Global Warming

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 degree (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.

Learn more:

• The Greenhouse Effect
• Types of Greenhouse Gases
• Measuring Global Warming
• Effects of Global Warming
• Efforts To Control Global Warming

Global Scale Pollution

Air pollution can expand beyond a regional area to cause global effects. The stratosphere is the layer of the atmosphere between 16 km (10 mi) and 50 km (30 mi) above sea level. It is rich in ozone, the same molecule that acts as a pollutant when found at lower levels of the atmosphere in urban smog. Up at the stratospheric level, however, ozone forms a protective layer that serves a vital function: It absorbs the wavelength of solar radiation known as ultraviolet-B (UV-B). UV-B damages deoxyribonucleic acid (DNA), the genetic molecule found in every living cell, increasing the risk of such problems as cancer in humans. Because of its protective function, the ozone layer is essential to life on Earth.

Ozone Depletion

Several pollutants attack the ozone layer. Chief among them is the class of chemicals known as chlorofluorocarbons (CFCs), formerly used as refrigerants (notably in air conditioners), as agents in several manufacturing processes, and as propellants in spray cans. CFC molecules are virtually indestructible until they reach the stratosphere. Here, intense ultraviolet radiation breaks the CFC molecules apart, releasing the chlorine atoms they contain. These chlorine atoms begin reacting with ozone, breaking it down into ordinary oxygen molecules that do not absorb UV-B. The chlorine acts as a catalyst—that is, it takes part in several chemical reactions—yet at the end emerges unchanged and able to react again. A single chlorine atom can destroy up to 100,000 ozone molecules in the stratosphere. Other pollutants, including nitrous oxide from fertilizers and the pesticide methyl bromide, also attack atmospheric ozone.

Scientists are finding that under this assault the protective ozone layer in the stratosphere is thinning. In the Antarctic region, it vanishes almost entirely for a few weeks every year. Although CFC use has been greatly reduced in recent years and will soon be prohibited worldwide, CFC molecules already released into the lower atmosphere will be making their way to the stratosphere for decades, and further ozone loss is expected. As a result, experts anticipate an increase in skin cancers, more cataracts (clouding of the lens of the eye), and reduced yields of some food crops.

Global Warming

Humans are bringing about another global-scale change in the atmosphere: the increase in what are called greenhouse gases. Like glass in a greenhouse, these gases admit the Sun’s light but tend to reflect back downward the heat that is radiated from the ground below, trapping heat in the Earth’s atmosphere. This process is known as the greenhouse effect. Carbon dioxide is the most significant of these gases, the result of our burning coal and fuels derived from oil. Methane, nitrous oxide, and CFCs are greenhouse gases as well.

Scientists predict that increases in these gases in the atmosphere will make the Earth a warmer place. They expect a global rise in average temperature of 1.4 to 5.8 Celsius degrees (2.5 to 10.4 Fahrenheit degrees) in the next century.

Warmer temperatures are expected to partially melt the polar ice caps, leading to a projected sea level rise of 9 to 100 cm (4 to 40 in) by the year 2100. A sea level rise at the upper end of this range would flood coastal cities, force people to abandon low-lying islands, and completely inundate coastal wetlands. Diseases like malaria, which at present are primarily found in the tropics, may become more common in the regions of the globe between the tropics and the polar regions, called the temperate zones. For many of the world’s plant species, and for animal species that are not easily able to shift their territories as their habitat grows warmer, climate change may bring extinction.

Measuring Global Warming

GOES Weather Satellite
Broadcasters use data from meteorological satellites to predict weather and to broadcast storm warnings when necessary. Satellites such as the Geostationary Operational Environmental Satellite (GOES) collect meteorological and infrared information about the atmosphere and the ocean. A camera on the GOES is continuously pointed at Earth, broadcasting satellite images of cloud patterns both day and night. Here, the GOES-C satellite is being encapsulated inside its payload fairing aboard a Delta rocket.

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.

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. 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.

Effects of Global Warming: Weather

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 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 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
C. Agriculture
D. Animals and Plants
E. Human Health

Effects of Global Warming: 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 Greenland, further swelling the sea. Sea levels worldwide rose 10 to 25 cm (4 to 10 in) during the 20th century, and IPCC scientists predict a further rise of 9 to 88 cm (4 to 35 in) in the 21st century.

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 Bangladesh, and most or all of many islands. Erosion of cliffs, beaches, and dunes will increase. Storm surges, in which winds locally pile up water and raise the sea, will become more frequent and damaging. As the sea invades the mouths of rivers, flooding from runoff will also increase upstream. Wealthier countries will spend huge amounts of money to protect their shorelines, while poor countries may simply evacuate low-lying coastal regions.

Effects of Global Warming: Agriculture

A warmed globe will probably produce as much food as before, but not necessarily in the same places. 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. Desert farm regions that bring in irrigation water from distant mountains may suffer if 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.

Effects of Global Warming: 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.

Effects of Global Warming: 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.

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 United States, still have the chance to help plant and animal species survive by preserving habitat corridors, strips of relatively undeveloped land running north and south. Species can gradually shift their ranges along these corridors, moving toward cooler habitats.

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.