1. Introduction

Global Warming or Climate Change, measurable increases in the average temperature of Earth’s atmosphere, oceans, and landmasses. Scientists believe Earth is currently facing a period of rapid warming brought on by rising levels of heat-trapping gases, known as greenhouse gases, in the atmosphere.

Greenhouse gases retain the radiant energy (heat) provided to Earth by the Sun in a process known as the greenhouse effect. Greenhouse gases occur naturally, and without them the planet would be too cold to sustain life as we know it. Since the beginning of the Industrial Revolution in the mid-1700s, however, human activities have added more and more of these gases into the atmosphere. For example, levels of carbon dioxide, a powerful greenhouse gas, have risen by 35 percent since 1750, largely from the burning of fossil fuels such as coal, oil, and natural gas. With more greenhouse gases in the mix, the atmosphere acts like a thickening blanket and traps more heat.

II. Global Warming in the Past

Earth has warmed and cooled many times since its formation about 4.6 billion years ago. Global climate changes were due to many factors, including massive volcanic eruptions, which increased carbon dioxide in the atmosphere; changes in the intensity of energy emitted by the Sun; and variations in Earth’s position relative to the Sun, both in its orbit and in the inclination of its spin axis.

Variations in Earth’s position, known as Milankovitch cycles, combine to produce cyclical changes in the global climate. These cycles are believed to be responsible for the repeated advance and retreat of glaciers and ice sheets during the Pleistocene Epoch (1.8 million to 11,500 years before present), when Earth went through fairly regular cycles of colder “glacial” periods (also known as ice ages) and warmer “interglacial” periods. Glacial periods occurred at roughly 100,000-year intervals.

An interglacial period began about 10,000 years ago, when the last ice age came to an end. Prior to that ice age, an interglacial period occurred about 125,000 years ago. During interglacial periods, greenhouse gases such as carbon dioxide and methane naturally increase in the atmosphere from increased plant and animal life. But since 1750 greenhouse gases have increased dramatically to levels not seen in hundreds of thousands of years, due to the rapid growth of the human population combined with developments in technology and agriculture. Human activities now are a powerful factor influencing Earth’s dynamic climate.

The ice of the polar regions furnishes clues to the makeup of Earth’s ancient atmosphere. Ice cores that scientists have bored from the ice sheets of Greenland and Antarctica provide natural records of both temperature and atmospheric greenhouse gases going back hundreds of thousands of years. Layers in these ice cores created by seasonal snowfall patterns allow scientists to determine the age of the ice in each core. By measuring tiny air bubbles trapped in the ice and properties of the ice itself, scientists can estimate the temperature and amount of greenhouse gases in Earth’s past atmosphere at the time each layer formed. Based on this data, scientists know that greenhouse gases have now risen to levels higher than at any time in the last 650,000 years.

Greenhouse gases are rising, and temperatures are following. Before the late 1800s, the average surface temperature of Earth was almost 15°C (59°F). Over the past 100 years, the average surface temperature has risen by about 0.7 Celsius degrees (1.3 Fahrenheit degrees), with most of the increase occurring since the 1970s. Scientists have linked even this amount of warming to numerous changes taking place around the world, including melting mountain glaciers and polar ice, rising sea level, more intense and longer droughts, more intense storms, more frequent heat waves, and changes in the life cycles of many plants and animals. Warming has been most dramatic in the Arctic, where temperatures have risen almost twice as much as the global average.

III. Global Warming in the Future

Scientists project global warming to continue at a rate that is unprecedented in hundreds of thousands or even millions of years of Earth’s history. They predict considerably more warming in the 21st century, depending on the level of future greenhouse gas emissions. For a scenario (possible situation) assuming higher emissions—in which emissions continue to increase significantly during the century—scientists project further warming of 2.4 to 6.4 Celsius degrees (4.3 to 11.5 Fahrenheit degrees) by the year 2100. For a scenario assuming lower emissions—in which emissions grow slowly, peak around the year 2050, and then fall—scientists project further warming of 1.1 to 2.9 Celsius degrees (1.9 to 5.2 Fahrenheit degrees) by the year 2100.

Melting polar ice and glaciers, as well as warming of the oceans, expands ocean volume and raises sea level, which will eventually flood some coastal regions and even entire islands. Patterns of rainfall are expected to change, with higher latitudes (closer to the poles) projected to receive more rainfall, and subtropical areas (such as the Mediterranean and southern Africa) projected to receive considerably less. Changes in temperature and precipitation patterns may damage food crops, disrupting food production in some parts of the world. Plant and animal species will shift their ranges toward the poles or to higher elevations seeking cooler temperatures, and species that cannot do so may become extinct. Increasing levels of carbon dioxide in the atmosphere also leads to increased ocean acidity, damaging ocean ecosystems.

Human beings face global warming with a huge population at risk. The potential consequences are so great that many of the world’s leading scientists—and increasingly, politicians, business leaders, and other citizens—are calling for international cooperation and immediate action to counteract the problem.

IV. The Greenhouse Effect

The energy that lights and warms Earth comes from the Sun. Short-wave radiation from the Sun, including visible light, penetrates the atmosphere and is absorbed by the surface, warming Earth. Earth’s surface, in turn, releases some of this heat as long-wave infrared radiation.

Much of this long-wave infrared radiation makes it back out to space, but a portion remains trapped in Earth’s atmosphere, held in by certain atmospheric gases, including water vapor, carbon dioxide, and methane. Absorbing and reflecting heat radiated by Earth, these gases act somewhat like the glass in a greenhouse, and are thus known as greenhouse gases.

Only greenhouse gases, which make up less than 1 percent of the atmosphere, offer the Earth any insulation. All life on Earth relies on the greenhouse effect—without it, the average surface temperature of the planet would be about -18°C (0°F) and ice would cover Earth from pole to pole.

A. Types of Greenhouse Gases

Greenhouse gases occur naturally in the environment and also result from human activities. By far the most abundant greenhouse gas is water vapor, which reaches the atmosphere through evaporation from oceans, lakes, and rivers. The amount of water vapor in the atmosphere is not directly affected by human activities. Carbon dioxide, methane, nitrous oxide, and ozone all occur naturally in the environment, but they are being produced at record levels by human activities. Other greenhouse gases do not occur naturally at all and are produced only through industrial processes. Human activities also produce airborne particles called aerosols, which offset some of the warming influence of increasing greenhouse gases.

A.1. Carbon Dioxide

Carbon dioxide is the second most abundant greenhouse gas, after water vapor. Carbon dioxide constantly circulates in the environment through a variety of natural processes known as the carbon cycle. It is released into the atmosphere from natural processes such as eruptions of volcanoes; the respiration of animals, which breathe in oxygen and exhale carbon dioxide; and the burning or decay of plants and other organic matter. Carbon dioxide leaves the atmosphere when it is absorbed into water, especially the oceans, and by plants, especially trees. Through a process called photosynthesis, plants use the energy of light to convert carbon dioxide and water into simple sugars, which they use as food. In the process, plants store carbon in new tissue and release oxygen as a byproduct.

Humans are significantly increasing the amount of carbon dioxide released to the atmosphere through the burning of fossil fuels (such as coal, oil, and natural gas), solid wastes, and wood and wood products to heat buildings, drive vehicles, and generate electricity. At the same time, the number of trees available to absorb carbon dioxide through photosynthesis has been greatly reduced by deforestation, the widespread cutting of trees for lumber or to clear land for agriculture.

Human activities are causing carbon dioxide to be released to the atmosphere much faster than Earth’s natural processes can remove it. In addition, carbon dioxide can remain in the atmosphere a century or more before nature can dispose of it. Before the Industrial Revolution began in the mid-1700s, there were about 280 molecules of carbon dioxide per million molecules of air (abbreviated as parts per million, or ppm). Concentrations of carbon dioxide have risen since then as industrial production and fossil fuel-based transportation and electricity generation have spread around the world, accelerating in the last 50 years. In 2007 the Intergovernmental Panel on Climate Change (IPCC), a major scientific organization, reported that levels of carbon dioxide had risen to a record high of 379 ppm and are increasing an average of 1.9 ppm per year.

To stabilize atmospheric concentrations of carbon dioxide, global emissions would need to be cut significantly—on the order of 70 to 80 percent. If efforts are not made to reduce greenhouse gas emissions, carbon dioxide is projected to reach concentrations more than double or even triple the level prior to the Industrial Revolution by 2100. In a higher-emissions scenario carbon dioxide is projected to reach 970 ppm by 2100, more than tripling preindustrial concentrations. In a lower-emissions scenario, carbon dioxide is projected to reach 540 ppm by 2100, still almost doubling preindustrial concentrations. (For a description of these two emissions scenarios, see the Introduction: Global Warming in the Future section of this article.

A.2. Methane

Methane is emitted into the atmosphere during the mining of coal and the production and transport of natural gas and oil. Methane also comes from rotting organic matter in landfills, rice paddies, and wetlands, as well as from certain animals, especially cows, as a byproduct of digestion. Live plants also emit small amounts of methane.

Scientists are increasingly concerned about the release of methane and carbon dioxide from melting permafrost, areas of frozen ground in the tundra (Arctic plains) of Alaska, Siberia, and other subpolar regions. Temperatures in the top layer of permafrost have increased, leading to a decrease in the area of seasonally frozen ground. Methane released from these areas as they melt would contribute to further warming and further melting, in what scientists call a feedback process.

Since the beginning of the Industrial Revolution, the amount of methane in the atmosphere has more than doubled. Methane traps nearly 30 times more heat than the same amount of carbon dioxide. Compared to carbon dioxide, methane appears in lower concentrations in the atmosphere and remains in the atmosphere for a shorter time. In total, methane contributes about a third as much as carbon dioxide to global warming.

A.3. Nitrous Oxide

Nitrous oxide is a potent greenhouse gas that is released primarily by plowing farm soils and burning fossil fuels. Nitrous oxide traps about 300 times more heat than does the same amount of carbon dioxide. The concentration of nitrous oxide in the atmosphere has increased 18 percent over preindustrial levels. Nitrous oxide contributes about a tenth as much as carbon dioxide to global warming.

A.4. Ozone

Ozone is both a natural and human-made greenhouse gas. Ozone in the upper atmosphere is known as the ozone layer and shields life on Earth from the Sun’s harmful ultraviolet radiation. This ozone is formed by the action of ultraviolet light from the Sun on molecules of ordinary oxygen. Some chemical compounds are known to destroy ozone molecules in the upper atmosphere. This can break down, or deplete, the ozone layer. Depletion of the ozone layer actually causes a slight cooling, offsetting a small part of the warming from greenhouse gases.

However, ozone in the lower atmosphere is a component of smog, a severe type of air pollution. Nitrogen oxides and volatile organic gases emitted by automobiles and industrial sources combine to form the ozone in smog. This ozone is a poison that damages vegetation, kills trees, irritates lung tissues, and attacks rubber. It is also a greenhouse gas that contributes about a fourth as much as carbon dioxide to global warming. Unlike the greenhouse gases discussed above, which are well-mixed throughout the atmosphere, ozone in the lower atmosphere tends to be limited to industrialized regions.

A.5. Synthetic Chemicals

Manufacturing processes use or generate many synthetic chemicals that are powerful greenhouse gases. Although these gases are produced in relatively small quantities, they trap hundreds to thousands of times more heat in the atmosphere than an equal amount of carbon dioxide does. In addition, their chemical bonds make them exceptionally long-lived in the environment.

Human-made greenhouse gases include chlorofluorocarbons (CFCs), a family of chlorine-containing gases that were widely used in the 20th century as refrigerants, aerosol spray propellants, and cleaning agents. Scientific studies showed that the chlorine released by CFCs into the upper atmosphere destroys the ozone layer. As a result, CFCs are being phased out of production under a 1987 international treaty, the Montréal Protocol on Substances that Deplete the Ozone Layer. CFCs were mostly banned in industrialized nations beginning in 1996 and will be phased out in developing countries after 2010. New chemicals have been developed to replace CFCs, but they are also potent greenhouse gases. The substitutes include hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs).

Although HCFCs are less damaging to the ozone layer than CFCs, they also contain chlorine and are scheduled to be completely phased out by 2030 under amendments made in 2007 to the Montréal Protocol. Developed countries must end their use of HCFCs by 2020 under the amended protocol.

Although HFCs and PFCs do not destroy the ozone layer, they are powerful greenhouse gases. In addition, they last longer in the atmosphere than CFCs, which have an average lifespan of 120 years. PFCs are exceptionally long-lived chemicals—they can persist in the atmosphere between 2,600 and 50,000 years, depending on the specific compound. Their accumulation in the atmosphere is therefore essentially irreversible. PFCs are used in the production of aluminum, in the manufacture of semiconductors, and as refrigerants.

Another human-made chemical, sulfur hexafluoride, is one of the most potentially destructive greenhouse gases ever produced. This synthetic gas compound has nearly 24,000 times the warming effect of an equal amount of carbon dioxide over a period of 100 years. It is an exceptionally stable gas with an estimated lifespan of 3,200 years once it is released in the atmosphere. Sulfur hexafluoride is used as insulation for high-voltage electrical equipment and in the production and casting of magnesium.

B. Aerosols

Fuel combustion, and to a lesser extent agricultural and industrial processes, produce not only gases but also tiny solid and liquid particles called aerosols that remain suspended in the atmosphere. Although aerosols are not considered greenhouse gases, they do affect global warming in several ways.

Diesel engines and some types of biomass burning produce black aerosols such as soot, which absorb the Sun’s energy and therefore contribute to warming. Conversely, coal-fired power plants burning high-sulfur coal emit sulfate aerosols, which are light-colored aerosols that reflect incoming solar energy back to space. In this way, they have a cooling effect. Natural aerosols that also have a cooling effect are produced during volcanic eruptions and the evaporation of seawater. Aerosol particles also have an indirect cooling influence by acting as “seeds” for the condensation of water vapor into cloud masses. In general, the amount of solar energy reflected back to space is greater on cloudy days.

Overall, aerosols may roughly offset the net warming influence of non-carbon dioxide greenhouse gases, half through their direct cooling effect and half through their indirect cooling effect. However, considerable uncertainty in aerosol processes means that their cooling influence could be much larger or much smaller. Aerosols are one of the least-understood factors in climate change and their effects are still being debated. Scientists are more certain, however, about the net effect of all greenhouse gas and aerosol emissions, which is estimated to be roughly equal to the warming influence of carbon dioxide alone.

V. Measuring Global Warming

As early as 1896 scientists suggested that burning fossil fuels might change the composition of the atmosphere and that an increase in global average temperature might result. The first part of this hypothesis was confirmed in 1957, when researchers working in the global research program called the International Geophysical Year sampled the atmosphere from the top of the Hawaiian volcano Mauna Loa. Their instruments indicated that carbon dioxide concentration was indeed rising. Since then, the composition of the atmosphere has been carefully tracked. The data collected show undeniably that the concentrations of greenhouse gases in the atmosphere are increasing.

Measuring warming of the global climate (the long-term average pattern of temperature) is a complex process. Temperatures vary widely all the time and from place to place, and a local warming trend may simply be due to the natural variability of the climate. But using many years of climate observations from around the world, scientists have detected a warming trend beyond such random fluctuations.

Records going back to the late 1800s show a warming trend, but these statistics were spotty and untrustworthy. However, since 1957 data have been gathered from more reliable weather stations, located far away from cities, and since 1979 from satellites. These data have provided new, more accurate measurements, especially for the 70 percent of the planetary surface that is ocean water. These more accurate records indicate that a clear surface warming trend exists and that temperatures have risen particularly sharply in the last few decades.

Eleven out of the twelve warmest years on record have occurred since 1995, with 2001-2006 all in the top six. Not every place in the world is warming at the same rate, or even warming at all—in fact, some parts of the world cooled over the 20th century. For this reason, many scientists use the term climate change rather than global warming. However, taking all of the local measurements together, the world is warming significantly, and many more places are warming than are cooling.

A. Debates Over Global Warming

While the behavior of the climate system and the processes that cause global warming are well understood and grounded in basic scientific principles, scientists are still working to understand certain details of the climate system and its response to increasing greenhouse gases. Scientific uncertainty is inevitable with a system as complex as Earth’s climate. However, advancements in measuring, analyzing, and modeling techniques have helped clarify many uncertainties in recent years.

For example, there had been uncertainty regarding why the warming trend stopped for three decades in the middle of the 20th century. Records even showed some cooling before the climb resumed in the 1970s. The lack of warming at mid-century is now attributed largely to the sulfate aerosols in air pollution, which have a cooling effect because they reflect some incoming sunlight back to space. Continued warming has now overcome this effect, in part because pollution control efforts have made the air cleaner.

Satellite measurements of atmospheric temperature, which became available around 1980, originally were thought to measure much less warming in the lower region of the atmosphere than surface thermometers. This led to some doubt about the accuracy of the warming detected at the surface. Eventually, other researchers reanalyzed the satellite data using more advanced techniques and concluded that the satellites were detecting warming quite similar to surface measurements. While there is still some uncertainty, scientists examining the satellite data now agree that the record is consistent with a warming world.

For many years global warming was portrayed in the media as an issue with two sides, with some scientists arguing that global warming is occurring and others arguing that it is not. However, this portrayal was an oversimplification of the scientific debate. Skeptics of global warming, including some scientists, pointed to lingering scientific uncertainties to question whether global warming is actually occurring. However, there is now undeniable evidence that global temperatures are increasing, based on direct temperature measurements and observations of other impacts such as melting glaciers and polar ice, rising sea level, and changes in the lifecycles of plants and animals. As the scientific evidence on rising global temperature became indisputable, skeptics focused their argument on whether human activities are in fact the cause of global warming. They argued that the observed warming could be caused by natural processes such as changes in the energy emitted by the Sun. However, the Sun’s influence has been found to have contributed only slightly to observed warming, particularly since the mid-20th century. In fact, there is overwhelming evidence that greenhouse gas emissions from human activities are the main cause of the warming.

In 1988 the United Nations Environment Program (UNEP) and the World Meteorological Organization (WMO) established the Intergovernmental Panel on Climate Change (IPCC). The panel comprises thousands of the top climate scientists from around the world and releases a report every six years describing the state of scientific knowledge on global warming. The IPCC’s Fourth Assessment Report, released in 2007, offered the strongest scientific consensus to date on global warming. The panel concluded that it is “very likely” (more than 90 percent probability) that human activities are responsible for most of the warming since the mid-20th century; that it is “extremely unlikely” (less than 5 percent probability) that the warming is due to natural variability; and that it is “very likely” the warming is not due to natural causes alone. This level of certainty is extremely high, given the complexity of the climate system and of the influence of human activities on the climate.

B. Global Warming Projections

In its 2007 report the IPCC projected temperature increases for several different scenarios, depending on the magnitude of future greenhouse gas emissions. For a “moderate” scenario—in which emissions grow slowly, peak around the year 2050, and then fall—the IPCC report projected further warming of 1.1 to 2.9 Celsius degrees (1.9 to 5.2 Fahrenheit degrees) by the year 2100. For a “high-emissions” scenario—in which emissions continue to increase significantly and finally level off at the end of the century—the IPCC report projected further warming of 2.4 to 6.4 Celsius degrees (4.3 to 11.5 Fahrenheit degrees) by the year 2100.

The IPCC cautioned that even if greenhouse gas concentrations in the atmosphere ceased growing, the climate would continue to warm for an extended period as a result of past emissions, and with more dramatic effects than were observed during the 20th century. If greenhouse gas emissions continue to increase, scientists project severe climate changes.

In October 2007 a study published in the Proceedings of the National Academy of Sciences warned that climate models used to project future global warming may have been overly optimistic. The study found that atmospheric carbon dioxide levels had increased 35 percent from 1990 to 2006, a rate of increase far higher than most climate models had assumed. The researchers reported that the average rate of growth in carbon dioxide levels was 1.3 percent during the period from 1990 to 1999, but 3.3 percent from 2000 to 2006. In 2000 an estimated 7 billion metric tons of carbon were released into the atmosphere from burning fossil fuels; by 2006 that number had grown to 8.4 billion metric tons, according to the study. Scientists pointed to the unexpectedly rapid melting of sea ice in the Arctic Ocean during the summer of 2007 as evidence that climate models were failing to predict how quickly the climate was changing.

VI. Effects of Global Warming

Scientists use elaborate computer models of temperature, precipitation patterns, and atmosphere circulation to study global warming. Based on these models, scientists have made many projections about how global warming will affect weather, glacial ice, sea levels, agriculture, wildlife, and human health. Many changes linked to rising temperatures are already being observed.

A. Weather

Scientists project that the polar regions of the Northern Hemisphere will heat up more than other areas of the planet, and glaciers and sea ice will shrink as a result. Regions that now experience light winter snows may receive no snow at all. In temperate mountains, snowlines will be higher and snowpacks will melt earlier. Growing seasons will be longer in some areas. Winter and nighttime temperatures will tend to rise more than summer and daytime temperatures. Many of these trends are already beginning to be observed. Arctic temperatures, for example, have increased almost twice as much as the global average over the past 100 years.

A warmer world will be generally more humid as a result of more water evaporating from the oceans. A more humid atmosphere can both contribute to and offset further warming. On the one hand, water vapor is a greenhouse gas, and its increased presence would further increase warming. On the other hand, more water vapor in the atmosphere will produce more clouds, which reflect sunlight back into space, thereby slowing the warming process (see Water Cycle). It is uncertain which of these effects will be greater in the future, and scientists factor in both possibilities when projecting temperature increases. This is one of the main reasons that projections include ranges of high and low temperatures for different emissions scenarios.

Storms are expected to be more frequent and more intense in a warmer world. Water will also evaporate more rapidly from soil, causing it to dry out faster between rains. Some regions might actually become drier than before. Overall, higher latitudes are projected to receive more rainfall, and subtropical areas are projected to receive less. Shifting patterns of precipitation (both snow and rain) have been observed in many regions since 1900. Significantly wetter conditions have been recorded in the eastern parts of North and South America, northern Europe, and northern and central Asia. Drier conditions have prevailed in the Sahel region of western Africa, southern Africa, the Mediterranean, and parts of southern Asia. Droughts are projected to become longer and more intense; in fact, this has already been observed since the 1970s, particularly in the tropics and subtropics.

Weather patterns are expected to be less predictable and more extreme. Storm tracks are projected to move toward the poles, shifting wind, rainfall, and temperature patterns. Heat waves will continue to become more frequent and intense, a trend already observed. Hurricanes, violent storms that draw their force from warm ocean water, are likely to become more severe. The intensity of hurricanes has already increased since the 1970s.

B. Ice Sheets and Glaciers

Warming temperatures are already causing significant changes to mountain glaciers around the world, ice sheets in Greenland and the Antarctic, and polar sea ice in the Arctic. From Europe to Africa to Asia to North America, mountain glaciers have receded over the 20th century, and melting is becoming more rapid. The large-scale melting of ice may accelerate the pace of global warming in what is known as a feedback process. Because ice reflects sunlight back out to space, it has a cooling effect. Water and land, which are darker than ice, absorb and retain more heat.

Glaciers on Kilimanjaro, the highest mountain in Africa, have lost 82 percent of their ice since 1912 and are estimated to be gone completely by 2020. Glaciers in the lofty Himalayas of Asia are melting at a rate of 9 to 15 m (30 to 50 ft) per year. Annual runoff from these glaciers feeds major rivers such as the Ganges, Yangtze, and Mekong. Glacier National Park in Montana is projected to have no glaciers left by 2030, and the number of glaciers has already dropped from an estimated 150 in 1850 to 26 in 2007.

In the Arctic annual average temperature has increased at almost twice the global rate over the past few decades. The area covered by sea ice during summer has declined by 15 to 20 percent in the last 30 years, and is projected to disappear almost completely late in the 21st century. Many species, including polar bears, seals, and walrus, depend on sea ice for their survival. The rapid loss of Alaskan glaciers represents almost half of the total loss of ice in glaciers worldwide, and makes a significant contribution to observed sea level rise. Melting of the Greenland ice sheet, which could raise sea level by 7 m (23 ft) if it melted completely, is also accelerating. The area that is experiencing at least some melting increased by 16 percent from 1979 to 2002, and scientists estimate that warming of more than a few degrees Celsius could cause widespread and possibly unstoppable melting, leading to significant sea level rise.

Fresh water flowing from melting Arctic ice into the North Atlantic Ocean could disrupt ocean circulation patterns, which have a significant influence on the global climate. According to scientific projections, a collapse of ocean circulation patterns is unlikely to occur by 2100. However, scientists do expect there to be a weakening and slowing of the thermohaline circulation, also known as the ocean conveyor belt. In addition, a disruption of surface circulation patterns in the North Atlantic, known collectively as the Gulf Stream, could lead to cooling in Europe.

In Antarctica the situation is somewhat different than in the Arctic. The Antarctic Peninsula, the “tail” of land reaching toward South America, has experienced dramatic warming at a rate several times the global average over the past 50 years. However, other parts of Antarctica have not shown similar trends, with some areas warming and some cooling. Overall, Antarctica is estimated to be warming at about the global average rate. Unlike the Arctic, there has been no clear general trend in sea ice. In the Antarctic Peninsula, however, ten floating ice shelves have lost more than 14,000 sq km (5,400 sq mi) of ice, and probably have not been at such a low level in the past 10,000 years. As in Greenland, scientists estimate that warming of more than a few degrees Celsius could lead to widespread melting of the West Antarctica ice sheet. This melting alone would raise sea level by as much as 5 m (16 ft).

C. Sea Level

As the atmosphere warms, the surface layer of the ocean warms as well, expanding in volume and thus raising sea level. The melting of glaciers and ice sheets, especially around Greenland, further swells the sea. Sea level rose 10 to 25 cm (4 to 10 in) during the 20th century. (The range is due to measurement uncertainties and regional variation.) By the end of the 21st century, sea level is projected to rise another 28 to 58 cm (11 to 23 in) if greenhouse gas emissions continue to increase significantly. The projection is somewhat less—a rise of 19 to 37 cm (8 to 15 in)—for a scenario in which greenhouse gas emissions peak around the year 2050 and then decrease. These projections do not incorporate possible large-scale melting of the Greenland or Antarctic ice sheets, which could begin in the 21st century with warming of a few degrees Celsius.

Rising sea level will complicate life in many island and coastal regions. Storm surges, in which winds locally pile up water and raise the sea, will become more frequent and damaging. Erosion of cliffs, beaches, and dunes will increase. As the sea invades the mouths of rivers, flooding from runoff will also increase upstream.

Small island nations such as Tuvalu and Kiribati, where the highest land is only a few meters above sea level, are already experiencing saltwater intrusion, which is making groundwater undrinkable, and increased impacts from typhoons and heavy surf. These nations could literally cease to exist as the rise in sea level continues, and their governments are negotiating with other nations to transplant their populations.

Even a modest rise in sea level will have huge impacts on coastal ecosystems. For example, a 50-cm (20-in) rise would submerge about half of the present coastal wetlands of the United States and other low-lying areas such as parts of New Orleans and the Louisiana coast. Much of the Florida Everglades would be lost to the sea. New marshes would eventually form in many places, but not where urban areas and developed landscapes block the way.

Damage can be curbed locally in various ways. Coastlines can be armored with dikes, levies, and other barriers to block encroachment of the sea. Alternatively, governments can assist coastal populations in moving to higher ground, although such a process is extremely costly, especially in heavily populated areas. Some extremely low-lying countries would face rising sea level with huge populations at risk. Wealthy countries like The Netherlands may need to spend huge amounts of money to protect their shorelines, while poor countries like Bangladesh may be forced to simply abandon low-lying coastal regions.

D. Agriculture

Global warming of a few degrees may increase agricultural production, but not necessarily in the same places where crops are grown now. Southern Canada, for example, may benefit from more rainfall and a longer growing season. At the same time, the semiarid tropical farmlands in some parts of Africa may become further impoverished. Farming regions such as California’s Central Valley that bring in irrigation water from distant mountains may suffer as the winter snowpack, which functions as a natural reservoir, melts before the peak growing months. Crops and woodlands may also be afflicted by more insects and plant diseases. Agricultural areas will need to adapt to changing conditions, such as by shifting the types of crops grown or investing in drought-tolerant or heat-tolerant varieties. Scientists estimate that warming of up to about 3 Celsius degrees (5.4 Fahrenheit degrees) could increase global agricultural potential, but that further warming is likely to decrease this potential.

E. Plants and Animals

Plants and animals will find it difficult to escape from or adjust to the effects of global warming. Scientists have already observed shifts in the lifecycles of many plants and animals, such as flowers blooming earlier and birds hatching earlier in the spring. Many species have begun shifting where they live or their annual migration patterns due to warmer temperatures.

With further warming, animals will tend to migrate toward the poles and up mountainsides toward higher elevations. Plants will also attempt to shift their ranges, seeking new areas as old habitats grow too warm. In many places, however, human development will prevent these shifts. Species that find cities or farmland blocking their way north or south may become extinct. Species living in unique ecosystems, such as those found in polar and mountaintop regions, are especially at risk because migration to new habitats is not possible. For example, polar bears and marine mammals in the Arctic are already threatened by dwindling sea ice but have nowhere farther north to go.

Projecting species extinction due to global warming is extremely difficult. Some scientists have estimated that 20 to 50 percent of species could be committed to extinction with 2 to 3 Celsius degrees (3.6 to 5.4 Fahrenheit degrees) of further warming. The rate of warming, not just the magnitude, is extremely important for plants and animals. Some species and even entire ecosystems, such as certain types of forest, may not be able to adjust quickly enough and may disappear.

Ocean ecosystems, especially fragile ones like coral reefs, will also be affected by global warming. Warmer ocean temperatures can cause coral to “bleach,” a state which if prolonged will lead to the death of the coral. Scientists estimate that even 1 Celsius degree (1.8 Fahrenheit degrees) of additional warming could lead to widespread bleaching and death of coral reefs around the world. Also, increasing carbon dioxide in the atmosphere enters the ocean and increases the acidity of ocean waters. This acidification further stresses ocean ecosystems.

F. Human Health

In a warmer world, scientists predict that more people will get sick or die from heat stress, due not only to hotter days but more importantly to warmer nights (giving the sufferers less relief). More frequent and intense heat waves will further contribute to this trend. At the same time, there will be some decreases in the number of cold-related deaths. Diseases such as malaria, now found in the tropics and transmitted by mosquitoes and other animal hosts, are projected to widen their range as these animal hosts move into regions formerly too cold for them. Other tropical diseases may spread similarly, including dengue fever, yellow fever, and encephalitis. Scientists also project rising incidence of allergies and respiratory diseases as warmer air grows more charged with pollutants, mold spores, and pollens.

VII. Efforts to Control Global Warming

Responding to the challenge of controlling global warming will require fundamental changes in energy production, transportation, industry, government policies, and development strategies around the world. These changes take time. The challenge today is managing the impacts that cannot be avoided while taking steps to prevent more severe impacts in the future.

Reducing emissions of greenhouse gases, also called greenhouse gas mitigation, is a necessary strategy for controlling global warming. There are two major approaches to slowing the buildup of greenhouse gases. One is to reduce the consumption of fossil fuels, thereby reducing greenhouse gas emissions. The other is to keep carbon dioxide out of the atmosphere by storing the gas or its carbon component somewhere else, a strategy known as carbon sequestration or carbon capture.

A. Carbon Capture

One way to keep carbon dioxide emissions from reaching the atmosphere is to preserve and plant more trees. Trees, especially young and fast-growing ones, soak up a great deal of carbon dioxide from the atmosphere and store carbon atoms in new wood. Worldwide, forests are being cleared at an alarming rate, particularly in the tropics. In many areas, there is little regrowth as land loses fertility or is changed to other uses, such as farming or housing developments. In addition, when trees are burned to clear land, they release stored carbon back into the atmosphere as carbon dioxide. Slowing the rate of deforestation and planting new trees can help counteract the buildup of greenhouse gases.

Carbon dioxide gas can also be captured directly. Carbon dioxide has traditionally been injected into depleted oil wells to force more oil out of the ground or seafloor. The same process can be used to store carbon dioxide released by a power plant, factory, or any large stationary source. For example, since 1996 this process has been used at a natural gas drilling platform off the coast of Norway. Carbon dioxide brought to the surface with the natural gas is captured, compressed, and then injected into an aquifer deep below the seabed from which it cannot escape. In most cases, the process of carbon capture would also involve transporting the gas in compressed form to suitable locations for underground storage. Deep ocean waters could also absorb a great deal of carbon dioxide, although the environmental effects may be harmful to ocean life. The feasibility and environmental effects of these options are under study by international teams.

B. Energy Sources

The total worldwide consumption of fossil fuels is increasing by several percent per year. However, energy use around the world is slowly shifting away from fuels that release a great deal of carbon dioxide toward fuels that release somewhat less of this heat-trapping gas.

Wood was the first major source of energy used by humans. With the advent of the Industrial Revolution in the mid-1700s, coal became the dominant energy source. By the mid-1800s oil had replaced coal in dominance, fueling the internal combustion engines that were eventually used in automobiles. By the 1900s, natural gas began to be used worldwide for heating and lighting. In this progression, combustion of natural gas releases less carbon dioxide than oil, which in turn releases less of the gas than do either coal or wood. However, a reversal of this trend may be seen as reserves of oil are used up. Other fuel sources such as tar sands (also known as oil sands) are beginning to be utilized. Producing oil from tar sands involves extraction and refining processes that release carbon dioxide. In addition, the relative abundance of coal reserves in countries such as China and the United States may lead to a new upswing in the use of coal for generating electricity. Newer technologies for cleaner coal-burning power plants may help offset the effects.

Significant reductions in carbon dioxide emissions can only be achieved by switching away from fossil-fuel energy sources. Nuclear power plants release no carbon dioxide at all, but nuclear energy is controversial for reasons of safety, security, and the high costs of nuclear waste disposal. Solar power, wind power, and hydrogen fuel cells also emit no greenhouse gases. These energy sources can be practical, low-pollution alternatives to fossil fuels. Other alternatives include fuels made from plants, such as biodiesel (made from used and new vegetable oil) and ethanol (a plant-based gasoline additive). Use of these fuels can help reduce total carbon dioxide emissions from automobiles. The hybrid electric vehicle (HEV), which uses both an electric motor and a gasoline or diesel engine, emits less carbon dioxide than conventional automobiles (see Electric Car). See also World Energy Supply.

C. International Agreements

International cooperation is required for the successful reduction of greenhouse gases. The first international conference addressing the issue was held in 1992 in Rio de Janeiro, Brazil. At the United Nations Conference on Environment and Development, informally known as the Earth Summit, 150 countries pledged to confront the problem of greenhouse gases by signing the United Nations Framework Convention on Climate Change (UNFCCC). To date, more than 180 nations have ratified the UNFCCC, which commits nations to stabilizing greenhouse gas concentrations in the atmosphere at a level that would avoid dangerous human interference with the climate. This is to be done so that ecosystems can adapt naturally to global warming, food production is not threatened, and economic development can proceed in a sustainable manner.

The nations at the Earth Summit agreed to meet again to translate these good intentions into a binding treaty for emissions reductions. In 1997 in Japan, 160 nations drafted an agreement known as the Kyōto Protocol, an amendment to the UNFCCC. This treaty set mandatory targets for the reduction of greenhouse gas emissions. Industrialized nations that ratify the treaty are required to cut their emissions by an average of 5 percent below 1990 levels. This reduction is to be achieved no later than 2012, and commitments to start achieving the targets are to begin in 2008. Developing nations are not required to commit to mandatory reductions in emissions. Under the Kyōto rules, industrialized nations are expected to take the first steps because they are responsible for most emissions to date and have more resources to devote to emissions-reduction efforts.

The protocol could not go into effect unless industrialized nations accounting for 55 percent of 1990 greenhouse gas emissions ratified it. That requirement was met in November 2004 when Russia approved the treaty, and it went into force in February 2005. By the end of 2006, 166 nations had signed and ratified the treaty. Notable exceptions included the United States and Australia.

In 1998 the United States—then the world’s single largest contributor to greenhouse gas emissions—became a signatory to the Kyōto Protocol. However, in 2001 U.S. president George W. Bush withdrew support for the treaty. He claimed that the treaty’s goals for reducing carbon dioxide emissions would be too costly and would harm the U.S. economy. He also claimed the treaty put an unfair burden on industrialized nations. Opposition to the treaty in the United States was spurred by the oil industry, the coal industry, and other enterprises that manufacture or depend on fossil fuels. These opponents claimed that the economic costs to carry out the Kyōto Protocol could be as much as $300 billion, due mainly to higher energy prices. Proponents of the Kyōto Protocol believed the costs would prove more modest—$88 billion or less—much of which would be recovered as Americans switched to more efficient appliances, vehicles, and industrial processes.

The Kyōto Protocol, which expires in 2012, is only a first step in addressing greenhouse gas emissions. To stabilize or reduce emissions in the 21st century, much stronger and broader action is required. In part this is because the Kyōto provisions did not take into account the rapid industrialization of countries such as China and India, which are among the developing nations exempted from the protocol’s mandatory emissions reductions. However, developing nations are projected to produce half the world’s greenhouse gases by 2035. Leaders of these nations argue that emissions controls are a costly hindrance to economic development. In the past, prosperity and pollution have tended to go together, as industrialization has always been a necessary component of an economy’s development. Whether or not an economy can grow without increasing greenhouse gas emissions at the same time is a question that will be critical as nations such as China and India continue on the path of industrialization.

In 2007 the European Union (EU) took the initiative in coming up with a new international plan to address global warming. At a “green summit” held in March, the 27 nations of the EU reached a landmark accord that went above and beyond the Kyōto Protocol in setting targets to reduce greenhouse gas emissions. The agreement set ambitious targets for the EU overall, but goals for individual EU nations and rules of enforcement were to be determined through additional negotiations.

In the accord EU leaders agreed to reduce emissions by 20 percent from 1990 levels by 2020—or by as much as 30 percent if nations outside the EU joined in the commitments. They also agreed that renewable sources of energy, such as solar and wind power, would make up 20 percent of overall EU energy consumption by 2020 (an increase of about 14 percent). The accord also called for a 10 percent increase in the use of plant-derived fuels, such as biodiesel and ethanol. In addition to these targets, EU leaders agreed to work out a plan to promote energy-saving fluorescent light bulbs, following the example of countries such as Australia and Chile that are officially phasing out less-efficient incandescent light bulbs.

D. Programs in the United States

At a national level, the United States has so far relied on voluntary programs to reduce emissions. For example, the Department of Energy, the Environmental Protection Agency, product manufacturers, local utilities, and retailers have collaborated to implement the Energy Star program. This program rates appliances for energy use and gives some money back to consumers who buy efficient machines.

The U.S. government has also focused on targets for greenhouse gas intensity, which is the ratio of emissions per unit of economic output. For the economy as a whole, greenhouse gas intensity is usually expressed as emissions per dollar of gross domestic product (GDP). Greenhouse gas intensity targets contrast with absolute targets, which limit total emissions (as in the Kyōto Protocol). Greenhouse gas intensity can decline even when total emissions rise. In other words, if the economy grows faster than emissions, greenhouse gas intensity goes down while the total amount of emissions goes up. This has already been the trend in the past few decades in the United States. Emissions intensity has decreased due to improvements in energy efficiency and rapid economic growth in relatively clean sectors, such as information technology and services. However, total U.S. emissions have grown steadily. For example, the emissions intensity of carbon dioxide in the United States decreased by 17 percent from 1990 to 2002, even as total carbon dioxide emissions grew by 18 percent over the same period. This trend of decreasing emissions intensity is expected to continue in the future.

In 2007 the U.S. Supreme Court issued a landmark environmental ruling—and its first relating to the issue of global warming—that greenhouse gases are air pollutants as defined by the Clean Air Act. The court also ordered the Environmental Protection Agency (EPA) to reevaluate its policy of not regulating carbon dioxide emissions from automobiles. The lawsuit, Massachusetts et al. v. Environmental Protection Agency et al., was filed against the EPA by 12 states and 13 environmental groups that had grown frustrated with the agency’s inaction on global warming issues.

Apart from the national government, many state and local governments are also working to curb greenhouse gas emissions. In 2005 three major initiatives were announced. First, the government of California committed to return to 1990 levels by 2020, and reduce emissions 80 percent below 1990 levels by 2050. Second, seven Northeastern and Mid-Atlantic states established the Regional Greenhouse Gas Initiative, a mandatory program to limit emissions from power plants (while allowing emitters to trade allocations). Third, the mayor of Seattle announced the Climate Protection Initiative, committing Seattle to meet the original Kyōto Protocol target for the United States (before it withdrew support for the treaty) of 7 percent reductions below 1990 levels. Since then, hundreds of other city mayors representing about 50 million Americans have committed to this initiative.

Individuals, too, can take steps to curb their own emissions. The same choices that reduce other kinds of pollution work against greenhouse gases. Every time a consumer buys an energy-efficient appliance, uses energy-saving light bulbs, adds insulation to a house, recycles materials, chooses to live near work, or commutes by public transportation, he or she is fighting global warming.

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