Is present global warming due more to human activity than to natural geologic trends

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Is present global warming due more to human activity than to natural geologic trends?

Viewpoint: Yes, there is strong evidence that most of the global warming observed over the last 50 years is due to human activities.

Viewpoint: No, present global warming is not due more to human activity than to natural geologic trends.

Of all the issues relating to earth science, none is more politically volatile than that of global warming. Even the name is controversial, since it is not entirely clear, from the scientific data, that Earth really is warming. And even if it is, the causes are open to debate. Typically, when the term global warming is mentioned in the media, the implication is "global warming due to human activities," but this may only be part of the picture.

In the debate over whether humans are primarily to blame for global warming, the "yes" side has long been far more visible and vocal. This is due in large part to the prominence of the environmental movement, which, as it has gained strength in the period since the late 1980s, has exerted enormous influence through the media, entertainment, and popular culture in general. Overexposure of the "yes" viewpoint is thus a mixed blessing for scientists who hold to it purely on the basis of data, and not because it is the fashionable position. Yet as we shall see, there is a strong argument for the view that, first of all, global warming is proceeding in a more or less linear fashion, and secondly, that human activities are responsible for it.

The idea that humans are causing a radical change in the planet's temperature is not a new one; in fact, an early manifestation of this viewpoint occurred in the mid-1970s, at a time when the environmental movement was just starting to gain in political influence. At that time, however, winters were unseasonably cold—for example, the winter of 1976-1977 saw numerous cases of snowfall even in the southern United States—and the fear at the time was that human activities were causing Earth to cool down, not warm up. This illustrates just how difficult it is to analyze exactly what the climate is doing, and which direction it is headed. One thing is certain, however: Earth's climate is continually changing.

The planet has long been subject to ice ages, and to interglacial periods, which, as their name suggests, are relatively short segments between ice ages. All of recorded human history, which dates back only about 5,500 years, to approximately 3500 b.c. in Sumer, takes up just half of the current inter-glacial period, which has lasted for approximately 11,000 years. Regardless of whether Earth is warming up or cooling down now, or whether humans are responsible for it, it seems clear that within the next few thousand years, another ice age will befall the planet.

In the meantime, though, what is happening to the climate, and what role are humans playing? It is unquestionable that humans are pouring unprecedented amounts of pollutants into the atmosphere, including greenhouse gases such as carbon dioxide and carbon monoxide. We are also cutting down our forests at an alarming rate. Before humans began chopping down forests, Earth's combined vegetation stored some 990 billion tons (900 billion metric tons) of carbon, of which 90% appeared in forests. Today, only about 616 billion tons (560 billion metric tons) of carbon are stored in Earth's vegetation, and the amount is getting smaller as time passes. At the same time, the amount of carbon dioxide in the atmosphere has increased from about 270 parts per million (ppm) in 1850 to about 360 ppm in 2000, and again, the increase continues.

There is also the threat to the ozone layer that has resulted from the misuse of a particular type of catalyst, a substance that speeds up a chemical reaction without taking part in it. In the upper atmosphere of Earth are traces of ozone, a triatomic (three-atom) molecular form of oxygen that protects the planet from the Sun's ultraviolet rays. During the latter part of the twentieth century, it became apparent that a hole had developed in the ozone layer over Antarctica, and many chemists suspected a culprit in chlorofluorocarbons, or CFCs, which had long been used in refrigerants and air conditioners, and as propellants in aerosol sprays. Chlorine acts as a catalyst to transform the ozone to elemental oxygen, which is not nearly as effective for shielding Earth from ultraviolet light.

Due to concerns about the danger to the ozone layer, an international agreement called the Montreal Protocol on Substances That Deplete the Ozone Layer, signed in 1987, banned the production of CFCs and the coolant Freon that contains them. Even proponents of the global-warming theory (that is, those who support the position for scientific, and not political or emotional reasons) concede that since the signing of the protocol, the growth in the ozone-layer hole has decreased.

One of the biggest areas of disagreement over the global-warming issue is with regard to the greenhouse effect, which is also one of the least-understood natural phenomena under debate in the present context. Contrary to popular belief, the greenhouse effect is not something originally caused by human activity, nor is it necessarily detrimental; on the contrary, if there were no greenhouse effect, there could be no human life on this planet.

The Sun radiates vast amounts of energy, part of which is in the direction of Earth, but only a portion of which Earth actually absorbs. The planet then re-radiates a dissipated form of the same energy, known as long-wavelength radiated energy, to space. Water vapor, carbon dioxide, and carbon monoxide, as well as methane, nitrous oxide, and ozone, all absorb long-wavelength radiated energy as the latter makes its way up through the atmosphere, and when heated, these "greenhouse gases" serve to slow the planet's rate of cooling. Without the greenhouse effect, surface temperatures on Earth would be about 59° Fahrenheit cooler than they are, and much too cold for Earth's biological processes. Thus the greenhouse effect literally preserves life on the planet.

With the greenhouse effect placed in its proper context, it is still reasonable to question whether human activities are placing an excessive amount of carbon dioxide, carbon monoxide, and other greenhouse gases into the atmosphere. There is certainly a body of evidence to indicate that we are, and that this activity is having a deleterious effect on the planet's climate by causing undue heating. On the other hand, a single volcano can thrust more natural pollutants into the atmosphere than a thousand cities of the size of Los Angeles. The debate over global warming thus continues, with no obvious conclusion in sight.

—JUDSON KNIGHT

Viewpoint: Yes, there is strong evidence that most of the global warming observed over the last 50 years is due to human activities.

"We have met the enemy and he is us!" Walt Kelly's philosophical cartoon possum named Pogo was sadly remarking on the ways humans were trashing their environment on Earth Day 1971. The possum was staring at heaps of junk thrown on the forest floor. If Pogo reflected that comment today, he might not see as much trash on the ground, but he would feel the warmer weather that is also the unfortunate result of human activities. According to the Third Assessment Report of the Inter-governmental Panel on Climate Change (IPCC) released in 2001, "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities."

In 1988 the United Nations set up the IPCC to figure out whether global warming was actually happening. The group has produced three reports, each more certain than the one before that global warming is in fact happening, and that the cause is due more to human activities than to natural events. Greenhouse gases accumulating from burning fossil fuels, and other gas-producing activities of industry, are blanketing the Earth and so causing the warming trend.

At the bidding of President George W. Bush, the National Research Council of the National Academy of Sciences studied the IPCC's findings and issued its own report in the summer of 2001. The news was grim. "Greenhouse gases are accumulating in Earth's atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise. Temperatures are, in fact, rising." In the United States, regional assessments were also made, and the results were the same. That is not to say global warming is uniform, however, because local variables, such as the proximity of the ocean or an ice field, play a role.

The Greenhouse Effect

The greenhouse effect is not all bad. With the blanketing of greenhouse gases (water vapor, carbon dioxide, methane, nitrous oxide, and tropospheric ozone) the planet is kept about 59°F (15°C) warmer than it would be without the cover. Water vapor is by far the most abundant greenhouse gas. The greenhouse effect gets its name because a layer in the atmosphere acts like the glass in a greenhouse, letting the Sun's light energy in, but preventing reflected lower energy heat radiation from escaping.

All the greenhouse gases that occur naturally are also produced by human activities, thus increasing their levels in the atmosphere at an accelerating pace. In addition to the "natural" greenhouse gases are the fluorocarbons, which only appeared in the atmosphere when humans introduced them. Over the millennium before the Industrial Era, the atmospheric concentrations of greenhouse gases remained relatively constant, according to the IPCC report. The contribution of a particular gas to radiative forcing of climate change depends on the molecular radiative properties of the gas, the amount introduced into the atmosphere, and the residence time of the gas in the atmosphere. Gases that last a long time in the atmosphere can affect the atmosphere for decades, centuries, or millennia before they are naturally removed.

The concentration of carbon dioxide in the atmosphere has increased by more than one third since the start of the Industrial Era in the late eighteenth century. The atmospheric lifetime of carbon dioxide ranges from 5 to 200 years. Methane was at a pre-industrial concentration of about 700 parts per billion (ppb) and had risen to 1745 ppb by 1998. Methane has an atmospheric lifetime of 12 years. Nitrous oxide was about 270 ppb and rose to 314 ppb during the same period. It has a lifetime of 114 year. Three fluorocarbon compounds were not in the atmosphere until recent years, but one of them, chlorofluorocarbon, was at 268 ppb in 1998, with a lifetime of 45 years. Another, hydrofluoro-carbon, reached 14 ppb and has a lifetime of 260 years. The third, perfluoro-methane, reached 80 ppb by 1998. It has a lifetime of over 50,000 years, which, in human time, seems close to forever.

Chlorofluorocarbons are believed by most scientists to be responsible for depleting the ozone layer that exists at the top of the stratosphere, the second layer in the Earth's atmosphere. The upper stratospheric ozone layer shields the Earth from intense ultraviolet radiation that causes skin cancer. The troposphere, the region from the ground up to about 8.6 mi (14 km), is the first layer. All the weather takes place in the troposphere.

There was one very small bit of good news from the United States Environmental Protection Agency's (EPA's) Global Warming Site in January 2002: the growth rate of chlorofluoro-carbons in the atmosphere has declined since the Montreal protocol was signed in 1987. This international treaty, signed by numerous countries across the world, was designed to phase-out the use of chlorofluorocarbons to protect the upper ozone layer.

Natural Factors That Affect Climate Changes

The climate has always been influenced by various factors. However, before the Industrial Era the changes were all due to natural causes. Solar irradiance is one example. The Sun has 11-year cycles of activity, with quiet periods and turbulent ones with active sunspots emitting storms of radiation. During periods of active sunspots, radio communications are often affected. Spectacular aurora displays in far northern and southern latitudes are also earthly evidence of solar storms.

Volcanoes play a part in temporary climate changes. Enormous ejections of toxic gases are produced as aerosols from explosive volcanoes such as the Mount Pinatubo eruption in the Philippines in 1991. That eruption produced some 20 million tons of sulfuric acid aerosols ejected into the stratosphere. The resulting upper atmosphere cloud covered the entire planet, reflecting sunlight and cooling the Earth by nearly 1°F (-17°C).

In 1815 Mount Tambora in Indonesia erupted, sending a cloud into the atmosphere that was four times the amount ejected by Mount Pinatubo. The year following the eruption has been described as "The Year without a Summer." Probably the greatest volcanic eruption to date is the 1883 Krakatoa eruption, which caused the Moon to appear blue for two years.

According to the IPCC, solar variation and volcanic aerosols are the two major natural factors that affect global warming. There are some anthropogenic aerosols, but they are short-lived. The major sources of anthropogenic aerosols are fossil fuel and biomass burning. These aerosols are associated with air pollution and acid rain near the Earth's surface.

There is one more natural factor to mention: the El Niño-Southern Oscillation (ENSO) phenomenon. A distinctive pattern of sea surface temperature changes in the Pacific Ocean is associated with large variations in weather around the world from year to year. The oceans act like a sink for carbon dioxide. The oceans absorb about 50% of the carbon dioxide released into the atmosphere. The exact amount depends on the ocean temperature. Therefore, the ENSO plays a role in modulating exchanges of carbon dioxide with the atmosphere.

Anthropogenic Factors That Affect Climate Changes

Anthropogenic factors impacting the global climate are human population growth, urbanization, the energy supply mix and the efficiency with which energy is produced and consumed, deforestation rates, and emission rates of chlorofluorocarbons and other synthetic greenhouse gases.

According to Edward O. Wilson in The Future of Life, the world population reached 6 billion by October 1999. The rate of growth is a phenomenal 200,000 people each day. People born in 1950 will see the human population double in their lifetime, from 2.5 billion to over 6 billion. In 1800 there were only about 1 billion people, and as the Industrial Era went into full swing in 1900, the population was still only at 1.6 billion.

With the population expected to reach 8 billion by mid-twenty-first century, all the human activities that affect climate will increase proportionally. Already at the start of the twenty-first century, atmospheric carbon dioxide is at the highest level it has ever been in at least 200,000 years.

Likewise, urban influence cannot be ignored. As masses of people move into cities, the asphalt roads, tar roofs, and other urban features trap more heat than surrounding countryside. Some researchers see urbanization as a major cause for global warming in the past 100 years. Deforestation, the clearing of trees and vegetation to make cities, has further exacerbated the problem.

As populations increase, there is greater demand for fuel for heating and cooling, transportation, and for industry. Many countries' growing dependence on fossil fuels has produced not only political and economic problems, but environmental problems as well, increasing the amount of synthetic greenhouse gases in the atmosphere.

What Is in the Future?

This is a difficult question to answer. Models developed by the IPCC and other research groups indicate that global warming will profoundly affect our quality of life. The factors that influence global climate are extremely complex and interrelated. There are some scientists who minimize our role in global warming. They contend that natural forces—such as solar energy output and the tilt of the Earth's axis—which are all beyond human control, dictate major climate changes.

In the near term, most scientists agree that we have been the cause for much of the change observed in the global climate since the start of the Industrial Era. If there is no moderation in population growth, fossil fuel use, and urban sprawl, global warming will continue to increase, weather events will be more extreme, and ocean levels will rise as glaciers melt. However, the alarm bells have been sounded and scientists from all over the world are working to address the problem. Citizens are also becoming more involved and knowledgeable about how they affect the environment.

—M. C. NAGEL

Viewpoint: No, present global warming is not due more to human activity than to natural geologic trends.

Human activities such as clear-cutting rain forests, burning fossil fuels, and driving cars contribute toward global warming. That is not in dispute. But human effects on climate are negligible compared with the effects naturally produced by the Earth, the atmosphere, and the Sun.

The term global warming can be misleading or even alarmist. It refers to only half the story of climate change. Gradual modifications of average worldwide surface temperature (AWST) over long periods of time are not only upward. Equally important to a thorough understanding of the dynamics of the worldwide climate is the concept of "global cooling." A warming climate is a complex and erratic phenomenon, which may include or even pass over into long or short periods of global or regional cooling. Similarly, periods of global cooling can include or pass over into warming periods. Science clearly shows that AWST oscillates, sometimes in a warming trend, sometimes in a cooling trend. In nature, climate is never stable over long periods of time, and modifications of climate are seldom inexorably progressive.

Until fairly recently in geologic time, there was a long period when the Earth was too warm to allow the formation of polar ice caps. Since their appearance, the Earth has gone through continuously oscillating periods of warming and cooling. Ocean levels naturally fall and rise as the Earth alternately cools and warms, and as the size of the polar ice caps consequently increases and decreases. Sometimes the Earth is so cold that glaciers cover vast areas of otherwise inhabitable land. These periods are called ice ages. In the last 700,000 years, there have been at least seven ice ages, some lasting as long as 80,000 years. In between the ice ages are the interglacial periods, which are almost always shorter than the ice ages.

The current interglacial period began about 11,000 years ago. Since that time AWST has been about 59°F (15°C), plus or minus about two degrees. But 18,000 years ago, in the depth of the Great Ice Age, AWST was about 50°F (10°C).

In 1993 Danish geophysicist Willy Dansgaard reported that average Arctic surface temperature fluctuations of as much as 41° or 42°F (5° or 6°C) per hundred years were not uncommon during the Great Ice Age. The Arctic ice core drill sample shows that, about 10,700 years ago in a famous geologic incident called the Younger Dryas Event, just after the beginning of the current interglacial period, the average Arctic surface temperature dropped 45°F (7°C) in just 50 years.

Several periods of significant temperature fluctuation have occurred naturally during the current interglacial period. Two major warming trends were the Great Climatic Optimum, about 4000 b.c., when AWST reached about 63°F (17°C), and the Little Climatic Optimum, from about 800 to 1250, when AWST was about 61°F (16°C). AWST began dropping rapidly about 1300. By 1400 the world was in the Little Ice Age, from which it did not emerge until about 1850. AWST was about 57°F (14°C), except for a brief warming trend in the late fifteenth and early sixteenth centuries. The Thames froze regularly in winter, Greenland became uninhabitable, and in northeastern North America, 1816 was called "The Year without a Summer." Since the end of the Little Ice Age, the Earth has been generally warming, but not uniformly. AWST is difficult to measure accurately, but in 2000 it was between 59° and 60°F (15° and 15.5°C).

History of Iceland

The recorded human history of Iceland is a clear illustration of how natural geological and climatological changes can affect people and their culture. The settlement of Iceland began with Norse landings around 874, during the Little Climatic Optimum, when the growing season was long enough to support several kinds of basic crops, and pasture was abundant for sheep and cattle. The Vikings named the place "Iceland" from sightings of drift ice in a fjord, not from the presence of glaciers. The Icelandic classic, Njal's Saga, written in the last half of the thirteenth century about events that happened in the tenth and eleventh centuries, speaks matter-of-factly of the existence of forests and lumbering in Iceland. Such statements would not be possible today. Trees still exist in Iceland, but they are maintained only with great difficulty and care. Because the average Icelandic surface temperature is lower now than in the Saga Age (930-1150), Iceland no longer has any natural stands of woods from which timber might be felled.

The Settlement Age (874-930) through the Saga Age was a golden era for Iceland in terms of both culture and climate. Civil strife in the Sturlung Age (1150-1264) resulted in Iceland's loss of independence, first to Norway in 1262, then to Denmark in 1397. The abrupt decline of Iceland was intensified by the onset of the Little Ice Age. The volcano Hekla erupted throughout the fourteenth century, most notably in 1389, darkening the skies, blocking the summer sun, and chilling the land. Pastures and farmland became unproductive, famine reigned, and many Icelanders starved or froze to death. The years from 1262 to 1874, which encompassed the Little Ice Age, held almost uninterrupted misery for Iceland. No Icelandic cultural, political, or economic renaissance was possible until the Little Ice Age began to abate in the mid-nineteenth century. Led by Jon Sigurdsson (1811-1879), Iceland recovered fairly quickly once global warming began, and the country regained limited independence from Denmark in 1874. Iceland remains vulnerable to its climate, and global cooling threatens it more than global warming.

AWST Fluctuations

Statistical meteorological data did not become reliable until about 1880. Since then, AWST has increased about 33°F (0.5°C). This upward trend has not been consistent. From about 1880 to 1940 AWST rose, but from about 1940 to 1970 it fell, and since about 1970 it has been rising. There are also regional variations. The upper Northern Hemisphere cooled from 1958 to 1987, even while AWST increased.

No human activity began or ended the Great or the Little Ice Age or the Great or the Little Climatic Optimum. Anthropogenic, or "humanly caused," modifications of AWST did not begin until the Industrial Revolution during the late eighteenth century. These modifications have always been well within the range of natural fluctuations of AWST. Nothing that humans have ever done to AWST has approached even the maximum AWST of the Little Climatic Optimum. Measurements and predictions of anthropogenic warming of the Earth fall not only within the natural range of AWST fluctuations, but also within the relative error for such measurements and their subsequent predictions. In other words, the data are inconclusive that anthropogenic global warming is at all significant.

What causes the onsets and ends of ice ages and interglacial periods? No one knows for sure. Probably the greatest single factor in determining global cooling and warming cycles is the fluctuation of the Sun's energy output, over which humans obviously have no control. There may be a correlation between weather patterns on Earth and 11-year cycles of sunspots, which are indicative of changes in solar energy output.

Solar energy occurs in many forms, including visible light, ultraviolet radiation, and infrared radiation. Ultraviolet radiation is filtered by the layer of ozone in the stratosphere. The recent anthropogenic damage to the ozone layer above Antarctica by the emission of chlorofluorocarbons (CFCs) and other chlorine compounds is serious, but mostly irrelevant to the question of global warming. The portion of solar energy responsible for heating the Earth is infrared. Every physical body emits heat as infrared radiation.

Some of the earliest and most cogent investigations of variations in solar energy reaching Earth's surface were done by a Serbian engineer, Milutin Milankovitch (1879-1958). He theorized that cycles of global warming and cooling are due to a correlation of three factors: (1) the elliptical orbit of the Earth around the Sun, which varies from more elliptical to more circular on a period of 100,000 years; (2) precession of the axis on a 23,000-year cycle; and (3) variations of the angle of the Earth's axis between 21.6 degrees and 24.5 degrees every 41,000 years.

About half the solar energy directed at the Earth is reflected back into space by the atmosphere. Another 10% is absorbed by atmospheric gases. Of the 40% that reaches the Earth's surface, about two-thirds is absorbed and used as heat and about one-third is reflected back upward.

The so-called greenhouse effect is the absorption of infrared radiation, i.e., the trapping of heat near the Earth's surface, by atmospheric gases. The greenhouse effect is one important reason that the Earth is warm enough to support life. Contrary to misunderstandings popular since the 1980s, the greenhouse effect is natural and almost entirely beneficial to humans. It makes the planet inhabitable by keeping AWST about 59°F (15°C) warmer than it would be without this action.

Human Activity and the Greenhouse Effect

A major question in the global warming dispute concerns the extent to which human practices may exacerbate the greenhouse effect. The so-called greenhouses gases, such as water vapor, carbon dioxide, methane, ozone, and others, are mostly natural components of the atmosphere. The greenhouse effect works because of their natural properties and natural concentrations. Whatever humans may add to them hardly changes their relative concentrations. Humans have little effect on the concentration of the most abundant greenhouse gas, water vapor. The combustion of fossil fuels creates greenhouse gases, but it also creates by-products that contribute toward global cooling, such as sulfur dioxide and sulfate aerosols.

Anthropogenic emissions of carbon dioxide, one of the most significant greenhouse gases, increased dramatically in the twentieth century, chiefly as by-products of industry. Total annual industrial carbon dioxide emissions rose from about 0.5 GtC/yr (gigatons of carbon per year) to over 7 GtC/yr, i.e., about 1,300% from 1900 to 1992; but, in the same period, total atmospheric carbon dioxide concentration rose from about 295 ppmv (parts per million by volume) to about 360 ppmv, i.e., about 22%. Humans released twice as much carbon dioxide into the atmosphere in 1990 as in 1958, but very little of this carbon dioxide remained in the atmosphere to increase the overall concentration of greenhouse gases.

The main reason that atmospheric carbon dioxide concentration has not kept pace with industrial carbon dioxide emissions is that carbon dioxide is soluble in water. At 59°F (15°C), the solubility of carbon dioxide is 0.197 grams per 100 grams of water. Oxygen, by contrast, at the same temperature dissolves only 0.004802 grams in 100 grams of water. Most of the carbon dioxide emitted by human activities quickly dissolve in the upper few hundred feet of the world's oceans, and thus has no chance to accumulate in the atmosphere. The oceans are far from saturated with carbon dioxide. They serve as a "carbon dioxide sink," maintaining a nearly optimal level of carbon dioxide in the atmosphere.

A more troublesome way that humans increase carbon dioxide levels is by deforestation, which increases the amount of carbon dioxide in the atmosphere by reducing the number of trees and other green plants that use atmospheric carbon dioxide in photosynthesis. The forests serve as the second carbon dioxide sink. Equatorial deforestation may have contributed to the southward extension of the Sahara Desert and to the rapid shrinking of Mount Kilimanjaro's ice cap, which lost over 80% of its area from 1912 to 2001. But, however drastic, these anthropogenic modifications of tropical forests, deserts, and isolated mountain ice caps are just regional events from a climatological point of view, and have only insignificant effects on AWST.

Another climatological factor beyond human control is volcanic activity, a widespread, powerful, and completely natural phenomenon that generally has the effect of lowering atmospheric temperature after each eruption. When Mount Pinatubo in the Philippines erupted in 1991, it spewed such a quantity of radiation-blocking ash, gases, and sulfate aerosols into the air that it lowered AWST until 1994. The monstrous explosion of Krakatoa, between Java and Sumatra, in 1883 darkened skies across the world and is believed to have lowered global temperature for as long as 10 years. The eruption of another Indonesian volcano, Tambora, on the island of Sumbawa, in April 1815 contributed to "The Year without a Summer" in North America in 1816. We have already mentioned Hekla's continuing effect on Iceland.

None of the above arguments denies the danger facing such places as Venice, New Orleans, much of the Netherlands, and much of Bangladesh, which may all disappear under the sea within the next few hundred years. That danger is quite real, but little or nothing can be done about it, as natural causes far outstrip anthropogenic causes of the current global warming trend.

—ERIC V.D. LUFT

Further Reading

Adger, W. Neil, and Katrina Brown. Land Use and the Causes of Global Warming. Chichester, England: Wiley, 1994.

Budyko, Mikhail Ivanovich. The Earth's Climate, Past and Future. New York: Academic Press, 1982.

Burroughs, William James. Does the Weather Really Matter? The Social Implications of Climate Change. Cambridge: Cambridge University Press, 1997.

Christianson, Gale E. Greenhouse: The 200-Year Story of Global Warming. New York: Penguin USA, 2000.

Drake, Frances. Global Warming: The Science of Climate Change. London: Arnold; New York: Oxford University Press, 2000.

Horel, John, and Jack Geisler. Global Environmental Change: An Atmospheric Perspective. New York: Wiley, 1997.

Houghton, John. Global Warming: The Complete Briefing. Cambridge: Cambridge University Press, 1997.

Intergovernmental Panel on Climate Change[cited July 16, 2002]. <http://www.ipcc.ch>.

Johansen, Bruce E. The Global Warming Desk Reference. Westport, CT: Greenwood Press, 2001.

Kondratyev, Kirill Yakovlevich. Climate Shocks: Natural and Anthropogenic. New York: Wiley, 1988.

Lin, Charles Augustin. The Atmosphere and Climate Change: An Introduction. Dubuque, IA: Kendall/Hunt, 1994.

Mannion, Antoinette M. Natural Environmental Change: The Last 3 Million Years. London; New York: Routledge, 1999.

Newton, David E. Global Warming: A Reference Handbook. Santa Barbara, CA: ABC-CLIO, 1993.

Nilsson, Annika. Greenhouse Earth. Chichester, United Kingdom: Wiley, 1992.

Parsons, Michael L. Global Warming: The Truth Behind the Myth. New York: Plenum, 1995.

Philander, S. George. Is the Temperature Rising? The Uncertain Science of Global Warming. Princeton: Princeton University Press, 1998.

Ruddiman, William F. Earth's Climate: Past and Future. New York: W.H. Freeman, 2000.

Schneider, Stephen H. Global Warming: Are We Entering the Greenhouse Century? New York: Vintage Books, 1990.

Singer, S. Fred, ed. Global Climate Change: Human and Natural Influences. New York: Paragon House, 1989.

———. Hot Talk, Cold Science: Global Warming's Unfinished Debate. Oakland, CA: The Independent Institute, 1997.

Stevens, William K. The Change in the Weather: People, Weather, and the Science of Climate. New York: Delacorte, 1999.

United States Environmental Protection Agency Global Warming Site [cited July 16, 2002]. <www.epa.gov/globalwarming/>.

Wilson, Edward O. The Future of Life. New York: Random House, 2002.

Wuebbles, Donald J., and Jae Edmonds. Primer on Greenhouse Gases. Chelsea, MI: Lewis, 1991.

KEY TERMS

AEROSOL:

Extremely small particles of liquid or dust ejected into the atmosphere.

ANTHROPOGENIC:

Caused by humans, as opposed to naturally occurring.

BIOMASS:

Total amount of living organisms in a particular area.

CLIMATE CHANGE:

A broader and more accurate term than either "global warming" or "global cooling" to describe shifts in average weather over long periods of time.

GREENHOUSE EFFECT:

Natural phenomenon that traps heat near the Earth's surface by the action of greenhouse gases and that makes the Earth inhabitable by keeping its average surface temperature about 59°F (15°C) warmer than it would be without this action.

GREENHOUSE GAS:

Any gas, such as carbon dioxide, methane, water vapor, or ozone, which has the property of absorbing infrared radiation within the Earth's atmosphere, thus retaining heat near the Earth's surface and creating the greenhouse effect.

INFRARED RADIATION:

Energy emitted from all substances above absolute zero. We feel it as heat but do not see it.

OZONE:

An unstable form of oxygen that has three atoms per molecule rather than the normal two.

RADIATIVE FORCING:

The influence on the atmosphere produced by the greenhouse effect. Negative radiative forcing causes colder temperatures, positive causes warmer temperatures.

SOLAR IRRADIANCE:

The amount of sunlight striking a particular area.

SULFATE AEROSOLS:

Atmospheric particles created by the reaction of sulfur dioxide and other sulfide gases with a variety of substances. Sulfate aerosols reflect infrared radiation into space and thus contribute to global cooling.

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Is present global warming due more to human activity than to natural geologic trends