Climate

Elements of climate
Temperature
Precipitation
Climates of the world
Humid tropical climates
Rain forest
Monsoon
Savanna
Dry climates
Desert
Steppe
Subtropical climates
Humid subtropical
Marine
Mediterranean
Temperate climates
Subpolar climate
Polar climate
Tundra
Arctic
Mountain climates
History of climate change
Precambrian era
Paleozoic era
Mesozoic era
Cenozoic era: Pre-Holocene
Cenozoic era: Holocene
Reasons for climate change
Continental drift
Earth's orbit
Volcanoes
Asteroids and comets
Solar variability
Ways to measure climate change
Rocks and rock formations
Ice cores
Ocean and lake sediments
Tree rings
For More Information

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. It also tells whether these conditions prevail year-round or if they change with the seasons.

Throughout history, 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.

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. Certain parts of Asia and Africa have monsoon climates, meaning they are warm year-round and relatively dry for half the year yet experience heavy rainfalls over the other six months. In other regions of the world severe blizzards or hurricanes are irregular, yet not uncommon, occurrences.

The climates of the world are differentiated by many 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 also defined by the plants that exist in a region. What's more, climate is not a fixed property of a region. The climate has changed considerably over the 4.6 billion years since Earth was formed. It continues to change today, as it will in the future.

WORDS TO KNOW

acid precipitation:

rain and snow that are made more acidic by sulfuric and/or nitric acid in the air.

air pressure:

the pressure exerted by the weight of air over a given area of Earth's surface. Also called atmospheric pressure or barometric pressure.

continental drift:

the theory that over the last 200 to 250 million years, forces deep within Earth's core have caused the single huge continent to break apart and continents to drift around the globe.

cosmic rays:

invisible, high-energy particles that bombard Earth from space.

dendrochronology:

the study of the annual growth of rings of trees.

dew point:

the temperature at which a given parcel of air reaches its saturation point and can no longer hold water in the vapor state.

eccentricity:

the alternating change in shape of Earth's orbit between a circle and an ellipse.

equinoxes:

the days marking the start of spring and fall and the two days of the year in which day and night are most similar in length.

evaporation:

fog that is formed when water vapor evaporates into cool air and brings the air to its saturation point.

extratropical cyclones:

a storm system that forms outside of the tropics and involves contrasting warm and cold air masses.

front:

the dividing line between two air masses.

global warming:

the theory that average temperatures around the world have begun to rise, and will continue to rise, due to an increase of certain gases, called greenhouse gases, in the atmosphere. Also called enhanced greenhouse effect.

haze:

the uniform, milky-white appearance of the sky that results when humidity is high and there are a large number of particles in the air.

horse latitudes:

a high-pressure belt that exists at around 30° latitude, north and south, where air from the equatorial region descends and brings clear skies.

Elements of climate

The two criteria that are most significant in defining climate type are temperature and precipitation (water that originates in the atmosphere and falls to the ground). These criteria, in turn, are influenced by a number of atmospheric, oceanographic, and topographic factors, such as: uneven heating of Earth by the Sun; distribution of land and water; ocean currents; wind circulation patterns; the locations of high- and low-pressure systems; mountain ranges; and altitude. Elements of secondary importance to defining climate are winds, humidity, air pressure, and sunshine versus cloud cover.

Temperature

There are two temperature statistics that are relevant to climate: annual mean (average) temperature and annual temperature range. To calculate the annual mean temperature, you need a year's worth of daily mean temperatures. A daily mean is the average of a day's maximum and minimum temperatures. For instance, if a day's high temperature is 60°F (15°C) and the low is 30°F (?1°C), the average is 45°F (7°C). Then add up the average temperature for every day throughout a year and divide by 365 to get the annual mean. A given location's annual mean temperature can be compared to the world's average, 59°F (15°F), to determine the relative warmness or coldness of that location's climate.

The annual temperature range is found by subtracting the year's lowest mean monthly temperature from the year's highest mean monthly temperature. Thus, if a city averaged 80°F (27°C) in July, its hottest month, and 40°F (4°C) in January, its coldest month, it would have an annual temperature range of 40 degrees. Some places in the United States, such as San Francisco, California, have a temperature range as small as 10 degrees. The annual temperature range reveals whether or not a location experiences different seasons, which is just as important as the annual mean temperature in identifying climate type.

Standardized global temperature information has been corrected for the influences of topography by a process known as "adjustment to sea level." Temperature drops at a rate of 3.6°F for every 1,000 feet (300 meters) you ascend above sea level. Thus, when taking the temperature on a mountain at a height of 4,000 feet (1,200 meters), you would add 14.4 degrees from the reading on the thermometer to find the temperature adjusted to sea level.

If you plot out mean annual sea level temperatures throughout the world, you find bands (called isotherms) that roughly correspond to latitude. Isotherms are imaginary lines connecting areas of similar temperature. 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. The Sun strikes Earth most directly near the equator and most indirectly (at the steepest angle) at the poles. The atmosphere, through the movement of global winds, strives to even out the distribution of heat but does not totally erase these differences.

The isotherms veer from their paths where landmasses meet the sea. This is due to the different heating and cooling behaviors of land and water (water takes longer to heat up and retains heat longer than land does). The bending of isotherms along the coasts is also caused by ocean currents, which carry warm water toward the poles and cold water toward the equator. A third cause of this temperature differential at the coasts is upwelling, the rising up of cold waters from the depths of the ocean.

Precipitation

Two measures of precipitation are relevant to determining climate: total annual precipitation and a month-by-month breakdown of when precipitation occurs. The former statistic is an indicator of the overall wetness or dryness of a region, while the latter indicates whether or not a region experiences distinct rainy and dry seasons.

Tropical rain forests, the world's wettest places, receive 60 inches (150 centimeters) or more of rain annually, while deserts and polar regions, the world's driest places, typically receive less than 10 inches (25 centimeters).

The amount of rainfall determines the type of plant and animal life a region can support. However, the distribution of precipitation throughout the year is also an important factor—particularly in relatively dry regions. For instance, if an area receives little total annual rainfall, but most comes during the summer months, that region may be suitable for agriculture. However, if the limited precipitation of a relatively dry region is spread more or less evenly throughout the year, it may not be able to sustain crops.

Wet and dry regions are scattered across the globe. There are some general trends, however, that correspond to global air circulation patterns. For instance, 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, about 60°, in the middle latitudes (the regions of Earth that lie between 30° and 60° latitude), where the westerlies meet the polar easterlies and air rises. Westerlies are global-scale surface winds that travel in the middle latitudes from southwest to northeast in the Northern Hemisphere (the half of Earth that lies north of the equator) and from northwest to southeast in the Southern Hemisphere (the half of Earth that lies south of the equator). Polar easterlies are cold global winds that travel across the polar regions, from the northeast to the southwest in the Northern Hemisphere and from the southeast to the northwest in the Southern Hemisphere.

In the subtropical regions around 30° latitude, where the trade winds meet the westerlies and air sinks, conditions are much drier. Most of the world's deserts are located there. The polar regions, where the polar easterlies converge and air falls, are also characterized by dryness. It should be noted that these latitudes are not fixed. As Earth makes its yearly revolution around the Sun, the amount of sunlight received at each

WORDS TO KNOW

isotherm:

an imaginary line connecting areas of similar temperature.

leeward:

the eastward side of the mountain, on which cold air descends, producing dry conditions.

middle latitudes:

the regions of the world that lie between the latitudes of 30° and 60°, north and south. Also called temperate regions.

monsoon:

rainy season that occurs during the summer on tropical continents, during which the land becomes warmer than the sea beside it.

obliquity:

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

ocean currents:

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

orographic lifting:

the upward motion of warm air that occurs when a warm air mass travels up the side of a mountain.

ozone layer:

the layer of Earth's atmosphere, between 25 and 40 miles (40 and 65 kilometers) above ground, that filters out the Sun's harmful rays. It consists of ozone, which is a form of oxygen that has three atoms per molecule.

paleoclimatologist:

a scientist who studies climates of the past.

permafrost:

a layer of subterranean soil that remains frozen year-round.

polar easterlies:

cold, global winds that travel across the polar regions, from the northeast to the southwest in the Northern Hemisphere and from the southeast to the northwest in the Southern Hemisphere.

radioactive dating:

a technique used to determine the age of rocks that contain radioactive elements, which works on the principle that radioactive nuclei emit high-energy particles over time.

rain shadow effect:

the uneven distribution of precipitation across a mountain, with most of the precipitation falling on the windward side and very little falling on the leeward side.

semipermanent highs and lows:

the four large pressure areas (two high-pressure and two low-pressure), situated throughout the Northern Hemisphere, that undergo slight shifts in position, and major changes in strength, throughout the year.

smog:

common name for photochemical smog, a layer of hazy, brown air pollution at Earth's surface comprised of surface ozone.

sunspots:

areas of magnetic disturbance on the surface of the Sun, sometimes referred to as "storms."

topography:

the shape and height of Earth's surface features.

trade winds:

dominant surface winds near the equator, generally blowing from east to west and toward the equator.

transpiration:

the process by which plants emit water through tiny pores in the underside of their leaves.

tropical storm:

a tropical cyclone weaker than a hurricane, with organized bands of rotating thunderstorms and maximum sustained winds of 39 to 73 mph (63 to 117 kph).

upwelling:

the rising up of cold waters from the depths of the ocean, replacing the warm surface water that has moved away horizontally.

virga:

rain that falls from clouds but evaporates in mid-air under conditions of very low humidity.

westerlies:

global-scale surface winds that travel from the southwest to the northeast in the Northern Hemisphere, and from the northwest to the southeast in the Southern Hemisphere, between about 30° and 60° latitude.

windward:

the westward side of a mountain, on which warm air ascends, forms clouds, and yields precipitation.

latitude changes, causing northward and southward shifts in wet and dry latitudes.

Precipitation is also connected with temperature in that warm air can hold more moisture than cold air. In the polar regions, the air holds so little moisture that precipitation is a rare event. This situation stands in contrast to the tropics, where warm air holds an abundance of water and rain is plentiful.

Another factor influencing precipitation is topography. As air travels over a mountain, orographic lifting, the upward lifting of warm air that occurs when a warm air mass travels up the side of a mountain, results in cloud formation and precipitation. Thus, the west (windward) side of a mountain range receives ample rainfall while the east (leeward) side stays relatively dry.

The positions of large-scale high and low pressure systems around the globe are a final factor influencing precipitation. These storm-producing and storm-directing zones are influential in determining the westerly and easterly (horizontal) patterns of precipitation.

Climates of the world

Although most people are familiar with certain climate types, such as tropical rain forest, desert, and tundra, no universally accepted, official system of climate designation exists. The first system to be used widely among scientific circles was the Köppen system, developed in 1918 by German meteorologist Wladimir Köppen (1846–1940). Köppen divided the world into five climate types, based primarily on precipitation and secondarily on temperature. They are humid tropical; arid (dry); humid subtropical; humid temperate; humid cold; and polar. Köppen then broke each of these climatic regions down into subclimates.

In the early 1930s, American climatologist C. Warren Thornthwaite (1899–1963) proposed another system, the Thornthwaite classification. He later revised the system in 1948 and again in 1955. Thornthwaite's system is based largely on precipitation levels but also takes into account the rate at which moisture returns to the atmosphere by evaporation (the process of water changing from a liquid to a gas) and transpiration (water lost through plant leaves and stems). He developed an index using the ratio of precipitation to evaporation and added a factor called potential evapotranspiration, the total amount of water that would be produced by a combination of evaporation and transpiration, given sufficient precipitation. He translated this formula into five different climate types named for their characteristic vegetation: rain forest, forest, grasslands, steppe, and desert.

Most of the classification schemes found in textbooks today, including the one presented here, are some combination of Köppen's and Thornthwaite's systems. In this chapter, the climates of the world will be described in the following general categories, some with subdivisions: humid tropical; dry; subtropical; temperate; subpolar; polar; and mountain.

Note: It would be helpful to have a world atlas beside you while reading this section to better understand the locations of the regions described.

Humid tropical climates

Humid tropical climates, as the name implies, are warm and wet. The mean temperature for any month seldom falls below 64°F (18°C), so there is no winter. But there is plenty of rainfall in these climates. They receive on average about 60 inches (150 centimeters) of rain per year, which may be concentrated over a few months or spread throughout the entire year.

A key reference to: Climate types (hot/wet to cold/dry)

Rain forest:

warm and rainy all year long. Covers portions of South America, eastern Central America, Caribbean Islands, central Africa, East Indies, eastern Madagascar, and Myanmar.

Monsoon:

warm year-round with very rainy (flood-prone) summers and relatively dry winters. Covers much of southern and southeastern Asia, the Philippines, coastal regions of northern South America, and slices of central Africa.

Savanna:

warm year-round with rainy summers and drought-prone dry winters; receives less rainfall than monsoon climates. Covers much of south-central and eastern Africa, portions of India, western Central America, Brazil, southern Florida, southeast Asia, and northern Australia.

Desert:

world's driest regions, with less than 10 inches (25.4 centimeters) of rainfall annually; warm and cold varieties. Examples of warm deserts include: the Sahara (Africa), the Great Sandy (Australia), and the Mojave and Sonoran (southwestern United States). Examples of cold deserts include: the Atacama (Chile), the Great Basin (Nevada), and the Gobi (Asia).

Steppe:

semidry grasslands (receive less than 20 inches [50.8 centimeters] of rainfall annually) that occur in the rain shadow of a mountain range or on the edge of a desert. Covers portions of central Mexico, southern South America, northwestern Africa, west-central Asia, and the western United States.

Humid subtropical:

hot, muggy summers and mild, wet winters. Covers the southeastern United States, eastern China, southern Japan, southeastern South America, and the southeastern coasts of Africa and Australia.

Marine:

long, cool summers and mild winters; coastal climate characterized by low clouds, fog, and drizzle for much of the year. Covers the coastal areas of the northwestern United States, the British Isles, New Zealand, western France, and southeastern South Africa.

Mediterranean:

relatively dry climate with dry summers and rainy, mild winters. Covers the following coastal lands: those surrounding the Mediterranean Sea, central and southern California, southwestern Africa, southeastern and southwestern Australia, and central-western Chile.

Temperate:

four distinct seasons with warm, humid summers and cold, snowy winters. Covers the northeastern United States and southern Canada, and parts of central Europe, central Asia, southeastern Chile, southern Australia, and New Zealand.

Subpolar:

cold northern climate with long, harsh winters and short, cool summers. Covers Alaska and most of central and northern Canada, northern Europe, and northern Asia.

Tundra:

bitterly cold winters and cool summers; for at least one month of the year the average temperature is above freezing. Covers the northern portions (except for the northernmost extremes) of North America, Europe, Asia, Iceland; and the coasts of Greenland and Antarctica.

Arctic:

barren lands of snow and ice that never thaw. Covers Ellesmere Island and the northern section of Baffin Island (Canada), and the interiors of Greenland and Antarctica.

Humid tropical climates are located in belts that are 20- to 40-degrees wide on either side of the equator. They cover the largest portion of Earth of any climate zone. They account for nearly 20 percent of the land and 43 percent of the oceans, totaling 36 percent of Earth's surface. This category is subdivided into three specific climate types: rain forest, monsoon, and savanna.

Rain forest

Rain forests are warm regions in which the rainy season usually lasts all year. Average rainfall for each month is over 2 inches (5 centimeters), and annual total rainfall usually exceeds 60 inches (150 centimeters). In some areas it may exceed 160 inches (400 centimeters) annually. On most days, heavy showers fall in the afternoon and skies clear by the evening.

Tropical rain forests exist at low elevations in regions very close to the equator. They cover the Amazon lowland of South America, portions of eastern Central America and the Caribbean Islands, the Congo River basin of Africa, the coastal area of western Africa, the East Indies from Sumatra to New Guinea, and a slice of Myanmar in southeastern Asia. Tropical rain forests also exist in isolated areas, such as the Atlantic coast of Brazil, the Pacific coast of Colombia, the Guiana highlands in northern South America, and eastern Madagascar.

Because of their proximity to the equator, rain forests experience about the same number of daylight hours year-round. Therefore, the annual temperature range is small, generally less than 6°F (3.3°C). The biggest change in temperature comes between day and night. The average daytime temperature is around 90°F (32°C), while the average nighttime temperature is about 72°F (22°C). The high humidity and cloud cover prevent the temperature of the rain forest from rising as high as it does on summer days in the subtropics and even in some parts of the temperate latitudes.

Tropical rain forests are also called tropical wet climates. They are noted for their abundant and varied plant and animal life. Whereas rain forests cover only 7 percent of the world's land, they contain 50 percent of all species of plants and animals. This total includes over 1.5 million species of plants and animals, the greatest biodiversity in the world.

Plants grow at all different heights throughout the rain forest, from the canopy (the uppermost spreading branchy layer of a forest) to the floor. The largest plants are the towering, broadleaf evergreens, such as mahogany and kapok trees. These trees block out most of the sunlight. Near the forest edge and in clearings where sunlight penetrates, vines and shrubs predominate. The undergrowth there is so thick that it's often impossible to walk through.

The great variety of plants provides sustenance for an equally impressive array of animals. In a rain forest, you will find animals occupying every possible niche: monkeys in the treetops; colorful birds such as toucans and parrots flying overhead; lizards scaling tree trunks; frogs, snakes, and insects on the ground; and termites tunneling underground. During the day, iguanas can be seen basking in the Sun while armadillos stay in their burrows to keep cool. Several species, such as fruit bats, moths, and caimans (cousins of the crocodile), become active at night.

Monsoon

In this type of tropical climate, the year is divided into dry and rainy seasons. While monsoon regions generally receive as much total annual rainfall as do rain forests, the precipitation is concentrated in the summer months. Typically, about 120 inches (300 centimeters) of rain falls during the rainy season, causing destructive flooding. Rainfall in the spring and fall is moderate while the months of December, January, and February are dry. The official designation of a monsoon climate is one in which monthly precipitation drops below about 2 inches (6 centimeters) for one or two months of the year.

The name monsoon comes from the Arabic word "mausim" for "season." A monsoon is actually a seasonal reversal of wind patterns. During winter, when the monsoon area is tilted away from the Sun, the winds that sweep across the region are primarily trade winds, blowing out toward the sea. These winds bring warm, dry air to the region. When spring arrives, the winds shift and carry moisture from the Indian Ocean over the land. At this time of year, the monsoon area is almost directly beneath the Sun. The land is heated intensely, which causes the surface air to rise. This air is replaced by the moist winds, which produce heavy rains that last throughout the summer months.

While monsoon climate is limited mostly to southeast Asia, a full 50 percent of the world's population lives in that area. The monsoon region encompasses most of India; the Indus Valley of Pakistan; portions of Nepal, Bhutan, Bangladesh, Myanmar, Malaysia, Vietnam, Thailand, Laos, and the Philippines; and the southeast tip of China. Some Atlantic coastal regions of northern South America also experience monsoon climate, as do parts of central Africa.

The total annual rainfall in monsoon regions is generally sufficient to sustain forests, even through the dry period. In some areas, these forests resemble rain forests, while in other areas the forests are less dense and

merge into scrublands. These lands are cultivated primarily with rice, a crop that thrives on flooding.

Animals that have evolved to survive in both wet and dry seasons live in monsoon regions. One example is the collared anteater, which eats ants in the wet season and termites in the dry season. Another example is the iguana, a reptile that can regulate temperature by changing its skin color and water loss by concentrating its urine.

Savanna

A savanna (sometimes spelled "savannah") is a relatively flat land covered with coarse grasses that are several feet tall, and scattered, stunted deciduous trees such as baobabs and acacias. Like a monsoon climate, this climate is tropical, with wet and dry seasons. Yet the dry periods in savanna regions last longer than in monsoon regions, and annual precipitation is much lower.

Savannas are located farther from the equator than are rain forests and monsoon climates. While savannas are most common in south-central and eastern Africa, they also exist in the following locations: central and eastern India, western Central America, areas of Brazil north and south of the Amazon basin, southern Florida, parts of southeast Asia, and northern Australia.

Savannas usually receive between about 30 and 60 inches (75 and 150 centimeters) of rainfall annually, much less than rain forests and monsoon regions receive. In general, the farther a savanna is from the equator, the lower its annual rainfall. The amount of rainfall in savannas is not at all predictable, either from month to month or from year to year. In a single year, a devastating flood may be followed by a severe drought. A relatively wet year may be followed by one that is quite dry.

For at least five months out of each year, the savanna rainfall typically totals less than 2 inches (6 centimeters) per month. The dry season occurs during the winter months (December, January, and February), when high pressure systems are prominent in the region. In the summer, low-pressure systems predominate, and rainfall is abundant.

In addition to wet and dry seasons, a savanna has seasonal shifts in temperature. While the mean temperature stays above about 70°F (21°C) even in the coolest months, it hovers closer to 80°F (27°C) in the warmest months (March, April, and May). In the warm months, afternoon temperatures often range between 90 and 100°F (32 and 38°C).

African savannas are teeming with wildlife. Those grasslands are regions where lions, leopards, wild dogs, and other carnivores relentlessly

pursue speedy and well-camouflaged herbivores like antelopes, gazelles, and zebras.

Dry climates

There are two types of climate considered to be dry or arid: deserts and steppes. While deserts are the drier of the two, in neither climate does annual rainfall keep pace with the rate at which water is lost due to evaporation and transpiration. Dry climates are generally found in the region between 15° and 30° latitude (although sometimes at higher latitudes) and are sometimes bordered by mountain ranges. They cover about a quarter of the world's landmass. That total is greater than for any other climate type.

A climate's designation as dry depends on both its annual rainfall and its temperature. The mean temperature must be high enough to result in the evaporation of what little rainfall there is. To illustrate this point, consider the annual rainfall received by a hot New Mexico desert. If that same amount of precipitation were to fall on cold northern Canada, it would be enough to sustain a conifer forest.

Dry climates usually receive their sparse rainfall during the summer months, when temperatures are higher and evaporation occurs more readily. However, this rainfall is often quite irregular. One location may go two years without a drop of rain, then receive four inches in a single downpour.

Desert

Deserts are the world's driest regions. These true arid climates receive less than 10 inches (25 centimeters) of rainfall a year. Often, precipitation over these lands takes the form of virga, streamers of water that evaporate into the dry air before they even reach the ground.

Deserts come in hot and cool varieties. Hot deserts, which have no cool season, are the hottest places on Earth. Their mean temperature remains above 65°F (18°C) year-round. Cool deserts, on the other hand, have an annual mean temperature below 65°F and, for at least one month out of the year, a mean temperature below 45°F (7°C).

The daily temperature in both hot and cool deserts fluctuates greatly. The reason for this fluctuation is that little humidity is available to absorb incoming sunlight during the day and virtually no cloud cover to trap the heat escaping from the surface at night. In hot deserts, daytime temperatures commonly hover between 105 and 115°F (41 and 46°C) and sometimes exceed 120°F (49°C), while at night it cools off to around 75°F (24°C). Occasionally, during the winter months, the temperature at night in hot deserts drops below freezing. In cool deserts, afternoon temperatures in the summer often reach 105°F. However, it is not uncommon for nighttime temperatures in winter to dip below 30°F (™1°C).

The world's hot deserts are located in the subtropics between 15° and 30° latitude north and south, near the Tropic of Cancer and the Tropic of

Capricorn. Also known as the horse latitudes, these regions contain a high-pressure belt where air from the equatorial region descends, bringing warmth and clear skies. During the summer months, the Sun is directly overhead and produces the searing heat for which these deserts are famous. Some of the deserts in this region include the Sahara of Africa; the Great Sandy Desert of Australia; and the deserts of the North American southwest and Mexico, such as the Mojave and the Sonoran.

Cool deserts are located far inland and removed from any source of water in the middle latitudes, which lie between the latitudes of 30° and 60°. Many of these are in the rain shadow of tall mountain ranges, where precipitation is often less than 5 inches (13 centimeters) annually. Examples of deserts in this category include the Atacama Desert in Chile at the base of the Andes Mountains; the Great Basin in Nevada at the base of the Sierra Nevada; the deserts of Nepal and northern India at the base of the Himalaya Mountains; and the Gobi in Asia.

Most deserts do have some vegetation although not a great deal. This vegetation takes the form of drought-resistant plants such as cacti, which store water in their stems and have waxy coverings that diminish transpiration, and scrubby plants like the creosote bush, which has an extensive root system. There are also a variety of wildflower species that pop up quickly following a rain only to drop their seeds and whither away once the soil dries.

Animals of the desert have adapted to the limited supply of water and food in many different ways. The camel, for instance, stores water and fat in its hump, reabsorbs moisture from exhaled air, and sweats only when its body reaches extremely high temperatures. The angulate tortoise of Africa stores water in its bladder, and reptiles retain moisture due to their thick, scaly skin. Other animals, such as snakes, spend the heat of the day in a burrow, while rabbits seek out the shade of a bush. Perhaps the ultimate example of an animal adapted to desert life is the kangaroo rat. Found in deserts of the American southwest, this rodent gets all the moisture it needs from solid food and can go its entire life without drinking a drop of water.

Steppe

A steppe is a semiarid climate, meaning that it receives somewhat more rain than a desert. Steppes can be found along the edges of deserts and serve as transition zones between arid and moist areas. Annual precipitation in steppes generally measures between 8 and 20 inches (20 and 50 centimeters), all of which falls during a short wet season. This precipitation is enough to sustain short, coarse bunch grass called steppe (from which the climate gets its name), clumps of low bushes, sagebrush, and isolated trees.

Expansive steppes can be found in central Mexico, southern South America, northwest Africa, west central Asia, and surrounding the desert of central Australia. In the United States, regions with steppe climate are called prairies. These regions include the Great Plains (east of the Rocky Mountains), valleys on the northern boundaries of the Great Basin, and along the southern coasts of California.

An example of a city with a steppe climate is Denver, Colorado, which has an annual mean temperature of 50°F (10°C). Its monthly mean temperature fluctuates from 30°F (™1°C) in January to 75°F (24°C) in July, giving it an annual temperature range of 45°F (7°C).

Denver receives 15 inches (38 centimeters) of precipitation yearly, most of which falls between April and July.

The large animals living in steppes are primarily migratory, grazing mammals. Examples of these include deer and antelope in the United States; gazelles and ostriches in Africa; camels in Asia; and rodents that live on or under the ground, such as prairie dogs in the United States, gerbils in Africa, and hamsters in Asia.

Subtropical climates

Subtropical climates are those with distinct summer and winter seasons and sufficient rainfall to keep them from being classified as dry. At least eight months of the year they have mean temperatures above 50°F (10°C) and at least one month has a mean temperature below 65°F (18°C). An important criterion of a subtropical climate is that the winters are mild. The mean temperature of the coldest month always stays above 25°F (?4°C).

These climates are situated within a band running from about 25° to 40° north and south of the equator. This band begins at the edge of the tropics and extends through the warmer half of the middle latitudes, to the edge of the temperate zone. There are three distinct types of moist subtropical climates: humid subtropical, marine, and Mediterranean.

Humid subtropical

The most outstanding characteristic of a humid subtropical climate is its hot, muggy summer. The mean temperature during the summer months is about 75° to 80°F (24 to 27°C). The nighttime temperature seldom drops below 70°F (21°C) and during the day it often rises to more than 100°F (38°C). Intense heat waves are not uncommon and sometimes last for weeks. The relative humidity is high, even during the heat of the day, which makes for even more oppressive conditions. The dew point (the temperature at which water condenses out of the air) is often above 75°F.

Winters are generally mild and, on the lower-latitude end (closest to the equator) of a humid subtropical region, it is rare for temperatures to fall below freezing. On the higher-latitude (poleward) end, frost and snow are more common, but heavy snowstorms are still rare. Conditions change rapidly in winter. One day's warm, sunny weather can give way to the next day's cold rain.

Rainfall is plentiful throughout the year, totaling between 30 and 65 inches (about 75 and 165 centimeters) annually. Afternoon thunderstorms are common in the summer, as is precipitation from the edge of tropical storms. Much of the rain and snow in winter is associated with eastward-moving storm fronts.

This type of climate is found along the east coasts of most continents, between 25° and 37° latitude. It covers the southeastern United States, eastern China, southern Japan, southeastern South America (Uruguay, Paraguay, and southern Brazil), and the southeastern coasts of Africa and Australia. These locations are mostly on the western sides of subtropical high-pressure systems, placing them in the path of maritime tropical air that originates near the equator and is directed toward the poles.

Since the growing season lasts from eight months to a year, depending on location, this climate is well suited for many types of agriculture. The natural plant cover consists of thick deciduous and coniferous woodlands at the eastern edge of these zones, graduating to prairies on the western edge.

There is a minor subset of humid subtropical climate, examples of which include Portugal and the Canadian Pacific coast, that have mild, rainy winters, and cool, dry, short summers. During winter in these places, the ocean winds keep the air warmer than it is farther inland. The average temperature for the warmest months in these regions never exceeds 72°F (22°C) and three times as much rain falls during the winter as during the summer.

Marine

Marine climate, also known as Marine West Coast climate, features cool summers and mild winters, yet the summers are longer than they are in the humid subtropical climate of Pacific Canada. Precipitation is plentiful year-round in marine climates, and there is a relatively small annual temperature range. Some parts of the world with a marine climate are the west coasts of northern California, Washington, and Oregon; the British Isles; New Zealand; the west coast of France; and the southeast coast of South Africa.

This climate is characterized by the presence of low clouds, fog, and drizzle for much of the year, as well as regular rainfall and occasional snowfall. The temperatures are generally high enough that if snow does fall, it melts quickly. These conditions, plus relatively stable temperatures year-round, are largely the result of the prevailing westerly winds that bring ocean air into these regions.

On the U.S. northwest coast, rainfall is higher in the winter than in the summer. The reason for this pattern is the subtropical Pacific high-pressure system that directs storms away from the coast in the summer. It is common to find thick stands of coniferous and deciduous trees in marine climates.

Mediterranean

The primary factor that distinguishes Mediterranean climates from other subtropical climates is that it has dry summers. In areas with a Mediterranean climate, there are four to six months each year with little or no rainfall. At least three times as much rain falls in the wettest (winter) month as in the driest (summer) month. Total rainfall for the year is between about 15 and 35 inches (38 and 89 centimeters).

The summer dryness is caused by subtropical high pressure systems located over the water, which produce sinking air and direct storms poleward, away from the coasts. The position of these highs shifts towards the equator in the summer, allowing mid-latitude storms (with plenty of rain) to enter the region.

The summer high temperatures are lowest at points nearest to the water's edge. This effect is caused by upwelling, the churning up of cold water from the depths of the ocean, which chills the coastal air. As one moves inland, the summers are hotter (and the winters are colder, too).

The largest region with a Mediterranean climate consists of the coastal lands surrounding the Mediterranean Sea. Specifically, this region includes the Iberian Peninsula (Spain and Portugal), southern France, most of Italy, western Serbia and Montenegro, Greece, southern

Bulgaria, the coasts of Turkey and Israel, and the Mediterranean coast of northern Africa. This region's reputation for warm, sunny summers attracts crowds of tourists from around the world each year.

Other parts of the world with a Mediterranean climate include the central and southern California coast, the southwestern tip of Africa, the southeastern and southwestern edges of Australia, and a small area on the west coast of central Chile.

The irregular rainfall and preponderance of summer wildfires means that the only types of vegetation that can survive in this climate are shrubs, dwarf trees, and grasses. The shrubs have extensive root systems, enabling them to gather what little moisture exists in the soil. The leaves of Mediterranean plants also have moisture-conserving designs. For instance, some leaves have hard, waxy surfaces; some are hairy; and others curl up when it's dry. The olive tree has gray-colored leaves, which reflect sunlight.

Mediterranean vegetation is also adapted to periodic wildfires. After a fire, many will resprout from their roots. Some plant species, such as the knobcone pine, actually depend on fire for their propagation. The cones of these trees open and release seeds only in the extreme heat of a fire. If there is any doubt about the ability of scrubland vegetation to withstand wildfires, one only has to witness the rapid growth that follows in the wake of the burn.

The native animal inhabitants of these lands are deer, rabbits, wolves, and numerous rodents such as squirrels, voles, and mice. In some regions, like California and southern Europe, cows, goats, sheep, and other domesticated grazing animals are now the dominant species.

Temperate climates

Temperate climates are those that have four distinct seasons. The criterion that differentiates them from subtropical (the next warmest) climates is that for a least one month of the year the mean temperature in a temperate climate is below 25°F (?4°C). Temperate climates can be distinguished from subpolar (the next coldest) climates in that they have at least one month in the year in which the mean temperature is above 70°F (21°C).

All temperate areas have warm, humid summers and cold, snowy winters, although the average monthly temperatures and precipitation levels vary considerably from one place to another. Total annual precipitation ranges from 20 to 40 inches (50 to 100 centimeters) and is steady throughout the year.

Temperate climates are situated mostly between 40° to 60° latitude, primarily in the Northern Hemisphere. They cover the northeastern United States, southern Canada, and parts of central Europe and central Asia. In the Southern Hemisphere, where there is very little land mass south of the 40th parallel, temperate climates exist only in the southeastern portion of Chile, the southern tip of Australia, and New Zealand.

Toward the lower-latitude end of a zone of temperate climate, summers are hotter and more humid than they are in the higher latitudes. High temperatures in the summer are often over 90°F (32°C) in the lower latitudes, and there are five or six months of the year during which temperatures never dip below freezing. This means that the growing season is long enough to cultivate a variety of crops. In those same locations, the winter is cold and blustery with significant snowfall.

In the colder temperate areas, winter is harsher and longer, summer is milder and shorter, and spring and fall are merely quick transitions between winter and summer. Typically, there are only three to five months a year with no frost. During the summer, high temperatures rarely exceed 90°F (32°C) while in the winter the temperature may drop below ?22°F (?30°C) and can remain below zero degrees for weeks on end.

The most common type of vegetation found in temperate areas is the deciduous forest. A deciduous forest contains trees such as oak, hickory, maple, beech, ash, and birch. In some areas there are also stands of conifers (evergreens) such as pine, spruce, and fir. In the autumn, a chemical transformation occurs within the leaves of deciduous trees, causing them to turn brilliant shades of red, orange, and yellow. In the spring, before the thick blanket of leaves has fully emerged, wildflowers dot the forest floor.

A great variety of birds, mammals, reptiles, and insects inhabit temperate areas. While many species of birds and some mammals migrate to warmer regions for the winter, other animals have developed methods of surviving through the cold months. Squirrels, for instance, store enough food in the summer and fall to last through the winter. Turtles find a deep, muddy burrow in which to hibernate. With many insect species, the adults die before the onset of winter, and their eggs hatch the following spring.

Subpolar climate

The subpolar climate zone is a cold, northern climate, found generally between 50 and 70° north latitude. It is dominated by a nearly continuous coniferous forest called the "taiga" or "boreal forest." Specifically, this region includes Alaska, central and northern Canada (except for the northernmost fringe), northern Europe (except for the north of Scandinavia), and northern Asia (except for the northern and southern fringes of Siberia). There is no subpolar land in the Southern Hemisphere because, except for the very southernmost tip of South America, no land exists at these latitudes.

Subpolar climate is characterized by long, harsh winters and short, cool summers, with a wide range of temperature variation between the coldest and warmest months. The factor that distinguishes subpolar climates from neighboring, colder polar climates is that subpolar climates have one to three months in which the mean temperature is above 50°F (10°C). There is no limit, however, defining the cold end of the subpolar temperature range. In northern Siberia, for instance, the coldest monthly average temperature sometimes drops below ?35°F (?37°C).

The cold air of subpolar regions retains little moisture. Thus there is little cloud formation and hence, light precipitation. Most subpolar lands receive less than 20 inches (50 centimeters) of precipitation annually. While snowfall is light, the air is so cold that once the snow falls, it remains for months. Because of the low temperatures, large tracts of subpolar land contain permafrost, a layer of subterranean soil that remains frozen year-round.

The short growing season rules out agriculture in the region. The main type of vegetation is conifer trees, such as fir, spruce, larch, and pine, plus a few varieties of deciduous trees, such as birch and alder. Conifers are well-suited to this environment. Their tall, narrow, conical shape

encourages the snow to slide off them, and their needles don't require much moisture to grow.

Large tracts of subpolar land are covered with highly acidic wetlands called bogs. Bogs are comprised primarily of sphagnum moss, hardy grasses, insectivorous plants, pondweeds, water lilies, and algae, as well as a smattering of flowering plants such as blueberry and azalea. When the moss dies, it accumulates as peat, which in some bogs forms a bouncy, carpetlike layer several feet thick.

The mammals of the boreal forest include the lynx and various members of the weasel family (wolverine, fisher, pine martin, mink, ermine, and sable). These animals prey on snowshoe hares, red squirrels, lemmings, and voles. Beaver are abundant, and their dam-building habit is an important part of forest succession. Large mammals of the boreal forest include moose and elk. Insect-eating birds such as wood warblers are migratory, but seed-eating birds (finches and sparrows, evening grosbeak, pine siskin, and red crossbill) and omnivores (ravens) may be year-round residents. All of these birds may move south if food is scarce. Mosquitoes are abundant in the spring.

Polar climate

The polar climate, which covers the extreme northern and southern portions of Earth, is the coldest in the world. The warmest month in these regions has an average temperature below 50°F (10°C). It is so cold near the poles because the sunlight strikes the surface there at a very steep angle and remains low in the sky even in the summer. This condition is in stark contrast to the world's hottest regions, where the Sun sometimes shines directly overhead.

The polar area extends, on its warmest boundary, to between 60° and 70° latitude. The coldest points are at 90° latitude, capping the North and South Poles. Lands with polar climates include: the northern reaches of North America, Europe, and Asia; all of Greenland; the northern half of Iceland; and Antarctica.

Polar regions receive less than 8 inches (20 centimeters) of precipitation annually, which is equivalent to that received by a desert. In a desert the air is warm enough for evaporation to exceed precipitation. In polar regions, however, the air is so cold that the meager rate of precipitation is still greater than the rate of evaporation, and the ground never dries out.

The polar climate is divided into two categories: tundra, which experiences a yearly thaw, and arctic, which remains a permanently frozen region.

Tundra

Tundra comprises the majority of the polar land in the Northern Hemisphere, with the arctic being restricted to areas close to the North Pole. Due to the virtual absence of land between 50° and 70° latitude south, the only tundra in the Southern Hemisphere lies in narrow strips on the coasts of Antarctica.

In the tundra, the mean temperature of the warmest month is between 30 and 50°F (?1 and 10°C). While the tundra summers are cool, the winters are bitterly cold. An example of a tundra climate is Barter Island, Alaska, at 70° north latitude. The coldest month of the year at Barter Island is February, with an average monthly temperature of ?20°F (?29°C). It has three months in which the average temperature is above freezing: in June the temperature reaches 35°F (2°C); and in July and August, 40°F (4°C). Throughout an average year, Barter Island receives about 7 inches (18 centimeters) of precipitation.

Beneath the tundra surface lies a layer of permafrost, frozen subterranean soil, hundreds of yards deep. In the summer the top few feet of soil thaw. Since the moisture can't penetrate the frozen ground below, the surface becomes muddy and swampy. The growing season is so short that only a limited number of small plant species dot the landscape. These include mosses, lichens, sedges, and dwarf trees. The trees are species of willow and birch; in warmer climates they grow several feet tall but in the tundra creep along the ground and are only a few inches tall.

What to watch

Moviegoing audiences were captivated in 2005 by the astonishing documentary March of the Penguins, a film about the annual mating and child-rearing cycle of Antarctica's emperor penguins. Aside from the incredible appeal of the penguins themselves, one of the most astonishing things about the film was the Antarctic landscape itself, so different from those humans inhabit as to seem almost like another planet. The climate plays a central role in the lives of these unusual birds, and director Luc Jacquet provides a look at the climate rarely before seen. March of the Penguins won an Academy Award for Best Documentary Feature, and is available on DVD.

A number of animal species have adapted to life in the tundra. Among them are waterfowl, most of which migrate south for the winter, and mammals with thick layers of insulating fat and fur. The ptarmigan and snowy owl are two birds that remain in the far north year-round. These birds each have a thick covering of feathers all over their body. The ptarmigan even has feathers on its feet. For protection from predators,

their feathers, like the fur of many animals, change color from brown in the summer to white in the winter.

Some small mammals, such as the arctic ground squirrel, survive the winter by hibernating. Heartier mammals, like the arctic fox, musk ox, caribou, and polar bear, remain active all winter.

Arctic

The arctic is a barren land of snow and ice located around the poles in both hemispheres. There the average temperatures for every month are below freezing. In the Northern Hemisphere arctic lands include Ellesmere Island and the northern section of Baffin Island (these islands are part of the Canadian Archipelago) and the interior of Greenland, plus the seas between these lands. In the Southern Hemisphere it includes nearly the whole of Antarctica.

While temperatures vary greatly throughout the arctic, the coldest weather is found in Antarctica. Earth's lowest temperature, ?128.6°F (?89°C), was recorded on July 26, 1983, at the Soviet research station Vostok in Antarctica. The station is located at 78° south latitude with an elevation of 11,400 feet (3,475 meters) above sea level. During Vostok's warmest month, December, the average temperature is ?30°F (?34°C) and during its coldest month, August, the average temperature is ?90°F (?68°C). The warmest temperature ever recorded there was ?5.8°F (?21°C).

Antarctica is warmest on its outskirts and coldest in its interior. In the summer months, average temperatures range from 14 to 20°F (?10 to ?7°C) near the seas and from about ?5 to ?20°F (?21 to ?29°C) near the center of the continent. In the coldest months the outer areas have average temperatures about ?20°F, while average temperatures in the interior are about ?95°F (?71°C).

Conditions in the arctic region of the Northern Hemisphere are not much more hospitable. Average temperatures there are about ?45°F (?43°C) in January, the coldest month, and about 10°F (?12°C) in July, the warmest month. The coldest temperature ever recorded in Greenland was ?87°F (?66°C) on January 9, 1954, at Northice, which is at a latitude of 78° north and an altitude of about 7,685 feet (2,340 meters). Farther south, at a latitude of 71° north, is Eismitte (which means "middle of the ice"). At this location, about 9,940 feet (3,030 meters) above sea level, the average temperature in February, the coldest month, is about ?55°F (?48°C) and the average temperature in July, the warmest month, is about 10°F (?12°C).

Weather report: Emperor penguins

Emperor penguins are the only animals to breed in the winter on the coast of Antarctica. They can withstand colder temperatures than any other species. These birds maintain a body temperature of 100°F (38°C) even during the winter months, when the air temperature drops as low as ?80°F (?62°C).

An emperor penguin is insulated by a thick layer of blubber. It also has waterproof feathers all over its body (even its bill and feet) except for its toes. The penguin is covered with more than seventy-seven feathers per square inch. A fluffy down at the base of each feather traps body heat.

The emperor penguin is the largest of any penguin species, standing 3 feet (just under 1 meter) tall and weighing about 88 pounds (40 kilograms). It also has the highest percentage of body fat of any penguin species and can survive up to four months without eating. The emperor penguin is so well insulated that it actually runs the risk of overheating during periods of physical activity!

Perhaps the only climatic factor more remarkable about the arctic than temperature is its patterns of sunlight. Because of the tilt of Earth's axis, day and night at the poles are each six months long. During the summer (March 21 to September 23 in the Northern Hemisphere and September 23 to March 21 in the Southern Hemisphere), the Sun never sets nor does it ever rise much above the horizon. The six months of winter, however, are shrouded in complete darkness. The exception to this pattern is the six weeks of twilight, centered around the spring and fall equinoxes, the days marking the start of spring and fall that are the two days of the year in which day and night are most similar in length. During the six weeks of twilight (from February 7 to March 21 and from September 23 to November 6), the Sun is constantly just below the horizon.

The bitter arctic cold is accentuated by blustery winds, sometimes over 100 mph (160 kph). Snowfall, however, is very light. These regions receive less than 4 inches (10 centimeters) of precipitation annually, most of that during the summer months.

The surface of arctic lands is a permanent ice layer, extending to a depth of thousands of feet. Thus, there is no plant growth. There are, however, some robust animal species that inhabit the mildest fringes of this icy world. Principal among them are emperor penguins, insulated from the elements by a thick layer of fat and a dense coat of feathers. This animal is able to withstand conditions colder than any other and is the only animal species that breeds during the Antarctic winter. Fifteen whale species and six seal species, as well as numerous species of fish, populate the seas surrounding Antarctica. The only animals to inhabit the arctic interior are tiny insects such as springtails and microscopic invertebrate worms such as nematodes and rotifers.

Mountain climates

There is no inclusive definition of a mountain climate, also called a highland climate. The reason is that a mountain climate encompasses a whole series of climate types that are found at various points on the way up a mountain. The primary reason that conditions change as you ascend a mountain is that temperature decreases with altitude. Air pressure, humidity, precipitation, and winds also change with altitude. The climates on a mountain form pockets within the general climate of the surrounding region.

Mountain climates can resemble climates found at higher latitudes. In general, each 1,000-foot (300-meter) increase in elevation will produce a change in climate similar to traveling about 185 miles (298 kilometers) towards one of Earth's poles. For every 1,000 feet (300 meters) gained in altitude, the temperature falls about 3.6°F (2.0°C). However, mountain climates are not exact matches to climate zones at higher latitudes because many factors are different. For example, the effects of global atmospheric circulation, extratropical cyclones, and other weather systems are not as significant in mountain climates.

A 13,000-foot-tall (3,960-meter-tall) mountain in the Sierra Nevada range, for example, encompasses five different climate types. A semiarid grassland surrounds the base of this mountain. Along the "foothills," the first 2,000 feet (610 meters), the grassland is gradually replaced by Mediterranean scrubland. From 3,000 to 6,000 feet (915 to 1,825 meters), there is the "montane zone," the climate resembles that of a northern temperate zone, with thick conifer forests. Above 6,000 feet, in the "subalpine zone," thinner stands of stunted conifers resemble a subpolar climate. This zone gives way to tundra at 9,000 feet (2,740 meters), in the "alpine zone." Finally, the mountain is capped with a permanent expanse of snow and ice.

Another notable feature of mountain climates is the uneven distribution of precipitation across a mountain or mountain range, known as rain shadow effect. Mountains have a relatively wet side, called the windward side, and a relatively dry side, called the leeward side. The windward side is the one encountered by approaching air masses. The windward side is on western slopes in the Northern Hemisphere and on eastern slopes in the Southern Hemisphere. The air rides upward along the mountainside, and as it rises, it cools. Once air reaches the dew point, condensation (the process by which water changes from gas to liquid) occurs and clouds form.

The animals that inhabit mountains are, for the most part, migratory. However, they do not follow typical north-south migratory paths. Rather they migrate up and down the mountain. They climb to higher elevations in the summer, when food is more abundant, and escape the harsh alpine winters by climbing down to warmer elevations. Among the wide variety of mountain animals are deer, bears, chipmunks, mountain goats, marmots, and many species of birds. Some animals are specially adapted to survive in the oxygen-thin air found at high altitudes. One such animal is the guanaco, a relative of the llama, which has an unusually large heart and lungs.

Mountain climates are scattered throughout North America, South America, Africa, Europe, Asia, and the islands north of Australia. Examples of these climates include the Sierra Nevada and the Rocky Mountain ranges in the United States and Canada, the Sierra Madre Range in Mexico and Central America, the Andes in Chile and Peru, the Alps in Europe, and the Himalayas in southern Asia.

History of climate change

Throughout Earth's 4.6 billion year existence, the global climate has undergone continuous change. Most of what scientists know about the history of climate change is limited to the most recent 10 percent of Earth's lifetime. The farther back in time, the fewer clues there are about the climate. However, by using the clues that do exist and drawing conclusions based on more recent data, scientists have been able to construct a climatic picture spanning the entire history of Earth.

There have been times when Earth has been alternately warmer and 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, periods called ice ages. Each ice age has brought about the extinction of numerous species of plants and animals. The result is that the process by which plants and animals have evolved has not been a smooth one. Rather it has been halted many times and jump-started with each new warm period.

A key reference to: Climates of the United States

Within the continental United States, every climate is represented except the tropical, subpolar, and polar varieties. With 3,623,420 square miles (9,384,658 square kilometers) stretching from 49° latitude on the northernmost edge to 26° latitude on the southernmost edge, the United States contains perhaps a wider assortment of climates than any other nation. They are as follows:

• Temperate: northeastern states, from New York to Maine, Michigan, Iowa, Wisconsin, Minnesota, and the eastern portion of the Dakotas.
• Humid subtropical: southeastern states, from Pennsylvania to Missouri and every state to the south.
• Semiarid: states in the rain shadow of the Rocky Mountains, including Nebraska south through Texas and portions of Montana, Idaho, Wyoming, Colorado, and New Mexico.
• Arid: desert areas of New Mexico, Arizona, California, Utah, and Nevada.
• Marine: northwestern California and the western portions of Oregon and Washington.
• Mediterranean: west coasts of central and southern California.
• Mountain: portions of the west coast states (Washington through California) containing the Cascades and Sierra Nevadas, as well as portions of western and plains states containing the Rockies (Montana, Wyoming, Colorado, and New Mexico).

The history of global climate change can be broken down into four main periods: Precambrian, Paleozoic, Mesozoic, and Cenozoic.

Precambrian era

The Precambrian era began with the formation of Earth about 4.6 billion years ago and ended 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. Volcanic eruptions spewed forth gases, such as nitrogen, argon, water vapor, sulfur dioxide, and carbon dioxide, trapped beneath the rocky surface. These gases rose and mingled with hydrogen gas, a byproduct of Earth's formation from solar gases, to create the early atmosphere. The water vapor condensed into rain, which fell to the surface forming vast oceans. The rain washed most of the sulfur dioxide and carbon dioxide out of the atmosphere, and oceans have covered much of Earth's surface since.

Soon after the formation of the early atmosphere, the first forms of life appeared. These were anaerobic (capable of living without oxygen) bacteria. Then, about 3.5 billion years ago, these bacteria evolved into blue-green algae, which had the ability to photosynthesize. Photosynthesis is a process driven by the heat of the Sun in which an organism combines water and carbon dioxide to form a simple sugar. Oxygen is given off as a byproduct.

Over time, oxygen accumulated in the atmosphere. 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 sunlight broke apart oxygen molecules, to form ozone (O3). The ozone formed a separate atmospheric layer and fulfills the important function of absorbing the Sun's dangerous ultraviolet rays. The oxygen in the atmosphere, plus the protection provided by the ozone layer (the layer of Earth's atmosphere, 25 to 40 miles above ground, that filters out the Sun's harmful rays), encouraged the burst of biological diversity that marked the end of the Precambrian era.

The Precambrian saw four ice ages. The first occurred somewhere between 2.7 billion and 2.3 billion years ago. Then Earth warmed up and was free of ice for almost a billion years. The second ice age took place between 950 and 890 million years ago; the third between 820 and 730 million years ago; and the fourth between 640 and 580 million years ago. In each case, some portion of Earth was iced over for about 100 million years.

Paleozoic era

The Paleozoic era began with a dramatic increase in the number of marine animal species. It lasted from 570 to 225 million years ago. The Paleozoic era was significantly warmer than the Precambrian era, with the exception of two glacial periods. At the end of the first glacial period, around 400 million years ago, plants began to take hold on land.

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.

During the last portion of this era, from 330 to 245 million years ago, temperatures fell again. About 250 million years ago, Earth's two super-continents, 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 super-continent, Pangea, extended from the North Pole almost all the way to the South Pole.

Who's who: Jean Louis Agassiz, discoverer of ice ages

Swiss American naturalist, educator, and fossil expert Jean Louis Rodolphe Agassiz (pronounced "A-guh-see") (1807–73) was a pioneer of the theory that ice ages have occurred in Earth's history. He was not the first, however, to come up with this idea. That honor belongs to Scottish geologist James Hutton (1726–97), whose ideas were shunned by other scientists. Agassiz, who was more persuasive in his arguments about ice ages, is generally credited with developing the theory.

Agassiz grew up in the Swiss Alps and spent much of his time exploring their great heights. He studied the huge boulders on the mountains and felt that they must have been

deposited there by glaciers. Thus, he began seeking evidence to prove that glaciers move.

A real breakthrough came in 1839 when Agassiz discovered that a house built upon a glacier had moved one mile during a ten-year period. This discovery prompted him to drive a line of stakes into the ice in order to trace their movement. In just a year he found that the stakes had moved into a U shape, with the stakes in the middle of the line showing the greatest change in position. Agassiz concluded that the glacier had moved the most slowly on the edges, where it was slowed by friction with the mountainside.

In 1840, Agassiz published his findings in a book entitled Studies on Glaciers. He wrote that within the past several thousand years, northern Europe had been covered by glaciers, in what he called a Great Ice Age. This notion was initially met with a great deal of skepticism in the scientific community.

In 1848, Agassiz became a professor of natural history at Harvard University. There he pursued many areas of study, principal among which was marine science. In 1859, he founded Harvard's Museum of Comparative Zoology. Meanwhile, Agassiz continued to search for evidence of ice ages in New England and around the Great Lakes. By the end of his lifetime, Agassiz's ice-age theory had gained a modest level of acceptance. It wasn't until the turn of the century that this concept became widely accepted.

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, Pangea split into two continents, one containing North America and the other containing Eurasia (Europe and Asia combined). About 100 million years ago, these landmasses 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 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. Landmasses, 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 the 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. According to one theory, an asteroid collided with Earth, creating a dust cloud that blocked out the Sun and brought about the ice age.

Cenozoic era: Pre-Holocene

The Cenozoic era began 65 million years ago and continues to the present. Throughout this era, the continents have continued to drift, moving into their present configuration. The continental plates continue to move, and the process of super-continent formation and breakup will likely repeat several more times. This era is also characterized by the emergence of mammals, including humans, as the dominant animal group.

The Cenozoic era is divided into two parts: the pre-Holocene epoch, which stretches from the beginning of the Cenozoic until ten thousand years ago; and the Holocene epoch, which covers the most recent ten thousand years.

The pre-Holocene epoch, the early part of the Cenozoic era, was warmer than it is today, and there were no polar ice caps. Beginning about 55 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 50 million years ago, and another about 38 million years ago. The most recent was about 15

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.

Geological Time Scale
Periods
Phanerozoic	Cenozoic	Neogene	24 million
		Paleogene	65 million
	Mesozoic	Cretaceous	144 million
		Jurassic	206 million
		Triassic	248 million
	Paleozoic	Permian	290 million
		Carboniferous	354 million
		Devonian	417 million
		Silurian	443 million
		Ordovician	490 million
		Cambrian	540 million
Precambrian		Proterozoic	2.5 billion
		Archean	3.8 billion

Over the last 2.4 million years there have been two dozen ice ages—times when the global temperature has plummeted sharply downward. At seven different points over the last 1.6 million years, ice covered up to 32 percent of Earth's surface. Scientists estimate that throughout this era new ice ages have begun about every 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 between twenty thousand years ago, when there were glaciers up to 10,000 feet (3,000 meters) thick over most of North America, northern Europe, northern Asia, as well as the southern portions of South America, Australia, and Africa. The sea level fell and exposed 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 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 warmer than it is today. The sea level rose and the current shape of continents emerged.

Cenozoic era: Holocene

The Holocene epoch began approximately ten thousand years ago. Extensive climatic data exist for this time period. The history of human civilization begins during this post-ice age 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 (now part of Iraq). There was a relatively cool period that began about 900 bce and lasted until about 500 ce, which resulted in crop failures. There are also indications that beginning about 800 ce there was a prolonged drought, which in all likelihood contributed to the fall of the great Mayan civilization in Mexico and Central America.

In the Middle Ages (500 to 1500 ce), the global climate was similar to today's. During that time, the civilizations of Europe flourished, and the Vikings colonized Iceland and Greenland. However, a cold spell began at the end of the thirteenth century. 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 continuously covered with layers of ice several inches thick. Food became scarce throughout Europe, the mountain glaciers increased in size, 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 increase in global temperature. Scientists now think that this increase constitutes part of a trend of human-influenced global warming and is not merely a natural part of Earth's continually changing climate.

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 most 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. They have, however, been largely unsuccessful. The proposed cycles either don't apply to all times in the past or don't 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 recent past and present. Deforestation, the burning of fossil fuels, and acid precipitation (rain and snow that are made more acidic when carbon, sulfur, and nitrogen oxides in the air dissolve into water) and smog (a layer of hazy, brown air pollution at Earth's surface) caused by industrial emissions are among the real or potential agents of climate change.

What follows is a discussion of a handful of forces that have affected the climate of Earth 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 continental drift theory was first suggested by German meteorologist Alfred Wegener in 1915. That theory states that all land on Earth was joined together in one huge continent 200 to 250 million years ago. Then, over the years, forces deep within Earth's core caused the land to break apart. Continents moved away from one another and drifted around the globe. This motion is predicted to continue indefinitely into the future.

Evidence of continental drift can be found in the fossils of dinosaurs and other mammals that migrated across once-connected landmasses and in the fossilized remains of tropical plants beneath polar ice sheets. Another piece of evidence that landmasses 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 are able to record the subtle, extremely slow movements of the continents today.

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 one 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 landmasses 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 dispersed 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 landmasses come together, the land is forced upward. Examples of mountain ranges formed in this way are the Rocky Mountains, the Andes, the Tibetan Plateau, and the Himalayas. 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. Formed 50 million years ago by the collision of the Indian and Asian continents, this plateau is one of the world's tallest and widest mountain ranges. 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 landmasses 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 Milankovitch (pronounced "muh-LAN-kuh-vich") (1879–1958) proposed a 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 nearly a perfect circle. At other times, it has a more elliptical shape (like an egg). The alternating change in the shape of Earth's orbit is called eccentricity. The eccentricity of Earth's orbit occurs 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's 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 will differ by up to 20 percent. In addition, a more elliptical orbit means the summers and winters are longer, and springs and falls are shorter.

The second orbital factor that affects climate is the wobbling of Earth about its axis of rotation. Earth's axis is tilted in relation to the plane of its orbit around the Sun. This effect is what causes the Northern and Southern Hemispheres to each receive different amounts of sunlight throughout the year.

Earth spins like a top in slow motion, so that its axis traces the path of a cone. It wobbles its way through one complete revolution every

twenty-six thousand years, a phenomenon called the precession of the equinoxes. This means that every thirteen thousand years the seasons are gradually reversed. Eventually, unless the calendar is adjusted, 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's 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 22 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. That is, winters are milder and summers are cooler. 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 100,000 year cycles over the last 800,000 years. Within those cycles, glacier formation occurs in secondary cycles, reaching peaks every 26,000 and 41,000 years. These time intervals correspond to the cycles of the three types of orbital variation described here.

Volcanoes

In the early stages of Earth's history, thousands of volcanoes dotted the planet's surface. These volcanoes underwent frequent, large eruptions that 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 very 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 fewer in number and intensity 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. For example, 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 a portion of the incoming solar radiation. The result is a global decrease in temperature.

Asteroids and comets

Throughout Earth's history, there have been five very abrupt and 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 large, rocky bodies from space, such as an asteroid or comet (together known as bolides) crashed into Earth. Many scientists believe that the extinction of the dinosaurs, which came about 65 million years ago, was caused by a collision with an object from space.

It has been calculated that 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 similar to those dropped on Hiroshima, Japan, heating the atmosphere 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 object from space were to fall on land, a thick dust cloud would rise up and potentially block out all sunlight for several months.

Following an early wave of wildfires caused by the high temperatures, any surviving plants and animals would be killed off during a long, dark period of cold weather during which virtually all sunlight would be blocked from Earth by a thick dust cloud.

It is more likely that a bolide would fall into the ocean, since oceans cover almost three-quarters of Earth's surface. In this case, the bolide impact would stir up carbonate-rich sediments and produce vast quantities of carbon dioxide. An increase of carbon dioxide in the air serves to trap reradiated infrared radiation from Earth's surface, leading to an increased greenhouse effect, which is the warming of Earth due to the presence of gases that trap heat.

The possibility of such a collision happening within our lifetime 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 established 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. It has been shown that 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 on board satellites by special instruments called radiometers 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. (A sunspot maximum is a period of the greatest number of sunspots; a sunspot minimum is a period in which there are the smallest number of sunspots.) The length of sunspot cycles varies over time, from seven to seventeen years. A growing body of evidence supports a link between the length of sunspot cycles and temperature patterns around the world. It has been shown over the last century that 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 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 between 1645 to 1716 was also the coldest part of the Little Ice Age.

Ways to measure climate change

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

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

Rocks and rock formations

The oldest rocks on Earth are about 3.9 billion years old. These rocks are the only objects that still exist from the earliest period in the 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 developed in the late 1800s called radioactive dating. 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 predict how long ago the sample was entirely made of parent atoms—in other words, when the sample was formed.

A similar technique is called carbon dating. This involves the analysis of radioactive carbon, which is produced in small amounts continuously over time in the atmosphere by cosmic rays (invisible, high-energy particles that bombard Earth from space). Radioactive carbon, like normal carbon, becomes assimilated into green plants through photosynthesis. Radioactive carbon is unstable because its nuclei contain too many neutrons. To achieve stability, the nucleus transmutes into nitrogen. Given the rate of decay of radioactive carbon it's possible to determine the age of a given sample.

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 away. 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.

Fossils of primitive organisms begin showing up in rocks dated about 3.8 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 few 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 point in history. Of particular interest are the fossils within each layer, which constitute a timeline of the emergence of various species.

The presence of marine animals in a given layer indicates that at that time the region was covered with water. By studying the chemical composition of fossilized shells, the primary ingredient of which is calcium carbonate, it's 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 gives an approximation of the water temperature.

Ice cores

Ice cores drilled into the 2-mile-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 for 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 to the air temperature at the time of the precipitation 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's 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 landmasses.

In addition, air bubbles can be analyzed for the composition of the atmosphere. The timing of volcanic eruptions can be inferred from 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 as rock formations and ice do. The age of sediment layers is determined using chemical analysis.

Embedded within the layers of sediment are fossils of tiny marine organisms that have evolved and become extinct over time. 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 recent study of a bog in northern Minnesota turned up 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 habitats, 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 date back to the time of Julius Caesar.

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.

In order to assess the overall conditions of a given year, the width of that year's growth ring is measured. In warm, wet years, trees grow more (and have wider growth rings) than in cool, dry years. Hence, 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.

Experiment: Create a weather "log"

You, too, can be a dendrochronologist. All you have to do is locate a slice of a tree trunk. You can use the stump of a recently cut tree or seek out a log that hasn't yet been split, from a firewood seller. Just be certain that you know the year in which the tree was cut and, if you're using a log, find out where it's from.

First count the rings on the face of the log to determine the age of the tree (a tree adds a new ring of growth each year, with the most recent year's growth in the center.) Then study the width of each ring. Remember that trees grow more in warmer and wetter years, resulting in wider growth rings.

Now you're ready to construct a basic climatic history for the area in which the tree grew. On a piece of paper, create two columns: one for the year and the other for the conditions. For years in which the ring is skinny, write "cold/dry." For years in which the ring is wide, write "warm/wet."

[See AlsoClimate Change and Global Warming; Human Influences on Weather and Climate ]

For More Information

BOOKS

Ahrens, C. Donald. Meteorology Today. 8th ed. Belmont, CA: Thomson, Brooks, and Cole, 2006.

Botkin, Daniel B. Environmental Science: Earth as a Living Planet. 5th ed. New York: John Wiley, 2005.

Lydolph, Paul E. The Climate of the Earth. Lanham, MD: Rowman & Littlefield, 1985, pp. 182-372.

Weigel, Marlene, ed. U*X*L Encyclopedia of Biomes. Detroit, MI: U*X*L, 2000.

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EdGCM. Educational Global Climate Modeling. ?http://edgcm.columbia.edu/? (accessed August 17, 2006).

Exploratorium. Global Climate Change Research Explorer. ?http://www.exploratorium.edu/climate/index.html? (accessed August 15, 2006).

National Oceanic and Atmospheric Administration (NOAA). Climactic Data Center. ?http://www.cpc.noaa.gov/ncdc.html? (accessed August 15, 2006).

National Resources Conservation Center's National Water and Climate Center. National Water and Climate Center. ?http://www.wcc.nrcs.usda.gov/? (accessed August 15, 2006).