Climate Change and Global Warming

Climate is the weather experienced by a given location, averaged over several decades. A region's climate tells how hot or cold, wet or dry, windy or still, and cloudy or sunny it generally is. Climate is determined not only by average weather conditions, but also by seasonal changes in those conditions and weather extremes. Thus, for example, a climate can be described as hot and wet year-round, frigid and dry year-round, or warm and rainy in the summer and cold and dry in the winter.

Climate has been an important factor in determining where groups of people choose to settle. While humans are resourceful enough to survive almost anywhere on the planet, most population centers are in areas where temperature and rainfall are adequate to sustain some form of agriculture. There are fewer settlements in regions of extreme dryness or cold, such as deserts or the arctic. Climate also influences how people live. It largely defines choices of architecture, clothes, food, occupation, and recreation.

The climates of the world are distinguished by several factors, including latitude (distance north or south of the equator), temperature (the degree of hotness or coldness of an environment), topography (the shape and height of land features), and distribution of land and sea.

Climate is not a fixed property. The global climate has changed continuously and dramatically over the 4.6 billion years since the Earth was formed. It continues to change today, and it will in the future. Understanding climate change and its causes will help us to anticipate future climates.

Elements of Climate

The two factors that are most significant in defining climate type are temperature and precipitation. These criteria, in turn, are influenced by a number of atmospheric, oceanographic, and topographic factors.

Temperature

Climate is affected by annual mean (average) temperature and annual temperature range. High mountain deserts such as in central Asia and the high mountain plateaus of South America may experience extreme temperature variation, with daytime temperatures exceeding 100°F (38°C) and nighttime temperatures plunging below freezing. At the opposite extreme, regions in the tropics may have little daily or yearly variation in temperature.

WORDS TO KNOW

air pollutant:

any harmful substance that exists in the atmosphere at concentrations great enough to endanger the health of living organisms.

Cenozoic era:

the historical period from sixty-five million years ago to the present.

deforestation:

the removal of all or most of the trees from an area.

desert climate:

the world's driest climate type, with less than 10 inches (25 centimeters) of rainfall annually.

desertification:

the process by which semiarid lands turn to desert (also called land degradation). It is caused by prolonged drought, during which time the top layers of soil dry out and blow away.

drought:

an extended period during which the amount of rain or snow that falls on an area is much lower than usual.

eccentricity:

a measure of how much an orbit deviates from a circle. A circular orbit has zero eccentricity. An ellipse has eccentricity between zero and one.

ecosystem:

a community of plants and animals, including humans, and their physical surroundings.

El Niño:

means "the Christ child" in Spanish. A period of unusual warming of the Pacific Ocean waters off the coast of Peru and Ecuador. It usually starts around Christmas, which is how it got its name.

equinoxes:

the days on which the Sun appears to cross Earth's equator in its yearly motion.

flood:

the inundation of normally dry land with water.

food chain:

the transfer of food energy from one organism to another. It begins with a plant species, which is eaten by an animal species; it continues with a second animal species, which eats the first, and so on.

fossil fuels:

coal, oil, and natural gas—materials composed of the remains of plants or animals that covered Earth millions of years ago and are today burned for fuel.

global warming:

the observed global increase in atmospheric temperature. Also called global climate change.

The annual temperature range is found by subtracting the year's lowest average monthly temperature from the year's highest average monthly temperature. The annual temperature range reveals whether or not a location experiences different seasons, which is just as important as the annual average temperature in identifying climate type.

Average annual sea-level temperatures plotted on a map roughly correspond to latitude. That is, temperatures are highest at the equator, decline with increasing latitudes, and are lowest at the poles. This is due to the uneven heating of Earth by the Sun due to the varying angle of the Sun's rays.

WORDS TO KNOW

greenhouse effect:

the warming of Earth due to the presence of greenhouse gases, which trap upwardly radiating heat and return it to Earth's surface.

greenhouse gases:

gases that trap heat in the atmosphere. The most abundant greenhouse gases are water vapor and carbon dioxide. Others include methane, nitrous oxide, and chlorofluorocarbons.

heat wave:

an extended period of high heat and humidity.

Holocene:

the most recent part of the Cenozoic era, from ten thousand years ago to the present.

ice age:

a period during which significant portions of Earth's surface are covered with ice.

interglacial period:

a relatively warm period that exists between two ice ages.

Maunder minimum:

a period of time from 1645 to 1715, during which sunspot activity was almost nonexistent.

Mesozoic era:

the historical period from 225 million years ago to 65 million years ago, best known as the age of the dinosaurs.

Milankovitch theory:

the theory stating that the three types of variation in Earth's orbit, taken together, can be linked with warm and cold periods throughout history. These variations include: the shape of Earth's orbit, the direction of tilt of its axis, and the degree of tilt of its axis.

obliquity:

the angle of the tilt of Earth's axis in relation to the plane of its orbit.

ocean current:

the major routes through which ocean water is circulated around the globe.

paleoclimatologist:

a scientist who studies climates of the past.

Paleozoic era:

the historical period from 570 million years ago to 225 million years ago.

precession of the equinoxes:

the reversal of the seasons every thirteen thousand years. This occurs because Earth spins about its axis like a top in slow motion and wobbles its way through one complete revolution every twenty-six thousand years.

season:

a period of the year characterized by certain weather conditions, such as temperature and precipitation, as well as the number of hours of sunlight each day.

An imaginary line connecting locations with the same temperature is called an "isotherm." Because water takes longer to heat up and retains heat longer than does land, isotherms veer where landmasses meet the sea. Ocean currents, which are the major routes that carry warm water toward the poles and cold water toward the equator, also cause the bending of isotherms along the coasts.

Precipitation

In addition to temperature, precipitation (water in any form falling out of the air) is the other factor determining climate. Both the total precipitation and the distribution of precipitation throughout the year have an impact. For example, a city that receives all of its precipitation during a few torrential rainfalls during the hottest summer months might have an arid climate, while a city that receives the same amount of precipitation distributed evenly through the year might have a climate able to support trees and grasses.

Wet and dry regions are scattered across the globe. However, there are some general trends corresponding to global air circulation patterns. Around the equator, where the trade winds (dominant surface winds that blow from east to west) converge and air rises, precipitation is relatively high. Precipitation is also high around 60° latitude, in the middle latitudes, where the warm westerly air currents rise as they meet the cold polar easterlies.

In the subtropical regions, around 30° latitude, conditions are much drier. It is there that most of the world's deserts exist. The polar regions are also characterized by dryness. However, these latitudes are not fixed. As Earth makes its yearly revolution around the Sun the amount of sunlight received at each latitude changes, causing northward and southward shifts in "wet" and "dry" latitudes.

History of Climate Change

Throughout Earth's 4.6-billion-year existence, the global climate has changed continuously. Most of what scientists know of the history of climate change is based on the most recent 10 percent of Earth's history—the last 500 million years or so. There are fewer clues about the climate in the more distant past. However, by using the clues that do exist plus drawing conclusions from more recent data, scientists have been able to construct a climatic picture spanning the entire existence of Earth.

There have been times when Earth has been alternately warmer or colder than it is today. There have also been several periods during which significant portions of the planet's surface have been covered with ice. These are popularly known as ice ages. Each ice age has brought about the extinction of numerous species of plants and animals as well as dramatic shifts in the distribution of plant and animal species, which means that the process by which plants and animals have evolved has not been a smooth one. Rather, it has been halted many times and begun anew with each new warm period.

The geologic history of Earth is conventionally divided into six main eras: Hadean, Archaean, Proterozoic, Paleozoic, Mesozoic, and Cenozoic.

Precambrian era (Hadean, Archaean, Proterozoic)

The Precambrian period begins with the formation of Earth around 4.6 billion years ago, along with the rest of our solar system, and ends 570 million years ago. Earth began its existence as a ball of molten liquid rock. Within about 900 million years, its surface cooled and solidified. Earth's atmosphere was first produced from gases of volcanoes and possibly with additional gases from comets, which are chunks of rock and ice that orbit the Sun. These gases included nitrogen compounds, water vapor, carbon dioxide, sulfur dioxide, and argon.

Comets

Another theory of the origin of Earth's atmosphere suggests that the atmospheric gases were deposited at least in part by comets colliding with Earth. Debris from comets is known to have carbon dioxide and nitrogen in roughly the same proportion as found in the early atmosphere. There are numerous impact craters on Earth from past collisions with asteroids, comets, and other objects from space. The role of comets in the formation of Earth's early atmosphere and life remains an area of vigorous scientific debate.

At first, the atmosphere contained a lot of water vapor. The water vapor condensed into rain, which fell to the surface forming vast oceans. The rain washed most of the carbon dioxide and sulfur dioxide out of the atmosphere. The carbon dioxide and sulfur dioxide eventually formed into limestone rocks. This left primitive Earth with an atmosphere composed of nitrogen, argon, water vapor, and a small amount of carbon dioxide. The small amount of carbon dioxide and other gases produced a modest greenhouse effect, which is the warming of Earth due to the presence of gases that trap heat, making the planet capable of supporting life. However, there was no free oxygen in Earth's early atmosphere. Since there was no free oxygen, there was also no ozone in Earth's upper atmosphere.

Soon after the formation of the early atmosphere, the first forms of life appeared. Geologists mark this as the beginning of the Archaean era. Since there was no free oxygen in Earth's atmosphere at this time, the organisms were probably some form of anaerobic bacteria known as Archaeans. (Anaerobic means capable of living without oxygen.) Arch-aeans of today are known to thrive in extremely harsh conditions—they were first discovered in the hot springs of Yellowstone National Park—so they were likely to be capable of thriving in the high temperatures and methane-rich environment of early Earth.

About 3.5 billion years ago, the first photosynthesizing organisms appeared. Photosynthesis is the process in which an organism combines water and carbon dioxide to form a simple sugar, driven by the heat of the Sun. Oxygen is given off as a byproduct.

Over time, oxygen accumulated in the atmosphere. By 2 billion years ago, it accounted for 1 percent of its present concentration; by 700 million years ago, it accounted for 10 percent; and by 350 million years ago, the concentration of oxygen in the atmosphere reached its present value of 21 percent. Oxygen molecules (O₂) then combined with free oxygen atoms, which were formed when oxygen molecules broken apart by sunlight, to form ozone (O₃). The ozone formed a separate atmospheric layer, 25 to 40 miles above ground, that filters out the harmful ultraviolet rays. Together with the protection provided by the ozone layer, the oxygen in the atmosphere encouraged the burst of biological diversity that marked the end of the Precambrian era.

Paleozoic era

The Paleozoic era, lasting from 570 to 225 million years ago, began with a dramatic increase in the number of marine animal species. The Paleozoic era saw dramatic temperature changes. Much of the time it was significantly warmer than the Precambrian era, but there were two glacial periods. The first was relatively short and began around 430 million years ago. At the end of this cold period, around 400 million years ago, plants began to take hold on land.

The first land plants were little more than rigid stems. They reproduced by releasing spores, the same reproduction mechanism used today by ferns and mushrooms. Within fifty million years, however, plants had begun producing seeds, which are capable of being dispersed over far greater distances than spores.

The spread of plants across the land had a significant impact on climate. Primarily, it reduced the albedo (reflectivity) of Earth's surface by 10 to 15 percent. As a result, more sunlight was absorbed, which raised the planet's surface temperature. The second cold period occurred during the last portion of the Paleozoic era, from 330 to 225 million years ago.

Around 250 million years ago, Earth's two supercontinents, called Laurussia (containing Greenland, North America, Scandinavia, and most of Russia) and Gondwana (containing most of the rest of the landmass), joined together. The newly formed supercontinent, Pangaea, extended from the North Pole almost all the way to the South Pole.

Mesozoic era

The Mesozoic era, which is often known as the age of the dinosaurs, lasted from 225 to 65 million years ago. In the latter half of this era, Pangaea split into two continents, Laurasia and Gondwana. Laurasia included North America, Europe, and Asia. Gondwana included most of the southern continents. About 100 million years ago, these land masses further subdivided, roughly into the present continents. However, the landmasses were still situated very close together.

Throughout the Mesozoic era, temperatures everywhere on Earth were, on average, 11 to 18°F (6 to 10°C) warmer than they are today. They were also relatively uniform across the planet. This was most likely due to the efficient distribution of heat from the equator to the poles by ocean currents and global winds. Land masses, even by the end of the Mesozoic era, were not as widely dispersed across the globe as they are today. Thus, ocean currents (the major routes through which ocean water is circulated around the globe) and winds had a relatively clear path between the equator and poles.

The Mesozoic era experienced a number of temperature swings, culminating in a sudden, brief ice age. This coincided with the extinction of about 70 percent of Earth's life forms, including dinosaurs. One theory is that the ice age was brought about by the collision of an asteroid with Earth that created a dust cloud that blocked out the Sun's rays.

Cenozoic era

The Cenozoic era began sixty-five million years ago and continues to the present. Throughout this era, the continents have continued to drift apart, moving to their present configuration. The process of supercontinent formation and breakup will likely repeat several more times as the continental plates continue to move. This era is also characterized by the emergence of mammals, including humans, as the dominant animal group.

The early part of the Cenozoic era was warmer than it is today, and there were no polar ice caps. Beginning around fifty-five million years ago, a long cooling trend began. This cooling occurred both gradually over time, and through a series of extremely cold periods. One of these cold spells took place about fifty million years ago, and another about thirty-eight million years ago. The most recent was about fifteen million years ago, the results of which can still be seen in the polar ice caps and the glaciers nestled in protected areas of tall mountains.

Over the last 2.4 million years there have been two dozen ice ages (periods during which the global temperature plummeted). At seven different times over the last 1.6 million years, up to 32 percent of Earth's surface has been covered with ice. Scientists estimate that throughout the Cenozoic era, new ice ages have occurred about every one hundred thousand years and have been interspersed with warmer, interglacial periods, each lasting at least ten thousand years.

The most recent ice age peaked about twenty thousand years ago, when there were glaciers up to 10,000 feet (3,000 meters) thick over most of North America, northern Europe, and northern Asia, as well as the southern portions of South America, Australia, and Africa. The sea level fell, exposing large areas of land that are currently submerged, such as the Bering land bridge, which connected the eastern tip of Siberia with the western tip of Alaska.

This era was followed by a warm period, beginning around fourteen thousand years ago. By eight thousand years ago, most of the ice had melted, and between seven and five thousand years ago, the world was about 5°F (3°C) warmer than it is today. The sea level rose and the current shape of continents emerged.

Holocene epoch

The most recent division of the Cenozoic is called the Holocene epoch. It began approximately ten thousand years ago and continues to today. Extensive climatic data exist for this time period. The history of modern civilization begins during this postglacial period, about six thousand years ago.

About five thousand years ago, when Earth was slightly warmer and wetter than it is today, agriculture was developed, and the earliest cities were established in Egypt and Mesopotamia, which is part of modern Iraq. There was a relatively cool period that began around 900 bce and lasted until about 500 bce, which resulted in crop failures. There are also indications that, beginning around 800 ce, there was a prolonged drought, or period of extreme dryness, which may have contributed to the decline of the great Mayan civilizations in Mexico and Central America.

In the Middle Ages (500 to 1500 ce), the global climate was similar to today. During that time, the civilizations of Europe flourished and the Vikings colonized Iceland and Greenland. However, the end of the thirteenth century saw the beginning of a cold spell. Summer after summer was cold and wet, which caused famine throughout Europe.

Conditions were more moderate during the fifteenth century and then became colder again between about 1500 and 1850, a period referred to as the "Little Ice Age." Rather than being continuously cold, the Little Ice Age consisted of a series of cold spells, each up to thirty years long, separated by warmer years. For the most part, this period was characterized by bitterly cold winters and cold, wet summers. The canals of Holland, as well as the Baltic Sea and the River Thames in England, were often covered with layers of ice several inches thick. Food became scarce throughout Europe, the mountain glaciers grew, and the colonists in Greenland and Iceland perished.

After the Little Ice Age, temperatures warmed. In the years since 1850, however, significant fluctuations have occurred in global temperatures. About a dozen cool periods have alternated with warmer periods. From 1900 to the present, there has been a 1°F (0.5°C) increase in global temperature. Scientists now think that this increase is not merely a natural part of Earth's continually changing climate, but rather constitutes part of a trend of human-influenced global warming, or temperature increase.

There are various ways of looking at the Holocene epoch. Some consider it a "warm period," since ice exists only at the polar regions, covering 10 percent of the planet. Others believe the world is still in the

final stages of the most recent ice age. What many scientists do agree on, however, is that another ice age is in store for the future.

Reasons for Climate Change

In an attempt to find some order in the series of climatic shifts that describes the history of Earth, scientists have sought to define a pattern of warm-cool cycles that repeat after a given period of time. However, they have been largely unsuccessful. The proposed cycles either do not apply to all times in the past or do not hold true for all parts of the world.

One problem in determining patterns of climatic change is that many factors are involved. Some of those factors affect Earth's climate for millennia while others affect it only for decades. In addition, some factors, such as Earth's shifting orbit around the Sun, are predictable and regular, while others, such as collisions with large objects in space, are not.

Human activity constitutes a whole category of factors affecting climate change in the very recent past, present, and future. Among the possible agents of climate change are deforestation, which is the removal of trees; the burning of fossil fuels, such as coal, oil, and natural gas; acid

precipitation, which is rain and snow that are made more acidic when carbon, sulfur, and nitrogen oxides in the air dissolve in the water; and smog, a layer of hazy, brown air pollution at Earth's surface caused by industrial emissions. Though the impact of human activity on climate change has be the subject of much debate in recent years, a February 2007 report by the United Nations' Intergovernmental Panel on Climate Change made one of the strongest arguments yet that recent sharp rises in global temperature, also known as global warming, are a direct result of human actions. The Intergovernmental Panel on Climate Change was established in 1988 to study the global climate. Its 2007 report stated that "most of the observed increase in globally averaged temperatures since the mid-twentieth century is very likely due to the observed increase in anthropogenic (human-generated) greenhouse gas concentrations."

There are a handful of forces that have affected Earth's climate throughout its history: continental drift, shifts in Earth's orbit, volcanoes, asteroids and comets, and solar variability. Another phenomena that is currently being studied as a possible factor in long-term climate change is El Niño/Southern Oscillation (ENSO).

Continental drift

The theory of continental drift was first suggested by German meteorologist Alfred Wegener (1880–1930) in 1915. That theory states that 200 to 250 million years ago, all land on Earth was

joined together in one supercontinent that Wegener named Pangaea. Then, over the years, the supercontinent broke up and the pieces drifted to where they are today. Although there was substantial evidence supporting the theory of continental drift, Wegener could not suggest a mechanism causing the breakup and drift of the continents. The theory was not considered for decades. Then, in the late 1950s and early 1960s, a new theory incorporating the ideas of continental drift was developed. This new theory, plate tectonics, explained the motion of the plates as being due to convection currents (the transfer of heat by the mass movement of heated particles into an area of cooler fluid) in Earth's upper mantle, the layer beneath the crust, which is the planet's thin outer-most layer.

Evidence of continental drift can be found in the fossils of dinosaurs and other mammals that migrated across once-connected land masses, and in the fossilized remains of tropical plants beneath polar ice sheets. Another piece of evidence that land masses were once connected is that shapes of the continents fit together like pieces of a jigsaw puzzle. The Atlantic coastlines of Africa and South America are the most striking examples of this phenomenon. In addition, satellites today are able to record the subtle, extremely slow movements of the continents.

Continental drift is believed to affect both the climates of individual continents and the climate of the entire planet. The climates of the individual continents have been altered by their gradual, but radical, change in position around Earth. For instance, parts of Europe and Asia that once sat on the equator are now at high latitudes in the Northern Hemisphere. India moved to its current low-latitude Northern Hemisphere position from deep in the Southern Hemisphere. Glaciers once covered parts of Africa, and Antarctica gradually slid from warmer latitudes to the South Pole.

On a global level, the position of land masses affects how the heat from the Sun is distributed around Earth. When the continents were joined together, ocean currents and global winds produced a different pattern of global heat distribution than they do at present. Global climatic conditions 200 to 250 million years ago were more uniform than they are today. As the continents separated and moved out around the globe, greater extremes in climatic conditions began to appear around the world.

Another consequence of continental drift has been the formation of mountain ranges. When land masses come together, the land is forced upward. Examples of mountain ranges formed in this way are the Rocky Mountains, the Andes, the Himalayas, and the Kunlun and Qilan mountain ranges bordering the Tibetan Plateau. Mountain ranges affect temperatures, winds, and rainfall over limited areas. Very tall ranges, particularly those with north-south configurations, can influence air circulation patterns over very large areas.

The Tibetan Plateau is a prime example of this phenomenon. Called "the roof of the world," it was formed fifty million years ago by the collision of the Indian and Asian continents and is one of the world's tallest and widest plateaus. It affects wind patterns across the entire Northern Hemisphere. Fossil records show that almost immediately following the formation of the Tibetan Plateau, the climate of the Northern Hemisphere cooled and large glaciers formed. The presence of glaciers led to further cooling. Snow accumulates on the ice, which reflects sunlight rather than absorbing it.

Another aspect of continental drift that affects global climate is the distribution of land masses at various latitudes. For instance, as land moved away from the tropics and toward the poles, tropical oceans replaced the land. These bodies of water absorbed huge amounts of incoming heat, which led to global cooling. Also, the movement of continents into polar regions provided a surface on which ice layers could accumulate.

Earth's orbit

In the 1930s, the Yugoslavian astronomer Milutin Milan-kovitch (1879–1958) proposed another theory to explain changes in Earth's climate. He listed three factors that could affect the planet's climate: the shape of Earth's orbit and the angle and direction of Earth's axis.

The shape of Earth's orbit around the Sun changes over long periods of time. At times, the orbit is almost circular. At other times, it has a more elliptical shape. The eccentricity of Earth's orbit changes in a regular cycle—from circular to elliptical to circular again—that takes about one hundred thousand years.

When the orbit is circular, there is less variation in the levels of solar energy received by Earth throughout the year than when it is elliptical. At present, the orbit is in a stage of low eccentricity, meaning that it is nearly circular. Earth receives only 7 percent more solar energy in January, when Earth is closest to the Sun, than July, when it is farthest from the Sun. In contrast, when the orbit is highly eccentric, the solar energy received at points closest to and farthest from the Sun differ by up to 20 percent. In addition, a more elliptical orbit means the relative lengths of the seasons are different. Springs and falls are shorter, while a particular hemisphere might have a long summer and short winter, or vice versa.

The second orbital factor that affects climate is the wobble in Earth's axis of rotation. This effect is what causes the Northern and Southern Hemispheres to receive different amounts of sunlight throughout the year.

Earth spins like a top in slow motion, so that its axis follows a cone-shaped path. It wobbles its way through one complete revolution every twenty-six thousand years, a phenomenon called the precession of the equinoxes. Equinoxes are the days on which the Sun appears to cross Earth's equator in its yearly motion, and "precession" refers to the motion of Earth's axis of rotation. The precession of the equinoxes means that every thirteen thousand years the seasons gradually reverse. Eventually, unless the calendars are adjusted to compensate, the Northern Hemisphere will experience winter in July and the Southern Hemisphere, in January. Precession also influences Earth's distance from the Sun at different seasons. When it is winter in the Northern Hemisphere in July, Earth will also be at its closest point to the Sun in that month.

The third variable affecting Earth's climate is the angle of the tilt of Earth's axis compared to the plane of its orbit. This angle is called obliquity. Over the course of forty-one thousand years, this angle fluctuates between 21.5 degrees and 24.5 degrees. It is currently 23.5 degrees. When the angle is smaller, sunlight strikes various points on Earth more evenly and the seasonal differences are smaller, bringing milder winters and cooler summers. Yet when the angle is larger, there is a greater variation in the level of solar radiation received across Earth and seasons are more pronounced.

The smaller angle of tilt tends to favor the formation of glaciers in polar regions. The reason for this effect is that when winters are warmer, the air holds more water and snowfall is heavier. That snow then has a greater probability of remaining on the ground during the cool summer.

Evidence has been found in deep ocean sediments that strongly support Milankovitch's theory. By analyzing the chemical composition of these sediments, scientists have deduced that glaciers have advanced and retreated in roughly one-hundred-thousand-year cycles over the last eight hundred thousand years. Within those cycles, glacier formation occurs in secondary cycles, reaching peaks every twenty-six thousand and forty-one thousand years. These time intervals correspond to the cycles of the three types of orbital variation described here. However, there are many difficulties in correlating climatic change to the Milankovitch cycles, which remains an active field of scientific inquiry.

Volcanoes

In the early stages of Earth's history, thousands of volcanoes dotted the surface. These volcanoes underwent frequent, large eruptions, which had a significant impact on the climate. In addition to releasing gases that rose up and formed Earth's atmosphere, these eruptions sometimes spewed out ash and dust so thick that they nearly blocked out the Sun. Volcanic eruptions have probably been the catalysts for some periods of glaciation.

While volcanic eruptions still occur today, they are far less frequent and intense than they once were. A very large volcanic eruption today affects global climate only for a few years. For example, in 1815 the Indonesian volcano Mount Tambora erupted. Dust from the eruption was carried around the world by upper-air winds. In conjunction with smaller eruptions of other volcanoes over the preceding four years, the Tambora event led to a decrease in global temperature. A severe cold spell in 1816 earned that year the nickname "the year without a summer."

The eruptions with the gravest climatic consequences are those rich in sulfur gases. Even after the ash and dust clears from the atmosphere, sulfur oxides continue to react with water vapor to produce sulfuric acid particles. These particles collect and form a heavy layer of haze. This layer can persist in the upper atmosphere for years, reflecting away a portion of the incoming solar radiation. The result is a global decrease in temperature.

Asteroids

Throughout Earth's history, there have been at least five abrupt, dramatic changes in global climate, occurring 500 million years ago, 430 million years ago, 225 million years ago, 190 million years ago, and 65 million years ago. One possible explanation for these changes is that a large body from space, such as an asteroid or a comet, crashed into Earth. Many scientists think that the extinction of the dinosaurs, which occurred around 65 million years ago, was caused by a collision with an asteroid.

The impact of an asteroid at least 6 miles (10 kilometers) across, traveling at a speed of at least 12 miles (20 kilometers) per second, would produce a crater about 95 miles (150 kilometers) in diameter. It would release energy equivalent to that of four billion atomic bombs such as those dropped on Hiroshima, Japan, during World War II, heating the atmosphere near the impact to temperatures of 3,600 to 5,400°F (about 2,000 to 3,000°C). Another result of this energy release would be the production of huge concentrations of nitric and nitrous acids. These acids would react with and destroy the ozone layer. They would fall to the ground as highly acidic rain, destroying plants and animals.

If the asteroid were to fall on land, a thick dust cloud would rise and could block out all sunlight for several months. Following an early wave of wildfires caused by the high temperatures, any surviving plants and animals would die during a long, dark winter.

It is more likely that an object from space would fall into the ocean, since oceans cover almost three-quarters of Earth's surface. In this case, the impact would stir up carbonate-rich sediments and produce vast quantities of carbon dioxide. Increase of carbon dioxide in the air traps reradiated infrared radiation from Earth's surface, leading to an increased greenhouse effect.

The possibility of such an asteroid impact happening within our lifetimes is remote. However, it has become more of a concern following the July 1994 crash of fragments of Comet Shoemaker-Levy into Jupiter. These collisions, which occurred over several days, caused disturbances on an Earth-sized area of the giant gaseous planet's atmosphere. Had the same impact occurred on Earth, the results would probably have been disastrous.

Solar variability

It has long been understood that the amount of energy emitted by the Sun varies slightly over the years. In recent years, scientists have made correlations between changes in cycles of solar output and particular weather patterns. While there have been several theories linking solar variation to long-term climate change, many more years of data collection will be necessary before such links can be proven. However, the evidence collected thus far presents a compelling case for the link between solar variability and climate change—at least climate change on the scale of decades.

The variation in solar output is primarily based on cycles of sunspot activity. Sunspots are dark areas of magnetic disturbance on the Sun's surface. When the number and size of sunspots is at a maximum, which occurs roughly every eleven years, the Sun's energy output is highest. This heightened solar output is due to an increase in bright areas, called faculae, which form around the sunspots.

Measurements taken by special instruments called radiometers on board satellites have shown that 0.1 percent, and possibly up to 0.4 percent, more solar energy reaches Earth during a sunspot maximum than during a sunspot minimum. The length of sunspot cycles varies over time, from seven to seventeen years. There is a growing body of evidence supporting a link between the length of sunspot cycles and temperature patterns around the world. It has been shown that over the last century, global temperatures, in general, are higher during shorter sunspot cycles than during longer sunspot cycles. In addition, a reduction in the amount of sea ice around Iceland, another sign of warming, has been noted during shorter sunspot cycles.

Another piece of evidence linking sunspots and global temperatures is that the period of lowest sunspot activity in several centuries coincided with the coolest period in several centuries. Between 1645 and 1715, the stretch of years known as the Maunder minimum (named for the British solar astronomer E. W. Maunder who discovered it in the late 1880s), sunspot activity was at a very low level. It is even possible there were no sunspots at all during this period. The period also coincides with the coldest part of the Little Ice Age.

Ways to Measure Climate Change

A paleoclimatologist, a scientist who studies climates of the past, uses many methods. The first step in learning about climates of the past is to discover objects that were formed long ago and to determine an accurate date for those objects. Then the problem is to extract information from the objects that describe the climate at that time.

There are many types of materials, both on Earth's surface and embedded far underground, that have been preserved over thousands and millions of years that provide clues about Earth's past. Paleoclimatologists use these materials in a variety of ways.

Rocks and rock formations

The oldest exposed rocks on Earth are found in northwestern Canada near Great Slave Lake. They are about 4.0 billion years old. These rocks are the only objects that still exist from the earliest period in Earth's history, the Precambrian era. Thus, scientists must rely entirely on rocks to learn about the climate of that time.

To determine the age of rocks, paleoclimatologists use a technique called radiometric dating, which was developed in the late 1800s. This can be used for rocks that contain radioactive elements, such as uranium, radium, and potassium. Radioactive nuclei exist in an unstable configuration and emit high-energy particles (alpha particles or positrons) over time, to achieve greater stability. When the parent nuclei shed alpha particles, or "decay," they transform into daughter nuclei (in the case of a uranium parent, the daughter is lead).

The age of a sample is determined by comparing the percentage of parent nuclei to daughter nuclei. Since scientists know the rate at which radioactive elements decay, they can determine how long ago the sample was entirely made of parent atoms; in other words, when the sample was formed.

Once scientists have established the age of a sample, they can study it for clues about the climate at that time. First, the shape of a rock tells them about the medium in which it once existed. For example, rocks with rounded surfaces probably once existed in a body of water, and rocks with eroded (worn-away) surfaces were probably once covered by glaciers.

To take this a step further, if rounded rocks from the same time period are found all over Earth, it can be assumed that the average temperature at that time was above freezing. The presence of surface water also indicates the existence of some form of atmosphere, without which water would quickly evaporate. By the same token, if eroded rocks of the same age are found at far-flung locations, this signals that an ice age was in progress.

Evidence of primitive organisms begins showing up in rocks dated about 4.0 billion years ago. Studying fossils provides information not only about the progression of life forms at different time periods, but also about the climate. For instance, rocks formed during cooler times, such as ice ages, have little or no fossils embedded within them. In contrast, rocks that were formed during warmer times contain fossils in far greater numbers.

A particularly valuable source of evidence of climate change is rock formations made from layers of particles, deposited incrementally and hardened over time. An analysis of samples of each layer of a rock formation provides information about Earth's climate at that time. Of particular interest are the fossils within each layer, which make up a timeline of the emergence of various species.

The presence of marine animals in a given layer indicates that the region was once covered with water. By studying the chemical composition of fossilized shells, which are primarily composed of calcium carbonate, it is even possible to determine the relative warmth of the water. Oxygen in calcium carbonate exists in two forms: oxygen-16, which is by far the most plentiful, and oxygen-18 (the number refers to how many neutrons the atom possesses). It has been found that during periods of glaciation, the concentration of oxygen-18 increases in the oceans. Thus, the ratio of oxygen-18 to oxygen-16 in fossilized shells can be used to estimate water temperature.

Ice cores

Ice cores drilled into the 2-mile-thick (3-km-thick) polar ice caps provide unique insights into climates of the past. The ice has been accumulating in layers, one year at a time, for thousands of years. Though individual layers of ice can be counted for the last 50,000 years in Greenland, patterns can be discerned from this ice that yield information as far back as 250,000 years. In Antarctica precipitation is so meager that annual layers of ice cannot be distinguished from one

another. However, information contained in the thick layer describes conditions from as far back as 500,000 years ago.

The layers of ice contain many types of climatic evidence, all perfectly preserved. For instance, the thickness of a layer tells how much precipitation fell during a given year. That, in turn, yields information about temperature, since greater levels of precipitation fall during warmer times.

Chemicals detected within the ice are also clues as to the air temperature at the time the precipitation fell. The approximate temperature during a particular period can be determined by comparing ratios of oxygen-16 and oxygen-18 present in a given medium.

By determining the nature of the dust contained within ice cores from the North Pole, it is possible to determine the origin of that dust. That, in turn, provides information about the wind patterns at that time. This type of evidence is not useful in South Pole ice cores, since Antarctica is surrounded by oceans, and is a great distance from all other land masses.

In addition, air bubbles in ice can be analyzed for the composition of the atmosphere. In addition, the timing of volcanic eruptions can be guessed at based on the presence of high levels of acidity in the ice. When volcanoes erupt, they emit dust and gases that are highly acidic.

Ocean and lake sediments

The sediments collected from the bottoms of oceans and lakes contain information about the global climate dating back millions of years. The sediment accumulates in layers, much the same as rock formations and ice. The age of sediment layers is determined using chemical analysis.

The fossils of tiny marine organisms that have evolved and become extinct over time are embedded within the layers of sediment. Each species was adapted only to a narrow range of water temperatures. Therefore, the presence of a species in a given layer of sediment reveals the ocean temperature at that time.

Pollen has also settled in layers on ocean and lake floors. Pollen provides clues to past conditions because every plant species requires a particular set of conditions for survival. The first step in pollen analysis is to identify the pollen species. The next step is to determine its age by finding the age of the surrounding sediment. Studying the times in which particular plant species inhabited a given location teaches scientists about that location's climatic history.

For example, a study of a bog in northern Minnesota identified pollen of fourteen plant types dating back eleven thousand years. In the oldest layer, the greatest pollen concentration was spruce. Since spruce trees inhabit cold climates, it could be inferred that at that time conditions were cold. The next layer yielded primarily pine pollen. Since pine trees grow in warmer regions than do spruce, conditions must have been warmer then. In the next layer, dated eighty-five hundred years ago, oak pollen was widespread. Oak grows in drier conditions than does either spruce or pine, which means that conditions must have been drier at that time. By examining the pollen within each layer, it was possible to construct a climatological history of the bog.

Tree rings

Dendrochronology, the study of the annual growth of rings of trees, is another important component of paleoclimatology. Trees are the oldest living entities on Earth. Some bristlecone pines that are still alive today are more than 4,700 years old.

As trees grow, they add new cells to the center of their trunk each year. These cells force the previous years' growth outward, forming concentric rings with the oldest ring on the outer edge. A tree's woody material acts like a library of climatic data, creating a record for each year of its lifetime. This information can be found in living trees, dead (but not rotten) trees, and tree stumps.

Dendrochronologists measure the width of a given year's growth ring in order to assess the overall conditions of that year. In warm, wet years, trees grow more (and therefore have wider growth rings) than in cool, dry years. Therefore, the difference in the width of growth rings is an indicator of climate.

In order to separate the effect of temperature from that of precipitation, it is necessary to examine trees growing at the edge of their temperature or rainfall range. An example of this is the fir trees growing at the edge of the subpolar zone in Canada. Since temperatures are quite cold every year, an increase in growth from one year to the next would be due almost entirely to precipitation. To study temperature, consider the case of Joshua trees in the heart of a North American desert. There, rainfall is continuously very light, so variations in the width of tree rings would be caused by temperature changes.

Global warming

Global warming is the name given to the recently observed phenomenon of rising global temperatures believed to be caused by human, not natural, activities. A better name is "global climate change," because some places on Earth may actually experience lower temperatures during a period of global warming. According to many atmospheric and planetary scientists, the rapid rise in global temperatures is primarily due to the corresponding rapid increase of gases in Earth's atmosphere called "greenhouse gases." The most important greenhouse gases in Earth's atmosphere are water vapor, carbon dioxide, and methane. Nitrous oxide and chlorofluorocarbons are also greenhouse gases but they contribute an insignificant amount to the global greenhouse effect.

The rapid increase of carbon dioxide in the atmosphere during the nineteenth and twentieth centuries is thought to be the main reason for global warming. Although this remains an area of vigorous scientific debate, the February 2007 report "Climate Change 2007" issued by the United Nations Intergovernmental Panel on Climate Change, which incorporates the input of hundreds of scientists from around the world, offered strong evidence that human-generated pollution is indeed the primary cause of global warming. Carbon dioxide is produced by burning fossil fuels, such as coal, fuel oil, gasoline, and natural gas, and is emitted into the air from homes, factories, and motor vehicles. During the last century, the amount of carbon dioxide in the atmosphere increased by 30 percent. During that same period, the planet has become, on average, slightly more than 1.0°F (0.5°C) warmer.

Since 1880, when accurate temperatures were first recorded around the world, the years 1998 and 2005 were on record as the world's warmest years, and every other year from 1995 to 2005 were among the warmest years on record. In fact, tree rings and sea coral growth indicate that the 1990s was the hottest decade in the last one thousand years and the twenty-first century looks to be even warmer. (Trees and sea corals grow outward from the center, depositing concentric rings every year; each ring yields information about rainfall and temperature for that year.)

In the November 6, 2000, issue of U.S. News & World Report, David Rind, a researcher with National Aeronautics and Space Administration's Goddard Institute for Space Studies, commented that warming in the twentieth century was "a magnitude of change that has not been seen for thousands of years." According to a February 2007 United Nations report, if present trends continue, we can expect an average global temperature increase, heat waves and droughts to become more persistent and severe, and tropical cyclones (hurricanes and typhoons) to become more intense and destructive. Some scientists studying global warming using sophisticated computer programs predict that Earth's climate will undergo as great a change in the next century as it has in the last ten thousand years.

Watch this: An Inconvenient Truth

Former vice president Al Gore's 2006 film An Inconvenient Truth explores the political and environmental consequences of unchecked global warming, and calls on viewers to take immediate action to learn how they can help reverse destructive climate trends. The film won an Academy Award for best documentary feature.

Global warming is a significant concern because it has the potential to disrupt ecosystems—entire communities of plants and animals,—and contribute to the extinction of numerous species. Many scientists blame global warming for the increasing number of severe storms. It may also be a contributing factor to global extremes of droughts and floods. Rising sea levels, another consequence of global warming, threaten to put island nations and coastal cities underwater.

Rising sea levels

Because of global warming, ocean levels have increased 4 to 10 inches (10 to 25 centimeters) since 1900. The rate at which the sea level is rising is expected to increase in the coming century. Current projections have ocean levels climbing as much as 2.5 feet (0.8 meter) by the year 2100. Such an increase would put many coastal areas underwater.

The primary reason for rising water levels is that ocean water becomes less dense and expands as its temperature increases. Water from melting glaciers in Greenland, Alaska, and Antarctica, as well as in the Rockies, Urals, Alps, and Andes mountain ranges, also contributes to rising sea levels.

"The melting of glaciers is emerging as one of the least ambiguous signs of climate change," wrote science writer Fred Pearce in the March 31, 2000, Independent of London. "Amid arcane arguments about the meaning of yearly fluctuations in the weather, it is hard to argue with the wholesale melting of some of the largest glaciers in the world. Mankind, it seems, has hit the defrost button."

Melting in Antarctica

The melting of the West Antarctic ice sheet, which is an enormous glacier in Antarctica, poses the greatest threat to coastal cities and island nations. At its thickest point, the ice sheet is 9.75 miles (15.7 kilometers) deep—ten times the height of the tallest skyscraper in the United States. Since the ice sheet sits on land below sea level, ocean waters lap at its edges and make it vulnerable to melting.

If the West Antarctic ice sheet were to collapse and pour into the sea, it would raise global sea levels by 13 to 20 feet (4 to 6 meters). A sea-level rise of that size would flood coastal regions, including much of Florida and New York City. Global warming experts, however, believe that it would take five hundred to seven hundred years for the West Antarctic ice sheet to collapse if global warming continues at its present rate.

Melting in Greenland

The melting of Greenland's ice sheet, the world's second largest expanse of ice after all of Antarctica's ice, is also a grave concern. The Greenland ice is 2 miles (3.2 kilometers) thick on average and covers 708,000 square miles (1.84 million square kilo-meters)—almost all of Greenland.

Studies conducted by the National Aeronautics and Space Administration (NASA) between 1993 and 1999 show that Greenland's ice is thinning on about 70 percent of its margins—in some places by 3 feet (1 meter) per year. In total, more than 2 cubic miles (8.2 cubic kilometers) of Greenland's ice melts each year. That amount of melting accounts for about 7 percent of the yearly rise in sea levels. If Greenland's entire ice sheet were to melt, ocean levels would rise by about 25 feet (7.6 meters), and there would be massive flooding in many parts of the world.

Melting in the Himalayas

The Himalayan Mountains, one-sixth of which are covered with glaciers, contain more snow and ice than any other place in the world except for the polar regions. (The Himalayas cover 1,500 miles [2,340 kilometers] across northern India, Nepal, and Tibet.) In the summer of 1999, researchers from Jawaharlal Nehru University in Delhi, India, announced their findings that the Himalayan glaciers are melting faster than anywhere else in the world. Their study showed that the Gangotri glacier, situated at the head of the Ganges River—a 1,550-mile-long (2,418-kilometers-long) river flowing southeast from the Himalayas into the Bay of Bengal—is shrinking at a rate of approximately 90 feet (27 meters) per year. If melting in the Himalayas continues at present levels, those glaciers could disappear by the year 2035.

The meltwater from the Himalayan glaciers has formed dozens of lakes, many of which are in danger of bursting through their natural dams. These dams consist of walls of debris that were deposited by retreating glaciers over the last three hundred years. As the water level in these lakes continues to rise, it puts pressure on the natural dams. Eventually, the force of the water will grow too great, and the dams will give way, causing walls of water to surge down the mountainsides. Such catastrophes used to occur about once every ten years, but for the last decade they have been occurring once every three years. Geologists anticipate that by 2101, lake-bursts in the Himalayas will be annual events.

The worst lake-burst in recent history took place in 1985 in the Khumbal Himal region of Nepal. A wall of water 50 feet (15 meters) high swept downstream, killing villagers and destroying a hydroelectric plant. In 1994 a lake-burst in northern Bhutan killed twenty-seven people and ruined buildings and farmland. Tsho Rolpa, a lake at the edge of the Trakarding glacier, northeast of Katmandu, is also in danger of bursting. Engineers for the Tsho Rolpa Hazard Mitigation Project have installed a hazard warning siren on the lake that can be heard far down the valley and is linked to satellite communications. In addition, engineers cut a slot in the natural dam and installed a sluice gate to allow controlled discharge of the meltwater. However, there are many other glacial lakes in India, Nepal, Pakistan, and Tibet that are in imminent danger of collapse that feature no such warning systems.

The increased melting of the Himalayan glaciers also threatens to disrupt the region's supply of water for drinking and crop irrigation. Glacial meltwaters supply two-thirds of the flow of the Ganges River and other nearby waterways. If the glaciers melt entirely, those rivers will shrink and cease to supply the region. If that happens, almost five hundred million people in India would be at risk of starvation.

Island nations in trouble

The effects of the swollen seas are already being felt by small island nations. On the South Pacific islands of Kiribati and Tuvalu, for example, rising waters have destroyed roads and bridges and washed out traditional burial grounds. Many residents of those islands have had to move to higher ground. In Barbados, rising ocean levels have caused salt water to contaminate fresh-water wells near the coasts. The salination (process of making salty) of drinking water is a grave concern in Barbados, where drinking water is already in short supply.

At the global warming summit held in The Hague, the Netherlands, in November 2000, representatives of thirty-nine small island nations expressed their frustration at rising sea levels. They described the threat that rising waters pose to tourism and agriculture, which are concentrated on the coasts and are primary sources of income for island nations. "These are serious issues of economics and livelihood—issues that can disrupt the social fabric of countries," stated Leonard Nurse, a representative from Barbados in a news report of November 17, 2000.

Nurse and other delegates from island nations responded angrily to suggestions made that they should cope with rising sea levels by building surge barriers and storm drains. They blamed industrialized nations, foremost among them the United States, for emitting large amounts of carbon dioxide into the air and accelerating global warming. Those sentiments were underscored by Yumie Crisostomo of the Marshall Islands, who stated to the press: "Whoever caused the problem has to clear up the problem."

What causes global warming?

It is likely that global warming is partially the result of natural processes. For example, Earth could be still recovering from the most recent ice age. However, the rate of global warming is higher now than at any time in Earth's geologic history. This suggests some abnormal cause, such as the recent rapid increase of greenhouse gases in the atmosphere.

While the term "greenhouse effect" has a negative popular connotation because of its association with pollution and global warming, Earth's natural greenhouse effect keeps Earth at a suitable temperature for life on Earth. Without a modest greenhouse effect, the planet's average surface temperature would be about 0°F (−18°C).

How the greenhouse effect works

The greenhouse effect is so-named because of the resemblance of the heat-trapping function of Earth's atmosphere to that of a greenhouse. However, the similarity is

only superficial. In a greenhouse, the glass panels allow solar radiation to pass through. The plants and other objects in the greenhouse absorb the radiation and heat the surrounding air. Since the warm air cannot escape, the air in the greenhouse can become quite warm when the Sun is shining. This is the same phenomenon that can cause the interior of an automobile to become blistering hot in the summertime.

Earth's greenhouse effect is a different process. Solar radiation is primarily visible light. This shorter-wavelength radiation passes through Earth's atmosphere and reaches the surface. Some is reflected back to space by clouds and by Earth's surface itself. The rest is absorbed by the surface and by plants. As the surface warms, it begins to emit infrared radiation. Infrared is longer wavelength than visible light, and Earth's atmosphere does not allow infrared to pass. Consequently, much of this outward-bound radiation is either absorbed by the atmosphere or reflected back to the surface, raising the temperature even further. Eventually, Earth achieves heat balance, with as much energy radiated by the surface as is absorbed, but the presence of greenhouse gases in the atmosphere means heat balance is achieved at a higher temperature.

Why melting icebergs don't contribute to rising sea levels

Many people are under the false impression that melting of icebergs and the floating ice cap that covers the waters around the North Pole are contributing to the rise in sea level. In fact, the melting of floating ice has no effect on the sea level. Ice, with its crystalline configuration of molecules, takes up more space than liquid water. Ice floats because it displaces a weight of water equal to the weight of the ice. When the ice melts, it becomes water with the same weight and volume as the water it was displacing. Consequently, there is no change in water level. The following experiment demonstrates this principle.

1. Take a large clear bowl and fill it half way with water.
2. Empty a tray of ice cubes into the water and measure the height of the water.
3. Wait for the ice to melt, and then measure the height of the water again.

So why do scientists say melting ice caps will cause a rise in sea level? If ice sheets melt, ice and snow covering the polar regions will no longer have anything holding them in place. If Antarctic ice, for example, slides into the ocean and melts, then the sea level will rise.

The most plentiful, and most effective, greenhouse gas is water vapor. Nighttime temperatures in winter often do not drop as low when there is a cloud cover. All other things being equal, the surface temperature remains higher on cloudy nights than it does on clear nights. On clear, dry nights, most of the reradiated heat can escape and the surface temperature can drop to low temperatures. This happens in the high deserts of Asia and South America, where daytime temperatures can soar to over 100°F (40°C) during the day and plummet below freezing at night. Since clouds block incoming solar radiation, they have a cooling effect on the surface during the day.

Why has the greenhouse effect has been getting so much negative publicity in recent years, when it is necessary to sustain life? The answer is that too much of a good thing, in this case the warming of Earth, can be harmful. The natural systems on Earth exist in a delicate balance and require a specific temperature range. If the heat is turned up, the balance is disrupted. The concentration of some greenhouse gases has increased rapidly in recent years, meaning that more heat is being trapped and returned to Earth. This condition is technically called enhanced greenhouse effect, but is commonly known as global warming.

Carbon dioxide and global warming

A majority of planetary and atmospheric scientists now agree that an increase in the amount of carbon dioxide in the atmosphere is almost certainly the primary reason for the enhanced greenhouse effect and the resulting global warming. Carbon dioxide is an industrial byproduct that has been accumulating in the atmosphere since the start of the Industrial Revolution (around 1760–1830). Carbon dioxide is produced by the burning of coal, oil, gas, and wood, and is emitted by factory smokestacks and motor vehicles.

Levels of carbon dioxide, measured at the Mauna Loa Weather Observatory in Hawaii, rose from about 315 parts per million (ppm) in 1960 to about 350 ppm in 1990. During the twentieth century, the level of carbon dioxide in the air rose by 25 percent. In 2001, the rate of increase of carbon dioxide in the atmosphere was about 0.5 percent. Since then, the rate of increase has steadily grown. Atmospheric carbon dioxide levels were 381 ppm in March 2006. Measurements show that 2005 saw one of the largest increases on record, 2.6 ppm, which is a rate of 0.75 percent per year.

Another reason why levels of carbon dioxide are increasing is deforestation, which is the removal of the forests. Deforestation affects the atmosphere in two ways. First, trees naturally absorb carbon dioxide by converting it to oxygen through the process of photosynthesis. With fewer trees, less carbon dioxide is absorbed. Second, in clearing forests to allow for other land use (such as farming), many trees are burned; this places large amounts of carbon dioxide into the atmosphere.

Other greenhouse gases

Carbon dioxide is not the only pollutant responsible for enhancing the greenhouse effect. Concentrations of other greenhouse gases, such as chlorofluorocarbons (CFCs), nitrous oxides, and methane, are also on the rise. While the concentrations of each of these other gases is substantially smaller than the concentration of carbon dioxide, these gases are much more efficient than carbon dioxide at absorbing infrared radiation.

Chlorofluorocarbons are similar to hydrocarbons, except that some or all of the hydrogen atoms have been replaced by chlorine or fluorine atoms. CFCs can be liquids or gases. They are used in refrigerators and air conditioners; as propellants in aerosol spray cans (such as deodorants, spray paints, and hairsprays) and foam-blowing canisters; and in some cleaning solvents.

Nitrous oxides, like carbon dioxide, are emitted from industrial smokestacks and car exhaust systems. They are also components of some fertilizers sprayed on agricultural fields.

Methane is a product of anaerobic (in the absence of oxygen) decay of organic matter. Some sources of methane are swamps, rice paddies, garbage dumps, and livestock operations. Approximately 60 percent of atmospheric methane is the result of human activity. Of that portion, most is due to livestock. By growing rice for millions of people, dumping refuse in landfills, and raising livestock, humans contribute to a rising concentration of methane in the atmosphere. Since methane only survives fifteen to twenty years in the atmosphere, and is twenty times more potent than carbon dioxide as a greenhouse gas, reducing methane emissions could be an effective means of reducing climate warming in a relatively short time.

Consequences of global warming

Recent findings by scientists on the Intergovernmental Panel on Climate Change (IPCC) and others about the trend in global warming maintain that within the next century the world may reach its warmest point in the history of civilization. The effects, according many scientists, could be far-reaching. One consequence of global warming, as discussed above, is an increase in ocean levels around the world. The warmer weather is also expected to alter rainfall patterns, increase the severity of storms, and have negative effects on human health. By many accounts, global warming has already harmed certain animal species. A review of 866 scientific studies in the journal Annual Review of Ecology, Evolution and Systematics finds that many species of frogs are becoming extinct, and cold-dependent animal species such as penguins and polar bears are also threatened.

Drought, floods, and storms

Global warming is expected to increase the amount of rainfall in the tropics and produce drought throughout temperate regions (for example, the northern three-quarters of the United States, southern Canada, and much of Europe). In places suffering from a lack of rainfall, crop yields would be lower, and natural vegetation would suffer. Grazing animals would either have to migrate to find food and water or would die off.

Climatologists (scientists who study climate) point to the occurrence, over the last two decades, of two of the most extreme El Niños on record as further evidence of global warming. El Niño is the extraordinarily strong episode of the annual warming of the Pacific waters off the coast of Peru.

Some scientists also assert that global warming has been responsible for recent, unusually severe, weather such as strong blizzards, hurricanes, tornadoes, heat waves (extended periods of high heat and humidity), and wildfires. They warn that these weather disasters will intensify as global warming increases. Other scientists dispute the effect of global warming on the weather, pointing out that there has been no increase in the number of major hurricanes in recent decades.

Risks to human health

By many accounts, global warming is bad for human health. A group of researchers in the United States warns that the number of U.S. residents dying of heat stress may double by the year 2075 (presently two thousand to three thousand people per year die from heat stress). Sustained rains, predicted for warm regions, may produce flooding that causes drinking water to be contaminated with sewage; people drinking the contaminated water would become sick. Warmer air also increases ground-level ozone pollution (smog), which aggravates symptoms of asthma and other respiratory ailments.

Disruptions to animal life

Many scientists are also concerned about the harm global warming causes to wildlife. "Global climate change has the potential to wipe out more species, faster, than any other single factor," stated Patty Glick, coordinator of the Climate Change and Wildlife Program at the National Wildlife Federation, in a November/December 2000 International Wildlife interview. The World Wildlife Fund estimates that 20 percent of species in northern regions, from New England to the North Pole, could die out by the year 2100 because of the loss of habitat brought about by global warming.

As reported by the World Wildlife Federation in June 1999, tropical fish have been forced to migrate to colder waters in search of food. Animals that depend on fish for sustenance, such as sharks, sea lions, and marine birds, have also been forced to migrate or face starvation.

The reason for the migration is that the warming of ocean waters in recent years has led to a reduction of the fishes' food source, ocean plankton, in waters where plankton have traditionally thrived. Plankton are microscopic plants (phytoplankton) and animals (zooplankton) that occupy the bottom rung of the food chain, in which food energy is transferred from one organism to another one as each feeds on another species. Plankton are the beginning of the chain. Off the west coast of North America, plankton populations have decreased by 70 percent since 1977. As a result of that change, there has been a 90 percent decline in seabird populations since 1987.

Antarctic penguin species declining

An animal species suffering from higher temperatures in recent years is the Adelie penguin. The Adelies live on the Antarctic continent and on the great sheets of off-shore pack ice. They survive on krill, tiny shrimplike animals that live in the icy waters. The krill, in turn, feed on the algae that bloom within the layers of sea ice and are released into the water as the ice melts.

The Adelies' reproductive cycle is tied to the changing of the seasons. They give birth just at the start of the Antarctic summer, when algae fill the water and krill are plentiful. If the temperature is too high and the thawing begins too early, however, the algae are scattered far and wide (as are the krill) at the time the chicks are born. In that case, the Adelie parents are forced to travel great distances to gather food for their young, all the while leaving their young unattended. A study found that Adelies were spending sixteen hours a day to gather food, up from previous years' average of six hours a day. As a consequence, the young may not get enough food or may fall prey to predatory birds.

Over the last twenty-five years, Adelie penguin populations in areas under study by American researchers have declined dramatically. Numbers of breeding pairs in five large colonies dropped from 15,200 to 9,200, and several small colonies were entirely wiped out. In the last two years alone, the Adelie population in the study area dropped by 10 percent. The decline in Adelies corresponds to an increase in temperature over the last fifty years, during which time Antarctica has become warmer by 3 to 5°F (1.7 to 2.8°C) in the summer and 10°F (5.6°C) in the winter. Seals, whales, and other species of penguins are also threatened by dwindling krill supplies near Antarctica. For example, an average blue whale eats 4 to 6 tons (3.6 to 5.4 metric tons) of krill each day.

Effects on arctic wildlife

The effects of global warming on wildlife are seen most vividly in the arctic. In parts of that far northern region, temperatures increased 7 to 10°F (4 to 5.6°C) in the thirty-five-year span from 1965 to 2000. During the 1990s the number of salmon in Alaska's rivers decreased dramatically, as did the numbers of Stellar sea lions and harbor seals in the Bering Sea and Gulf of Alaska. More than one million seabirds starved to death in the Bering Sea in the years 1997 and 1998 alone, because of dwindling food supplies in the warmer waters.

The warmer weather also triggered much heavier snowfalls in Alaska. For some animals, such as the Peary caribou, the large quantities of snow make it difficult to get to buried food. This factor has led to a reduction in the numbers of this species of small caribou from 24,500 in 1961 to only about 1,000 in the year 2000.

The human factor

Since global warming was identified as a problem in the 1970s, there has been vigorous debate over what role, if any, human activity plays in the trend. Since 1995, however, many global climate scientists agree that increased carbon dioxide levels and other forms of atmospheric pollution are a major factor in contributing to global warming, and that immediate action is necessary. Scientists from a variety of public and private agencies recommend that governments regulate greenhouse gas emissions now, instead of waiting until the problem is worse and the remedy more costly. They warn that even if the emission of pollutants were curbed today, it would take many years for the global warming trend to stop (primarily because heat, stored in the oceans, would continue to be slowly released). There is also debate over the projections of how fast climate changes will occur in the future and the effects of those changes.

Is global warming a natural phenomenon?

Some scientists and others remain skeptical that humans are the primary cause of global warming. They point out that annual average temperatures in the continental United States have varied from decade to decade, with no significant upward or downward trend throughout the century. It was relatively cool from the beginning of the century until 1920; warmed up in the 1920s through the 1950s; cooled down in the 1960s and 1970s; and began warming again in the 1980s. One theory suggested to explain the warm 1990s is that the heat given off by the Sun has increased. In recent years, however, a growing number of corporations have abandoned the position that global warming is not a social problem.

Earth Summit addresses global warming

The debate over the role of human activity in global warming has been carried out all over the world. The first international meeting to address the problem of global warming was held in June 1992 in Rio de Janeiro, Brazil. Formally called the United Nations (U.N.) Conference on Environment and Development, but better known as the Earth Summit, the 1992 meeting was attended by representatives of 178 nations, including 117 heads of state.

One outcome of the Earth Summit was the drafting of a document called the Declaration on Environment and Development, also known as the Rio Declaration. The document spelled out twenty-seven guiding principles of environmentally friendly economic development. Conference attendees came to an informal agreement on the need to change energy policies in order to halt global warming.

Global warming and the United States

In June 2000, a report commissioned by the U.S. Congress painted a grim picture of the effect of global warming on the United States. The national assessment report, compiled by scientists from both within and outside of the government, gave a detailed summary of what will likely occur if average temperatures rise 5 to 10°F (2.8 to 5.6°C) over the next century. While recognizing the "significant uncertainties in the science underlying climate-change impacts," the study concluded that "based on the best available information, most Americans will experience significant impacts" from global warming.

The report predicts that as temperatures rise, entire ecosystems will move northward. For instance, as New England warms, that region's sugar maple forests will die off, to be replaced by forests in Canada. Salmon currently inhabiting the Columbia River (in the Pacific Northwest) will be unable to survive in the warmer water, while other warmer-water fish species will move in. The report also warned that rising sea levels may cause coastal marshes and wetlands to spread inland, completely submerging the barrier islands off the coast of the Carolinas.

Among the report's other projections were sweltering heat waves in urban areas, frequent droughts in the Midwest, the conversion of forests in the Southeast into grasslands, the reduction of water levels in the Great Lakes (because of increased evaporation), and damage to roads and buildings in Alaska due to the thawing of the ground.

The Kyoto conference

In December 1997, representatives of 166 nations gathered in Kyoto, Japan, for the U.N. Framework Convention on Climate Change. The conference focused on the growing problem of global warming and ways to reduce greenhouse emissions.

Delegates to the meeting produced the Kyoto Protocol, a document that called upon industrialized nations (relatively wealthy nations, such as those in North America and western Europe) to take the lead in reducing emissions of greenhouse gases. The protocol specifically directed thirty-six industrialized nations to reduce greenhouse-gas emissions between the years 2008 and 2012 to 5.2 percent below 1990 levels. These nations are the largest producers of greenhouse gases. The United States, for instance, is responsible for 25 percent of global carbon dioxide emissions—more than any other nation. Poorer, developing nations (in Africa, Latin America, parts of Asia, and the Middle East) were spared the treaty's requirements. Conference participants agreed that it would pose too great an economic burden on developing nations to greatly reduce greenhouse emissions.

In November 1998 the administration of President Bill Clinton endorsed the Kyoto Protocol. The signing of the document, however, was largely symbolic since the Senate did not give its approval. (The U.S. Constitution states that all treaties are subject to ratification by two-thirds of the Senate.) Given the treaty's certain defeat in the Senate, President Clinton chose not to put it to a vote.

Senate leaders made their concerns about the Kyoto Protocol clear in a 1997 resolution, passed by a vote of 95-0 shortly after the convention. Senators vowed to oppose the agreement unless it required developing countries to reduce greenhouse emissions during the same time period as mandated for industrialized nations. They pointed out that developing nations, especially India and China, are rapidly increasing their use of fossil fuels.

Despite the United States' refusal to participate in the Kyoto Protocol, it has been signed by 169 countries and other governmental entities and became part of the United Nations Framework Convention on Climate Change on February 16, 2005.

Talks in The Hague end in stalemate

In November 2000, delegates from more than 180 countries met in The Hague, the Netherlands, with the goal of implementing the Kyoto accord. They sought to develop a method for monitoring greenhouse gas emissions and to devise penalties for countries that did not reduce their emissions.

The two-week-long meeting, however, ended without an agreement. The talks broke down over a demand by the United States and some other industrialized nations to receive credit for carbon dioxide "sinks," which are areas covered with vegetation that naturally absorb carbon dioxide, such as forested land and farmland. The credits would have partially offset the amount by which those nations were required to cut their emissions.

The future of global warming

If global warming continues at the current rate, dramatic changes in the global climate are likely. Forests in the U.S. Southeast could be converted to grasslands. Deserts might spread around the globe at middle latitudes. The tropics might become virtually uninhabitable. Paradoxically, some areas might become cooler. For example, if the Gulf Stream (a warm water circulation pattern in the North Atlantic) is diverted to the south by meltwater from the Arctic ice, the British Isles could experience much colder weather.

At the same time, much of the frozen north would thaw and become habitable. The great coniferous forests of Asia, Europe, and North America could be converted to agriculture. The polar regions themselves might be habitable. This would lead to much conflict in areas such as Antarctica, where land would be at a premium. Whatever the ultimate consequences, it is clear that unchecked global climate change will have a profound effect on Earth.

[See AlsoClimate; El Niño; Flood; Forecasting; Human Influences on Weather and Climate; La Niña; Weather: An Introduction ]

For More Information

BOOKS

Flannery, Tim. The Weather Makers: How Man Is Changing the Climate and What It Means for Life on Earth. New York: Atlantic Monthly Press, 2006.

Gore, Al. An Inconvenient Truth 2nd ed. New York: Rodale Books, 2006.

Kolber, Elizabeth. Field Notes from a Catastrophe: Man, Nature, and Climate Change. New York: Bloomsbury USA, 2006.

Linden, Eugene. The Winds of Change: Climate, Weather, and the Destruction of Civilizations. New York: Simon & Schuster, 2006.

Pearce, Fred. When the Rivers Run Dry: Water—The Defining Crisis of the Twenty-First Century. Boston, MA: Beacon Press, 2006.

PERIODICALS

Appenzeller, Tim, and Dennis R. Dimick. "Signs from Earth." National Geographic (September, 2004): pp. 2-12.

Hayden, Thomas. "Super Storms: No End in Sight." National Geographic (August, 2006): pp. 66-77.

Parmesan, Camille. "Ecological and Evolutionary Responses to Recent Climate Change." Annual Review of Ecology, Evolution, and Systematics, vol. 37 (December, 2006): pp. 637-669.

Trenberth, Kevin. "Uncertainty in Hurricanes and Global Warming." Science (June 17, 2005): pp. 1753-1754.

Zwingle, Erla. "Meltdown: The Alps under Pressure." National Geographic (February, 2006): pp. 96-115.

WEB SITES

"Climate Change." United States Environmental Protection Agency. 〈http://www.epa.gov/climatechange/〉 (accessed March 24, 2007).

"Climate Change Impacts: Feeling the Heat." Nature Conservancy. 〈http://www.nature.org/initiatives/climatechange/issues〉 (accessed March 24, 2007).

"Climate Change." World Wildlife Fund. 〈http://www.panda.org/about_wwf/what_we_do/climate_change/index.cfm〉 (accessed March 24, 2007).

"Climate Change." New Scientist: Environment. 〈http://environment.newscientist.com/channel/earth/climate-change〉 (accessed March 25, 2007).

De Roy, Tui. "Caught In a Melting World: Adelie Penguins May Be the Canaries in the Coal Mines of Global Warming." International Wildlife(November-December, 2000). 〈http://findarticles.com/p/articles/mi_m1170/is_2000_Nov-Dec/ai_66309910/pg_1〉 (accessed April 25, 2007.)

Intergovernmental Panel of Climate Change. 〈http://www.ipcc.ch/〉 (accessed April 25, 2007).

"Working Together." The Pew Center on Global Climate Change. 〈http://www.pewclimate.org〉 (accessed March 25, 2007).