484 Lesson 1

LESS

ON 1 Our Dynamic Climate

OF ALL THE ENVIRONMENTAL issues, global climate change may be the one that will have the greatest impact on your future. Recall that while weather refers to the daily changes in temperature, precipita-tion, air pressure, and so forth, climate is an area’s average weather condi-tions over a long period. To understand why Earth’s climate is changing, you first need to know what determines climate. Three factors have more influence on Earth’s climate than all others combined—the sun, global wind patterns, and the oceans.

Energy From the Sun The heating of Earth’s atmosphere by the sun is influenced by

the greenhouse effect, latitude, and sunspot cycles.

The sun is the source of the energy that determines weather and climate on Earth. Energy from the sun is transferred to Earth by means of radia-tion. About 30 percent of this incoming radiation is reflected back into space when it strikes land, water, or clouds. This reflected radiation is lost to space and does not contribute to the heating of Earth. The rest of the incoming solar radiation is not reflected. This radiation may be absorbed by Earth and the atmosphere and converted to heat.

The Greenhouse Effect in the Atmosphere If Earth did not have an atmosphere, most of the energy from the sun would be reflected back into space. The greenhouse effect is a natural process in which certain gases in the atmosphere keep heat near Earth and prevent it from radiat-ing into space. The gases that do this are called greenhouse gases. The major greenhouse gases are water vapor (H2O) and carbon dioxide (CO2). Other greenhouse gases include tropospheric ozone (O3), nitrous oxide (N2O), and methane (CH4).

The term greenhouse effect is a bit misleading. Greenhouses used for plants hold heat in place by preventing warm air from escaping. In contrast, greenhouse gas molecules in the atmosphere do not trap air or anything else. Instead, they absorb heat and release it slowly.

• Describe factors that affect how the sun warms Earth.

• Discuss the role of wind patterns in determining climate.

• Explain how the oceans affect climate.• Describe how climate is affected by topography,

volcanoes, regional vegetation, and periodic changes in Earth’s orbit.

Reading Strategy Before you read, preview Figure 1. Write two questions about the process shown in the illustra-tion. As you read, write answers to the questions.

Vocabulary greenhouse effect, greenhouse gas, thermohaline circulation, El Niño, topography

Guiding Question: What factors determine Earth’s climate?

FOCUS Make a cluster diagram on the board. Write the phrase Factors That Determine Climate in the center of the cluster diagram. Have students identify factors that they think affect Earth’s climate and add these factors to the cluster diagram. As students read the les-son, ask them to add to or revise the cluster diagram.

GUIDING QUESTION

16.1 LESSON PLAN PREVIEWDifferentiated Instruction ELLs complete cloze prompts to better understand lesson concepts.Inquiry Students model ther-mohaline circulation.Real World Students iden-tify topographical features that may influence their local climate.

16.1 RESOURCESModeling Lab, Effects of Greenhouse Gases • Lesson 16.1 Worksheets • Lesson 16.1 Assessment • Chapter 16 Overview Presentation

Sun’s energy reaches Earth.

Some heat is radiated into space.

Some radiated heatis absorbed by greenhouse gases in the atmosphere.

Earth’s surface is heated.

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▶ Sunlight and Heat If greenhouse gases keep heat in the atmosphere, why don’t they also block solar energy from getting to Earth? The answer lies in what happens to solar energy when it reaches Earth. Greenhouse gases do not stop sunlight from getting through. However, after sunlight hits the surface of Earth, much of its energy is converted to heat. Much of this heat radiates back into the atmosphere. Greenhouse gases absorb some of the heat radiated from Earth’s surface. Figure 1 shows how the greenhouse effect works.

Greenhouse gases prevent some heat from radiating into space as rapidly as it otherwise would. They release the heat slowly, and this slow release of heat warms the troposphere.

▶ No Life Without Greenhouse Gases The greenhouse effect is a natu-ral process that is generally beneficial to living things. Greenhouse gases have been present in our atmosphere for billions of years. Without them, life on Earth would be impossible because the surface would be too cold. However, as you will learn, human activities are adding greenhouse gases to the atmosphere and increasing the greenhouse effect.

The Effect of Latitude Latitude has a significant effect on climate. Latitude is a measurement of a place’s distance from the equator. The equator is located at 0° latitude. The farther you move from the equator, the greater the number of a place’s latitude. For example, the latitude of Charlotte, North Carolina, is 35° N. In contrast, Anchorage, Alaska, is located at 61° N latitude, indicating that Anchorage is farther from the equator than Charlotte is. The N in the latitude numbers indicates that both Charlotte and Alaska are north of the equator. An S would indicate that a place is south of the equator. In general, the farther a place is from the equator, the cooler its climate. Areas close to the equator are generally warm.

FIGURE 1 Greenhouse Effect Greenhouse gases, such as water vapor and carbon dioxide, trap some of the heat that radiates from Earth’s surface. Without the greenhouse effect, living things could not survive.

What are the causes and conse-quences of a warming Earth?Self-Knowledge Students may have already heard about the greenhouse effect and greenhouse gases in the context of global warming from the media and other sources. Because of this, they may assume that the greenhouse effect is completely caused by humans and is always harmful to the environment. Rein-force that the greenhouse effect is not only a natural process, but a nec-essary one to support life on Earth. Have pairs discuss why greenhouse gases and the greenhouse effect are necessary for life. Then, have a class discussion to summarize this topic.

BIG QUESTION

Less direct

Less direct

Mostdirect Tropical

Temperate

Temperate

Polar

Polar

66.5° N

66.5° S

23.5° N

0°

23.5° S

90° N

90° S

486 Lesson 1

▶ Unequal Heating The relationship between climate and lati-tude happens because the sun’s radiation strikes regions of Earth at different angles. The difference in angles causes unequal heating on Earth. You can see this in Figure 2. Notice that there are three general climate regions: tropical, temperate, and polar.

▶ The Tropics and the Poles Tropical areas are generally hot. That is because, all year round, the sun’s rays hit the equator most directly compared to other parts of Earth. Because of the angle at which the rays strike the polar areas, the energy that polar regions receive is spread out over a larger area than the energy received by regions near the equator. Therefore, polar regions are generally colder than other areas.

▶ The Temperate Climate Zones Regions between the poles and the equator are in temperate climate zones. Temperate zones generally have climates that are cool during some parts of the year and warm in others.

▶ Changing Seasons The seasons change because, as Earth orbits the sun, the angle at which the sun’s rays strike parts of Earth changes. In the Northern Hemisphere in June, the northern end of Earth’s axis is tilted toward the sun, and the Northern Hemisphere experiences summer. In December, the northern end of Earth’s axis is tilted away from the sun. It is then winter in the Northern Hemisphere.

21 3 4 65 7 8 9 Move the flashlight up and aim the beam at the mid-latitudes. Keep the flashlight horizontal. Have your partner mark the lighted area.

21 3 4 65 7 8 9 Repeat Step 4, but shine the light on the North Pole area of the strip.

Analyze and Conclude1. Observe What shape was the lighted area

when you pointed the flashlight at the equator?

2. Compare and Contrast How did the size and shape of the lighted area change when you moved the flashlight beam to the mid-latitudes and the North Pole?

3. Relate Cause and Effect Why did the shape of the lighted area change as you moved the flashlight?

4. Use Models How does this activity show why areas near the equator are warmer than areas at the poles?

Does Latitude Affect the Sun’s Rays?21 3 4 65 7 8 9 Work with a partner. Begin by taping a strip of paper to a

globe from the North Pole to slightly below the equator.21 3 4 65 7 8 9 Divide the paper into three parts. Label the top part North

Pole, the middle part mid-latitudes, and the bottom part equator.

21 3 4 65 7 8 9 Tape the end of a toilet-paper roll to the light end of a flash-light and turn the flashlight on. The flashlight represents the sun. Hold the flashlight about 30 cm (12 in.) from the equator. On the paper strip, your partner should draw lines indicating the area the light shines on.

FIGURE 2 Climate Zones The sun’s rays hit different locations on Earth at different angles. As a result, different parts of Earth have different climates.

Westerlies

60º N

30º N

30º S

60º S

EQUATOR

Westerlies

Northeast trade winds

Southeast trade winds

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Sunspot Cycles The sun varies slightly in the amount of radiation it emits, over both short and long periods. There is, for example, a relation-ship between sunspots and radiation emitted. A sunspot is a dark spot on the surface of the sun. The more sunspots present on the surface of the sun, the more energy the sun gives off. The number of sunspots rises and falls in cycles that last about 11 years. Although sunspot cycles have some effect on global climate, scientists think that they do not have a major, long-term effect.

ReadingCheckpoint

What accounts for the change in seasons in temperate zones?

Wind Patterns in the Atmosphere Winds distribute heat and moisture globally.

Recall that if air becomes warm, it usually rises. In contrast, if air becomes cool, it sinks. Rising warm air and sinking cool air form convection cur-rents. Convection currents that result from unequal heating produce air currents, or winds. Winds transport both heat and moisture, affecting both temperature and precipitation in the regions they pass over.

Winds and Heat Because the regions near the equator are warm, air rises above them. In contrast, the North and South poles are cold, and air moves downward, toward Earth’s surface. The rising of air in equatorial regions and the sinking of air in polar regions help create global wind pat-terns, called prevailing winds, shown in Figure 3. The prevailing winds move huge air masses around the surface of Earth. Warm air moves away from the equator and toward the poles, and cold air moves in the opposite direction.

Winds and Moisture Moisture in the atmosphere occurs in the form of water vapor, which is water in the form of a gas. In the water cycle, water vapor enters the atmosphere through evaporation from Earth’s sur-face from lakes, oceans, and soil. Plants also release water vapor into the atmosphere. In general, warm air can carry more water vapor than cooler air can. When warm, moist air is cooled, the water vapor condenses to form clouds. Rain, snow, or other forms of precipitation may then fall from the clouds.

Winds move moisture from one location to another. For example, when winds move over a large body of water, such as a lake or ocean, they pick up water vapor. The winds may then carry the water vapor a long distance over land, where it falls as precipitation.

FIGURE 3 Prevailing Winds Global wind currents show patterns related to latitude. Trade winds between the equator and 30° latitude blow westward, whereas westerlies between 30° and 60° latitude blow eastward. Because of Earth’s rotation, in general winds blow from east to west near the equator and from west to east over temperate zones.

ANSWERS

Quick Lab 1. circle2. The lighted area became larger

and more elongated.3. The angle at which the light rays

hit the globe changed.4. Like the light from the flashlight,

the sun’s rays are more concentrat-ed at the equator and more spread out at the poles, so areas near the equator are warmer than areas at the poles.

Reading Checkpoint The changing angle at which the sun’s rays strike Earth as Earth orbits around the sun

Convection loop

WindsEquator

EQUATOR

Indonesia

Movementof warm water

Movementof warm water

Increased convection

Peru

(a) Normal conditions (b) El Niño conditions

Winds

Winds

PACIFICOCEAN

Upwelling of deep, cold water Peru

Deep, cold water stays below surface.

Equator

Indonesia

Adapted from National Oceanic and Atmospheric Administration, Tropical Atmospheric Ocean Project.

Movementof warm water

Movementof warm water

488 Lesson 1

The Oceans and Climate Oceans affect climate by transporting heat and absorbing

carbon dioxide.

Like winds, ocean currents transport heat over long distances. In addi-tion, ocean water absorbs carbon dioxide, and this has a cooling effect.

Ocean Circulation Ocean water exchanges huge amounts of heat with the atmosphere, and ocean currents move heat energy from place to place. A worldwide system of ocean currents is caused by a combination of unequal heating of water and unequal salinity (salt concentration).

▶ Thermohaline Circulation Cool water generally has a greater density than warm water. Saltier water is denser than water with a lower salinity. Therefore, warmer, less salty water moves along the surface of the ocean, and colder, saltier water moves deep beneath the ocean’s surface. This pat-tern is called the thermohaline circulation. As part of this pattern, cooler, saltier water at the poles sinks, and warmer, less salty water from the equator moves to take the place of the cooler water. In the Gulf Stream in the Atlantic Ocean, warm surface water flows northward from the equa-tor. The warm water keeps Europe warmer than it would otherwise be.

▶ El Niño and La Niña The interactions between ocean and atmosphere called El Niño and La Niña affect climate. El Niño is a change in air pressure, wind patterns, ocean temperature, and ocean circulation in the Pacific Ocean. Normally, prevailing winds blow from east to west along the equator. The winds help move warm surface waters westward. During El Niño, however, equatorial winds weaken, and the surface water in the eastern Pacific Ocean becomes warmer than usual. Figure 4 diagrams this pattern. El Niño has a major effect on weather worldwide. For example, it causes rainstorms and floods in areas that are usually dry, such as south-ern California. The pattern known as La Niña is the opposite of El Niño. During La Niña, temperatures in the eastern Pacific Ocean are colder than average. Like El Niño, La Niña disrupts weather worldwide.

FIGURE 4 El Niño (a) Normally, winds push warm waters toward the western Pacific. (b) In contrast, under El Niño conditions, the winds weaken and the warm water flows back across the Pacific toward South America. El Niño changes precipitation patterns all over the world.

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Ocean Absorption of Carbon Dioxide Carbon dioxide moves back and forth between the atmosphere and ocean water. Oceans can hold 50 times more carbon dioxide than is found in the atmosphere. Since carbon dioxide is a greenhouse gas, the absorption of carbon diox-ide by the ocean has a cooling effect on the atmosphere. However, the oceans absorb carbon dioxide more slowly than it is being added to the atmosphere. Therefore, carbon absorption by the oceans is slowing global warming but not preventing it.

Other Factors That Affect Climate Global climate may be affected by factors such as topography,

volcanic eruptions, regional vegetation, and changes in Earth’s orbit.

Other factors besides the sun, the atmosphere, and the oceans affect cli-mate. Four of these factors are topography, volcanoes, regional distribu-tion of vegetation, and changes in Earth’s path around the sun.

Topography A region’s topography describes the surface character-istics of the area, including its elevation and features such as mountains, rivers, and lakes. The characteristics of a region’s topography affect its climate. Two of these are altitude and the presence of mountain ranges.

▶ Altitude Mount Cayambe in South America is located right on the equator. However, its peak is always covered with snow, as are the tops of many high mountains near the equator. In general, the greater the altitude, or elevation, the cooler the air temperature will be. That is why the kinds of plants at the bottom of mountains are usually not the same varieties as those found higher up.

▶ Mountain Ranges As winds pass over mountains, the rising air cools and clouds often form. Then, precipitation may fall from those clouds. By the time the air has moved to the other side of the mountains, however, it has usually lost much of its moisture. In general, precipitation falls on the windward side of mountain ranges, which is the side that wind first passes over. The leeward side of a mountain range, or the downwind side, gets relatively little precipitation.

ReadingCheckpoint

What is topography?

FIGURE 5 Snow-Covered Mountain Even though Mount Cayambe is on the equator, its peak is always covered with snow.

ANSWERS

Reading Checkpoint The surface characteristics of a region

490 Lesson 1

11. Relate Cause and Effect Why does a region’s lati-

tude affect its climate? In your answer, mention the equator, the poles, and the regions in between the equator and the poles.

2. Compare and Contrast How is the behavior of warm air different from that of cold air? Relate this difference to global wind patterns.

3. Explain What happens during El Niño?

4. Apply Concepts What is the relationship between altitude and climate? How does this account for the differences in ecosystems at the base of a mountain and at its peak?

5. Explore the BIGQUESTION Imagine that you have taken a trip, via spaceship, to a planet that has very little atmosphere, and therefore very little green-house effect. Describe what the planet’s climate might be like, and how it would probably be differ-ent from Earth’s climate.

Volcanoes An erupting volcano may expel huge amounts of gases and particles, as Figure 6 shows. Winds can carry these materials to areas that are a long way from the volcano. If the eruption is large enough, the gases and particles may block some sun-light from entering Earth’s atmosphere. This blocking of sunlight may, in turn, cool the atmosphere. The cooling is temporary, however, because the volcanic materials remain in the atmosphere for a limited time.

Regional Vegetation Plant life, or vegetation, can influence climate when it covers a large area. For example, the abundant vegetation of the Amazon rain forest promotes cloud formation and rainfall. In

addition, plants affect the amount of carbon dioxide in the atmosphere because they use carbon dioxide in the process of photosynthesis. Huge forests take in an especially large amount of carbon dioxide. This intake decreases the amount of carbon dioxide in the atmosphere. However, when large sections of forests are cut down, the loss of trees means that the carbon dioxide they would have used remains in the atmosphere. Therefore, the destruction of forests in one area can increase the tempera-ture of the atmosphere worldwide.

Changes in Earth’s Orbit Evidence indicates that during Earth’s his-tory climate has changed many times. For example, in the last three mil-lion years, Earth has gone through a series of ice ages. During these ice ages, huge glaciers covered large parts of the Northern Hemisphere. Many large mammals that are now extinct, such as woolly mammoths, survived in the frigid climate.

One cause of these climate changes is the periodic variation in Earth’s movement and position in space in relation to the sun. Minor changes in Earth’s orbit, and in the tilt of Earth’s axis, occur in regular cycles. These cyclic changes affect the distribution of solar radiation over Earth’s sur-face. This change in the distribution of sunlight can affect climate. Cli-mate changes caused by these variations may last for thousands of years.

FIGURE 6 Volcanoes and Climate A volcano erupts in New Guinea. Volcanoes spew out materials that may prevent some sunlight from entering Earth’s atmosphere.

ANSWERS

Lesson 1 Assessment For answers to the Lesson 1 Assessment, see page A–25 at the back of the book.