Under normal conditions the greenhouse effect is beneficial, being essential to maintaining a global environment conducive to life on earth. Without it the planet's average surface temperature would be approximately 55° F (30° C) cooler than it is today and consequently inhospitable to most life-forms. However, data from multiple studies indicate that the continued release of CO2 and other aforementioned "greenhouse" gases from human-created (anthropogenic) sources, such as cars and factories, is causing an unnatural increase in atmospheric temperatures. Many scientists believe that this heat buildup, known as global warming, has already begun to produce massive and potentially harmful environmental changes.
Causes of Global Warming
All greenhouse gases come from a combination of natural and anthropogenic sources (with the exception of CFCs, which are artificially created, commercial compounds). For example, volcanic eruptions and forest fires both emit carbon dioxide—as do the combustion of fossil fuels, such as coal, oil, gasoline, and natural gas—to generate electricity and power vehicles.
Among the greenhouse gases, CO2, in fact, has attracted the greatest concern owing to the huge volumes of carbon dioxide that are generated by human activities. According to a report from the Earth Policy Institute, global carbon emissions in 2005 grew to a record high of 7.9 billion tons, more than half of which were contributed by only five countries: the United States (21%), China (18%), Russia (6%), Japan (5%), and India (5%). Other significant producers were Germany (3%), Canada (2%), the United Kingdom (2%), South Korea (1%), and Italy (1%).
Although the combustion of fossil fuels is the main anthropogenic source of carbon dioxide, the fact that plants remove CO2 from the environment for use in photosynthesis means that deforestation and other vegetation loss also are important factors in the atmospheric buildup of this gas. Consequently, the clearing of rain forests and many other types of woodlands for timber harvesting and agricultural, industrial, and residential development is believed to account for approximately 13% of the current global warming trend.
Evidence for Global Warming
In a 1995 report the Intergovernmental Panel on Climate Change (IPCC)—created by the United Nations Environment Programme and the World Meteorological Organization—stated that the bulk of scientific evidence indicates that human activities are having measurable climatic effects. Similarly, in a joint 1999 letter to the British newspaper The Independent, the directors of the U.S. National Oceanic and Atmospheric Administration and the British Meteorological Office stated that the evidence in favor of the theory of anthropogenically caused climate change is "almost incontrovertible."
These assertions are supported by a growing body of research. For example, a 1999 report from the universities of Massachusetts and Arizona reveals strong indications of a late-20th-century warming trend. Using evidence culled from the examination of ice cores, tree rings, corals, and both historical and instrumental data, the university scientists reconstructed centuries of climate variations. Their study suggests that the 1990s was the hottest decade in at least 1,000 years, with 1998 specifically being the hottest year in a millennium.
Indications of rising greenhouse gas levels also include a climb in regional CO2 concentrations at the atmospheric monitoring facility in Mauna Loa, Hawaii. Since beginning operation in the late 1950s, the site, home of the longest continuous carbon dioxide monitoring program in the world, has recorded a rise of about 19% in mean annual CO2 levels. Other research indicates that between the late 19th and the early 21st century, global carbon dioxide concentrations increased by more than 30%.
A 2001 report from the IPCC predicted that between 1990 and 2100 the earth's surface temperature would increase by between 2.5° and 10.4° F (1.4° to 5.8° C). According to the study this warming could produce a number of dramatic environmental changes, including an acceleration in the melting of glaciers and polar ice caps. The IPCC further predicted that by the year 2100 melting ice, combined with a general heat-induced expansion of marine waters, could raise sea levels by 20 inches (50 cm).
Using data gathered from weather stations and ship-based observations, the study also reported that greenhouse gases appear to have already initiated a warming trend, with global mean surface air temperatures having risen between approximately 0.7° and 1.5° F (0.4° and 0.8° C) over the course of the 20th century. These results are consistent with a 1984 report by the marine physicist Tim P. Barnett, of the Scripps Institution of Oceanography; his analysis of over 150 tide gauges indicated that globally between 1881 and 1980 sea levels had risen about 5.5 inches (14 cm), with the greatest increases occurring in the decades after 1930.
In February 2005 a study by scientists from both Scripps and the Lawrence Livermore National Laboratory provided what may be the most convincing evidence to date for global warming. Using millions of marine temperature readings from the National Oceanic and Atmospheric Administration, the researchers compared the world's ocean temperatures with figures generated using computer models designed to predict the effect of human activity on the earth's climate. The correspondence between the predicted and actual temperatures was so compelling that the researchers believed they had conclusively proved the existence of anthropogenically induced climate change.
The IPCC's report in May 2007 used even stronger language. The panel of scientists and government officials concluded that human activity is "very likely" a factor of global climate change—that is, they were at least 90% certain that human emissions of greenhouse gases are warming the planet's surface. The level of certainty was influenced not only by the results of scientific research but also by 29,000 observational reports that were included in the analysis, which revealed regional impacts on every continent and most oceans. The panel projected that by 2100, average global temperatures will probably rise between 1.8 C degrees (3.2 F degrees) and 4 C degrees (7.2 F degrees), but possibly as much as 6.4 C degrees (11.5 F degrees).
The environmental impacts of global warming are expected to be varied and complex. Scientific evidence suggests that such warming-induced changes have already begun.
If sea levels rise as predicted over the course of the 21st century, increased coastal flooding would be expected, affecting towns and cities around the world, including Bangkok; Shanghai; Sydney; St. Petersburg, Russia; Hamburg, Germany; London; New Orleans; Miami; and New York City. Less-developed nations would be especially hard hit, with many coastal rice-producing areas becoming inundated. Low-lying island nations, many of them home to economically valuable tropical resorts, also could suffer profound damage, as might coastal wetlands supporting numerous commercially important fish and wildlife species. While about 10 million people worldwide are currently at risk from annual flooding, it has been estimated that a 16-inch (40-cm) rise in sea level would increase this figure to between 88 and 241 million. Various strategies, such as the construction of dikes and bulkheads, could help to curtail flood hazards but would prove expensive; safeguarding the East Coast of the United States, for example, could cost between $75 and $110 billion, according to the U.S. Environmental Protection Agency.
Based on geographic location and the various routes water follows as it cycles through the atmosphere, global warming is expected to cause a dramatic rise in precipitation in some areas but drier conditions in others. For example, as temperatures climb, ocean waters will evaporate at an increased rate, resulting in even greater amounts of rainfall in already wet coastal regions.
Warming temperatures will also increase the evaporation of moisture from the ground. This means that in such areas as the Midwest, which does not experience the same precipitation patterns as coastal regions, moisture may leave the soil faster than it can be replaced, causing droughtlike conditions. Areas affected by extended dry periods may not only suffer agriculturally but also could see an increased incidence of forest and grassland fires and the consequent destruction of valuable wildlife habitats, timber resources, grazing lands, and recreational areas.
Vanishing Sea Ice
In its 2001 climate-change assessment, the IPCC suggested that global warming was significantly affecting polar ecology; by 2007 they were convinced. The panel cited data indicating that over 13,000 sq km (5,000 sq mi) of sea ice has been lost from the Antarctic Peninsula over the last 50 years. Since 1995, three major sections of Antarctic ice totaling 16,200 sq km (6,200 sq mi) have broken off. In the Arctic the depth of summer permafrost melting has increased, and during the late summer and early autumn, Arctic sea ice is now about 40% thinner than it was during those same months in the late 1950s. A winter thickness decline, albeit a much smaller one, appears to have occurred over the past several decades as well. Moreover, since the 1950s the number of square feet covered by Arctic sea ice during the spring and summer has apparently shrunk by 10% to 15%.
Loss of polar sea ice is expected to produce numerous impacts, according to a 2004 report from the Arctic Climate Impact Assessment research group (assembled by the intergovernmental Arctic Council and the International Arctic Science Committee). Arctic wildlife are already suffering from resulting habitat changes: in 2007, polar bears became the first species proposed for listing under the Endangered Species Act because of global warming. Indigenous people will experience decreased food sources and a loss of traditional ways of life, but may benefit from reduced heating costs and more navigable sea routes. Economic benefits may also be derived from greater access to Arctic oil resources and improved shipping conditions.
Melting Glaciers and Snow
Of the earth's freshwater, 70% is contained in glaciers, with populations in many parts of the world dependent on the melting glacier ice that feeds river systems during summer months. However, as global warming permanently reduces ice volumes, causing glaciers to melt faster than they could be replenished by snowfall, serious water shortages are predicted for glacier-fed regions.
A 2004 report from the European Environment Agency states that between the years 1850 and 1980, glaciers in the European Alps shrank by about one-half of their mass and one-third of their area. By the early 21st century another 20% to 30% of the ice was gone. Other research indicates that global warming could cause the glaciers of the Andes mountains to disappear by 2015, a critical concern for Peru, which depends, during the dry season, on river water supplied solely from glacial runoff.
In Africa, Mt. Kilimanjaro has lost 82% of its glacial cover. Scientists at the American Association for the Advancement of Science project that the mountain will be completely free of ice by the year 2020, leading to drought in the region and economic loss due to reduced tourism and crop production.
According to a 2005 study from the World Wide Fund for Nature (WWF, formerly called the World Wildlife Fund), based in Switzerland, 67% of glaciers found in the Himalayan mountain system are receding at a "startling" rate. These declines are expected to cause water shortages within a few decades for hundreds of millions of people in Nepal, China, and India, with severe impacts on these nations' agriculture and industry.
Another important water source, packed mountain snow, also is threatened by rising temperatures. According to a 2004 report funded by the U.S. Department of Energy, the western United States could face severe water shortages by 2050 as the volume of mountain snow draining into the Colorado and Columbia rivers is diminished. This would affect water supplies required for hydroelectric power, perhaps cutting electricity generated by the Colorado River, for example, by up to 40%.
Increased Storm Intensity
Because hurricanes derive power from heat energy, ocean warming is expected to raise the intensity of these storms. Consequently, hurricane-related flooding and property damage may become more extensive. Death tolls from such storms also are likely to increase.
Impact on Plants and Animals
Rising temperatures, changing rainfall patterns, and an increase in violent storms could together affect virtually every ecosystem on earth and most of the planet's species. Computer models suggest, for instance, that rapidly increasing temperatures would outstrip most species' ability to adapt to them, causing dramatic declines in many plant and animal populations or even driving numerous species to extinction. In its 2007 report, the IPCC predicted that 20–30% of species will be at high risk for irreversible extinction.
Damage to plant life, however, is not expected to be universal. Research suggests that although some plant populations will suffer in an altered environment, others will thrive owing to increases in atmospheric CO2 (since carbon dioxide is essential to plant growth).
Certain animal species also may flourish and expand their range as temperatures rise. This would likely include, however, certain disease-carrying warm-weather organisms. For example, vampire bats, which can transmit rabies to the cattle on which they feed, might extend northward from Mexico, causing substantial damage to the Texas livestock industry. Insects that transmit malaria and dengue fever also could expand into previously inhospitable environments.
Uncertainties in the Study of Global Climate Change
Although a growing body of evidence indicates that anthropogenically induced climate change is under way, the ultimate extent of global warming remains a matter of debate. Some scientists, for instance, believe that the IPCC's global-warming projections are conservative, since increasing temperatures could lead to a variety of feedback cycles that accelerate the heating process.
For example, a 2004 study—the culmination of a ten-year project employing researchers from around the world—reported that since the Industrial Revolution of the early 19th century, the oceans have absorbed approximately half of all CO2 introduced by humans into the atmosphere. Other research, however, indicates that this will not remain the case, with rising global temperatures reducing the oceans' ability to dissolve and retain carbon dioxide. These waters could consequently begin to discharge large quantities of stored CO2 into the atmosphere, promoting a more rapid increase in global temperature and so, in turn, triggering even greater releases of ocean-bound carbon dioxide.
Similarly, the melting of glaciers may not only result from global warming but also accelerate it. The reduction of light-reflecting ice cover in various regions of the world would enable the planet to absorb more sunlight, the energy from which would be converted to heat. Moreover, rising global temperatures may lead to a greater use of air conditioning, expanding consumption of CO2-emitting fossil fuels, while an increase in forest fires, a source of carbon dioxide, and the resulting loss of CO2-absorbing trees also would exacerbate the greenhouse effect.
Some scientists, however, assert that several factors could mitigate human-produced climate changes. For instance, as rising temperatures boost the amount of water evaporating from the planet's surface, cloud cover may increase, reducing the amount of sunlight striking the earth. This in turn could slow the rise in global temperatures.
Numerous solutions are available to reduce greenhouse gases. International bans on ozone-depleting CFCs enacted in the 1980s and early 1990s, for example, are diminishing one source of the problem.
To address global climate change adequately, however, the IPCC believes that global carbon dioxide emissions need to be cut by 50–85% by 2050. Yet by the mid-21st century, as developing nations continue to seek prosperity through industrialization, CO2 emissions from these countries are projected to quadruple. Additionally, such emissions from industrialized nations are expected to increase by 30% over the same period. It is essential, therefore, that to reduce global warming, a livable balance must be struck between economic needs and environmental prudence.
Reduction of CO2 Production
Energy experts have estimated that global carbon dioxide emissions could be cut by 3 billion tons per year within two decades through the use of cost-effective, energy-efficient technologies. The large-scale use of renewable energy systems employing, for example, wind generators and photovoltaic modules, also could dramatically reduce CO2 emissions.
In addition to technological strategies, reduction of atmospheric carbon dioxide levels will also require massive global reforestation. The British ecologist and tropical forest expert Norman Myers argues that replanting 1 million square miles (2.6 million sq km) of tropical rain forest—about one-third of the area that has been deforested—would reduce atmospheric carbon dioxide by 2.5 billion tons annually. Although such replanting would cost approximately $100 billion, the economic savings from a reduction in global climate change would at least partially offset the expense.
In 1997 the United Nations sponsored a meeting in Kyoto, Japan, at which nations from around the world negotiated an international agreement on addressing global climate change. According to the pact, known as the Kyoto Protocol, industrialized nations participating in the agreement must reduce levels of six major greenhouse gases—including CO2, methane, and nitrous oxide—between 2008 and 2012 (when the treaty expires). The cuts for all of these industrialized countries combined will, if the agreement's goals are accomplished, reduce their overall emissions to 5.2% below 1990 levels. As of early July 2006, after the initial signing by their representatives, the governments of 164 countries, including Russia and those of the European Union, had formally approved participation in the treaty.
The agreement went into effect on Feb. 16, 2005. It had taken several years for it to come into force because of the treaty's guidelines. These stated that the protocol could not be considered valid until it had been approved by countries who together produced greenhouse gas emissions equaling at least 55% of the 1990 worldwide emission level. This goal was reached when, in November 2004, Russia ratified the treaty.
Owing to federal resistance, however, the United States has not taken part in the Kyoto Protocol. The U.S. Senate passed a resolution in July 1997 opposing such participation, stemming from the fact that the agreement's requirements are binding for industrialized nations but not for developing countries. Legislators were also concerned that adherence to the protocol would be economically harmful to American industry. Following his election to the presidency in 2000, George W. Bush also rejected U.S. participation in the agreement, claiming that there was not yet enough scientific evidence to support the treaty's guidelines.
Even before the Kyoto Protocol went into effect, some nations, such as the United Kingdom, France, and Germany, had already seen decreased greenhouse gas emissions, largely as a result of energy-efficiency policies and increased reliance on renewable energy. Russia and many members of the former Soviet Union also had posted dramatic reductions in CO2 emissions in advance of the treaty's activation, albeit primarily because of declines in industrial production following the dissolution of the Soviet Union.
A separate pact to cut greenhouse-gas emissions was signed in July 2005 by six nations: Australia, China, India, Japan, South Korea, and the United States. (Along with the United States, Australia also did not sign the Kyoto Protocol.) Known as the Asia-Pacific Partnership on Clean Development and Climate, its aim is to encourage the reduction of greenhouse gases through the employment of new technologies. Critics have asserted that the treaty is unlikely to prove beneficial, since it does not impose mandatory limits or reductions on emissions.
Lacking a federal initiative, many state and local officials in the United States have collaborated to set emission reduction agendas at local, state, and regional levels. In 2003 the governors of 10 northeastern states announced that they would participate in a regional cap-and-trade program to limit CO2 emissions. By July 2006, 29 states and Puerto Rico had completed or were working on action plans to significantly reduce carbon emissions. In June 2007, 6 western states and 2 Canadian provinces agreed to combat global warming by developing their own regional system to reduce greenhouse gases.
Daniel D. Chiras
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