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.
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).
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, 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.
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.
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.
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 long-lived greenhouse gases complicates projections of future climate changes.
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 atmosphere-ocean-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.
Estimates of Impacts of Global Warming
Estimates of future global warming are made by computer models of the climate system. These models use various scenarios of human-caused emissions of carbon dioxide and other greenhouse gases, along with other assumptions. The latter include estimates of population, economic development, energy intensity, standards of living, and so on. Of course, there are uncertainties in the emission scenarios and in how the climate system in the models responds to the scenarios. The range of global mean temperature change over the next 100 years predicted by current models for plausible emission scenarios is 1 to 6 C degrees (1.8 to 10.8 F degrees); values are more likely to be in the intermediate range. These are numbers to be concerned about, especially at the high end. Climate changes of this magnitude in the past have had profound effects on life, including human life.
Civilizations have risen and fallen as temperatures and rainfall have shifted from favorable to unfavorable patterns to support agriculture. The significance of a decrease in temperature of 6 C degrees or more associated with the major ice ages is easy to understand. But what about smaller changes of only a few degrees? One of the difficulties in getting many people to take global warming seriously is the projection of an apparently small degree of global warming of about 0.2 C degrees (0.36 F degrees) per decade. People who often experience daily fluctuations of 20 C degrees (36 F degrees) or more have difficulty envisioning the importance of a few degrees of global warming. Climate models and history show, however, that when the Earth's mean temperature changes by only a degree or so, the regional changes in temperature can be much larger—perhaps five times as much. Other important aspects of climate may be affected as well. For example, patterns of precipitation, floods, droughts, and severe storms may shift thousands of kilometers and increase or decrease in intensity and frequency. The Little Ice Age, for instance, was associated with a global cooling of only a few tenths of a C degree, although the cooling was much more pronounced in Europe. The storms, cold, and floods that ravaged Europe during this period had enormous disruptive effects on agriculture, fisheries, economies, health, politics, and wars in this region.
The consequences of global warming are difficult to predict because climate models, although agreeing in general with the global response to increasing greenhouse gases, show great variability in regional responses where the magnitude of the response is likely to be much greater. One area about which models do agree is the Arctic; there a positive feedback occurs between higher temperatures due to the greenhouse effect and where less snow and ice occur. Records show that parts of Alaska have warmed by more than 3 C degrees (5.4 F degrees) since 1970. This has caused the thawing of permafrost, with resulting sinking and damaging of roads and buildings. There has been significant damage to ecosystems, including forests, wildlife, fisheries, and the lives of aboriginal people. Some models predict as much as 10 C degrees (18 F degrees) additional warming in the next century. Such warming would melt all the ice in the Arctic and cause profound changes in the life in this region.
As the Arctic example shows, global warming of even a small amount can have very significant effects locally and regionally. A major impact is likely to be in the frequency and severity of extreme events such as floods, droughts, heat waves, and severe storms, including hurricanes. As the atmosphere warms, it can hold more water vapor; the hydrologic cycle is thus intensified. Precipitation over mid-latitude and high-latitude continents in the Northern Hemisphere increased by about 1% per decade in the 20th century. Floods and droughts are likely to increase in both frequency and severity. Thus the frequency of severe floods may increase from one every 100 years to one every 30 years. The number of days each summer in which temperatures exceed 38° C (100° F) may increase from one or two to ten or more. Tropical and extratropical cyclones and thunderstorms derive part of their energy from the latent heat of water vapor. Therefore, these storms are likely to increase in intensity.
Although some aspects of climate change can be beneficial, the continual state of change tends to make planning difficult. Warming may help to reduce winter heating costs, for example. But the decreased frequency and magnitude of freezing temperatures in some areas will make insects and diseases (such as fungal diseases that affect wheat and cotton) viable for the following season. Warming in summer can lead to greater discomfort and increased heat stress and cooling costs. Although the details of these changes are uncertain, they are all plausible. Their impact on society would be enormous.
Despite uncertainties, most scientists maintain that the rise in global temperatures in recent decades is a result of an enhanced greenhouse effect caused by human activities. In 1992, at the Earth Summit in Rio de Janeiro, a climate-change treaty set voluntary goals for industrial nations to lower greenhouse-gas emissions to 1990 levels by 2000. Since this proved relatively ineffective, the 1997 UN Conference on Climate Change was convened in Kyoto, Japan, to establish binding limits. The industrial nations finally agreed, on Dec. 11, 1997, to a commitment to reduce emission of six greenhouse gases to 5.2% below 1990 levels by 2012. (The United States promised a reduction of 7%; the European Union, of 8%; and Japan, of 6%.) In March 2001, though, U.S. president George W. Bush formally backed out of the Kyoto Protocol; he claimed that it would harm the U.S. economy. In July 2001, 178 signers from other countries agreed to salvage the protocol. The United States, thus isolated in the position it took, promised to come up with its own proposals for dealing with global warming. In February 2002, however, the vague proposals then advanced by Bush called only for voluntary efforts to slow the growth of greenhouse-gas emissions. Although this approach had yet to be debated in Congress, many climate experts already found it worrisome and unsatisfying.
Russia's endorsement of the treaty in 2004 removed the last obstacle to the pact's enactment. The treaty had already been ratified by 120 countries. Russia's approval was nevertheless required to put it over the threshold of being endorsed by nations accounting for at least 55% of the 1990-level emissions by the world's industrialized nations. The Kyoto Protocol came into effect on Feb. 16, 2005, 90 days after its ratification by Russia's legislature.
On July 28, 2005, Australia, China, India, Japan, South Korea, and the United States—which together accounted for nearly half of the world's greenhouse-gas emissions—signed a new pact, outside of the Kyoto Protocol. They agreed to cut greenhouse gases through the use of new technology. The new Asia-Pacific Partnership on Clean Development and Climate aimed to cut the 1990 level of greenhouse emissions by 5.2% by 2008–12 through the use of new technology. It did not, however, impose mandatory reduction targets on industrialized nations. Critics charged that the accord would weaken the Kyoto Protocol and was likely to be ineffective.
Since that time, many other studies on global warming and its mitigation have been launched. During the International Polar Year (IPY), which began in March 2007, the study of climate change was to be a major priority. Because Antarctica contains about 90% of the world's ice, scientists are studying it for clues to the past that might help predict the future. By 2011, for example, members of the West Antarctic Ice Sheet Divide Ice Core Project plan to cut and retrieve a cylinder of Antarctic ice some 12 cm (4.7 in) wide and 3.5 km (2.2 mi) long that should provide the most accurate record of atmospheric carbon dioxide to date. In an interesting carbon-mitigation experiment, Iceland in 2007 announced plans to inject carbon dioxide–charged water into porous basalt rock to see if it would react with the rock to form a stable mineral that would remain there for millions of years. If successful, this experiment in carbon sequestration could help the world protect itself from the effects of CO2 emissions until they can be reduced in other ways.
Meanwhile, a major four-part report of the UN Intergovernmental Panel on Climate Change was issued in 2007. This report was designed to form the basis for a December 2007 conference in Indonesia that was to prepare the framework for controlling carbon dioxide emissions after 2012, when the Kyoto Protocol expired. Its findings were likely to guide policy makers for decades to come.
Nearly 90% of the scientists involved in producing the first part of the IPCC report, issued in February 2007, believed that their research results were consistent with a warming world. They also concluded that it was "at least 90% likely" that the warming observed since 1950 was due primarily to greenhouse-gas emissions generated by human activities.
The second part of the report, released in April 2007, offered a bleak assessment of Earth's future. It declared that billions of people, most of them poor, were likely to face shortages of food and water and greater risks of flooding due to global warming. An estimated 75 million to 250 million people in Africa might face water shortages by the year 2020; rain-fed agriculture could decrease by half in some African countries by that date. In Asia, crop yields might increase in East and Southeast Asia; they might be reduced, however, by as much as 30% in Central and South Asia. Nations depending on meltwater from glaciers and snow cover, such as Peru, would experience severe water shortages; in Asia, increased snowmelt in the Himalayas was likely to cause catastrophic flooding. North America would probably be hit by more frequent hurricanes, floods, droughts, heat waves, and wildfires; its crop yields were likely to increase initially, then decline dramatically. In all likelihood any plant and animal species would face extinction if temperatures rose.
The third part of the report, published in May 2007, focused on ways to curb greenhouse-gas emissions and the rise in global temperature. Part four, scheduled for release in November 2007, summarized all of the findings.
Richard A. Anthes
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