| I | | INTRODUCTION |
Global Warming, increase in the average temperature of the atmosphere, oceans, and landmasses of Earth. The planet has warmed (and cooled) many times during the 4.65 billion years of its history. At present Earth appears to be facing a rapid warming, which most scientists believe results, at least in part, from human activities. The chief cause of this warming is thought to be the burning of fossil fuels, such as coal, oil, and natural gas, which releases into the atmosphere carbon dioxide and other substances known as greenhouse gases. As the atmosphere becomes richer in these gases, it becomes a better insulator, retaining more of the heat provided to the planet by the Sun.
The average surface temperature of Earth is about 15°C (59°F). Over the last century, this average has risen by about 0.6 Celsius degrees (1 Fahrenheit degree). Scientists predict further warming of 1.4 to 5.8 Celsius degrees (2.5 to 10.4 Fahrenheit degrees) by the year 2100. This temperature rise is expected to melt polar ice caps and glaciers as well as warm the oceans, all of which will expand ocean volume and raise sea level by an estimated 9 to 100 cm (4 to 40 in), flooding some coastal regions and even entire islands. Some regions in warmer climates will receive more rainfall than before, but soils will dry out faster between storms. This soil desiccation may damage food crops, disrupting food supplies 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. The potential consequences of global warming are so great that many of the world's leading scientists have called for international cooperation and immediate action to counteract the problem.
| II | | THE GREENHOUSE EFFECT |
The energy that lights and warms Earth comes from the Sun. Most of the energy that floods onto our planet is short-wave radiation, including visible light. When this energy strikes the surface of Earth, the energy changes from light to heat and warms 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 all the way back out to space, but a portion remains trapped in Earth’s atmosphere. Certain gases in the atmosphere, including water vapor, carbon dioxide, and methane, provide the trap. Absorbing and reflecting infrared waves radiated by Earth, these gases conserve heat as the glass in a greenhouse does and are thus known as greenhouse gases. As the concentration of these greenhouse gases in the atmosphere increases, more heat energy remains trapped below. All life on Earth relies on this greenhouse effect—without it, the planet would be colder by about 33 Celsius degrees (59 Fahrenheit degrees), and ice would cover Earth from pole to pole. However, a growing excess of greenhouse gases in Earth’s atmosphere threatens to tip the balance in the other direction—toward continual warming.
| III | | 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.
Carbon dioxide is the next most abundant greenhouse gas. It flows into the atmosphere from many natural processes, such as volcanic eruptions; the respiration of animals, which breathe in oxygen and exhale carbon dioxide; and the burning or decay of organic matter, such as plants. Carbon dioxide leaves the atmosphere when it is absorbed into ocean water and through the photosynthesis of plants, especially trees. Photosynthesis breaks up carbon dioxide, releasing oxygen into the atmosphere and incorporating the carbon into new plant tissue.
Humans escalate the amount of carbon dioxide released to the atmosphere when they burn fossil fuels, 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 long-term destruction of forests by indiscriminate cutting of trees for lumber or to clear land for agricultural activities.
Ultimately, the oceans and other natural processes absorb excess carbon dioxide in the atmosphere. However, human activities have caused carbon dioxide to be released to the atmosphere at rates much faster than that at which Earth’s natural processes can cycle this gas. In 1750 there were about 281 molecules of carbon dioxide per million molecules of air (abbreviated as parts per million, or ppm). Today atmospheric carbon dioxide concentrations are 368 ppm, which reflects a 31 percent increase. Atmospheric carbon dioxide concentration increases by about 1.5 ppm per year. If current predictions prove accurate, by the year 2100 carbon dioxide will reach concentrations of more than 540 to 970 ppm. At the highest estimation, this concentration would be triple the levels prior to the Industrial Revolution, the widespread replacement of human labor by machines that began in Britain in the mid-18th century and soon spread to other parts of Europe and to the United States.
Methane is an even more effective insulator, trapping over 20 times more heat than does the same amount of carbon dioxide. Methane is emitted during the production and transport of coal, natural gas, and oil. Methane also comes from rotting organic waste in landfills, and it is released from certain animals, especially cows, as a byproduct of digestion. Since the beginning of the Industrial Revolution in the mid-1700s, the amount of methane in the atmosphere has more than doubled.
Nitrous oxide is a powerful insulating gas released primarily by burning fossil fuels and by plowing farm soils. 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 17 percent over preindustrial levels.
In addition, greenhouse gases are produced in many manufacturing processes. Per fluorinated compounds result from the smelting of aluminum. Hydro fluorocarbons form during the manufacture of many products, including the foams used in insulation, furniture, and car seats. Refrigerators built in some developing nations still use chlorofluorocarbons as coolants. In addition to their ability to retain atmospheric heat, some of these synthetic chemicals also destroy Earth’s high-altitude ozone layer, the protective layer of gases that shields Earth from damaging ultraviolet radiation. For most of the 20th century these chemicals have been accumulating in the atmosphere at unprecedented rates. But since 1995, in response to regulations enforced by the Montréal Protocol on Substances that Deplete the Ozone Layer and its amendments, the atmospheric concentrations of many of these gases are either increasing more slowly or decreasing.
Scientists are growing concerned about other gases produced from manufacturing processes that pose an environmental risk. In 2000 scientists identified a substantial rise in atmospheric concentrations of a newly identified synthetic compound called trifluoromethyl sulfur pentafluoride. Atmospheric concentrations of this gas are rising quickly, and although it still is extremely rare in the atmosphere, scientists are concerned because the gas traps heat more effectively than all other known greenhouse gases. Perhaps more worrisome, scientists have been unable to confirm the industrial source of the gas.
| IV | | 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
Scientists have long suspected that the global climate, the long-term average pattern of temperature, was also growing warmer, but they were unable to provide conclusive proof. Temperatures vary widely all the time and from place to place. It takes many years of climate observations to establish a trend. Records going back to the late 1800s did seem to show a warming trend, but these statistics were spotty and untrustworthy. Early weather stations often were located near cities, where temperature measurements were affected by the heat emitted from buildings and vehicles and stored by building materials and pavements. Since 1957, however, data have been gathered from more reliable weather stations, located far away from cities, and from satellites. These data have provided new, more accurate measurements, especially for the 70 percent of the planetary surface that is ocean water (see Satellite, Artificial). These more accurate records indicate that a surface warming trend exists and that, moreover, it has become more pronounced. Looking back from the end of the 20th century, records show that the ten warmest years of the century all occurred after 1980, and the three hottest years occurred after 1990, with 1998 being the warmest year of all.
Greenhouse gas concentrations are increasing. Temperatures are rising. But does the gas increase necessarily cause the warming, and will these two phenomena continue to occur together? In 1988 the United Nations Environment Program and the World Meteorological Organization established a panel of 200 leading scientists to consider the evidence. In its Third Assessment Report, released in 2001, this Intergovernmental Panel on Climate Change (IPCC) concluded that global air temperature had increased 0.6 Celsius degree (1 Fahrenheit degree) since 1861. The panel agreed that the warming was caused primarily by human activities that add greenhouse gases to the atmosphere. The IPCC predicted in 2001 that the average global temperature would rise by another 1.4 to 5.8 Celsius degrees (2.5 to 10.4 Fahrenheit degrees) by the year 2100.
The IPCC panel cautioned that even if greenhouse gas concentrations in the atmosphere ceased growing by the year 2100, the climate would continue to warm for a period after that as a result of past emissions. Carbon dioxide remains in the atmosphere for a century or more before nature can dispose of it. If greenhouse gas emissions continue to increase, experts predict that carbon dioxide concentrations in the atmosphere could rise to more than three times preindustrial levels early in the 22nd century, resulting in dramatic climate changes. Large climate changes of the type predicted are not unprecedented; indeed, they have occurred many times in the history of Earth. However, human beings would face this latest climate swing with a huge population at risk.
| V | | 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 several predictions about how global warming will affect weather, sea levels, coastlines, agriculture, wildlife, and human health.
| A | | Weather |
Scientists predict that during global warming, the northern regions of the Northern Hemisphere will heat up more than other areas of the planet, northern and mountain glaciers will shrink, and less ice will float on northern oceans. Regions that now experience light winter snows may receive no snow at all. In temperate mountains, snowlines will be higher and snow packs will melt earlier. Growing seasons will be longer in some areas. Winter and nighttime temperatures will tend to rise more than summer and daytime ones.
The warmed world will be generally more humid as a result of more water evaporating from the oceans. Scientists are not sure whether a more humid atmosphere will encourage or discourage further warming. On the one hand, water vapor is a greenhouse gas, and its increased presence should add to the insulating effect. On the other hand, more vapor in the atmosphere will produce more clouds, which reflect sunlight back into space, which should slow the warming process (see Water Cycle).
Greater humidity will increase rainfall, on average, about 1 percent for each Fahrenheit degree of warming. (Rainfall over the continents has already increased by about 1 percent in the last 100 years.) Storms are expected to be more frequent and more intense. However, water will also evaporate more rapidly from soil, causing it to dry out faster between rains. Some regions might actually become drier than before. Winds will blow harder and perhaps in different patterns. Hurricanes, which gain their force from the evaporation of water, are likely to be more severe. Against the background of warming, some very cold periods will still occur. Weather patterns are expected to be less predictable and more extreme.
| B | | Sea Levels |
As the atmosphere warms, the surface layer of the ocean warms as well, expanding in volume and thus raising sea level. Warming will also melt much glacier ice, especially around
Sea-level changes will complicate life in many coastal regions. A 100-cm (40-in) rise could submerge 6 percent of The Netherlands, 17.5 percent of
Even a modest rise in sea level will greatly change coastal ecosystems. A 50-cm (20-in) rise will submerge about half of the present coastal wetlands of the
| C | | Agriculture |
A warmed globe will probably produce as much food as before, but not necessarily in the same places.
| D | | Animals and Plants |
Animals and plants will find it difficult to escape from or adjust to the effects of warming because humans occupy so much land. Under global warming, animals will tend to migrate toward the poles and up mountainsides toward higher elevations, and plants will shift their ranges, seeking new areas as old habitats grow too warm. In many places, however, human development will prevent this shift. Species that find cities or farmlands blocking their way north or south may die out. Some types of forests, unable to propagate toward the poles fast enough, may disappear.
| E | | Human Health |
In a warmer world, scientists predict that more people will get sick or die from heat stress, due less to hotter days than to warmer nights (giving the sufferers less relief). Diseases now found in the tropics, transmitted by mosquitoes and other animal hosts, will widen their range as these animal hosts move into regions formerly too cold for them. Today 45 percent of the world’s people live where they might get bitten by a mosquito carrying the parasite that causes malaria; that percentage may increase to 60 percent if temperatures rise. Other tropical diseases may spread similarly, including dengue fever, yellow fever, and encephalitis. Scientists also predict rising incidence of allergies and respiratory diseases as warmer air grows more charged with pollutants, mold spores, and pollens.
| VI | | DEBATES OVER GLOBAL WARMING |
Scientists do not all agree about the nature and impact of global warming. A few observers still question whether temperatures have actually been rising at all. Others acknowledge past change but argue that it is much too early to be making predictions for the future. Such critics may also deny that the evidence for the human contribution to warming is conclusive, arguing that a purely natural cycle may be driving temperatures upward. The same dissenters tend to emphasize the fact that continued warming could have benefits in some regions.
Scientists who question the global warming trend point to three puzzling differences between the predictions of the global warming models and the actual behavior of the climate. First, the warming trend stopped for three decades in the middle of the 20th century; there was even some cooling before the climb resumed in the 1970s. Second, the total amount of warming during the 20th century was only about half what computer models predicted. Third, the troposphere, the lower region of the atmosphere, did not warm as fast as the models forecast. However, global warming proponents believe that two of the three discrepancies have now been explained.
The lack of warming at mid-century is now attributed largely to air pollution that spews particulate matter, especially sulfates, into the upper atmosphere. These particulates, also known as aerosols, reflect some incoming sunlight out into space. Continued warming has now overcome this effect, in part because pollution control efforts have made the air cleaner.
The unexpectedly small amount of total warming since 1900 is now attributed to the oceans absorbing vast amounts of the extra heat. Scientists long suspected that this was happening but lacked the data to prove it. In 2000 the U.S. National Oceanic and Atmospheric Administration (NOAA) offered a new analysis of water temperature readings made by observers around the world over 50 years. Records showed a distinct warming trend: World ocean temperatures in 1998 were higher than the 50-year average by 0.2 Celsius degree (0.3 Fahrenheit degree), a small but very significant amount.
The third discrepancy is the most puzzling. Satellites detect less warming in the troposphere than the computer models of global climate predict. According to some critics, the atmospheric readings are right, and the higher temperatures recorded at Earth’s surface are not to be trusted. In January 2000 a panel appointed by the National Academy of Sciences to weigh this argument reaffirmed that surface warming could not be doubted. However, the lower-than-predicted troposphere measurements have not been entirely explained.
| VII | | EFFORTS TO CONTROL GLOBAL WARMING |
The total consumption of fossil fuels is increasing by about 1 percent per year. No steps currently being taken or under serious discussion will likely prevent global warming in the near future. The challenge today is managing the probable effects while taking steps to prevent detrimental climate changes in the future.
Damage can be curbed locally in various ways. Coastlines can be armored with dikes and barriers to block encroachments of the sea. Alternatively, governments can assist coastal populations in moving to higher ground. Some countries, such as the
There are two major approaches to slowing the buildup of greenhouse gases. The first is to keep carbon dioxide out of the atmosphere by storing the gas or its carbon component somewhere else, a strategy called carbon sequestration. The second major approach is to reduce the production of greenhouse gases.
| A | | Carbon Sequestration |
The simplest way to sequester carbon is to preserve trees and to plant more. Trees, especially young and fast-growing ones, soak up a great deal of carbon dioxide, break it down in photosynthesis, and store the carbon in new wood. Worldwide, forests are being cut down 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 building housing developments. Reforestation could offset these losses and counter part of the greenhouse buildup.
Many companies and governments in the
Carbon dioxide gas can also be sequestered directly. Carbon dioxide has traditionally been injected into oil wells to force more petroleum out of the ground or seafloor. Now it is being injected simply to isolate it underground in oil fields, coal beds, or aquifers. At one natural gas drilling platform off the coast of
In an encouraging trend, energy use around the world has slowly shifted 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 dawn of the Industrial Revolution in the 18th century, coal became the dominant energy source. By the mid-19th century oil had replaced coal in dominance, fueling the internal combustion engines that were eventually used in automobiles. By the 20th century, 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.
Nuclear energy, though controversial for reasons of safety and the high costs of nuclear waste disposal, releases no carbon dioxide at all. Solar power, wind power, and hydrogen fuel cells also emit no greenhouse gases. Someday these alternative energy sources may prove to be practical, low-pollution energy sources, although progress today is slow.
| B | | National and Local Programs |
The developed countries are all working to reduce greenhouse emissions. Several European countries impose heavy taxes on energy usage, designed partly to curb such emissions.
In the
Many local governments are also working against greenhouse emissions by conserving energy in buildings, modernizing their vehicles, and advising the public. Individuals, too, can take steps. The same choices that reduce other kinds of pollution work against global warming. Every time a consumer buys an energy-efficient appliance; adds insulation to a house; recycles paper, metal, and glass; chooses to live near work; or commutes by public transportation, he or she is fighting global warming.
| C | | International Agreements |
International cooperation is required for the successful reduction of greenhouse gases. In 1992 at the Earth Summit in
In 1997 in
But in 2001 newly elected
Some critics find the Kyoto Protocol too weak. Even if it were enforced immediately, it would only slightly slow the buildup of greenhouse gases in the atmosphere. Much stronger action would be required later, particularly because the developing nations exempted from the
Behind the issue of cost lies a larger question: Can an economy grow without increasing its greenhouse gas emissions at the same time? In the past, prosperity and pollution have tended to go together. Can they now be separated, or decoupled, as economists say? In nations with strong environmental policies, economies have continued to grow even as many types of pollution have been reduced. However, limiting the emission of carbon dioxide has proved especially difficult. For example, The Netherlands, a heavily industrialized country that is also an environmental leader, has done very well against most kinds of pollution but has failed to meet its goal of reducing carbon dioxide output.
After 1997 representatives to the Kyoto Protocol met regularly to negotiate a consensus about certain unresolved issues, such as the rules, methods, and penalties that should be enforced in each country to slow greenhouse emissions. The negotiators designed a system in which nations with successful cleanup programs could profit by selling unused pollution rights to other nations. For example, nations that find further improvement difficult, such as The Netherlands, could buy pollution credits on the market, or perhaps earn them by helping reduce greenhouse gas emissions in less developed countries, where more can be achieved at less expense.
Contributed By:
John Hart
