* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Earth_Day - Caldwell County Schools
Climate sensitivity wikipedia , lookup
Global warming controversy wikipedia , lookup
Climate engineering wikipedia , lookup
Media coverage of global warming wikipedia , lookup
General circulation model wikipedia , lookup
Mitigation of global warming in Australia wikipedia , lookup
Global warming hiatus wikipedia , lookup
Effects of global warming on human health wikipedia , lookup
Scientific opinion on climate change wikipedia , lookup
Fred Singer wikipedia , lookup
Surveys of scientists' views on climate change wikipedia , lookup
Climate change, industry and society wikipedia , lookup
Climate change and poverty wikipedia , lookup
Instrumental temperature record wikipedia , lookup
Ministry of Environment (South Korea) wikipedia , lookup
Effects of global warming on humans wikipedia , lookup
Global warming wikipedia , lookup
Effects of global warming on Australia wikipedia , lookup
Politics of global warming wikipedia , lookup
Public opinion on global warming wikipedia , lookup
Attribution of recent climate change wikipedia , lookup
IPCC Fourth Assessment Report wikipedia , lookup
Years of Living Dangerously wikipedia , lookup
1. Earth Day On April 22, hundreds of millions of people in the United States, Canada, and more than 190 other countries will celebrate Earth Day. On this day, people everywhere come together with a single goal: to protect the health of our planet. Schools and community groups hold recycling drives. People work in teams to clean up parks and beaches. Others leave their cars at home on Earth Day, turn off lights and appliances, and otherwise avoid consuming energy or creating pollution. Most of all, Earth Day has been a very effective means for highlighting the greatest environmental issues of our time: climate change, energy efficiency, renewable energy, and the creation of a global green economy. 2013 is the 43rd year in a row that the world has paused to observe Earth Day. Earth Day is celebrated annually on April 22 to focus attention on the protection and conservation of natural resources. Arising from a student-led movement in the United States, Earth Day gained status and stature when, in 1969, U.S. senator Gaylord Nelson supported the notion that a day be set aside to honor the environment and to remind humankind of the importance of environmental protection. Now celebrated worldwide, Earth Day is marked by official and unofficial activities, large and small. Many government bodies and international organizations, including the United Nations, use the occasion to highlight programs, research reports, and problems—and their solutions—related to the world's natural environment. 2. Environmental Movements: Introduction to Environmental Movements Environmental movements are social and political movements that are concerned with the protection of natural surroundings from depletion, pollution and degradation by humans. They are primarily a post–Industrial Revolution phenomenon initiated mainly in the United States and Europe by educated members of society—scientists, politicians, economists, academicians—following major spurts of industrial expansion and resource exploitation. Such movements initially focused on population growth and resource depletion, but more recently they have been concerned with human-generated pollution (see pollution, environmental) and the preservation of natural environments. Three major environmental movements of the past 200 years are described below. 3. Environmental Movements: Malthus, Ricardo, and Mill Thomas Robert Malthus and David Ricardo were English economists who lived during the early years of the Industrial Revolution. They watched as plummeting death rates transformed Europe from an agrarian society to one more urban and more densely populated. Malthus put forth his ideas regarding the future of this technologically sophisticated yet crowded new world in his monumental treatise of 1798 entitled An Essay on the Principle of Population as It Affects the Future Improvement of Society. He believed that population would eventually outstrip food supplies with calamitous results. Only disease, wars, and voluntary reductions in birthrates could save the world. Ricardo wrote along similar lines in his 1817 book Principles of Political Economy and Taxation. Some 30 years later, political theorist John Stuart Mill wrote Principles of Political Economy (1848). Mill believed that humanity might be able to check population growth while discovering and better utilizing resources, but he also introduced the notion that personal solitude and natural beauty might be impaired through population and industrial growth. During the remainder of the 19th century, public interest in natural environments was further stimulated by such writers as Henry David Thoreau (Walden, 1854) and George Perkins Marsh (Man and Nature, 1864). 4. Environmental Movements: The Conservation Movement An American event that spanned the period 1890 to 1920, the conservation movement was closely identified with the personality and politics of President Theodore Roosevelt. By the end of the 19th century, Americans were concerned with the rapid pace of social progress. They had seen the United States move quickly from a frontier to an industrial society, and the nation's much-cherished natural resource endowment, especially its forests, appeared on the verge of extinction. Under Roosevelt's programs, the protection of forests, rangeland, and mineral and water resources began to evolve in piecemeal fashion. Perceiving this approach as inadequate, administration officials, such as Chief of the U.S. Forest Service Gifford Pinchot and Secretary of the Interior James R. Garfield, argued for a more unified policy of natural-resource planning. A successful appeal was made to the public for political support, and the drive for resource protection surged forward among middle- and upper-income Americans with the fervor of a moral crusade. Some people, such as the naturalist-founder of the Sierra Club, John Muir, favored resource preservation. The prevailing theme of the era, however, was one of conservation—the restrained and efficient use of natural resources. American businessmen were generally supportive. The conservation movement's goal—to provide for the sustained production of material goods at the lowest cost—was very much in tune with business thought of the time. The most significant legacy of the conservation movement was its application of science to natural-resource management problems, through programs administered by the federal bureaucracy. Just as philosophy had aided the earlier British economists, practical science was the ally of the conservation movement. In order to administer scientifically based management, major federal and state conservation laws were passed, and new agencies such as the U.S. Forest Service, the United States Geological Survey, the National Park Service (see national parks), and the Bureau of Reclamation were created. By 1920, political squabbling among the various participants, and changes in presidential administrations, had slowed the momentum of the conservation movement. Over the next three decades the Depression of the 1930s and World War II dominated American energies, and ideas about preserving nature seemed to have lost their relevance. 5. Environmental Movements: The Modern Environmental Movement The publication of Silent Spring by Rachel Carson in 1962 provides a notable event by which to mark the beginning of the modern environmental movement. Carson, a successful author and former U.S. Fish and Wildlife Service biologist, wrote about the dangers of such recently developed agricultural chemicals as DDT, sending the world very specific warnings about the risks of postwar technologies that were producing artificial pesticides and other new chemical products. Silent Spring inspired others to write about the threats that human actions posed to the environment. Notable examples of these writings include Paul Ehrlich's The Population Bomb (1968), Barry Commoner's The Closing Circle (1971), and Limits to Growth (1972), written by a research team from the Massachusetts Institute of Technology. Carson's works, and those which followed, engendered almost immediate grassroots sentiment for improving environmental quality. Support for change was perhaps nowhere stronger than on college campuses, where environmental issues often became intertwined with other concerns, such as protests against the Vietnam War. Earth Day, first held Apr. 22, 1970, was largely a campus-based event that represented perhaps the apogee of the early years of the movement. The heightened social concern led to the passage of important environmental legislation. These laws included the National Environmental Policy Act of 1969, the Clean Water Act of 1972, the National Forest Management Act of 1976, the Clean Air Act amendments of 1977, and the National Acid Precipitation Act of 1980, as well as the creation (1970) of the Environmental Protection Agency. The National Environmental Policy Act was particularly important. It established a process that forced federal agencies to examine the impacts that all their activities, such as dam and highway construction, could have on the environment. Today federal agencies routinely produce hundreds of environmental impact statements, which help to identify courses of action that will minimize adverse impacts on the environment. By the 1980s, public interest in the environment was further strengthened by a number of events such as the acid rain controversy, the Chernobyl nuclear catastrophe, the Exxon Valdez disaster (see also oil spill), tropical deforestation, the harvesting of old-growth timber in Alaska and the Pacific Northwest, the depletion of the ozone layer, and the discovery of the possibility of global warming. Many organizations are now involved in the environmental movement. Among the most notable are the Sierra Club, the Wilderness Society, the National Audubon Society, the Natural Resources Defense Council, the National Wildlife Federation, the Izaak Walton League of America, and Environmental Defense. Individual groups tend to specialize in particular environmental issues. They rely upon fund raising, lobbying, the political system, and the courts. In addition to these mainstream groups, there are radical environmental groups such as Earth First!, and Greenpeace International, which sometimes use confrontational techniques such as driving hidden metal spikes into forest trees, to prevent them from being processed in saw mills, and harassing whaling ships from small boats. "Deep ecology" is the motivating philosophy of many radical environmentalists. This philosophy embodies the notion that all of nature's creations are deserving of equal protection, and thus direct action must be taken to protect them. Political Green parties have also been formed, most notably in Europe, to support environmental candidates for political office. The modern environmental movement differs from earlier movements in that the concern is over human-caused pollution and degradation of the environment as well as population growth and resource scarcity. Also, many of the environmental problems faced today are global as well as local. However, despite apparent public interest in the environment, at least in the Western world, the barriers to a better environment seem formidable. The lack of progress on the resolutions of the United Nations–sponsored 1992 Earth Summit and on the 1997 Kyoto Protocol to minimize global climatic change are testimonies to the difficulties involved. The United States actually rejected the latter in 2001, while the activating signature (Russia) did not materialize until 2004; implementation of the protocol is still to be decided. Developing future consensus among the nations of the world and coordinated plans of action to combat environmental degradation, excessive population growth, and resource depletion will be quite challenging. 6. Conservation: Introduction to Conservation Conservation is the philosophy and policy of managing the environment to assure adequate supplies of natural resources for future as well as present generations. In the late 1800s and early 1900s, conservation usually referred to management of a single, economically valuable resource such as forests, soils, or wildlife. Today, reflecting an increasing understanding of ecology—the science of the interrelationships between living things and their environment—the use of the term conservation has been extended to consider the environment as a whole. Modern conservation, then, can be defined as the management of the human use of the environment so that it may yield the greatest sustainable benefit to present generations while maintaining its potential to meet the needs and aspirations of future generations. It is concerned with the quality as well as the basic support of human life. Conservationists recognize that human activities profoundly change the face of the Earth and can irreparably damage or destroy the natural resources on which human well-being and, indeed, human survival depend. Natural resources traditionally have been classified as renewable and nonrenewable. Renewable resources are those which, under proper management, regenerate and even improve their resource values, but which when misused can be depleted or lost entirely. They include plants and animals and other resources such as soils and inland waters. Nonrenewable resources are minerals and fossil and nuclear fuels, which are present on the Earth in fixed amounts and, once used, do not regenerate. Increasingly, elements of the environment, such as oceans, tidal lands, and the air itself, are also being recognized as natural resources. It has become increasingly clear that resources and resource uses are intimately interrelated. A forest contains lumber, which is a valuable economic commodity; it also, however, serves as a watershed, keeps soil from eroding, provides habitat for wildlife, provides recreation, and ameliorates local climate. Indiscriminate cutting of trees may destroy the forest, but it will also have corollary, and potentially far more serious, effects. A paramount principle of conservation is that the use of any resource requires consideration of the impacts of that use on associated resources, and on the environment as a whole. The goals of resource conservation are 1) the maintenance of essential ecological processes, which range from the global cycles of nitrogen (see nitrogen cycle), carbon dioxide, and water to the localized regeneration of soil, recycling of nutrients (see nutrient cycle), and cleansing of waters and air and life-support systems, such as agricultural systems, coastal and freshwater systems, and forests; 2) the preservation of genetic diversity; and 3) the assurance that utilization of species and ecosystems such as forests and grazing lands is sustainable. The consumption of nonrenewable resources should ensure that scarce minerals are used conservatively and recycled where possible, and that their mining and use have the least possible adverse impact on other resources, and on environmental quality. 7. Conservation: History: The United States The conservation movement began in the United States largely in response to the unparalleled damage the settlers had inflicted on natural resources. The original vast forests of eastern North America were devastated by clearing for agriculture, and by destructive lumbering and the massive forest fires that followed. Agricultural soils were depleted, farmers speaking proudly of having "wore out" several farms on their march westward. The grasslands of the west were so severely overgrazed that many have never recovered their productivity. Wildlife was particularly hard hit by unrestricted market hunting and predator control. The abundant herds of bison, deer, elk, and antelope were greatly reduced, and wolves, bears, and mountain lions were virtually eliminated. A number of local varieties of wildlife vanished utterly. Some bird species were virtually wiped out, and species that had existed in incredible numbers—the passenger pigeon and Carolina parakeet—were exterminated. Conservation in America grew out of the recognition of these destructive processes. Many far-sighted individuals contributed to conservation theory. In 1832 artist and author George Catlin proposed setting aside large areas of the western United States where the Indians, as well as wildlife, could survive. Geographer George Perkins Marsh published Man and Nature (1864), the first real consideration of the human impact on nature and its resources. Author and naturalist John Muir became a champion of wilderness preservation and was a founder and first president of the Sierra Club. The later 1800s saw the creation of several of the earliest private conservation organizations. In 1875 the American Forestry Association was founded, followed in 1883 by the American Ornithologists' Union, the Boone and Crockett Club in 1887, the Sierra Club in 1892, and the New York Zoological Society in 1895. These were pioneers among the nongovernmental organizations that have played a vital role in the development of conservation. 8. The Beginnings of Federal Involvement. In 1864, Congress gave the state of California Yosemite Valley for a public park and recreation area, effectively making it the precursor of today's national park system; in 1872, it established Yellowstone as America's first true national park. The Division of Forestry, now the U.S. Forest Service, was created in 1876. The U.S. Geological Survey was established in 1879, and its first director, John Wesley Powell, publicized the agricultural possibilities and limitations of the West. The U.S. Biological Survey, precursor of the Fish and Wildlife Service, was founded in 1885. In 1897, Gifford Pinchot was appointed chief of the reorganized Division of Forestry. Pinchot became a key architect of the conservation policies that developed under President Theodore Roosevelt. 9. The Roosevelt Administrations. Conservation of natural resources was firmly established as an important concern and priority of the federal government under Theodore Roosevelt's administration (1901–09). Through various initiatives, including the White House Governors' Conference on Conservation held in 1908, Roosevelt also persuaded state governments of the importance of conservation. The establishment of national parks, national forests (see National Forest System), and wildlife refuges (the first in 1903) firmly set out the conservation principle that certain public lands must be held in trust by the federal government and managed for the good of the country as a whole. For a time, other lands in the public domain received little attention, but in the 1930s it became evident that resource depletion due to overgrazing on these lands had reached crisis proportions. With the passage of the Taylor Grazing Act (1934), under President Franklin D. Roosevelt, Congress affirmed that all lands in the public domain were to be managed as part of the public trust. The catastrophe known as the Dust Bowl occurred in the early 1930s, when a series of dry years coincided with the extension of agriculture to unsuitable lands, and when poor agricultural practices had caused grossly deteriorated conditions on vast areas of midwestern and western farm lands. Dry and denuded lands simply blew away, and the clouds of dust reached as far east as Washington, D.C., dramatizing the severity of the crisis. With the establishment (1933) of the Soil Erosion Service, the country began to accept the principle that it was appropriate for the government to intervene to assist private landowners, especially in the areas of soil and water conservation. The year 1933 also saw the creation of the Civilian Conservation Corps and the Tennessee Valley Authority. 10. Post World War II. In the period following World War II, growing populations, advanced technological capabilities, and increased emphasis on economic development put new stresses on the environment and its resources. For example, DDT and other synthetic pesticides were developed in an attempt to reduce insect-borne diseases in humans and to increase food production. The initial results were dramatically successful. DDT was heavily used virtually worldwide, and in many areas malaria, carried by the anopheles mosquito, practically disappeared. Particularly in the United States, the new pesticides helped to produce bumper crops. Nevertheless, it became apparent that these substances were producing severe environmental effects, and in many cases creating problems that were worse than those the pesticides were intended to cure. The publication in 1962 of Rachel Carson's Silent Spring alerted Americans to the dangers of unwise pesticide applications, and eventually the use of DDT and related pesticides was severely restricted. 11. The Emergence of Environmentalism. As pollution increased throughout the 1950s and '60s, conservation problems were forcibly brought to public attention via television. People saw the ghastly effects of mercury poisoning at Minamata Bay in Japan in the 1950s, the Torrey Canyon oil spill (1967) in the English Channel, and the killer smog episodes in Los Angeles and London. One result was an unprecedented growth of public concern and the emergence of citizen conservation organizations. Many came to recognize that the narrow approaches to conservation that had marked earlier efforts were no longer appropriate and that a more comprehensive environmental approach was needed. Conservationists began to become environmentalists. Public environmental concerns led Congress to develop the National Environmental Policy Act of 1969 (NEPA) and to pass it unanimously over President Richard M. Nixon's objections. In the process of developing NEPA, Congress found that there were more than 80 government units whose activities affected the environment, yet there was no governmental mechanism to develop environmental policy, maintain an overview of governmental actions, or provide environmental coordination. To fill these needs, NEPA established the Council on Environmental Quality and mandated the environmental impact statement in an attempt to ensure that environmental factors received due consideration in any federal actions. Today, most states, many other countries, and international development institutions have adopted somewhat similar mechanisms to assess the environmental effects of proposed actions. The 1970s represent the high point in the passage of U.S. conservation-related legislation. Following NEPA and the establishment of the Environmental Protection Agency in 1970, a series of important pollution control measures were legislated: the Clean Air Act of 1970, the Water Pollution Control Act of 1972, the Toxic Substances Control Act of 1976, and the Clean Water Act of 1987, as well as comprehensive legislation on ocean dumping, fisheries conservation and management, wetlands and coastal-zone conservation, and protection of marine mammals and endangered species. The International Environment Protection Act of 1983 is landmark legislation. It incorporates wildlife and plant conservation as important objectives of U.S. development assistance to developing countries, and it requires the federal government to formulate U.S. strategy to conserve biological diversity in those countries. The 1980s were marked by a backlash against the environmentalism of the previous decade. Led by the administration of President Ronald Reagan and based on ideological, political, and economic concerns, attempts were made to weaken or repeal many of the conservation-related laws and regulations of the previous decade. Outspokenly anticonservation administrators were appointed to head government agencies such as the Department of the Interior and the EPA, and antienvironment citizen groups such as Wise Use Movement emerged. This in turn led to a revitalization of the environmental movement, and the country has made some additional progress on environmental issues, but conservation-related matters often have remained controversial. 12. Conservation: International Conservation The most effective early international conservation efforts involved agreements on migratory species, such as the 1911 treaty for the conservation of the northern fur seal (signed by Canada, Japan, Russia, and the United States) and the 1916 U.S.Canadian treaty for the conservation of migratory waterfowl. Only since World War II, however, has public awareness led to increasing international cooperation on such conservation issues as endangered species, national parks, conservation education, and conservation law. The United Nations Conference on the Human Environment, held in Sweden in 1972, firmly established conservation of natural resources as an important concern of governments throughout the world. Today, conservation is concerned with a small number of major global issues. Each of these affects the others, and all are basic to human survival. Although not strictly conservation issues, population growth and economic factors underlie virtually all conservation problems. 13. Deforestation. Tropical forests are being destroyed at an ever increasing rate. Estimates of the extent and rate of loss vary, but it appears that nearly half of the world's tropical forests already have been lost, and the remainder will all but disappear in the next two to three decades. The loss is incalculable. These forests provide habitat for an estimated half of the world's plant and animal species, provide water and fuel for much of the world's population, and influence regional and global climate. Commercial logging, clearance for agriculture, ranching, and fuel gathering are all responsible for the destruction. Solutions include the development of alternative fuelwood supplies through fuelwood plantations, the regulation of logging, and a consensus as to the value of forest conservation over commercial development. In contrast, temperate-zone forests have actually increased in recent decades. Their greatest threat is acid rain pollution, which is already severely affecting large areas of the conifer forests of Europe and northeastern Loss of Agricultural Land. 14. North America As the world's population increases, the lands needed to produce its food are disappearing, covered by buildings and roads, their topsoil lost through erosion, and their productivity destroyed by the salinization caused by irrigation. Large-scale commercial agriculture in parts of the United States results in severe and unsustainable rates of erosion soil loss. Overgrazing and firewood gathering denude vast areas of arid lands, resulting in the inexorable spread of deserts and desertlike conditions. Much of the problem in the developing countries is caused by unsound or ineffective development assistance efforts. (See also pollution, land.) 15. Loss of Biological Diversity The ever increasing loss of plant and animal species represents a major conservation concern. Habitat loss, especially in tropical forest areas, is the greatest threat. Overexploitation threatens some species, such as whales and the rhinoceros. The Convention on Trade in Endangered Species of Flora and Fauna has worked well to control trade in most species threatened by commercial exploitation. A more fundamental solution, however, must be the establishment of a global network of areas that protect and maintain representative samples of the world's ecosystems. Substantial progress has been made toward this goal, and there are protected areas—on paper at least—in virtually all nations. To strengthen and augment protected areas where people are excluded, increasing emphasis is being placed on sustainable development, or "community conservation" where people live; the objective here is to bring benefits to the people involved so that they will assist in biodiversity conservation. This approach represents a major new thrust in both national and international conservation endeavors, and it is now central to the conservation efforts of international development institutions such as the World Bank. From the standpoint of conservation, maintaining species of plants and animals in botanic gardens, zoos, or gene banks may be a last resort but is no substitute for maintaining species in their natural habitat. 16. Endangered Water Supplies Water supplies are threatened virtually worldwide with depletion and pollution. Globally, the major problem is loss of watershed areas through denudation of vegetation. The solution must come from better land use and the protection of critical watershed vegetation, along with water conservation and recycling. (See also pollution, water; water quality; water resources; water supply.) 17. Exhaustion of Nonrenewable Resources The primary conservation concern in this area is with fossil fuels, both in terns of exhaustion of supplies and with global warming caused by the carbon dioxide that is released when fossil fuels are burned. Solutions include improvements in the efficiency of fuel combustion and pollution control, as well as more-intensive explorations for alternatives, particularly from solar energy and other renewable resources. 18. Conservation: Conservation as a Political and Economic Issue The benefits of conservation are often long term and therefore accrue to future as well as to present generations. Many of the benefits (environmental quality, for example) do not neatly fit in conventional cost-benefit economic calculations. Thus conservation efforts nearly always run counter to the objectives of short-term economic gain. For example, a lumber company can usually make a greater shortterm profit from rapid, indiscriminate "mining" of the trees in a forest than it can from careful, selective logging, followed by replanting to assure a sustained yield. Fishing provides another example. Technological advances have provided modern fishing fleets with the means to harvest vast quantities of fish, providing short-term profit but severely depleting fish stocks worldwide. In developed countries, conservation issues are often clear-cut economic ones. From the start, the concept of public-trust management of U.S. public lands has provided a major focus for conflict between conservationists and the economic interests of loggers, ranchers, and miners, who virtually without cessation have sought control over the resources of public lands and continue to receive subsidized access to timber, minerals, and grazing privileges. Short-term economic interests often conflict with conservation interests where proposed development will create environmental damage. (An extreme example, but one that is fundamental to conservation issues, is where development threatens a species with extinction.) The benefits of development can be quantified; the costs of the anticipated damage often cannot. Because of the potency of economic motivations, however, the use of economic incentives to promote conservation represents another new thrust in conservation. Recreational use of public lands has increased dramatically, and while this has created a larger constituency for public-trust management, it has also caused new problems. Increased numbers of hikers, cyclists, horseback riders, and campers are overusing many fragile park and wilderness areas, and future access probably must be limited. Off-road vehicles pose the greatest immediate problem because of their impact on wild-country ecosystems. Other conflicts arise over quality-of-life issues, particularly those involving lowversus high-density commercial and residential additions to towns and suburban areas. With increasing population pressure, controlling growth has become a highprofile issue in much of the United States. Conservation issues can also have a potent political dimension, especially where they engender conflicts with those whose ideology favors "free enterprise" over government involvement. In developing countries it has become clear that conservation and sound development are mutually interdependent. Without development, conservation has little chance of being sustained. Where people live at subsistence level and rely on burning wood for heat and cooking, they must cut the rapidly diminishing trees even though they may know that the trees are needed to hold soil and moisture. Yet development that does not take environmental factors into account also has little chance of continuance. For example, unless the watershed above a new dam is protected, its trees may be cut down and the resultant erosion will silt up the dam, making it useless. The interdependence of conservation and development is well illustrated by those development projects which ignored environmental concerns and caused massive deforestation in Latin America, desertification in Africa, and salinization in Asia. Famines in Africa have been exacerbated, if not substantially caused, by development projects that did not incorporate environmental considerations. Conservation can maintain the resource base needed by developing countries, and it can contribute directly to economic development, as in Kenya and Costa Rica, where wildlife-based tourism earns a significant percentage of those nations' foreign currency. Increasingly, conservation concerns are being incorporated into economic development plans. At the U.N.-sponsored 1992 Earth Summit in Rio de Janeiro, the largest congregation of world leaders in history agreed on the broad principles that must guide environmental policies while still encouraging economic growth. Binding treaties commit most of the Earth's nations to curb the emission of greenhouse gases and to protect endangered species. 19. Global Warming: Introduction to Global Warming The term global warming is used by the general public to refer to the phenomenon of global change arising from human activities that result in an increase in greenhouse gases, notably carbon dioxide, in the atmosphere. One manifestation of this climate change is the observed rise in global mean temperature at the Earth's surface. This climate change involves much more than just increases in global temperatures, however. Changes in precipitation, drought, and water resources, for instance, are also involved. Such factors can have a profound impact on the environment and human endeavors. Estimates have been made of changes in the Northern Hemisphere's average surface temperature for the past 1,000 years. This record was constructed using data from tree rings, corals, ice cores, and historical records for the first 900 years of this period. Also used were data from instruments, including thermometers and satellite sensors, for the past 100 years. Significant fluctuations occur in mean temperature from year to year and from decade to decade. The variations before 1900 are almost completely natural; that is, human activities had little or nothing to do with them. Such variations arise from changes in the Sun and the effects of volcanic eruptions. Interactions among the components of the climate system—the atmosphere, the oceans, the land surface, and sea and land ice—also cause such variations. Notably, however, relatively rapid temperature increases started in about 1900. The rate and duration of this warming are greater than at any other time during the past 1,000 years. The decade of the 1990s was the warmest of the last millennium; the early years of the 21st century were even warmer. Human activities, mainly the burning of fossil fuels, almost certainly played a significant role in causing this warming. It has been estimated that the influence of human activities exceeded the bounds of natural variability from about 1980. The rapid warming over the past 100 years is demonstrated by a variety of other independent observations. These include the melting of glaciers around the world; rising sea level; reduced areal coverage of Arctic sea ice; warming of the surface, and of the upper layers, of the ocean; decreases in Northern Hemisphere snow cover; thawing of the Arctic permafrost; and shortening of the length of the freezing season in the Northern Hemisphere. As indicated by recent reports of the Intergovernmental Panel on Climate Change (IPCC), by far the majority of credible atmospheric scientists around the world agree that the observed global warming over the past 100 years is real. The IPCC is an international body of scientists convened by the United Nations (UN) jointly under the UN Environmental Programme and the World Meteorological Organization. Initiated in 1988, the IPCC was charged with providing policy makers with an objective assessment of the scientific basis for global climate change and its environmental and socioeconomic aspects. It was to focus particularly on assessing the effects of human activities. The IPCC's 2001 report stated that global warming is real. It declared that "there is new and stronger evidence that most of the warming observed over the past 50 years is attributable to human activities." It is hard to overemphasize the importance of global warming on the scale shown during the past 100 years. In particular, the recent changes are occurring at an unprecedented rate. They exceed anything seen in the past 10,000 years. Ecosystems are extremely sensitive to climate and evolve rather slowly as climate changes. When the changes are rapid, many ecosystems are stressed. Some cannot adapt. If the recent global warming continues or accelerates during the next 50 to 100 years, as is projected by most scenarios, the Earth will experience a climate hotter than any in the past million years. The consequences will affect all life, including humans, in ways that are highly uncertain but potentially disastrous. As the oceanographer Roger Revelle pointed out in 1957, humankind is engaging in an enormous geophysical experiment whose outcome is in doubt. The doubt comes from uncertainties as to how human activities will change and how the complex climate system, which is affected by human activities as well as by natural processes, will respond. The attribution of global warming to human activities is controversial because the Earth's climate system is complicated. It is therefore difficult to determine cause and effect with certainty. It is also controversial because proposals to mitigate the causes—such as reducing fossil-fuel burning and other forms of consumption—pose threats to some industries and some parts of the economy. Nevertheless, the continuing changes over the past several years have led almost all skeptics to agree that humans are causing global warming. Skeptics now tend to argue more about how much temperature increase is due to humans and whether it could also have some beneficial effects. 20. Global Warming: Factors That Cause Climate Change Long before the existence of humans, or before their numbers were large enough to affect weather and climate, climate changed. Many factors combine to produce climate variation on all scales of time and space. These range from the age of the Earth itself (4.5 billion years) to individual seasons and months and from local cities and regions to the size of the entire globe. Fundamental to determining climate and climate change is the radiation balance of the Earth. This balance indicates how much solar radiation is received; how much and where this solar radiation is absorbed by the Earth system (atmosphere, oceans, and land and ice surfaces); how much and where the components of the Earth system emit infrared radiation back into space; and how the atmosphere and oceans respond to these heating and cooling distributions and their variations in time. Factors that determine the radiation balance of the Earth system and the circulation of the atmosphere and oceans are listed here and discussed below: (1) the Earth's orbital parameters; (2) the solar output and its variations; (3) changes in the surface of the Earth (including continental drift, mountain building and decay, and changes in ocean basins and sea-bottom topography); and (4) changes in atmospheric composition (including volcanic gases and particulates, greenhouse gases, and aerosols and particulates). 21. Solar Variability and Earth's Orbit The Sun is by far the primary source of energy for the Earth. The variations in total energy output from the Sun are very small. They amount to about plus or minus 0.1% over the 11-year solar cycle. The direct heating effect of this magnitude of change is too small to explain the major fluctuations that have been reported; it is estimated, however, that warming by perhaps as much as 0.2 C degrees (0.36 F degrees) in the first part of the 20th century may have been due to changes in the Sun. Even though the variation in the total energy output from the Sun is small, the variation in the ultraviolet wavelengths is much larger (about 1% to 10%). This magnitude is enough to affect the amount of stratospheric ozone (see ozone layer), which is a strong absorber of solar radiation in the stratosphere. Changes in the stratospheric heating due to changes in ozone modify stratospheric circulation; moreover, through coupling with the troposphere and the oceans, they also modify the weather and climate near the Earth's surface. These positive feedbacks may amplify the direct effects of small variations in solar radiation. Paleoclimatology records over thousands of years have indeed shown a strong correlation of climate with solar variability. It is therefore likely that the Sun has contributed to events such as the Little Ice Age (from A.D. c.1300 to c.1850). Nevertheless, the details of the connection between solar variability and climate change are far from resolved. A definite and well-understood factor in changing climate is the slow variation in the Earth's orbit around the Sun. This factor causes significant changes in seasonal and latitudinal distribution of solar energy. These changes—which occur in cycles of about 21,000, 45,000, and 100,000 years—drive the huge variation in the climate associated with glacial and interglacial periods. 22. Changes in the Earth's Surface The response of the climate to solar radiation depends strongly on the characteristics of the surface of the Earth, whether involving water, snow, bare ground, or vegetation. This is because the different surfaces absorb and reflect the sunlight and infrared radiation received from the atmosphere in very different ways. The two principal factors that determine what happens to the radiation received at the surface are albedo (reflectivity) and moisture availability. Surfaces with high albedo, such as snow, reflect as much as 90% of the solar radiation back into space. Dark surfaces, such as water or dense forests, reflect as little as 5% of the solar radiation. What happens to the radiation that is absorbed depends greatly on the effective moisture availability. Over moist surfaces, most of the energy goes into evaporation and moistens rather than heats the lower atmosphere. Over dry surfaces such as deserts, most of the energy goes into heating the atmosphere. Therefore, changing the surface of the Earth through activities such as deforestation or irrigation can cause significant differences in the local, regional, and perhaps global climate. On very long time scales (millions of years), major geographic features of the Earth have varied. These variations—including the locations of the poles, the drift of continents, and the building and decay of mountains—have profound effects on the Earth's radiation balance. They also greatly affect the general circulation of the atmosphere, including important features of climate such as monsoons. For example, the Himalayas are quite effective in preventing the mixing of warm and cold air masses to their south and north. 23. Atmospheric Changes: The Greenhouse Effect The greenhouse effect is a popular term for the effect that certain variable constituents of the Earth's lower atmosphere have on surface temperatures. These trace gases—water vapor, carbon dioxide (CO2), methane, nitrous oxide, chlorofluorocarbons (CFCs), halocarbons, and others—keep ground temperatures at a global average of approximately 14° C (58° F). Without them, the average would be about −19° C (−2° F), and the oceans would be frozen. Greenhouse gases have this heating effect because infrared radiation emitted by the Earth's surface is absorbed, or trapped, by the gases and radiated both out into space and back toward the surface. (The effect was originally thought comparable to the way in which a greenhouse stays warm, hence the term. The main reason that the inside of a greenhouse is warmer than the outside is not radiative effects, however; it is the prevention of mixing of cold outside air with warm inside air.) An example of a runaway greenhouse effect is Earth's near-twin planetary neighbor, Venus. Because of Venus's thick CO2 atmosphere, the planet's cloud-covered surface is hot enough to melt lead. People are concerned that increases in greenhouse gases—particularly increases caused by human activities—could cause the Earth's surface to warm up to a dangerous level. Even a small rise in average surface temperature might lead to at least partial melting of the polar ice caps and hence a major rise in sea level. Sea level has already risen by about 15 cm (6 in) in the past 100 years. Other adverse environmental effects might include the destruction of ecosystems and wider weather and climate extremes. Water vapor is the most important greenhouse gas, contributing to roughly 60% of the total greenhouse effect. It is the main reason why humid regions experience less cooling at night than do dry regions. CO2 is the second most important, contributing another 26%. Variations in the atmosphere's CO2 content played a major role in past climatic changes. Since the beginning of the Industrial Revolution (about 1750), atmospheric CO2 has increased by approximately 31%, largely as the result of the burning of fossil fuels. This buildup comes about because carbon dioxide persists for a very long time, estimated at more than a century; further emissions hence lead to greater CO2 atmospheric concentration. Nitrous oxide, methane, and CFCs are also long-lived. If the many other determinants of the Earth's present global climate remain more or less constant, the CO2 increase will raise the average temperature at the Earth's surface. With the warming of the atmosphere, the amount of water vapor would probably also increase, because warm air can contain more water than cooler air. This cycle of warming caused by increased CO2, which causes more water vapor, which in turn causes further warming is an example of a positive feedback in the climate system. A negative feedback, and the most important "brake" on the system, is that the long-wave (infrared) radiation emitted by the Earth system increases rapidly as its temperature rises. Carbon dioxide is emitted not only by human activities but also by natural processes such as forest fires and decaying vegetation. It is also removed from the atmosphere by growing vegetation and absorption in the oceans. A great deal remains unknown about this cycling of carbon through the environment. In particular, little is known about the role of oceans in the atmospheric carbon cycle. 24. Atmospheric Changes: Aerosols Aerosols, tiny airborne particulates in the atmosphere, affect the climate in two different ways. Depending on their type, they can either absorb or reflect solar radiation. They thereby either cool or warm the atmosphere. Aerosols can also have an indirect influence on temperature through their effect on clouds and precipitation. Some aerosols act as nuclei upon which cloud drops form. They can therefore affect the number and size of water drops in clouds, which affects the albedo and the precipitation efficiency. For example, a large number of aerosols can cause clouds to contain many very small drops that are too light to fall out as precipitation. Thus through the indirect effect, aerosols cause brighter clouds that are less efficient in producing precipitation. Aerosols occur naturally when winds blow up dust from dry land areas or when volcanoes erupt and inject aerosols into the upper troposphere or lower stratosphere. Aerosols were injected into the stratosphere during the eruption of Mount Pinatubo in the Philippines in 1991; these aerosols reflected enough radiation back into space to cool the surface by up to 5 C degrees (9 F degrees) for two years following the eruption. Humans are now adding huge amounts of aerosols to the atmosphere, exceeding natural sources (except volcanoes). Satellite photos show massive clouds of aerosols blowing off the continents of Asia, North America, and South America. The net effect of these aerosols is a cooling off of the lower atmosphere, partially offsetting the warming effect of increasing greenhouse gases, and a reduction in rainfall. Aerosols emitted from the surface are washed out of the atmosphere by precipitation, however; they typically last only about a week. Therefore, unlike the greenhouse gases, they do not build up in the atmosphere. Their effects are primarily regional. This difference between aerosols and the behavior of the longlived greenhouse gases complicates projections of future climate changes. 25. Natural Variability, Feedbacks, and Abrupt Climate Change The many factors that cause the climate to change are all responsible to some degree for causing the fluctuations of the past. Even if every one of the external factors were constant (for example, the orbital parameters of the Earth), however, the climate would still vary over many time scales. This is because the atmosphereocean-land system is highly nonlinear and interactive on many time and space scales. The individual internal properties of the climate system oscillate from one state to another. Through feedbacks, they affect other properties in complex ways. One example is the El Niño–La Niña cycle; this involves a strong coupling between the ocean and the atmosphere in the equatorial Pacific. During an El Niño, equatorial Pacific Ocean temperatures are as much as 5 C degrees (9 F degrees) above normal. The easterly trade winds weaken and are replaced by westerlies, and the sea-level pressure patterns over the Pacific and many other parts of the world are modified. These variations are accompanied by significant changes in the atmospheric circulation patterns, jet streams, and storm tracks, with associated shifts in cold and warm anomalies and precipitation patterns. These shifts have an enormous impact on agriculture and fisheries in some parts of the world, notably North and South America, Australia, Indonesia, and parts of Africa and southern Asia. Because of the complex nonlinear interactions among the components of the climate system, the climate may not always change gradually. Abrupt changes (major transformations that occur within only a few years) are possible that are difficult to predict. An example of a possible abrupt climate change could be associated with a shutdown of the global ocean circulation. Called the thermohaline circulation, this is driven by differences in density associated with temperature and salinity. The thermohaline circulation is a giant conveyor belt of water that transports heat and salt around the world; it plays a key role in determining the climate. In the thermohaline circulation of the North Atlantic, warm, relatively light surface water in the Gulf Stream flows to high latitudes; there it cools and becomes saltier, because evaporation exceeds precipitation. The denser water sinks and returns southward in the deep ocean. The thermohaline circulation is relatively sensitive, so that small fluctuations in climate could cause considerable changes in the circulation or even shut it down completely. For example, melting of Arctic ice and increased precipitation over North America might freshen the surface water of the North Atlantic portion of the conveyor belt, thereby reducing its density; this would prevent the sinking branch of the circulation, causing the whole circulation to slow down or stop. The consequences for the climate in northern Europe would be severe. 26. Global Warming: Estimates of Impacts of Global Warming The Sun is by far the primary source of energy for the Earth. The variations in total energy output from the Sun are very small. They amount to about plus or minus 0.1% over the 11-year solar cycle. The direct heating effect of this magnitude of change is too small to explain the major fluctuations that have been reported; it is estimated, however, that warming by perhaps as much as 0.2 C degrees (0.36 F degrees) in the first part of the 20th century may have been due to changes in the Sun. Even though the variation in the total energy output from the Sun is small, the variation in the ultraviolet wavelengths is much larger (about 1% to 10%). This magnitude is enough to affect the amount of stratospheric ozone (see ozone layer), which is a strong absorber of solar radiation in the stratosphere. Changes in the stratospheric heating due to changes in ozone modify stratospheric circulation; moreover, through coupling with the troposphere and the oceans, they also modify the weather and climate near the Earth's surface. These positive feedbacks may amplify the direct effects of small variations in solar radiation. Paleoclimatology records over thousands of years have indeed shown a strong correlation of climate with solar variability. It is therefore likely that the Sun has contributed to events such as the Little Ice Age (from A.D. c.1300 to c.1850). Nevertheless, the details of the connection between solar variability and climate change are far from resolved. A definite and well-understood factor in changing climate is the slow variation in the Earth's orbit around the Sun. This factor causes significant changes in seasonal and latitudinal distribution of solar energy. These changes—which occur in cycles of about 21,000, 45,000, and 100,000 years—drive the huge variation in the climate associated with glacial and interglacial periods.