single one of which is burning fossil fuel (coal and oil and natural gas), which releases carbon dioxide." 15
Fossil fuels were formed hundreds of millions of years ago from the fossilized remains of plants and animals. After the organisms died and decomposed, they were eventually buried under hundreds or thousands of feet of mud, rock, and sand. Over time, pressure and heat from the earth compacted the material into layers of sedimentary rock. Different types of fossil fuels were formed based on the types of animals and plants that had decomposed, how long this material was buried, and the degree of temperature and pressure that existed.
Coal is a fossil fuel that has been used since about 1000 B . C ., but during the Industrial Revolution its use began to soar. Since coal was found to be both plentiful and cheap, it was burned to power the rapidly growing industry throughout Europe and America. Historian Gale Christianson describes the pollution of this period: "By the mid-1800s, the tall chimneys [of factories], each constructed of a million or more bricks and hundreds of tons of mortar, had far eclipsed the great cathedrals of medieval Europe, rising as high as 450 feet. They spewed their burden of gases and effluents high into the atmosphere round the clock, where it was believed the pollutants would disperse without harm." 16
For many years, coal was considered the primary fuel. It was used not only for manufacturing, but also for everything from heating homes to providing power for railroad trains and steamships. Today, coal is used by industry to manufacture such products as steel and cement, but its leading use is in making electricity. Power plants use coal to heat water to high temperatures until it turns into steam, which then rotates large turbines to create electricity. In the United States, more than 50 percent of all electrical plants use coal, which also provides power for about 40 percent of the total electricity generated throughout the world.
The other two fossil fuels, oil and natural gas, are also used to produce electricity, but not as often as coal. Oil and gas are primarily used to heat homes and factories, as well as fuel all forms of transportation from buses to ships and motorcycles to airplanes.
All fossil fuels release carbon whenever they are burned, but coal has a much higher carbon content than either oil or gas. The Union of Concerned Scientists says that coal is a main contributor toward global warming because so much electricity is produced from coal-burning power plants. The group also says that these power plants are the single largest source of atmospheric CO 2 . Each year, about 7 billion tons of carbon are released through the burning of fossil fuels; and when this reacts with oxygen, carbon dioxide is created—more than 20 billion tons of it.
Carbon dioxide is a colorless, odorless gas that is naturally present in the atmosphere, but only in tiny amounts. In fact, oxygen and nitrogen comprise about 99 percent of atmospheric gases, while all the other gases—including CO 2 —total just 1 percent. Yet even though carbon dioxide is only a trace gas, it is essential for life. Its powerful heat-trapping capabilities help keep the earth warm, and CO 2 is also necessary in order for all types of plants to live and grow. That is because of its role in photosynthesis, the process by which plants combine light energy from the sun with CO 2 and water to produce their own food.
Carbon dioxide is created naturally when all living things breathe. For example, the bodies of humans and animals contain about 18 percent carbon. Each time they inhale they take in oxygen, which mixes with the carbon in their bodies and is then exhaled as carbon dioxide in a process known as respiration. CO 2 is also formed naturally when living things die and decompose. The carbon that has been stored in the body of the plant, animal, or human is released into the soil over time. Eventually it reacts with oxygen in the soil and releases carbon dioxide into the air.
Together, all these processes make up a natural system that keeps carbon dioxide levels in balance. As long as the amount of CO 2 that is added to the air through respiration and decay is the same as the amount that is taken out, that balance is maintained. However, over the past hundred years, atmospheric concentrations of carbon dioxide have increased by about one-third—and in that same period of time the earth has warmed by about one degree Fahrenheit. Many scientists believe this is not a coincidence.
According to John J. Berger, carbon dioxide is more responsible for changing the earth's climate than any other gas, as he explains:
This is because we add more of it to the atmosphere—by far—than any other. Amazing as it may seem, by adding only a few hundredths of a percent of it to the air, we change our climate. The Earth's temperature and the concentration of carbon dioxide in the atmosphere have risen and fallen together for at least the past 420,000 years—as far back in time as our instruments can probe. . . . Only in the past 150 years, however, have human actions actually begun markedly raising the carbon dioxide levels in the atmosphere. 17
Berger says that more than 65 percent of the warming that has occurred over the past century has been caused by the carbon dioxide added to the atmosphere by humans.
The first scientist to propose that increased carbon dioxide could alter the atmosphere was a Swedish chemist named Svante Arrhenius. In the late 1890s, he studied the paper that had been written by Jean-Baptiste-Joseph Fourier about seventy years before. Arrhenius agreed with Fourier about the role of heat-trapping gases in the atmosphere, and he was intrigued with the scientist's theory that the earth acted like a giant glass vessel that trapped and held heat. Arrhenius took the theory one step further, though. He suspected that humans were causing the gases to accumulate at a faster-than-normal rate because of the burning of fossil fuels such as coal. Arrhenius thought it was logical that as more fossil fuels were burned, more carbon was released into the atmosphere. He believed this could cause atmospheric carbon dioxide levels to rise significantly—which, he reasoned, would trap more of the sun's energy and make the earth hotter. For this reason, he used the example of a hothouse, or greenhouse, model to describe the warming of the planet.
In 1895, Arrhenius presented a paper to a prominent scientific group in Stockholm, Sweden. The paper was called "On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground," and it expressed his belief that higher levels of carbon dioxide in the atmosphere could raise the earth's
During the following years, some scientists explored Arrhenius's theories about the relationship between carbon dioxide and climate. However, most scientists paid no attention. There was no way to prove that CO 2 was building up in the atmosphere because there were no instruments to measure it; and it was assumed the earth's oceans would prevent carbon dioxide from accumulating in the atmosphere because they could absorb gas. Then in the 1950s, Roger Revelle and Hans Suess, both scientists from Scripps Institute of Oceanography, published a research paper challenging that belief. They concluded that there were limits to how much CO 2 the oceans could absorb, and that the excess carbon dioxide produced by industry and automobiles would remain in the air and eventually warm the planet. Still, the majority of scientists found this theory hard to believe. These beliefs did not begin to change until Charles David Keeling designed and built his manometer. His ability to measure carbon dioxide levels piqued the interest of other scientists.
The real turning point came during the 1980s. Researchers at the Polar Plateau in Vostok, East Antarctica, drilled thousands of miles into a glacier and extracted an ice core that dated back more than one hundred thousand years. By analyzing air bubbles trapped in the ice, the scientists were able to confirm that carbon dioxide levels had risen steadily since the mid-1700s. At that point, more scientists began to pay attention.
Over the following years, measurements continued to be taken at the Mauna Loa Observatory, and carbon dioxide levels showed steady increases each year. By the year 2000, atmospheric concentrations of carbon dioxide had risen to more than 368 ppm—a 17 percent jump from the 1950s when the first measurements were taken. These findings meant that not only were CO 2 levels rising, they were rising fast.
Even scientists who doubt that global warming is a problem admit that humans have increased the amount of CO 2 in the atmosphere. For instance, Dr. Patrick J. Michaels says that predictions about global warming have been proven inaccurate because the earth has warmed at a much slower rate than some scientists said it would. For this reason, he believes that the global warming issue has been blown out of proportion. However, he also acknowledges that CO 2 levels have risen and that humans have played at least some role in the increase, as he explains:
It has been known since 1872 that water vapor and carbon dioxide are the principal "greenhouse" gases in the atmosphere, and that increasing their concentration should elevate the temperature in the lower atmosphere. What has been a subject of contention ever since, is the amount and character of the warming. Because of all of the atmospheric greenhouse gases emitted by human activity, we have progressed to roughly a 60% increase in the equivalent natural carbon dioxide greenhouse effect. 18
Burning fossil fuels is not the only human activity that releases carbon dioxide into the air. The destruction of the world's forests creates about 30 percent of all anthropogenic greenhouse gases. Living trees breathe in carbon and breathe out oxygen, and when they are cut down, huge concentrations of CO 2 are released into the air. When trees are burned, the effects are doubly harmful to the environment.
In many parts of the world, especially in tropical countries such as Brazil, Indonesia, Burma, and Thailand, rain forests are considered more of a problem than an asset. In order to clear land for planting crops or raising livestock, the forests are chopped down and burned, a practice called slash-and-burn agriculture. The environmental group Rainforest Action Network says that only about half of the world's forests that existed a thousand years ago remain today. NASA predicts what will happen if the destruction of forests is not stopped: "The loss of tropical rain forest is more profound than merely destruction of beautiful areas. If the current rate of deforestation continues, the world's rain forests will vanish within 100 years—causing unknown effects on global climate and eliminating the majority of plant and animal species on the planet." 19
Cutting down and burning forests releases carbon dioxide and other heat-trapping gases into the atmosphere. That is not the only damage that is done, however. The immense green canopies of the world's
Carbon dioxide accounts for most of the anthropogenic gases in the atmosphere, but other heat-trapping gases have been building up over time. Methane comprises about 20 percent of these, and concentrations of it are increasing at the rate of about 1 percent each year. Also, while methane is not as plentiful as CO 2 , it is about thirty times more powerful at absorbing heat in the atmosphere—which means its potential for contributing to global warming is greater than any other gas.
Methane is released in a number of ways. It is emitted into the atmosphere during the burning of fossil fuels, and also when forests are burned. It escapes from the ground during oil drilling and coal mining, and is often vented into the air to prevent underground mine explosions. Natural gas is about 90 percent methane, and when natural gas is extracted from the ground, methane can escape through cracked or leaking pipelines. Methane is also formed during the decay of garbage. In the United States alone, about 10 million tons of food waste is disposed of each year by commercial restaurants and households. When organic materials such as food scraps, grass and tree clippings, leaves, and wood debris are hauled to landfills, they are buried. Unlike carbon dioxide, methane forms without oxygen; so as the materials decompose beneath the ground, methane is eventually released into the atmosphere.
Many people are surprised to learn that a major contributor to methane gas is the raising of livestock. When animals such as cattle, sheep, goats, horses, pigs, and camels eat grass and hay, the food is broken down in their digestive systems by bacteria. Methane is expelled into the air when these animals belch, and when their manure decomposes. In countries where there is a high amount of agriculture, farm animals are often the largest source of methane. In Scotland, for instance, farm animals produce more than 45 percent of the country's total methane, while in New Zealand the number is much higher: nearly 90 percent. Currently, the worldwide cattle population is increasing faster than the human population, and as more cattle are raised, more methane gas is created.
Rice agriculture is another leading source of global methane emissions, and it is responsible for about 10 percent of the anthropogenic methane in the atmosphere. Rice is the staple food for more than half of the world's people, especially in Asian countries such as China and India. As the population of these countries continues to grow, more rice must be produced. For much of the growing season, rice farmers flood their rice paddies with water to help control bugs and weeds. Mud-dwelling bacteria and other organisms break down organic material in the waterlogged soil to produce methane. Then, as the hollow stems of the rice plants act as tubes, the methane gas moves up from the soil and into the air.
Besides carbon dioxide and methane, there are other gases that add to the atmosphere's heat-trapping ability. Chlorofluorocarbons, usually called CFCs, are created synthetically for use in refrigerators, air conditioners, foam, and insulation products, as well as for propellants in spray cans. CFCs are up to sixteen thousand times more effective than carbon dioxide at absorbing heat, and they contribute about 20 to 25 percent of the total anthropogenic greenhouse gases. Scientific studies have also connected CFCs with rapid destruction of the ozone layer, a protective shield of atmospheric gas that absorbs harmful ultraviolet radiation from the sun. In most parts of the world, including the United States, CFCs are rarely used because of international control agreements. However, they have been in use all over the world for more than sixty years, and some countries still use them today. This is especially troublesome to scientists because CFCs remain in the atmosphere for at least one hundred years, and possibly two or three times longer.
Nitrous oxide is a powerful heat-trapping gas that is two hundred times more heat absorbent than CO 2 and constitutes about 9 percent of the total anthropogenic greenhouse gases. It is created when fossil fuels are burned, as well as from slash-and-burn agriculture. However, the main source of nitrous oxide is chemical fertilizers, which are usually made from nitrogen. When these fertilizers are spread on lawns or golf courses, parks or farm fields, nitrogen is released into the soil. Once it reacts with oxygen in the air, nitrous oxide is formed. In the past, nitrous oxide was not considered one of the primary heat-trapping gases. But like carbon dioxide and methane, atmospheric concentrations of nitrous oxide have continued to increase, and many scientists now believe it is a contributor to global warming.
For as long as there has been a planet Earth, there have been fluctuations in temperature and this will continue in the future. Solid evidence has proven this to be fact, and it is the one area where all scientists agree. Where they differ is over the issue of human activities—specifically, whether or not anthropogenic greenhouse gases are causing the earth to warm at an unnaturally fast pace, and how much this will affect future climatic conditions. Scientists who believe humans are to blame say the evidence speaks for itself—that in the past several hundred years, humans have altered the atmosphere so much that irreparable damage has been done. Somerville sums up why he and other scientists have made these conclusions:
Climate has varied on every time scale to which we have any observational access. Ice ages come and go on time scales of tens of thousands of years, for example. . . . Climate changes. It changes on all time scales. What's different between our time and our grandparents' time is that now humankind, which has been a passive spectator at this great natural pageant, has become an actor and is up on the stage. And what we—all 6 billion of us—do can affect the climate. 20