The Greenhouse Effect and Climate Change

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Earth scientists are in the midst of a revolution in their understanding of how climate change occurred in the past. Researchers are discovering the geological and astronomical forces that have changed the planet's environment from hot to cold, wet to dry, and back again, over hundreds of millions of years. Dramatic climate change is nothing new for planet Earth. The climate of the past 10,000 years, during which civilization developed, is a mere blip in a much bigger history of climate change. In fact, Earth's climate will most certainly continue to go through dramatic changes—even without the influence of human activity. Most scientists believe that, if not for extreme climate shifts some 65 million years ago, most of the animals on Earth today, including humans, would probably not be here.

Astronomical cycles have caused the climate to fluctuate between long periods of cold lasting 50,000 to 80,000 years and shorter periods of warmth lasting about 10,000 years. Some scientists believe that these geologically rapid shifts between warm and cold were the catalyst behind human evolution, forcing humans to adapt physiologically and socially in order to cope with the changing climate.


Earth's climate is a delicate balance of energy inputs, chemical processes, and physical phenomena. Temperatures on Venus are too hot for the human body; on Mars, they are too cold. This difference in temperature is due to the varying composition of each planet's atmosphere. All three planets receive huge quantities of solar energy, but the amount radiated back into space as heat depends on the atmospheric composition of the particular planet. Some gases, such as carbon dioxide (CO2) and methane, absorb and maintain heat in the same way that glass traps heat in a greenhouse. These gases in Earth's atmosphere allow temperatures to build up, keeping our planet warm and habitable. That is why the increased buildup of these and other gases caused by pollution is often called the "greenhouse effect." (See Figure 2.1.)

It is necessary, however, to distinguish between the "natural" and a possible "enhanced" greenhouse effect. The natural greenhouse effect creates a climate in which life can exist, causing the mean temperature of Earth's surface to be about 33 degrees Celsius (91 degrees Fahrenheit) warmer than it would be if natural greenhouse gases were not present. Without this process, Earth would be frigid and uninhabitable. An enhanced greenhouse effect, sometimes called the anthropogenic effect, refers to the possible increase in the temperature of Earth's surface due to human activity.

The scorching heat of Venus is the result of an atmosphere that is composed largely of CO2. The atmosphere of Earth, however, is nitrogen- and oxygen-based and contains only 0.03 percent CO2. This percentage has varied little over the past million years, permitting a stable climate favorable to life. The blanket of air enveloping Earth moderates its temperature and sustains life.

A Revolutionary Idea

Earth's atmosphere was first compared to a glass vessel in 1827 by the French mathematician Jean-Baptiste Fourier. In the 1850s British physicist John Tyndall tried to measure the heat-trapping properties of various components of the atmosphere. By the 1890s scientists had concluded that the great increase in combustion in the Industrial Revolution had the potential to change the atmosphere's load of CO2. In 1896 the Swedish chemist Svante Arrhenius made the revolutionary suggestion that human activities could actually disrupt this delicate balance. He theorized that the rapid increase in the use of coal that came with the Industrial Revolution could increase CO2 concentrations and cause a gradual rise in temperatures. For almost six decades his theory stirred little interest.

In 1957 studies at the Scripps Institute of Oceanography in California suggested that, indeed, half the CO2 released by industry was being permanently trapped in the atmosphere. The studies showed that atmospheric concentrations of CO2 in the previous 30 years were greater than in the previous two centuries and that the gas had reached its highest level in 160,000 years.

Findings in the 1980s and 1990s provided more disturbing evidence. Scientists detected increases in other, even more potent gases that contribute to the greenhouse effect, notably chlorofluorocarbons (CFC-11 and -12), methane, nitrous oxide (N2O), and halocarbons (CFCs, methyl chloroform, and hydrochlorofluorocarbons). Atmospheric concentrations of these gases from the 1700s through the 1900s have increased drastically. As shown in Table 2.1, the atmospheric concentration of CO2 increased from 280 parts per million (ppm) in preindustrial times to 370.3 ppm in 2001.

Table 2.2 shows trends for the late 1990s and early 2000s in U.S. greenhouse gas emissions and sinks (repositories, such as forests, that absorb and store carbon). Major sources of greenhouse gas emissions are shown in Figure 2.2. In 2001 electricity generation at power plants continued to account for the largest share (33 percent) of these emissions, followed by transportation (27 percent), industry (19 percent), agriculture (8 percent), commercial sources (7 percent), and residences (5 percent).

Emissions from most sources have increased since 1995. During the late 1990s emissions from industry began to decline and continued that trend into the early 2000s. The Environmental Protection Agency (EPA) attributes the decline to a shift in the overall U.S. economy from a focus on manufacturing industries to service-based businesses. Agricultural emissions are predominantly nitrogen-based, rather than carbon-based. Residential emissions are mainly due to CO2 generated from combustion of fossil fuels (such as oil) for heating purposes.

Scientists know that atmospheric levels of greenhouse gases are increasing. Figure 2.3 shows the dramatic increase in concentrations of CO2 from 1981 through 2002 based on data collected by the National Oceanic and Atmospheric Administration (NOAA) from its Climate Monitoring and Diagnostics Laboratory (CMDL). This buildup of CO2 and other gases could possibly trap energy from the Sun. No one is certain how this accumulation affects Earth's climate.

Is the Earth Getting Warmer?

As of 2004 even experts are not sure whether the world has already experienced human-induced climate change. Scientists have been unable to provide a definitive answer because they do not know how much the global climate has varied on its own in the relatively recent past (about 1,000 years). Temperature records based on thermometers go back only about 150 years. Investigators have turned, therefore, to "proxy" (indirect) means of

Atmospheric variableCO2CH4N2OSF61CF41
Pre-industrial atmospheric concentration2800.7220.270040
Atmospheric concentration2370.31.8420.3164.780
Rate of concentration change31.540.00740.00080.241.0
Atmospheric lifetime50–200512611463,200>50,000
1Concentrations in parts per trillion (ppt) and rate of concentration change in ppt/year.
2Concentration for CO2 was measured in 2001. Concentrations for all other gases were measured in 2000.
3Rate is calculated over the period 1990 to 1999.
4Rate has fluctuated between 0.9 and 2.8 ppm per year for CO 2 and between 0 and 0.013 ppm per year for CH4 over the period 1990 to 1999.
5No single lifetime can be defined for CO2 because of the different rates of uptake by different removal processes.
6This lifetime has been defined as an "adjustment time" that takes into account the indirect effect of the gas on its own residence time.
Note: Atmospheric concentrations are in parts per million (ppm) and rate of concentration change is parts per billion (ppb) per year.
source: "Table 1-1: Global Atmospheric Concentration (ppm Unless Otherwise Specified), Rate of Concentration Change (ppb/year) and Atmospheric Lifetime (Years) of Selected Greenhouse Gases," in Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2001, U.S. Environmental Protection Agency, Office of Atmospheric Programs, Washington, DC, April 15, 2003

measuring past temperatures. These methods include chemical evidence of climatic change contained in fossils, corals, ancient ice, and growth rings in trees.

In 1998 Drs. Michael E. Mann and Raymond S. Bradley of the University of Massachusetts at Amherst and Dr. Malcolm K. Hughes of the University of Arizona at Tucson surveyed proxy evidence of temperatures in the Northern Hemisphere since 1400. They discovered that the twentieth century was the warmest century of the past 600 years. They concluded that the warming trend seems to be closely connected to the emission of greenhouse gases by humans. Some experts, however, question whether studies of proxy evidence will ever be reliable enough to yield valuable information on global warming.

Three international agencies have compiled long-term data on surface temperatures—the British Meteorological Office in Bracknell, United Kingdom, the National Climatic Data Center in Asheville, North Carolina, and the National Aeronautics and Space Administration (NASA) Goddard Institute for Space Studies in New York. Temperature measurements from these organizations reported that the 1990s were the warmest decade of the twentieth century and the warmest decade since humans began measuring temperatures in the mid-nineteenth century. The average global surface temperature was approximately 1 degree Fahrenheit warmer than at the turn of the twentieth century, and this rise increased more rapidly since 1980.

According to NASA, the 2002 meteorological year was the second warmest year recorded since the late 1800s. (See Figure 2.4.) A meteorological year runs from the beginning of winter to the end of autumn. Thus, the 2002 meteorological year ran from December 1, 2001, to November 30, 2002. The mean surface temperature for that year was 0.51 degrees Celsius (33 degrees Fahrenheit) warmer than the climatological mean (average for 1951–1980). The warmest temperature occurred in 1998. There has been a strong warming trend over the past three decades.

While some scientists are uncertain whether the greenhouse effect accounts for the change, most believe that it is the most likely explanation. Climate models suggest the potential for a warming from 2 to 6 degrees Fahrenheit over the next 100 years, warmer than Earth has been for millions of years.

In December 2003 the council of the American Geophysical Union (AGU) issued a statement declaring that Earth's climate was warming faster than expected and blamed the increase on anthropogenic (human-related) emissions of greenhouse gases. The statement from the AGU, an international scientific organization with more than 40,000 members, declared that "scientific evidence strongly indicates that natural influences cannot explain the rapid increase in global near-surface temperatures observed in the second half of the 20th century" (David Perlman, "Earth Warming at Faster Pace, Say Top Science Group's Leaders," San Francisco Chronicle, December 18, 2003).

However, some scientists observe that major climate events should be viewed in terms of thousands of years, not just a century. A record of only the past century may indicate, but not prove, that a major change has occurred. Is it caused by greenhouse gases, or is it natural variability? While some experts believe it is not possible to conclude that the warming is caused by greenhouse gases emitted by human activity, the rising temperature is roughly on track with that of computer models programmed to predict the course of greenhouse warming.


In 1990, at the first of several meetings of the Intergovernmental Panel on Climate Change (IPCC), several early signs of actual climate change were noted: the average warm-season temperature in Alaska had risen nearly three degrees Fahrenheit in the previous fifty years; glaciers had generally receded and become thinner on average by about thirty feet in the past forty years; there was about 5 percent less sea ice in the Bering Sea than in the 1950s; and permafrost was thawing, causing

Fossil fuel combustion4,814.85,141.55,325.85,400.05,420.55,488.85,692.25,614.9
Iron and steel production85.474.468.371.967.464.465.859.1
Cement manufacture33.336.837.138.339.
Waste combustion14.118.519.421.222.523.925.426.9
Ammonia manufacture & urea application19.320.520.320.721.920.619.616.6
Lime manufacture11.212.813.513.713.913.513.312.9
Natural gas flaring5.
Limestone and dolomite use5.
Aluminum production6.
Soda ash manufacture and consumption4.
Titanium dioxide production1.
Carbon dioxide consumption0.
Land-use change and forestry (Sink)1(1,072.8)(1,064.2)(1,061.0)(840.6)(830.5)(841.1)(834.6)(838.1)
International bunker fuels2113.9101.0102.3109.9112.9105.399.397.3
Natural gas systems122.0127.2127.4126.0124.0120.3121.2117.3
Enteric fermentation117.9123.0120.5118.3116.7116.6115.7114.8
Coal mining87.173.568.468.167.963.760.960.7
Manure management31.336.234.936.639.038.938.238.9
Wastewater treatment24.126.626.827.327.728.228.328.3
Petroleum systems27.524.223.923.622.921.621.221.2
Rice cultivation7.
Stationary sources8.
Mobile sources5.
Petrochemical production1.
Field burning of agricultural residues0.
Silicon carbide production********
International bunker fuels20.
Agricultural soil management267.5284.1293.2298.2299.2297.0294.6294.3
Mobile sources50.660.960.760.359.758.857.554.8
Manure management16.216.617.017.317.317.417.918.0
Nitric acid17.819.920.721.220.920.119.117.6
Human sewage12.713.914.114.414.615.115.115.3
Stationary combustion12.513.213.813.713.713.714.314.2
Adipic acid15.
N2O product usage4.
Field burning of agricultural residues0.
Waste combustion0.
International bunker fuels21.
HFCs, PFCs, and SF694.499.5113.6116.8127.6120.3121.0111.0
Substitution of ozone depleting substances0.921.730.437.744.550.957.363.7
HCFC-22 production35.
Electrical transmission and distribution32.127.527.725.220.916.415.415.3
Semiconductor manufacture2.
Aluminum production18.111.812.511.
Magnesium production and processing5.
Net emissions (sources and sinks)5,066.85,450.75,646.05,942.05,970.96,008.56,212.76,098.1
*Does not exceed 0.05 Tg CO2 Eq.
1For the most recent years, a portion of the sink estimate is based on historical and projected data. Parentheses indicate negative values (or sequestration).
2Emissions from international bunker fuels are not included in totals.
Note: Totals may not sum due to independent rounding.
source: "Table ES-1: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg CO2 Eq.)," in Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2001, U.S. Environmental Protection Agency, Office of Atmospheric Programs, Washington, DC, April 15, 2003

the ground to subside, opening holes in roads, producing landslides and erosion, threatening roads and bridges, and causing local floods. Ice cellars in northern villages have thawed and become useless. More precipitation now falls as rain than snow, and the snow melts faster, causing more running and standing water. While this could be natural variability, it is the kind of change expected of global warming, that is, the Arctic will warm more than the global average.

According to the EPA, global sea levels have risen by six to eight inches over the last century. The increase is attributed to melting mountain glaciers, expansion of ocean water in response to rising temperatures, melting of the polar ice sheets, and surface discharge of groundwater that has been pumped out of the ground. The sea level along much of the U.S. coast has risen by ten to twelve inches per century, although the rate varies by location.

Melting glaciers and collapsing ice shelves in Antarctica have been well publicized, particularly the breakup of the Larsen B Ice Shelf in early 2002. Scientists at the U.S. Geological Survey estimate that melting of Antarctica's entire ice sheet will increase the sea level by 73 meters (approximately 240 feet). However, it is unknown whether the ice sheet is actually shrinking or growing.

The Effects of Volcanic Activity on Climate

Volcanic activity, such as the 1991 eruption of the Mount Pinatubo volcano in the Philippines, can temporarily offset recent global warming trends. Volcanoes spew vast quantities of particles and gases into the atmosphere, including sulfur dioxide (SO2) that combines with water to form tiny supercooled droplets. The droplets create a long-lasting global haze that reflects and scatters sunlight, reducing energy from the Sun and preventing its rays from heating the Earth, thereby causing the planet to cool.

This also occurred in 1982, when the El Chichon volcano in Mexico depressed global temperatures for about four years. NASA reported that satellite sensors measured the SO2 cloud from Mount Pinatubo at 15 million tons, about twice the size of the one emitted by El Chichon. NASA found that the haze of sulfur from the eruption reflected enough sunlight to cool the Earth by about 1 degree Fahrenheit, as was predicted by computer models.

In 1815 a major eruption of the Tambora volcano in Indonesia produced serious weather-related disruptions, such as crop-killing summer frosts in the United States and Canada. It became known as the "year without a summer." On the other hand, for several years following the Tambora eruption people around the world commented about the beautiful sunsets, which were caused by the suspension of volcano-related particulate matter in the atmosphere.

The Effects of Clouds on Climate

Clouds may hold a key to understanding climate change. Although we see clouds virtually every day, surprisingly little is known about them—where they occur, their role in energy and water transfer, and their ability to reflect solar heat. Earth's climate maintains a balance between the energy that reaches Earth from the Sun and the energy that radiates back from Earth into space. Scientists refer to this as Earth's "radiation budget." The components of Earth's system are the planet's surface, atmosphere, and clouds.

Different parts of Earth have different capacities to reflect solar energy. Oceans and rain forests reflect only a small portion of the Sun's energy. Deserts and clouds, on the other hand, reflect a large portion of solar energy. A cloud reflects more radiation back into space than the surface would in the absence of clouds. An increase in cloudiness can also act like the panels on a greenhouse roof.

NASA's Earth Science Enterprise is a satellite-based program that includes numerous scientific studies of clouds. These studies have revealed that:

  • The effect of clouds on climate depends on the balance between the incoming solar radiation and the absorption of Earth's outgoing radiation.
  • Low clouds have a cooling effect because they are optically thicker and reflect much of the incoming solar radiation out to space.
  • High thin cirrus clouds have a warming effect because they transmit most of the incoming solar radiation while also trapping some of Earth's radiation and radiating it back to the surface.
  • Deep convective clouds have neither a warming nor a cooling effect because their reflective and absorptive abilities cancel one another.

Another Possible Climate Culprit—Solar Cycles

Scientists have known for centuries that the Sun goes through cycles; it has seasons, storms, and rhythms of activity with sunspots and flares appearing in cycles of roughly 11 years. Some scientists contend that these factors play a role in climate change on Earth. Some research, though sketchy and controversial, suggests that the Sun's variability could account for some, if not all, of global warming to date. The biggest correlation occurred centuries ago—between 1640 and 1720—when sunspot activity fell sharply and Earth cooled about 2 degrees Fahrenheit. (The Sun is brighter when sunspots appear and dimmer when they disappear.)

The Effects of the Oceans on Climate

Oceans have a profound effect on climate, because of their huge capacity to store heat and because they can moderate levels of atmospheric gases that regulate global temperatures. Covering more than 70 percent of Earth and holding 97 percent of the water on the planet's surface, oceans function as huge reservoirs of heat. Ocean currents transport this stored heat and dissolved gases so that different areas of the world serve as either sources or sinks (repositories) for these components. While scientists know a great deal about oceanic and air circulation, they are less certain about the ocean's ability to store additional CO2 or about how much heat it will absorb.

The top eight feet of the oceans hold as much heat as Earth's entire atmosphere. As ocean waters circulate globally, heat is transferred from low altitudes to high altitudes, from north to south, and vertically from surface to deep oceans and back. But how is this heat apportioned? If heat is able to circulate through the entire oceanic depth range, the process could take centuries and the world's oceans could serve to buffer or delay global warming. Researchers are working to determine that possibility, but it remains one of many unanswered questions.

The Effects of El Niño and La Niña on Climate

For centuries fishermen in the Pacific Ocean off the coast of South America have known about the phenomenon called El Niño. Every three to five years, during December and January, fish in those waters virtually vanish, bringing fishing to a standstill. Fishermen gave this occurrence the name "El Niño," which means "the Child," because it occurs around the celebration of the birth of Jesus, the Christ child. Although originating in the Pacific, the effects of El Niño are felt around the world. Computers, satellites, and improved data gathering have found that the El Niño phenomenon has been responsible for drastic climate change.

An El Niño occurs because of interactions between atmospheric winds and sea surfaces. In normal years, trade winds blow from east to west across the eastern Pacific. They drag the surface waters westward across the ocean, causing deeper, cold waters to rise to the surface. This upwelling of deep ocean waters carries nutrients from the bottom of the ocean that feed fish populations in the upper waters.

In an El Niño the westward movement of waters weakens, causing the upwelling of deep waters to cease. The resulting warming of the ocean waters further weakens trade winds and strengthens El Niño. Without upwelling, the nutrient content of deep waters is diminished, which in turn causes the depletion of fish populations. The warm waters that normally lie in the western waters of the Pacific shift eastward. This turbulence creates eastward weather conditions, in which towering cumulus clouds reach high into the atmosphere with strong vertical forces and the weakening of normal east-to-west trade winds. An El Niño is the warm phase of a phenomenon known as ENSO (El Niño/Southern Oscillation), which can also include a cold phase known as a La Niña.

The worldwide effects of El Niño can include torrential rains, tornadoes, hurricanes, mud slides, flash flooding, beach and cliff erosion, sewage spills, drought, increased snowfall, and disruption in the marine food chain. Such weather events often affect regional energy and economic markets. The El Niño of 1982–83 is estimated to have caused $13.6 billion in damage and killed 2,000 people around the world. In 1997 and 1998 El Niño–related storms in California caused more than 9,000 people to seek federal disaster assistance for property losses. An estimated $3.6 million in aid was distributed to approximately 1,689 people. The American Fisheries Society estimated that 1990s storms were the most devastating for marine life in more than a century.


Rising Sea Level

Some observers compare global warming to nuclear war in its potential to disrupt human and environmental systems. While some sources dispute the occurrence of human-induced climate change, if temperature increases of 1.5 to 4.5 degrees Celsius (35 to 40 degrees Fahrenheit) were to take place, substantial changes would occur on Earth's surface. If average temperatures rise this much by 2030, the global sea level could rise 20 to 140 centimeters (8 to 56 inches). The Climate Institute in Washington, D.C., forecasts a further rise of 26 inches by 2100. This rise would be caused by the expansion of seawater as it is warmed, along with melting glaciers and ice caps.

More than half the population of the United States lives within 50 miles of a coastline. A rising sea level would narrow or destroy beaches, flood wetland areas, and either submerge or force costly fortification of shoreline property. Figure 2.5 shows areas along the Southeast coast that could be vulnerable to rising sea levels because they lie at low elevations.

Higher water levels would increase storm damage and many coastal cities worldwide would be flooded. Some islands, such as the Philippines, have already seen encroachment. Residents of the Maldives, islands which lie on average three feet above sea level in the Indian Ocean, are already erecting artificial defenses, such as breakwaters made of concrete, to fend off rising seas.

Some 70 million people in low-lying areas of Bangladesh could be displaced by a one-meter (39.4 inches) rise. Such a rise would also threaten Tokyo, Osaka, and Nagoya in Japan, as well as coastal China and the Atlantic and Gulf Coasts of the United States. The rising waters would also intrude on inland rivers, threatening fresh water supplies and increasing the salt content of groundwater as the sea encroaches on freshwater aquifers (naturally occurring underground water reservoirs). Much of the increased rainfall would come not in the steady, gentle rains favored by farmers, but from heavy storms and flooding.

Increased Heat

Warmer temperatures may also increase the evaporation rate, thereby increasing atmospheric water vapor and cloud cover, which in turn may affect regional rainfall patterns. A warmer climate would likely shift the rain belt of the middle latitudes toward the poles as water-laden air of the tropics travels further toward the poles before the moisture condenses as precipitation. This would shift patterns of rainfall around the world. Wetter, more violent weather is projected for some regions. Forests, which are adapted to a narrow temperature and moisture range, would particularly be threatened by climate shifts.

The opposite problem—too little water—could worsen in arid areas such as the Middle East and parts of Africa. Some experts have suggested that the greenhouse effect and global warming are mild terms for a coming era that may be marked by heat waves that may make some regions virtually uninhabitable. Frequent droughts could plague North America and Asia, imperiling food production, as agriculture is particularly vulnerable to the effects of climate change. Most experts believe Africa will be the most vulnerable to climate change because its economy depends largely on rain-fed agriculture, and many farmers are too poor and ill equipped to adapt. Australia and Latin America could also be subject to severe drought.

NASA has begun using computer calculations in an attempt to predict the effects of global warming on some of the major cities in the United States.

The Effect of Increased Temperatures on Humans

Extra heat alone would be enough to kill some people. Some deaths would occur directly from heat-induced strokes and heart attacks. Air quality also deteriorates as temperatures rise. Hot, stagnant air contributes to the formation of atmospheric ozone, the main component of smog, which damages human lungs. Poor air quality can also aggravate asthma and other respiratory diseases. Increasing ultraviolet rays can increase the incidence of skin cancers, diminish the function of the human immune system, and cause eye problems such as cataracts. Higher temperatures and added rainfall could create ideal conditions for the spread of a host of infectious diseases by insects, including mosquito-borne malaria, dengue fever, and encephalitis.

Decreasing Biological Diversity

Biological diversity is also predicted to suffer from global warming. Loss of forests, tundra (arctic plains), and wetlands could irrevocably damage ecosystems. Some species that live in precise, narrow bands of temperature and humidity may find their habitats wiped out altogether. Rising seas would cover coastal mangrove swamps, causing the loss of many species, including the Bengal tiger. Plants and animals of the far north, like the polar bear and the walrus, would die out for lack of an acceptably cold environment. Many species cannot migrate rapidly enough to cope with climate change at the projected rate. Opportunistic species, such as weeds, often out-compete trees and other plants more valued by humans. Pests, such as certain insects, may survive where other species cannot.

The U.S. Forest Service believes that eastern hemlock, yellow birch, beech, and sugar maple forests would gradually shift their ranges northward by 300 to 600 miles but would be severely limited by the warming and largely die out, along with the wildlife they shelter. Studies by World Wildlife Fund International report that more than half the world's parks and reserves could be threatened by climate change. These include the Florida Everglades, Yellowstone National Park, the Great Smoky Mountains, and Redwood National Park in California. The EPA warned in 1988: "If current trends continue, it is likely that climate may change too quickly for many natural systems to adapt."


There are several possible positive consequences of global warming. Agriculture in the northern United States and southern Canada, on the West Coast, and in interior parts of the West could benefit, as could the evergreen forests of the West Coast. Milder winters could reduce the number of cold-weather deaths, as well as the cost of snow clearance and heating. Northern waters could remain open longer for navigation, and the Arctic Ocean might become ice-free, opening a new trade route between Europe and Asia.


The science of global atmospheric change is still in its infancy. Most of our images of the world's environmental future must come from computer and mathematical models. The most highly developed tools now available to project climatic changes are complex computer models called general circulation models (GCMs). Even these models are crude, imperfect representations of the real world and of the future world. They cannot indicate where changes will be the worst. Nor can they accurately calculate the heat reflected by clouds or absorbed by the ocean. As a result, scientists disagree over whether the forecasts of global warming are reliable.

Although computer models have become more accurate over the years, important uncertainties limit their predictive abilities. Even the most powerful computers are limited in their ability to store and analyze the vast quantity of data required to accurately simulate the global climate. Modelers have tried to overcome these limitations by introducing assumptions into their models that deliberately oversimplify some operations to free the GCMs' capacity for more critical operations. For example, modelers have assumed that the ocean was not warmed by emissions of greenhouse gases before 1985. Although this assumption increases GCM capacity, it increases the uncertainty of the computer's predictions because the ocean will reach its capacity to absorb emissions sooner. Scientists do not know, however, how much or by how long the predictions are distorted because of this error.

In Global Deception: The Exaggeration of the Global Warming Threat (Washington, DC: Center for the Study of American Business, Washington University, 1997), Patrick J. Michaels claims that the global warming crisis has been greatly overstated. He notes that newer GCMs, which are more physically realistic than the older models, tend to forecast less warming rather than more.


The Energy Information Administration of the U.S. Department of Energy (DOE) reported that CO2 accounted for 83 percent of greenhouse gas emissions in the United States in 2001. (See Figure 2.6.) Methane (CH4) was second with 9 percent of the total, followed by nitrous oxide (N2O) with 6 percent and other greenhouse gases, such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6), with 2 percent.

Carbon Dioxide

Carbon dioxide (CO2) is a gas that is a naturally occurring component of Earth's atmosphere. It is also produced by a variety of human and animal sources. These factors are a part of the carbon cycle. (See Figure 2.7.) As shown in Figure 2.6, CO2 was the most prevalent of the greenhouse gases emitted from 1990 to 2001.

The burning of fossil fuels by industry and motor vehicles is, by far, the leading source of CO2 in the atmosphere, accounting for 97 percent of the nation's emission of greenhouse gases in 2001. (See Table 2.2.) As populations and economies expand, they use ever-greater amounts of fossil fuels.

The DOE released its International Energy Annual 2001 in March 2003. The report presents data collected on CO2 emissions related to fossil fuel use around the world. The United States was responsible for the largest portion (23 percent) of such emissions in 2001, followed by Western Europe (16 percent) and China (13 percent). (See Figure 2.8.) The western European countries responsible for most of that region's CO2 emissions were Germany, the United Kingdom, Italy, and France.

The DOE predicts that CO2 emissions from developing countries could actually surpass those from industrialized countries before 2020. (See Figure 2.9.) This trend is attributed to increased use of coal in developing countries, particularly China and India, while developed countries rely increasingly on natural gas.


Forests act as sinks, or repositories, absorbing and storing carbon. Trees naturally absorb and neutralize CO2, although scientists do not agree on the extent to which forests can soak up excess amounts. The increasing levels of CO2 in the atmosphere might conceivably be tolerated in Earth's normal CO2 cycle if not for the additional complicating factor of deforestation. The burning of the Amazon rain forests and other forests has had a twofold effect: the immediate release of large amounts of CO2 into the atmosphere from the fires, and the loss of trees to neutralize the CO2 in the atmosphere. (See Figure 2.10.)


The oceans are, by far, the largest reservoir of carbon in the carbon cycle. The oceans hold approximately 50 times more carbon than the atmosphere and 20 times more than the terrestrial reservoir (land). Oceanographers and ecologists disagree over the carbon cycle–climate connection and over the ocean's capacity to absorb CO2. Some scientists believe that the oceans can absorb one to two billion tons of CO2 a year, about the amount the world emitted in 1950. Until scientists can more accurately determine how much CO2 can be buffered by ocean processes, the extent and speed of disruption in the carbon supply remains unclear.


Methane (CH4) is an important component of greenhouse emissions, second only to CO2. (See Figure 2.6.) While there is less methane in the atmosphere, scientists estimate that it may be 21 times more effective at trapping heat in the atmosphere than CO2. Over the past two centuries, methane's concentration in the atmosphere has more than doubled and scientists generally attribute those increases to human sources, such as landfills, natural gas systems, agricultural activities, coal mining, and wastewater treatment. However, according to the Energy Information Administration, U.S. emissions of methane from energy and waste management sources actually declined between 1990 and 2002, while agricultural emissions increased over the same time period. (See Figure 2.11.) The increase is blamed on domestic animals and the decomposition of their waste, which releases methane. Growing markets for beef and milk products are driving a booming livestock business.

Nitrous Oxide

Nitrous oxide (N2O) is formed by natural biological sources and by a number of human activities. Although N2O makes up a much smaller portion of greenhouse gases than CO2, it is much more (perhaps 310 times more) powerful than CO2 at trapping heat. Figure 2.12 shows that agriculture is and has been the major source of N2O emissions in the United States, followed by energy and industrial sources. Specific agricultural sources of N2O emissions are shown in Figure 2.13.


Other primary greenhouse gases are what the DOE calls "engineered gases." These are gases specially designed for modern industrial and commercial purposes. They include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6).

HFCs are chemicals that contain hydrogen, fluorine, and carbon. They are popular substitutes in industrial applications for chlorofluorocarbons (CFCs). CFCs are commonly used in cooling equipment, fire extinguishers, as propellants, and for other uses. They are blamed for depletion of the ozone layer in the stratosphere, which shields the Earth from deadly ultraviolet radiation.

PFCs are a class of chemicals containing fluorine and carbon. They also are increasingly used by industry as substitutes for ozone-depleting CFCs. SF6 is a colorless, odorless gas commonly used as an insulating medium in electrical equipment and as an etchant (an etching agent) in the semiconductor industry.

Although emissions of these chemicals are very small in comparison to other greenhouse gases, they are of particular concern because of their long life in the atmosphere. PFCs and SF6 have atmospheric lifetimes of thousands of years and are actually far more potent greenhouse gases than CO2 per unit of molecular weight.


Global warming was only acknowledged as an international problem at the end of the twentieth century. At the world's first ecological summit, the 1972 Stockholm Conference, climate change was not even listed among the threats to society.

Climate-change science has been developing rapidly, leading observers to recognize the complexity of the issues. In order to understand the issue of climate change and any possible global warming, scientists worldwide gathered together in 1990 to exchange information. They formed the International Panel on Climate Change (IPCC), which is sponsored jointly by the United Nations Environment Program (UNEP) and the World Meteorological Organization. The panel's mission was to advise the parties to the 1992 Global Warming Treaty. It received the input of nearly 400 of the world's climate experts from 25 nations.

The IPCC report was the most comprehensive summary of climate-change science to date. With regard to various statements about climate change, the scientists responded based on how sure they were of global warming: "virtually certain" (nearly unanimous agreement and no credible alternative), "very probable" (roughly a 90 percent likelihood of occurring), "probable" (a two out of three chance of happening), and "uncertain" (evidence does not support). The outcome indicated an international consensus on three basic statements related to global warming: that greenhouse gases do produce climate change, that there is a natural greenhouse effect, and that emissions resulting from human activities are contributing to the proliferation and effects of greenhouse gases.

A Landmark Judgment—The 1995 IPCC Report

In 1995 the IPCC reassessed the state of knowledge about climate change. The panel reaffirmed its earlier conclusions and updated its forecasts, predicting that, if no further action is taken to curb emissions of greenhouse gases, temperatures will increase 1.44 degrees to 6.30 degrees Fahrenheit by 2100. The panel concluded that the evidence suggested a human influence on global climate. The cautiously worded statement was a compromise following intense discussions. Nonetheless, it was a landmark conclusion because the panel, until then, had always said that global warming and climate changes could have been the result of natural variability. Despite acrimonious debate and challenges to the report, the conclusions have largely held up to scientific scrutiny.

The 2001 Update

The IPCC issued its third assessment report in 2001. It actually comprises four reports: Climate Change 2001: The Scientific Basis, Climate Change 2001: Impacts, Adaptation and Vulnerability, Climate Change 2001: Mitigation, and Climate Change 2001: The Synthesis Report. The IPCC's assessment covered the adaptability and vulnerability of North America to climate change impacts likely to occur from global warming. Among the suggested possible effects of global warming are:

  • Expansion of some diseases in North America
  • In coastal areas, increased erosion, flooding, and loss of wetlands
  • Risk to "unique natural ecosystems"
  • Changes in seasonal snowmelts, which would have effects on water users and aquatic ecosystems
  • Some initial benefits for agriculture, but those benefits would decline over time and possibly "become a net loss"

Next IPCC Update Due in 2007

The IPCC plans to publish its fourth major assessment report in 2007. At meetings held in Paris during 2003 seven themes were selected for focus in the report:

  • risk and uncertainty
  • regional integration
  • water
  • key vulnerabilities
  • adaptation and mitigation
  • sustainable development
  • technology


In the Swiss Review of World Affairs (July 1994), Dr. Richard Lindzen, professor of meteorology at the Massachusetts Institute of Technology, observed that a solid scientific foundation is lacking from claims that global warming is imminent. He believes that Earth's climate depends on more than the CO2 content. Instead, Earth cools by atmospheric movements upward and poleward rather than only by radiation; and climate changes since 1850 are indistinguishable from natural variability. He further claims that no large-scale model includes all the known major factors in the water vapor cycle. In 1997 Dr. Lindzen added that the natural system has built-in resilience and that the observed changes are insignificant and not urgent.

In October 2003 Anders Sivertsson, Kjell Aleklett, and Colin Campbell, researchers from the University of Uppsala in Sweden, disputed the IPCC's predictions of extreme temperature change due to CO2 emissions. According to Andy Coghlan in his article "'Too Little' Oil for Global Warming," in the journal New Scientist, the researchers noted that their data show that the world's oil and gas supplies will be depleted long before atmospheric CO2 concentrations build to sufficient levels to make a major difference in Earth's climate.

Among the claims of critics of global climate warming are:

  • The increase in the Earth's surface temperature during the past 150 years is less than the best existing climate models can explain. As the models improve, however, they predict less and less warming.
  • Climate has been known to change dramatically within a relatively short period without any human influence.
  • Temperature readings already showed increased temperatures before CO2 levels rose significantly (before 1940).
  • Natural variations in climate may exceed any human-caused climate change.
  • Some of the increase in temperatures can be attributed to sunspot activity.
  • If warming should occur, it will not stress Earth; it may even have benefits, such as for agriculture, and may delay the next ice age.
  • Reducing emissions will raise energy prices, reduce gross domestic product, and produce job losses.
  • While clouds are crucial to climate predictions, so little is known about them that computer models cannot produce accurate predictions.

In 2000 a handful of scientists at the Oregon Institute of Science and Medicine began a drive to collect the signatures of other scientists on a petition reading in part: "There is no convincing scientific evidence that human release of carbon dioxide, methane, or other greenhouse gasses is causing or will, in the foreseeable future, cause catastrophic heating of the Earth's atmosphere and disruption of the Earth's climate. Moreover, there is substantial scientific evidence that increases in atmospheric carbon dioxide produce many beneficial effects on the natural plant and animal environments of the Earth." As of April 2004 the so-called Petition Project claims to have collected more than 19,500 signatures, including 17,100 from scientists, mostly with advanced degrees.


United States

Many industrialized countries have committed themselves to stabilizing or reducing CO2 emissions. President George H. W. Bush's administration (1989–1993) opposed precise deadlines for CO2 limits, arguing that the extent of the problem was too uncertain to justify painful economic measures. However, in 1989 the U.S. Global Change Research Program (USGCRP) was established and later authorized by Congress in the Global Change Research Act of 1990.

When President Bill Clinton took office in 1993, he joined the European Community in calling for overall emissions to be stabilized at 1990 levels by 2000, but this goal was not met. In October 1993 the United States, under the United Nations (UN) Framework Convention on Climate Change, released The Climate Change Action Plan detailing the nation's response to climate change. The plan included a set of measures by both government and the private sector to lay a foundation for the nation's participation in world response to the climate challenge.

The measures called for under the Action Plan would reduce emissions for all greenhouse gases to 1990 levels by 2000. However, since the time the projections were prepared and the Action Plan was published, the economy grew at a more robust rate than anticipated, which led to increased emissions. Furthermore, the U.S. Congress did not provide full funding for the actions contained in the plan.

At the time, the U.S. global warming program was coordinated through the Committee on Earth and Environmental Science within the Office of Science and Technology Policy of the White House. Some eighteen federal agencies were represented in the multiyear, multibillion-dollar research program, nine of which received the bulk of funding. NASA received 66 percent, and the National Science Foundation (NSF) was provided 10 percent. Rounding out the top nine were the Smithsonian Institution and a half dozen federal agencies, including the EPA and the Departments of Agriculture, Commerce, Defense, Energy, and the Interior.

Even though the United States had a comprehensive global warming program in place, Congress was reluctant to take steps to reduce emissions. However, the Clinton administration implemented some policies that did not require congressional approval. In 1999 President Clinton made a number of executive orders and began several new initiatives. These included the Climate Change Technology initiative, a five-year, $6.3 billion program of tax incentives and investments focusing on improving energy efficiency and renewable energy technologies; the Wind Powering America initiative, seeking to supply 5 percent of the nation's electricity through wind technology by 2020; and a Brightfields initiative, aimed at using former industrial sites contaminated with toxic waste for producing pollution-free solar energy. Executive Order 13134 aimed at coordinating federal efforts to develop technology to grow the economy while simultaneously solving some environmental problems, such as converting crops, trees, and other "biomass" into fuels, power, and products. Executive Order 13123 required all federal government agencies to reduce greenhouse gas emissions below 1990 levels by 2010. The federal government was the largest energy consumer in the nation, and the Clinton administration wanted it to serve as an example to the nation's businesses and consumers, who ultimately could reap benefits from energy improvements. President Clinton also established the U.S. Climate Change Research Initiative to study areas of uncertainty about global climate change science and identify priorities for public investments.

After President George W. Bush took office in 2001 he established a new cabinet-level management structure to oversee government investments in climate change science and technology. Both the U.S. Climate Change Research Initiative and the USGCRP were placed under the oversight of the interagency Climate Change Science Program (CCSP), which reports integrated research sponsored by thirteen federal agencies. The CCSP is overseen by the Office of Science and Technology Policy, the Council on Environmental Quality, and the Office of Management and Budget.

In June 2002 the George W. Bush administration released the third National Communication of the United States of America under the UN Framework Convention on Climate Change. The U.S. Climate Action Report2002 acknowledges that greenhouse gases resulting from human activities are accumulating in the atmosphere and that they are causing air and ocean temperatures to increase. However, it does not rule out the still unknown role of natural variability in global warming. In addition, the report reiterates that the administration plans to reduce the nation's greenhouse gas intensity by 18 percent over the following decade through a combination of existing regulations and voluntary, incentive-based measures. In November of that year the CCSP publication Our Changing Planet was issued as a supplement to the President's 2003 fiscal year budget. According to this document the administration's overall approach to climate change is "to achieve both environmental protection and a healthy economy, based on the best possible information for action."

In July 2003 the CCSP published two major reports: Strategic Plan for the U.S. Climate Change Science Program and The U.S. Climate Change Science Program: Vision for the Program and Highlights of the Scientific Strategic Plan. Together these documents outline the approach the CCSP plans to take to achieve its five main scientific goals:

  • Improve knowledge of the Earth's past and present climate and environment, including its natural variability, and improve understanding of the causes of observed variability and change
  • Improve quantification of the forces bringing about changes in the Earth's climate and related systems
  • Reduce uncertainty in projections of how the Earth's climate and related systems may change in the future
  • Understand the sensitivity and adaptability of different natural and managed ecosystems and human systems to climate and related global changes
  • Explore the uses and identify the limits of evolving knowledge to manage risks and opportunities related to climate variability and change

The CCSP illustrates that climate change is a complex issue and involves a wide range of natural and anthropogenic (human-related) factors as shown in Figure 2.14.

Other Nations

Saudi Arabia and several other oil producing nations have resisted the setting of new targets for emissions because they fear this will reduce the demand for their oil. On the other hand, many environmentalists believe that reducing pollution depends on increasing energy efficiency and gradually switching from fossil fuels to renewable energy, an idea most industrialized countries have been slow to embrace.

China's ongoing economic revolution may lead that nation to become the world's largest contributor to global warming. China is burning increasing amounts of coal and is expected to become the single greatest creator of greenhouse gases in the coming decades. China's leaders indicate that they are well aware that coal burning causes pollution, but the Chinese government has made it clear that it will not sacrifice development for the environment. Chinese leaders argue that it is unfair to impose constraints on China or developing nations when Western countries have been willfully polluting the environment for more than a century, pointing out that China has only recently become a significant offender. Critics of China's argument note that China itself would be harmed by global warming, which could dry up crops, shrink water supplies, and cause the flooding of major coastal cities.

In 1992 representatives of thirty-seven island nations, which make up the Alliance of Small Island States, drafted an agreement to present to the Earth Summit. These nations, including Cyprus and Malta (in the Mediterranean) and the Caribbean islands, fear their existence is threatened by the rising sea level. They claim they will be the first victims of global warming, becoming a whole new category of environmental refugees. Tourist beaches are shrinking, dikes are being erected to protect reclaimed land, and some islands have already been evacuated. Since they are so vulnerable, these nations feel an urgency not felt by the Northern industrial countries and the larger developing nations. Nonetheless, larger countries may have similar incentives to stabilize sea levels. A three-foot rise in ocean levels would render an estimated 72 million people homeless in China, 11 million in Bangladesh, and 8 million in Egypt.

A Global Warming Treaty

In 1992, 143 countries approved a UN global warming treaty in Rio de Janeiro, Brazil, that committed them to reduce the amount of greenhouse gases emitted into the atmosphere. Many environmentalists criticized the treaty as too weak because it did not establish specific targets that governments must meet. The treaty did not include specific targets mainly because then-President George H. W. Bush, representing the United States, refused to accept them. President Clinton signed the treaty in 1994, although adherence to the environmental measures has largely been disappointing even among those nations that originally signed.

Supporters of a global warming strategy advocate limiting the emissions of the four main greenhouse gases and recommend a gradual transition away from fossil fuels, which currently provide about three-quarters of the world's energy.

In 1995, 120 parties to the global warming treaty met in Berlin in what is known as the Berlin Mandate to determine the success of existing treaties and to embark on discussions of emissions after 2000. Differences persisted along North-South lines, with developing countries making essentially a moral argument for requiring more of the richer nations. They pointed out that the richer nations are responsible for most of the pollution. The Berlin talks essentially failed to endorse binding timetables for reductions in greenhouse gases.

The Kyoto Protocol—Rich versus Poor Countries

In December 1997 delegates from 166 countries met in Kyoto, Japan, at the UN Climate Change Conference to negotiate actions to reduce global warming. The task was more complicated and difficult than was envisioned in 1995, when parties to the 1992 Rio de Janeiro treaty on climate change decided that stronger action was needed. What was originally envisioned as a matter of deciding on a reduction target and a timetable for industrialized countries once again broadened into a contentious debate between developed and developing countries as to the proper role of each.

Some developed nations, including the United States, wanted to require all countries to reduce their emissions. However, developing countries felt the industrialized nations had caused, and were still causing, most global warming and therefore should bear the brunt of economic sacrifices to clean up the environment. Even within the industrialized community, the European Union criticized the United States for lagging behind in reducing emissions, as it had previously pledged.

Different targets for different countries, tailored to their economic and social circumstances, emerged as a possible way to get around the impasse between nations. The final compromise, signed by the parties, was for the industrialized nations to cut emissions by an average of 5.2 percent between 2008 and 2012. The treaty also set up an emission trading system that would allow countries that exceed their pollution limits to purchase on an open market "credits" from countries that pollute less. This provision was viewed as necessary to U.S. congressional approval. The developing nations feared that such a trading system would allow rich countries to buy their way

Very wellFairly well %Not very well %Not at all %No opinion%
2004 Mar 8–111850266*
2003 Mar 3–51553275-
2002 Mar 4–71752256*
2001 Mar 5–715542461
1997 Nov 6–9164528101
1992 Jan114222223
source: "Next, thinking about the issue of global warming, sometimes called the 'greenhouse effect,' how well do you feel you understand this issue—would you say very well, fairly well, not very well, or not at all?," in Poll Topics and Trends: Environment, The Gallup Organization, Princeton, NJ, March 17, 2004 [Online] [accessed March 30, 2004]

into compliance rather than make unpopular emissions cuts. Enforcement mechanisms were not agreed to, nor did developing nations commit to binding participation.

The U.S. Congress never ratified the treaty. In March 2001 President George W. Bush indicated that the United States would pull out of the treaty because it would cost an estimated $400 billion and 4.9 million jobs to comply. The treaty was ratified by the European Union and Japan in June 2002.


In March 2004 the Gallup Organization conducted its annual poll on topics related to the environment. Participants were asked several questions about global warming and the Kyoto Protocol.

As shown in Table 2.3 more than a quarter of those asked admitted they did not have a good understanding of the global warming issue. Half of those asked said they understood the issue fairly well. Another 18 percent felt they had a very good understanding about the issue. In 1992, 22 percent of people responded that they did not understand the issue at all, but this had dropped to 6 percent in 2004.

Table 2.4 shows how poll participants view the timeliness of global warming. Just over half (51 percent) of those asked in 2004 believe that global warming has already begun, while 17 percent feel that global warming will occur within the next few years or within their lifetimes. Another 18 percent consider global warming a problem for the distant future, and 11 percent believe that it will never happen at all. This breakdown of opinions on the subject has remained fairly constant through the years.

Concern about global warming has wavered over the years but is generally lower in 2004 than in previous years. (See Table 2.5). In a poll from March 2004, 26 percent of respondents expressed a great deal of concern about global warming and 25 percent expressed a fair amount of concern. Together these opinions represent just over half of all the people polled. In 1989 nearly two-thirds of those asked felt a great deal or fair amount of concern about this issue. In 2000 nearly three-quarters did. Overall concern does appear to be on a downward trend. These results coincide with the data presented in Table 2.6. Poll participants were asked to rate their views about the seriousness of global warming based on the information they have seen in the media. In 2004 more people (38 percent) considered the seriousness to be exaggerated by the media rather than reported correctly (25 percent) or underestimated (33 percent).

In 2004 Gallup asked people about the response of the Bush administration to global warming. Pollsters pointed out that a group of "prominent" scientists have criticized the administration's approach to dealing with global warming, claiming that scientific evidence has been ignored and distorted. Only 8 percent of the poll respondents indicated they had heard a great deal about this criticism. Another 26 percent had heard a moderate amount of information about it. The largest fraction (40 percent) claimed they knew very little about the criticism and 26 percent knew nothing at all about it. When asked which side they tended to believe in this dispute (the Bush administration or the scientists) a majority (59 percent) sided with the scientists, while 32 percent sided with the Bush administration.

Despite the administration's well-publicized opposition to the Kyoto treaty, Gallup's 2004 poll shows that more people support the treaty than oppose it. Figure 2.15 illustrates that 42 percent of poll respondents indicated that the United States should agree to abide by the provisions of the treaty, compared to 22 percent who disagreed. A sizable percentage of those asked (36 percent) had no opinion on the matter.

People participating in Gallup's 2003 environment poll were asked whether increases in the Earth's temperature over the last century were due more to natural causes or to pollution associated with human activities. As shown in Figure 2.16, human activities received nearly twice the blame than did natural causes. This breakdown is identical to that reported by Gallup when the same question was asked in 2001.

Already begun %Within a few years %Within your lifetime %Not within lifetime, but affect future %Will never happen %No opinion %
2004 Mar 8–115151218113
2003 Mar 3–55161217104
2002 Mar 4–7535131793
2001 Mar 5–7544131874
1997 Nov 6–9483141997
source: "Which of the following statements reflects your view of when the effects of global warming will begin to happen—[ROTATED: they have already begun to happen, they will start happening within a few years, they will start happening within your lifetime, they will not happen within your lifetime, but they will affect future generations, (or) they will never happen]?," in Poll Topics and Trends: Environment, The Gallup Organization, Princeton, NJ, March 17, 2004 [Online] [accessed March 30, 2004]
Great deal %Fair amount %Only a little %Not at all %No opinion %
2004 Mar 8–11262528192
2003 Mar 3–5283023172
2002 Mar 4–7292923172
2001 Mar 5–7333022132
2000 Apr 3–9403215121
1999 Apr 13–14343418122
1999 Mar 12–14283123162
1997 Oct 27–28242629174
1991 Apr 11–14352722125
1990 Apr 5–8302720166
1989 May 4–7352818127
source: "Please tell me if you personally worry about this problem a great deal, a fair amount, only a little, or not at all. The 'greenhouse effect' or global warming?," in Poll Topics and Trends: Environment, The Gallup Organization, Princeton, NJ, March 17, 2004 [Online] [accessed March 30, 2004]
Generally exaggerated %Generally correct %Generally underestimated %Generally exaggerated %No opinion %
2004 Mar 8–113825334
2003 Mar 3–53329335
2002 Mar 4–73132325
2001 Mar 5–73034324
1997 Nov 6–9*3134278
*Based on half sample.
source: "Thinking about what is said in the news, in your view is the seriousness of global arming—[ROTATED: generally exaggerated, generally correct, or is it generally underestimated]?," in Poll Topics and Trends: Environment, The Gallup Organization, Princeton, NJ, March 17, 2004 [Online] [accessed March 30, 2004]