Section 3 • Climatic Changes 387

Black Sea

Alps

Caspian Sea

Sea ice

Aral Sea

Europe

China

Siberia

JapanArcticOcean

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Climatic Changes

MAIN Idea Earth’s climate is constantly changing on many differ-ent timescales.

Real-World Reading Link You might not notice changes in your friends’ physical appearance from day to day; however, if you only see someone once a year, he or she might appear to have changed a lot. Climate changes on long timescales with differences that might not be noticed day to day.

Long-Term Climatic ChangesSome years might be warmer, cooler, wetter, or drier than others, but during the average human lifetime, climates do not appear to change significantly. However, a study of Earth’s history over hundreds of thousands of years shows that climates have always been, and cur-rently are, in a constant state of change. These changes usually take place over long time periods.

Ice ages A good example of climatic change involves glaciers, which have alternately advanced and retreated over the past 2 million years. At times, much of Earth’s surface was covered by vast sheets of ice. During these periods of extensive glacial cover-age, called ice ages, average global temperatures decreased by an estimated 5°C. Global climates became generally colder and snowfall increased, which sparked the advance of existing ice sheets. Ice ages alternate with warm periods—called interglacial intervals—and Earth is currently experiencing such an interval. The most recent ice age, shown in Figure 14.14, ended only about 10,000 years ago. In North America, glaciers spread from the east coast to the west coast and as far south as Indiana.

Section 1 144..3 3

Objectives

◗ Distinguish between long-term and short-term climatic changes.

◗ Identify natural causes of climate change.

◗ Recognize why climatic changes occur.

Review Vocabularyglacier: large, moving mass of ice that forms near Earth’s poles and in mountainous regions at high elevations

New Vocabularyice ageseasonEl NiñoMaunder minimum

■ Figure 14.14 The last ice age covered large portions of North America, Europe, and Asia. Average global temperatures were roughly 5°C lower than they are today. Explain how decreased global tem-peratures can lead to an ice age.

SC.912.E.7.4: Summarize the conditions that contribute to the climate of a geographic area, including the relationships to lakes and oceans. SC.912.E.7.7: Identify, analyze, and relate the internal (Earth system) and external (astronomical) conditions that contribute to global climate change.

388 Chapter 14 • Climate

Short-Term Climatic ChangesWhile an ice age might last for tens of thousands of years, other climatic changes occur over much shorter time periods. Climatic change can affect seasons differently. Seasons are short-term peri-ods with specific weather conditions caused by regular variations in daylight, temperature, and weather patterns.

Seasons The variations that occur with seasons are the result of changes in the amount of solar radiation an area receives. As Figure 14.15 shows, the tilt of Earth on its axis as it revolves around the Sun causes different areas of Earth to receive different amounts of solar radiation. During winter in the northern hemi-sphere, the north pole is tilted away from the Sun, and this hemi-sphere experiences long hours of darkness and cold temperatures. At the same time, it is summer in the southern hemisphere. The south pole is tilted toward the Sun, and the southern hemisphere experiences long hours of daylight and warm temperatures. Throughout the year, the seasons are reversed in the northern and southern hemispheres. During the spring and fall, neither pole points toward the Sun.

El Niño Other short-term climatic changes include those caused by El Niño, a band of anomalously warm ocean temperatures that occasionally develops off the western coast of South America. Under normal conditions in the southeastern Pacific Ocean, atmo-spheric and ocean currents along the coast of South America move north, transporting cold water from the Antarctic region.

■ Figure 14.15 When the north pole is pointed away from the Sun, the northern hemisphere experiences winter and the southern hemisphere experiences summer. During spring and fall, neither pole points toward the Sun.

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Interactive Figure To see an animation of seasons, visit glencoe.com.

To read about how seasonal changes affect

the Okavango delta, go to the National Geographic Expedition on page 904.

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Warmwater Cool

water

Cold-watercurrent fromAntarctica

Australia

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Tradewinds

Equatorial currents

Cold-watercurrent

Australia

SouthAmerica

Weak tradewinds

Strong countercurrent

Section 3 • Climatic Changes 389

Meanwhile, the trade winds and ocean currents move westward across the tropics, keeping warm water in the western Pacific, as shown in Figure 14.16. This circulation, driven by a semiperma-nent high-pressure system, creates a cool, dry climate along much of the northwestern coast of South America.

Occasionally, however, for reasons that are not fully understood, this high-pressure system and its associated trade winds weaken drastically, which allows the warm water from the western Pacific to surge eastward toward the South American coast, as shown in Figure 14.17. These conditions are referred to as an El Niño event.

The sudden presence of this warm water heats the air near the surface of the water. Convection currents strengthen, and the nor-mally cool and dry northwestern coast of South America becomes much warmer and wetter. The increased convection pumps large amounts of heat and moisture into the upper atmosphere, where upper-level winds transport the hot, moist air eastward across the tropics. This hot, moist air in the upper atmosphere is responsible for dramatic climate changes, including violent storms in Califor-nia and the Gulf Coast, stormy weather to areas farther east that are normally dry, and drought conditions to areas that are nor-mally wet. Eventually, the South Pacific high-pressure system becomes reestablished and El Niño weakens.

Sometimes the trade winds blow stronger than normal and warm water is pulled across the Pacific toward Australia. The coast of South America becomes unusually cold and chilly. These condi-tions are called La Niña.

■ Figure 14.17 During El Niño, warm water surges back toward South America, changing weather patterns.

■ Figure 14.16 Under normal condi-tions, trade winds and ocean currents move warm water west across the Pacific Ocean.

VOCABULARYSCIENCE USAGE V. COMMON USAGEPressureScience usage: the force that a column of air exerts on the air below it

Common usage: the burden of physical or mental distress

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Earth

Circular orbit

Elliptical orbit

Sun

390 Chapter 14 • Climate

Natural Causes of Climatic ChangesMuch discussion has taken place in recent years about whether Earth’s climate is changing as a result of human activities. You will read more about this in Section 14.4. It is important to note that many cycles of climatic change occurred long before humans inhabited Earth. Studies of tree rings, ice-core samples, fossils, and radiocarbon samples pro-vide evidence of past climatic changes. These changes in Earth’s cli-mate were caused by natural events such as variations in solar activity, changes in Earth’s tilt and orbit, and volcanic eruptions.

Solar activity Evidence of a possible link between solar activity and Earth’s climate was provided by English astronomer Edward Walter Maunder in 1893. The existence of sunspot cycles lasting approximately 11 years had been recognized by German scientist Samuel Heinrich Schwabe in 1843. However, Maunder found that from 1645 to 1716, the number of sunspots was scarce to nonexis-tent. The Maunder minimum is the term used to describe this period of low numbers of sunspots. This period closely corresponds to an unusually cold climatic episode called the Little Ice Age. During this time, much of Europe experienced bitterly cold winters and below-normal temperatures year-round. Residents of London are said to have ice-skated on the Thames River in June. The rela-tionship between sea surface temperature, which is used as an indi-cator of climate, and periods of low sunspot numbers is illustrated in Figure 14.18. Studies indicate that increased solar activity coincides with warmer-than-normal sea surface temperatures, while periods of low solar activity, such as the Maunder minimum, coincide with colder sea surface temperatures.

Earth’s orbit Climatic changes might also be triggered by changes in Earth’s axis and orbit. The shape of Earth’s elliptical orbit appears to change, becoming more elliptical, then more cir-cular, over the course of a 100,000-year cycle. As Figure 14.19 shows, when the orbit elongates, Earth travels for part of the year in a path closer to the Sun. As a result, temperatures become warmer than normal. When the orbit is more circular, Earth remains in an orbit that is farther from the Sun, and temperatures dip below average.

■ Figure 14.19 Scientists hypothesize that a more elliptical orbit around the Sun could produce significant changes in Earth’s climate.

■ Figure 14.18 Scientists theorize that solar activity might be linked to cli-matic changes.Evaluate How is the number of sunspots related to changes in sea surface temperature?

Sunspotnumber

Differencefrom SSTS

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Decreased tilt

Axis with reduced angle

Equator

Sunlight

Sun

Earth

Existingaxis

PolarisVega

Earth

Section 3 • Climatic Changes 391

Earth’s tilt As you know, seasons are caused by the angle of the tilt of Earth’s axis. At present, the angle of the tilt is 23.5°. However, the angle of tilt varies from a minimum of 22.1° to a maximum of 24.5° every 41,000 years. Scientists theorize that these changes in angle affect the differences in seasons. For example, a decrease in the angle of the tilted axis, shown in Figure 14.20, might cause a decrease in the temperature difference between winter and sum-mer. Winters would be more warm and wet, and summers would be cooler. The additional snow in latitudes near the poles would not melt in summer because temperatures would be cooler than average. This could result in increased glacial formation and cover-age. In fact, some scientists hypothesize that changes in the angle of Earth’s tilted axis can cause ice sheets to form near the poles.

Reading Check Describe how a change to the angle of Earth’s tilt can lead to climate change.

Earth’s wobble Another movement of Earth might be respon-sible for climatic changes. Over a period of about 26,000 years, Earth wobbles as it spins around on its axis. Currently, the axis points toward the North Star, Polaris, as shown in Figure 14.21.Because of Earth’s wobbling, however, the axis will eventually rotate away from Polaris and toward another star, Vega, in about 13,000 years. Currently, winter occurs in the northern hemisphere when the direction of the tilt of Earth causes the northern hemi-sphere to receive more direct radiation from the Sun. However, in 13,000 years, the northern hemisphere will be tilted in the opposite direction relative to the Sun. So, during the time of year associated with winter today, the northern hemisphere will be tilted toward the Sun and will experience summer.

■ Figure 14.20 If the angle of the tilt of Earth’s axis decreased, there would be less temperature contrast between summer and winter.

■ Figure 14.21 Earth’s wobble determines the timing of the seasons. When the northern hemisphere points toward the star, Vega, in 13,000 years, the northern hemisphere will experience summer during the time now associated with winter.

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Self-Check Quiz glencoe.com392 Chapter 14 • Climate

Volcanic activity Climatic changes can also be triggered by the immense quantities of dust-sized particles, called aerosols, that are released into the atmosphere during major volcanic eruptions, as shown in Figure 14.22. Volcanic dust can remain suspended in the atmosphere for several years, blocking incoming solar radiation and thus lowering global temperatures. Some scientists theorize that peri-ods of high volcanic activity cause cool climatic periods. Climatic records from the past century show that several large eruptions have been followed by below-normal global temperatures.

For example, the ash released during the 1991 eruption of Mount Pinatubo in the Philippines resulted in slightly cooler tem-peratures around the world the following year. Generally, volcanic eruptions appear to have only short-term effects on climate. These effects, as well as the others you have read about thus far, are a result of natural causes.

Section 1 414..33 AssessmentSection Summary◗◗ Climate change can occur on a long-

term or short-term scale.

◗◗ Changes in solar activity have been correlated with periods of climate change.

◗◗ Changes in Earth’s orbit, tilt, and wobble are all associated with changes in climate.

Understand Main Ideas1. MAIN Idea Identify and explain an example of long-term climatic change.

2. Describe What are seasons? What causes them?

3. Illustrate how El Niño might affect weather in California and along the Gulf Coast.

4. Analyze How does volcanic activity affect climate? Are these effects short-term or long-term climatic changes?

Think Critically5. Assess What might be the effect on seasons if Earth’s orbit became more ellipti-

cal and, at the same time, the angle of the tilt of Earth’s axis increased?

Earth ScienceMATH in6. Study Figure 14.18. During which period were sunspot numbers lowest? During

which period were sunspot numbers highest?

■ Figure 14.22 After Mount Pinatubo’s eruption in the Philippines, aerosol concentration increased worldwide. High concentrations appear in white and low concentrations in brown. The first image was taken immediately after the eruption and the second was taken two months later.Infer How did this affect global climates?

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SC.912.E.7.4, SC.912.E.7.7, MA.912.S.1.2

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