ddiscoveryiscovery llabab model cloud cover - earth...

14.1 What is climate? 359

Model Cloud CoverDiscovery LabDiscovery Lab

OBJECTIVES

• Describe different typesof climate data.

• Recognize limits associ-ated with the use ofnormals.

• Explain why climatesvary.

VOCABULARY

climatology tropicsclimate temperate zonenormal polar zone

Fifty thousand years ago, the United States had much differentweather patterns than those that exist today. The average tempera-ture was several degrees cooler, and the jet stream was probably far-ther south. Understanding and predicting such climatic changes arethe basic goals of climatology.Climatology is the study of Earth’s cli-mate and the factors that affect past, present, and future climaticchanges.

CLIMATE: MORE THAN JUST AVERAGE WEATHERClimate, as you’ll recall from Chapter 12, describes the long-termweather patterns of an area. These patterns include much more thanaverage weather conditions. Climate also describes annual variationsof temperature, precipitation, wind, and other weather variables.Studies of climate show extreme fluctuations of these variables overtime. For example, climatic data can indicate the warmest and cold-est temperatures ever recorded for a location. This type of informa-tion, combined with comparisons between recent conditions and

What is climate?14.114.1

Some areas are generally morecloudy than others. This affects boththe temperature and the amount ofprecipitation that these areas receive.In this activity, which should be doneonly when the weather forecast callsfor clear, calm skies overnight, you’llmodel the effect of cloud cover onlocal temperatures.

1. On a calm, clear afternoon, lay twosheets of dark construction paperon the grass in an open area. Place arock on each sheet of paper to pre-vent them from blowing away.

2. Open an umbrella and prop it on

the ground over one of the sheetsof paper.

3. The next morning, observe what hashappened to the sheets of paper.

Observe In your science journal,describe any differences in dew for-mation that you observed. How is theumbrella in this activity similar toclouds in the atmosphere? Based onyour observations, infer how temper-atures during the night might differbetween climates with extensive cloudcover and climates with fewer clouds.

Fifty thousand years ago, the United States had much differentFift thweather patterns than those that exist today. The average tempera-ture was several degrees cooler, and the jet stream was probably far-ther south. Understanding and predicting such climatic changes arethe basic goals of climatology.Climatology is the study of Earth’s cli-mate and the factors that affect past, present, and future climaticchanges.

Climate, as you’ll recall from Chapter 12, describes the long-termweather patterns of an area. These patterns include much more thanaverage weather conditions. Climate also describes annual variationsof temperature, precipitation, wind, and other weather variables.Studies of climate show extreme fluctuations of these variables overtime. For example, climatic data can indicate the warmest and cold-

long-term averages, can be used by companies to decide where tolocate new facilities and by people who have medical conditions thatrequire them to live in certain climates.

NORMALSThe data used to describe an area’s climate are compiled from mete-orological records, which are continuously gathered at thousands oflocations around the world. These data include daily high and lowtemperatures, amounts of rainfall, wind speed and direction, humid-ity, and air pressure. Once the data are gathered, they are averaged ona monthly or annual basis for a period of at least 30 years to deter-mine the normals, or standard values, for a location. The Problem-Solving Lab below lists some normals for Jacksonville, Florida.

360 CHAPTER 14 Climate

Infer climatic conditions fromnormals Normals offer a comprehen-sive look at local weather conditions over

relatively long periods of time. Use the

data provided in the table to answer the

following questions about the climate of

Jacksonville, Florida.

Analysis

1. According to normal daily maximumtemperatures, during what months

would you expect the temperature to

reach at least 90°F?

2.What were the highest and lowest

temperatures ever recorded in this city,

and in what month and year?

Thinking Critically

3. Use graph paper to plot the monthlyvalues for normal daily maximum tem-

peratures which cover the 30-year time

period from 1966 through 1996. Next,

use the monthly values to calculate the

average daily maximum temperature

for the 30-year period.

4.Which months were warmer than theaverage daily maximum temperature

of the 30-year period? Which months

were colder?

Making and Using Tables

Time Period

Temperature F° (Years) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Normal Daily Maximum 30 64.2 67.0 73.0 79.1 84.7 89.3 91.4 90.7 87.2 80.2 73.6 66.8

Highest Daily Maximum 55 85 88 91 95 100 103 105 102 100 96 88 84

Year of Occurrence 1947 1962 1974 1968 1967 1954 1942 1954 1944 1951 1986 1994

Normal Daily Minimum 30 40.5 43.3 49.2 54.9 62.1 69.1 71.9 71.8 69.0 59.3 50.2 43.4

Lowest Daily Minimum 55 7 19 23 34 45 47 61 63 48 36 21 11

Year of Occurrence 1985 1996 1980 1987 1992 1984 1972 1984 1981 1989 1970 1983

Normals for Jacksonville, Florida

ity, and air pressure. Once the data are gathered, they are averaged ona monthly or annual basis for a period of at least 30 years to deter-mine the normals, or standard values, for a location. The

While normals offer valuable information, they must be used withcaution. Weather conditions on any given day might differ widelyfrom normals. For instance, the average high temperature in Januaryfor a city might be 0°C. However, it’s possible that no one day inJanuary had a high of exactly 0°C. Normals are not intended todescribe usual weather conditions. They are simply the average valuesover a long period of time.

Another issue complicates the use of normals. While climatedescribes the average weather conditions for a region, normals applyonly to the specific place where the meteorological data were col-lected. Most meteorological data are gathered at airports, which can-not operate without up-to-date, accurate weather information. Doyou know anyone who lives at an airport? Probably not. In fact, manyairports are located well outside city limits because of the noise andtraffic that they generate. When climatic normals are based on air-port data, they may differ quite a lot from actual weather conditionsin nearby cities.Why? Changes in elevation and other factors such asproximity to large bodies of water can cause climates to vary, as you’lllearn next.

WHAT CAUSES CLIMATES?One glance at the map shown in Figure 14-1 shows that climatesaround the country vary greatly. For example, average daily tempera-tures are much warmer in Dallas, Texas, than in Minneapolis,


Figure 14-1 This mapshows daily minimum tem-perature in January acrossthe United States. The lati-tudes of the cities areshown because, as you’lllearn on the next page, lati-tude greatly affects climate.

0°C

UnitedStates

Canada

Minneapolis44°N

Jan. –12°CJuly 22°C

Dallas33°N

Jan. 5°CJuly 29°C

San Francisco37°N

Jan. 9°CJuly 16°C

Wichita37°N

Jan. –1°CJuly 27°C

+10°C

+20°C

–10°C

–20°C

–30°C

–40°C

Minimum Temperatures for January

WHAT CAUSES CLIMATES?

Minnesota. There are several reasons forsuch climatic variations, including latitude,topography, closeness of lakes and oceans,availability of moisture, global wind pat-terns, ocean currents, and air masses.

Latitude Recall that different parts of Earth receive differentamounts of solar radiation. The amount of solar radiation receivedby any one place varies because Earth is tilted on its axis, and thisaffects how the Sun’s rays strike Earth’s surface. As Figure 14-2 shows,the area between 23.5° south of the equator and 23.5° north of theequator, known as the tropics, receives the most solar radiationbecause the Sun’s rays strike that area from almost directly overhead.As you might expect, temperatures in the tropics are generally warmyear-round. The temperate zones lie between 23.5° and 66.5° northand south of the equator. As their name implies, temperatures in theseregions are moderate. The polar zones are located from 66.5° northand south of the equator to the poles. Solar radiation strikes the polarzones at a low angle. Thus, polar temperatures are nearly always cold.

Topographic Effects Water heats up and cools down moreslowly than land. Thus, large bodies of water affect the climates ofcoastal areas. Many coastal regions are warmer in the winter andcooler in the summer than inland areas of similar latitude.

Also, temperatures in the lower atmosphere generally decreasewith altitude. Thus, mountain climates are usually cooler than thoseat sea level. In addition, climates often differ on either side of amountain. Air rises up one side of a mountain as a result of oro-graphic lifting. The rising air cools, condenses, and drops its mois-ture. The climate on this side of the mountain—the windwardside—is usually wet and cool. On the opposite side of the moun-tain—the leeward side—the air is dry, and it warms as it descends.For this reason, deserts are common on the leeward sides of moun-tains, as shown in Figure 14-3.


Polar zone

Polar zone

Temperatezone

Temperatezone

Tropics

66.5°

Tropic of Cancer

Tropic of Capricorn

66.5°

Equator

0°

23.5

°23.5

°

Figure 14-2 Latitude has a great effect on climate. The amount of solar radiationreceived on Earth decreases in intensity from the equator to the poles.

Topic: TropicsTo find out more about thetropics, visit the EarthScience Web Site at earthgeu.com

Activity: Design a Venndiagram to compare andcontrast the three majortypes of tropical climates.

Latitude Recall that different parts of Earth receive differentamounts of solar radiation. The amount of solar radiation receivedby any one place varies because Earth is tilted on its axis, and thisaffecaffaff ts how the Sun’s rays strike Earth’s surface. As

Topographic EffectsTT Water heats up and cools down moreslowly than land. Thus, large bodies of water affect the climates ofcoastal areas. Many coastal regions are warmer in the winter andcooler in the summer than inland areas of similar latitude.

Also, temperatures in the lower atmosphere generally decreasewith altitude. Thus, mountain climates are usually cooler than thoseat sea level. In addition, climates often differ on either side of amountain. Air rises up one side of a mountain as a result of oro-graphic lifting. The rising air cools, condenses, and drops its mois-ture. The climate on this side of the mountain—the windwardside—is usually wet and cool. On the opposite side of the moun-tain—the leeward side—the air is dry, and it warms as it descends.For this reason, deserts are common on the leeward sides of moun-tains, as shown in

Air Masses Two of the main causes of weather are the movementand interaction of air masses. Air masses affect climate, too. Theyhave distinct regions of origin, caused primarily by differences in theamount of solar radiation. The properties of air masses are alsodependent on whether they formed over land or water.

Average weather conditions in and near regions of air-mass for-mation are fairly similar to those exhibited by the air masses them-selves. For example, consider an island in the tropical Atlantic Ocean.Because this island is located in an area where maritime tropical (mT)air masses dominate the weather, the island’s average weather condi-tions, or climate, have maritime tropical characteristics.


1. Compare and contrast temperatures inthe tropics, temperate zones, and polarzones.

2. Infer how climate data can be used byfarmers.

3. What are some limits associated with theuse of normals?

4. Describe two topographic features thatcause variations in climate.

5. Thinking Critically Average daily temper-atures for one city, located at 15° southlatitude, are 5°C cooler than average daily

temperatures for a second city, located at30° south latitude. What might accountfor the cooler temperatures in the firstcity, which lies so near the equator?

SKILL REVIEW

6. Forming a Hypothesis Suppose thatmeteorological data for an area are nor-mally gathered at an airport located 10 kmfrom a large lake. Hypothesize how nor-mals for the area might change if the datawere gathered from the edge of the lake.For more help, refer to the Skill Handbook.

Moist air

Windward side Leeward side

Dry air

Figure 14-3 On the wind-ward side of a mountain,moist air is forced upward,cools, condenses, and dropsits moisture (A). The air andthe climate on the leewardside of the mountain aredry. Deserts such as theAtacama in Chile are com-mon on leeward sides ofmountains (B).

A B

earthgeu.com/self_check_quiz


OBJECTIVES

• Describe the criteriaused to classify climates.

• Compare and contrastdifferent climates.

VOCABULARY

Koeppen classification system

microclimateheat island

14.214.2 Climate Classification

Picture a parched desert with wind-blown dunes stretching towardthe horizon. Now, imagine a glistening iceberg floating amid a polarsea. These images represent vastly different climates. What criteriawould you use to classify them? Temperature is an obvious choice, asis amount of precipitation. The Koeppen classification system, awidely used classification system for climates, uses both of these cri-teria. Developed by Russian-born German climatologist WladimirKoeppen (1846–1940), the system is based on the average monthlyvalues of temperature and precipitation. It also takes into accountthe distinct vegetation found in different climates.

80°

40°

0°

40°

80°

Tropical climates

Tropical wet

Tropical wet and dry

Mild climates

Marine west coast

Mediterranean

Humid subtropical

Dry climates

Semiarid

Arid

Continental climates

Warm summer

Cool summer

Subarctic

Polar climates

Tundra

Ice cap

High elevation

Highlands

Uplands

Map of World Climates

Figure 14-4 Koeppen’s classification system, shown here in amodified version, is made up of six main divisions.

14.2 Climate Classification 365

KOEPPEN CLASSIFICATION SYSTEMKoeppen decided that a good way to distinguish among different cli-matic zones was by natural vegetation. Palm trees, for instance, are notlocated in polar regions; they are largely limited to tropical and sub-tropical regions. Koeppen later realized that quantitative values wouldmake his system more objective and therefore more scientific. Thus,he revised his system to include the numerical values of temperatureand precipitation. A map of global climates according to a modifiedversion of Koeppen’s classification system is shown in Figure 14-4.

Tropical Climates Constant high temperatures characterizetropical climates. In some tropical areas, the heat is accompanied byup to 600 cm of rain each year. The combination of heat and rainproduces tropical rain forests, which contain some of the most dra-matic vegetation on Earth. You saw an example of a tropical rain for-est in the photograph at the beginning of this chapter. Tropicalregions are almost constantly under the influence of maritime trop-ical air. The transition zones that border the rainy tropics north andsouth of the equator, known as tropical wet and dry zones, have dis-tinct dry winter seasons as a result of the occasional influx of drycontinental air masses. Tropical wet and dry zones include savannas,as shown in Figure 14-5. These tropical grasslands are found inAfrica, among other places.

Dry Climates Dry climates, which cover about 30 percent ofEarth’s land area, make up the largest climatic zone. Most of theworld’s deserts, such as the Sahara, the Gobi, and the Australian, areclassified as dry climates. In these climates, continental tropical (cT)air dominates, precipitation is low,and vegetation is scarce. Many ofthese areas are located near thetropics. Thus, intense amounts ofsolar radiation result in high ratesof evaporation and few clouds.Overall, evaporation rates exceedprecipitation rates. The resultingmoisture deficit gives this zone itsname. Within this classification,there are two subtypes: aridregions or deserts, and semi-aridregions or steppes. Steppes aremore humid than deserts; theygenerally separate arid regionsfrom bordering wet climates.

Estimating Use themap in Figure 14-4to determine theapproximate percent-age of land coveredby tropical wet climates.

Figure 14-5 This wateringhole in Botswana is foundin a savanna.


Mild Climates Mild climates can be classifiedinto three subtypes: humid subtropical climates,marine west coast climates, and mediterranean cli-mates. Humid subtropical climates are influencedby the subtropical high-pressure systems that arenormally found over oceans in the summer. Thesoutheastern United States has this type of climate.There, warm, muggy weather prevails during thewarmer months and dry, cool conditions predomi-nate during the winter. The marine west coast cli-mates are dominated by the constant inland flow ofair off the ocean, which creates mild winters and coolsummers, with abundant precipitation throughoutthe year. An example of this type of climate is shownin Figure 14-6. Mediterranean climates, found inItaly and parts of Spain, among other places, are

influenced by the Mediterranean Sea. Summers in these climates aregenerally warm because the lack of cool ocean currents in theMediterranean Sea results in relatively warm water temperatures.

Continental Climates Continental climates are also classifiedinto three subtypes: warm summer climates, cool summer climates,and subarctic climates. Located in the zone dominated by the polarfront, continental climates are battlegrounds for clashing tropical andpolar air masses. Thus, these zones experience rapid and sometimesviolent changes in weather. Both summer and winter temperatures canbe extreme because the influence of polar air masses is strong in win-ter, while warm tropical air dominates in summer. The presence ofwarm, moist air causes summers to be generally wetter than winters,especially in latitudes that are relatively close to the tropics.

Polar Climates To the north of continental climates lie the polarclimates, the coldest regions on Earth. Just as the tropics are known fortheir year-round warmth, polar climates are known for their constantcold—the mean temperature of the warmest month is less than 10°C.Precipitation is generally low because cold air holds less moisture thanwarm air. Also, the amount of heat radiated by Earth’s surface is toolow to produce the strong convection currents needed to release heavyprecipitation. Figure 14-7A shows an ice-cap polar climate.

A variation of the polar climate is found at high elevations. Thistype of climate includes parts of the Andes Mountains of SouthAmerica, shown in Figure 14-7B, which lie near the equator. Theintense solar radiation found near such equatorial regions is offset bythe decrease in temperature that occurs with altitude.

Figure 14-6 The GoldenGate Bridge in San Fran-cisco, California, is nearlyhidden beneath a denselayer of fog. Fog is charac-teristic of marine west coast climates.

14.2 Climate Classification 367

MICROCLIMATESSometimes, the climate of a small area can be much different fromthat of the larger area surrounding it. A localized climate that differs from the main regional climate is called a microclimate.If you climb to the top of a mountain, you can experience a type ofmicroclimate; the climate becomes cooler with increasing elevation.You’ll learn more about microclimates in the Design Your Own GeoLabat the end of this chapter. Figure 14-8 shows a microclimate in a city.

A B

Potomac River

–1.1

–0.6

City center

–1.7

–1.7

–2.2

–2.2

–2.7

–2.7

–3.3

–3.3

–3.9

–3.9

N

Figure 14-8 This diagram shows wintertemperatures in Washington, D.C. Thebuildings and paved surfaces of the citycreate a microclimate. The temperature inthe center of the city is –0.6°C, nearly 3°Cwarmer than temperatures in some partsof the surrounding area.

Figure 14-7 Penguins areone of the few species thatcan survive in Antarctica’sice-cap polar climate (A).Llamas are common in thehigh-elevation climates ofthe Andes Mountains (B).

MICROCLIMATESSometimes, the climate of a small area can be much different fromthat of the larger area surrounding it. A localized climate that differs from the main regional climate is called a microclimate.If you climb to the top of a mountain, you can experience a type ofyymicroclimate; the climate becomes cooler with increasing elevation.


Heat Islands The mere presence of a building can create microcli-mates in the area immediately surrounding it.How? The building castsshadows that lower air temperature. The presence of many concretebuildings and large expanses of asphalt can create heat islands,wherein the climate is warmer than in surrounding rural areas. Thiseffect was recognized as long ago as the early nineteenth century, whenLondoners noticed that the temperature in their city was noticeablywarmer than in the surrounding countryside.

The heat-island effect occurs because large areas of asphalt andconcrete radiate far more heat into the air than do grasslands,wooded areas, and bodies of water. This causes mean temperaturesin large cities to be significantly warmer than in surrounding areas,as shown in Figure 14-9. The heat-island effect also causes greaterchanges in temperature with altitude, which sparks strong convec-tion currents. This in turn produces increased cloudiness and up to15 percent more total precipitation in cities.

Heat islands are examples of climatic change on a small scale. Inthe next sections, we’ll examine large-scale climatic changes causedby both natural events and human activities.

1. Compare and contrast the five main climate types.

2. What criteria is the Koeppen climate classification system based on?

3. What are microclimates? What climaticeffects do heat islands have on large cities?

4. Describe the climate of your area. Whichzone do you live in? What type of airmasses generally affect your climate?

5. Thinking Critically Of the different typesof climates, which do you think would bemost strongly influenced by the polar jetstream? Why?

SKILL REVIEW

6. Making and Using Tables Make a table ofthe Koeppen climate classification system.Include major zones, subzones, and char-acteristics of each. For more help, refer tothe Skill Handbook.

Figure 14-9 These imagesshow differences in day-time temperatures betweenan urban area (A) and asuburban area (B). Thecoolest temperatures arerepresented by blue; thewarmest temperatures arerepresented by red.

A B


Heat Islands The mere presence of a building can create microcli-mates in the area immediately surrounding it.How? The building castsshadows that lower air temperature. The presence of many concretebuildings and large expanses of asphalt can create heat islands,wherein the climate is warmer than in surrounding rural areas. This

The heat-island effect occurs because large areas of asphalt andconcrete radiate far more heat into the air than do grasslands,wooded areas, and bodies of water. This causes mean temperaturesin large cities to be significantly warmer than in surrounding areas,

14.3 Climatic Changes 369

00.000.014.314.3 Climatic Changes

OBJECTIVES

• Distinguish amongdifferent types ofclimatic changes.

• Recognize why climaticchanges occur.

VOCABULARY

ice ageseasonEl NinoMaunder minimum

Some years may be warmer, cooler, wetter, or drier than others, butduring the average human lifetime, climates do not appear to changesignificantly. However, a study of Earth’s history over hundreds ofthousands of years shows that climates always have been, and cur-rently are, in a constant state of change. These changes usually takeplace over extremely long time periods. Geologic records show that inthe past, Earth was sometimes much colder or warmer than it is today.

ICE AGESA good example of climatic change involves glaciers, which havealternatively advanced and retreated over the past 2 million years. Attimes, much of Earth’s surface was covered by vast sheets of ice.During these periods of extensive glacial coverage, called ice ages,average global temperatures decreased by an estimated 5°C.Although this may not seem like a large decrease, global climatesbecame generally colder and snowfall increased, which sparked theadvance of existing ice sheets. Ice ages alternate with warm periodscalled interglacial intervals—we are currently experiencing such aninterval. The most recent ice age ended only about 10 000 yearsago. In North America, glaciers spread from the eastcoast to the west coast and as far south asIndiana, as shown in Figure 14-10. Theresults of this glacial period are apparentin the Great Lakes and the Finger Lakesof central New York, which werescoured out as the glaciers retreated.

Figure 14-10 The last ice age covered large portions ofNorth America, Europe, and Asia.Average global temperatures wereroughly 5°C lower than normal.

Black Sea

Alps

Caspian Sea

Sea ice

Aral Sea

Europe

China

Siberia

Japan

Arctic

Ocean

North

Atlantic

Ocean

North

Pacific

Ocean

Iceland

United

States

Alaska

Some years may be warmer, cooler, wetter, or drier than others, butSo b leduring the average human lifetime, climates do not appear to changesignificantly. However, a study of Earth’s history over hundreds ofthousands of years shows that climates always have been, and cur-rently are, in a constant state of change. These changes usually takeplace over extremely long time periods. Geologic records show that inthe past, Earth was sometimes much colder or warmer than it is today.

ICE AGESA good example of climatic change involves glaciers, which havealternatively advanced and retreated over the past 2 million years. Attimes, much of Earth’s surface was covered by vast sheets of ice.During these periods of extensive glacial coverage, called ice ages,average global temperatures decreased by an estimated 5°C.Although this may not seem like a large decrease, global climatesbecame generally colder and snowfall increased, which sparked theadvance of existing ice sheets. Ice ages alternate with warm periodscalled interglacial intervals—we are currently experiencing such aninterval. The most recent ice age ended only about 10 000 yearsago. In North America, glaciers spread from the east


SHORT-TERM CLIMATIC CHANGESWhile ice ages take place over many thousands of years,other climatic changes take place in much shorter timeperiods. The most obvious of these are seasons, which areshort-term periods of climatic change caused by regularvariations in daylight, temperature, and weather patterns.These variations are the result of changes in the amount ofsolar radiation an area receives. As Figure 14-11 shows, thetilt of Earth on its axis as it revolves around the Sun causesdifferent areas of Earth to receive different amounts ofsolar radiation. During summer in the northern hemi-sphere, the north pole is tilted toward the Sun, and thishemisphere experiences long hours of daylight and warmtemperatures.At the same time, it is winter in the southernhemisphere. The south pole is tilted away from the Sun,and the southern hemisphere experiences long hours ofdarkness and cold temperatures. Throughout the year, theseasons are reversed in the north and south hemispheres.

El Nino Other short-term climatic changes are causedby El Nino, a warm ocean current that occasionallydevelops off the western coast of South America. In theSoutheast Pacific Ocean, atmospheric and ocean currentsalong the coast of South America normally move north,

transporting cold water from the Antarctic region. Meanwhile, thetrade winds and ocean currents move westward across the tropics,keeping warm water in the western Pacific. This circulation, drivenby a semipermanent high-pressure system, creates a cool, dry climatealong much of the northwestern coast of South America.

Occasionally, however, for reasons that are not fully understood,this high-pressure system and its attendant trade winds weaken dras-tically, which allows the warm water from the western Pacific to surge eastward toward the South American coast. The suddenpresence of this warm water heats the air near the surface of thewater. Convection currents strengthen, and the normally cool anddry northwestern coast of South America becomes much warmerand wetter. The increased precipitation pumps large amounts of heatand moisture into the upper atmosphere, where upper-level windstransport the hot, moist air eastward across the tropics. This hot,moist air in the upper atmosphere is responsible for dramatic cli-matic changes. Sharp temperature differences in the upper air allowthe jet stream to shift farther south. This causes weather systems totake a more southerly track, bringing violent storms to Californiaand the Gulf Coast, which are usually south of the storm tracks.

Figure 14-11 When thenorth pole is pointedtoward the Sun, the north-ern hemisphere experiencessummer and the southernhemisphere experienceswinter (A). During springand fall, neither pole pointstoward the Sun (B).

A

B

While ice ages take place over many thousands of years,WWother climatic changes take place in much shorter timeperiods. The most obvious of these are seasons, which areshort-term periods of climatic change caused by regularvariations in daylight, temperature, and weather patterns.These variations are the result of changes in the amount ofsolar radiation an area receives. As Figure 14-11 FF shows, thetilt of Earth on its axis as it revolves around the Sun causesdifferent areas of Earth to receive different amounts ofsolar radiation. During summer in the northern hemi-


The effects of hot, moist upper air spread farther east, bringingstormy weather to areas that are normally dry and drought condi-tions to areas that are normally wet. The end result is extensive prop-erty damage and untold human suffering. This is especially true intropical regions, where the effects of El Nino are most pronounced.El Nino does have one positive effect—the strong upper winds it pro-duces keep tropical disturbances from increasing to hurricane-strength storms in the Atlantic Ocean. This results in fewerhurricanes in that region for the duration of El Nino. Eventually, theSouth Pacific high-pressure system becomes reestablished and ElNino weakens, but not before it causes the climatic effects shown inFigure 14-12. The warm water moves back across the Pacific Ocean,and conditions along the South American coast cool off.

CHANGE CAN BE NATURALMuch discussion has taken place in recent years about whether Earth’sclimate is changing as a result of human activities. We’ll discuss thisin the next section. For now, it’s important to note that climaticchanges occurred long before humans came on the scene. Studies oftree rings, ice-core samples, fossils, and radiocarbon samples provideevidence of past climatic changes. These changes in Earth’s climatewere caused by natural events such as variations in solar activity,changes in Earth’s tilt and orbit, and volcanic eruptions.

Figure 14-12 During El Niño, some areasof the world experience extreme droughtswhile other areas are ravaged by heavyfloods.

CHANGE CAN BE NATURALAA

Much discussion has taken place in recent years about whether Earth’sclimate is changing as a result of human activities. We’ll discuss thisin the next section. For now, it’s important to note that climaticchanges occurred long before humans came on the scene. Studies oftree rings, ice-core samples, fossils, and radiocarbon samples provideevidence of past climatic changes. These changes in Earth’s climatewere 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 activityand Earth’s climate was provided by English astronomer E. W.Maunder in 1893. The existence of sunspot cycles lasting approxi-mately 11 years had been recognized since the days of Galileo.However, Maunder found that from 1645 to 1716, sunspot activitywas scarce to nonexistent. This period of very low sunspot activity,called the Maunder minimum, closely corresponds to an unusuallycold climatic episode called the “Little Ice Age.” During this time,much of Europe experienced bitterly cold winters and below-normaltemperatures year-round. Residents of London are said to have ice-skated on the Thames River in June. The relationship between climate

and periods of low sunspot activity is illus-trated in Figure 14-13. Studies indicate thatincreased solar activity coincides withwarmer-than-normal climates, while peri-ods of low solar activity, such as theMaunder minimum, coincide with cold cli-matic conditions.

Earth’s Orbit Climatic changes mayalso be triggered by changes in Earth’s axisand orbit. The shape of Earth’s ellipticalorbit appears to change, becoming moreelliptical, then more circular, over thecourse of a 100 000-year cycle. As Figure14-14 shows, when the orbit elongates,Earth passes closer to the Sun, and tem-peratures become warmer than normal.When the orbit is more circular, Earth isfarther from the Sun and temperatures dipbelow average. The amount of radiation

1450

1500

Year

1550

1600

1650

1700

1750

1800

1850

Maunderminimum

Warm

Cold

Little Ice Age

1900

Severity of wintersin London and Paris

Sunspot number

Climate and Sunspots

Figure 14-14 Scientiststheorize that a more ellipti-cal orbit around the Suncould produce significantchanges in Earth’s climate.

Figure 14-13 Very fewsunspots occurred duringthe Maunder minimum, andtemperatures were lowerthan normal. Thus, scientiststheorize solar activity may belinked to climatic changes.

Earth

Sun

Circular orbit

Elliptical orbit


Earth receives when its orbit elongates is much higher than when itsorbit is more circular.

As you know, seasons are caused by the tilt of Earth’s axis. At pres-ent, the angle of the tilt is 23.5°. However, the angle of tilt varies froma minimum of 22.1° to a maximum of 24.5° every 41 000 years.Scientists theorize that these changes in angle cause seasons to becomemore severe. For example, a decrease in the angle of the tilted axis, asshown in Figure 14-15,might cause a decrease in the temperature dif-ference between winter and summer. Winters would be warmer andwetter, and summers would be cooler. The additional snow in lati-tudes near the poles would not melt in summer because tempera-tures would be cooler than average. This could result in expandedglacial coverage. In fact, some scientists hypothesize that changes inthe angle of Earth’s tilted axis cause ice ages.

Earth’s Wobble Another movement of Earth may be responsiblefor climatic changes. Over a period of about 26 000 years, Earth wob-bles as it spins on its axis. Currently, the axis points toward the NorthStar, Polaris, as shown in Figure 14-16. Because of Earth’s wobbling,however, the axis will tilt toward another star, Vega, by about the year14 000. Currently, winter occurs in the northern hemisphere whenEarth is closest to the Sun, and summer occurs when Earth is farthestfrom the Sun. When the axis tilts toward Vega, however, winter willoccur in the northern hemisphere when Earth is farthest from theSun, and summer will occur when Earth is closest to the Sun. This willcause warmer summers and colder winters than those that we nowexperience.

Figure 14-15 If the angle of tilt of Earth’s axisdecreased, there would beless temperature contrastbetween summer and winter.

Decreased tilt

Axis with reduced angle

Equator

Sunlight

Sun

Earth

Existingaxis

PolarisVega

23.5˚

Earth

Figure 14-16 By about theyear 14 000, Earth’s axis willpoint toward the star, Vega.Winter will then occur inthe northern hemispherewhen Earth is farthest fromthe Sun, causing winters tobe colder.


Volcanic Activity Climatic changescan also be triggered by the immensequantities of dust released into theatmosphere during major volcaniceruptions, shown in Figure 14-17.Volcanic dust can remain suspended inthe atmosphere for several years,blocking incoming solar radiation andthus lowering global temperatures.Some scientists theorize that periods ofhigh volcanic activity cause cool cli-matic periods. This theory is supportedby records over the past centurybecause several large eruptions havebeen followed by below-normal globaltemperatures. For instance, the ashreleased during the 1991 eruption ofMt. Pinatubo in the Philippines

resulted in slightly cooler temperatures around the world the follow-ing year. Generally, volcanic eruptions appear to have only short-term effects on climate. These effects, as well as the others you’ve readabout thus far, are a result of natural causes. In the next section,you’ll learn about climatic changes caused by human activities.

Figure 14-17 The dust and gases released by thisvolcanic eruption in NewGuinea blocked incomingsolar radiation and affectedglobal climates.

1. What three changes in Earth’s movementin space might result in long-term climatic changes?

2. What are seasons? What causes them?

3. Explain how El Nino might affect weatherin California and along the Gulf Coast.

4. Why are the greatest effects of El Ninoexperienced mainly in the tropics?

5. How does volcanic activity affect climate?Are these effects examples of short-termor long-term climatic change?

6. Thinking Critically What might be theeffect on seasons if Earth’s orbit becamemore elliptical and, at the same time, theangle of the tilt of Earth’s axis increased?

SKILL REVIEW

7. Concept Mapping Use the followingphrases to complete a concept map of theeffects of El Nino. For more help, refer tothe Skill Handbook.

1.upper-level

winds transportheated air across

the tropics

2.precipitation

increases

4.convection

currents strengthen

5.trade winds

weaken

7.high-pressure

system becomesreestablished

6.warm watersurges east-

ward, heating surface air

3.jet stream

shifts south


Volcanic Activity Climatic changescan also be triggered by the immensequantities of dust released into theatmosphere during major volcaniceruptions, shown in Volcanic dust can remain suspended inVVthe atmosphere for several years,blocking incoming solar radiation andthus lowering global temperatures.Some scientists theorize that periods ofhigh volcanic activity cause cool cli-matic periods. This theory is supportedby records over the past centurybecause several large eruptions havebeen followed by below-normal globaltemperatures. For instance, the ashreleased during the 1991 eruption ofMt. Pinatubo in the Philippines

resulted in slightly cooler temperatures around the world the follow-ing year. Generally, volcanic eruptions appear to have only short-term effects on climate. These effects, as well as the others you’ve readabout thus far, are a result of natural causes. In the next section,you’ll learn about climatic changes caused by human activities.

14.4 The Human Factor 375

Environmental ConnectionEnvironmental Connection

14.414.4

One of the most significant influences on Earth’s climate is theatmosphere. As you learned in Chapter 11, solar radiation that is notreflected by clouds passes freely through the atmosphere. It’s thenabsorbed by Earth’s surface and released as long-wavelength radia-tion. This radiation is absorbed by atmospheric gases such as watervapor, methane, and carbon dioxide. The atmospheric gases then reradiate the stored energy, so that Earth receives energy from twosources: the Sun and the atmosphere.

THE GREENHOUSE EFFECTThe retention of heat by the atmosphere results in the greenhouseeffect, which is the natural heating of Earth’s surface caused by cer-tain atmospheric gases called greenhouse gases. Without the green-house effect, which is illustrated in Figure 14-18, life as we know itcould not exist on Earth. Our planet would be cold, like Mars, whichhas an extremely thin atmosphere and surface temperatures that dipto –90°C. On the other hand, a marked increase in the greenhouseeffect might cause our planet to be hot, like Venus, which, because of

The Human Factor

OBJECTIVES

• Compare and contrastthe greenhouse effect andglobal warming.

• Identify how humansimpact climate.

VOCABULARY

greenhouse effectglobal warming

Figure 14-18 Solar radia-tion reaches Earth’s surfaceand is reradiated as long-wavelength radiation. Thisradiation cannot escapethrough the atmosphere,and is absorbed and re-released by atmosphericgases. This process is calledthe greenhouse effectbecause it is similar to theway that heat is trapped andreleased in a greenhouse.

THE GREENHOUSE EFFECTThe retention of heat by the atmosphere results in the greenhouseeffect, which is the natural heating of Earth’s surface caused by cer-tain atmospheric gases called greenhouse gases. Without the green-house effect, which is illustrated in Figure 14-18,FF life as we know itcould not exist on Earth. Our planet would be cold, like Mars, whichhas an extremely thin atmosphere and surface temperatures that dipto –90°C. On the other hand, a marked increase in the greenhouseeffect might cause our planet to be hot, like Venus, which, because of


its thick atmosphere, has surface tempera-tures of 470°C. You’ll model the greenhouseeffect in the MiniLab on this page.

Scientists theorize that it is possible toincrease or decrease the greenhouse effectby changing the amount of atmosphericgreenhouse gases, particularly carbon diox-ide (CO2). Any increase in the amount ofthese gases would theoretically result in theincreased absorption of radiation. Levels ofatmospheric carbon dioxide are increasing.This in turn can lead to a rise in global tem-peratures, known as global warming.

GLOBAL WARMINGTemperatures worldwide have indeed shownan upward trend over the past 200 years,with several of the warmest years on recordhaving occurred within the last two decades.If the trend continues, polar ice caps mightmelt, sea level might rise and flood coastalcities, deserts could spread into fertileregions, and the frequency and severity ofstorms could increase.

Based on available evidence, many scien-tists agree that global warming is occurring.They disagree, however, about what is caus-ing this warming. As you’ve learned, naturalcycles of Earth and the Sun can affect cli-mate. Some scientists hypothesize that thesenatural changes adequately explain theincreased temperatures. Mounting evidenceindicates, however, that the warming trendis a result of increases in atmospheric car-bon dioxide. Global warming remains acontroversial issue. Neither viewpoint canbe proven or disproven conclusively; itmight very well be that there are several fac-tors involved. However, if increased carbondioxide is responsible, two logical questionsfollow: What is causing the increase? Cananything be done to stop it?

How does the atmosphere affectthe transfer of energy?

Model the greenhouse effect.

Procedure

1. On a clear day, place a cardboard box

outside in a shaded area. Prop two

thermometers against the box. Make sure

the thermometers are not in direct sun.

2. Cover one thermometer with a clean

glass jar.

3. Observe and record the temperature

changes of each thermometer every two

minutes over a 30-minute period.

Analyze and Conclude

1.Make a graph showing how the tempera-

tures of the two thermometers changed

over time.

2. Based on your graph, which thermometer

experienced the greatest increase in

temperature? Why?

3. Relate your observations to the green-

house effect in the atmosphere.

its thick atmosphere, has surface tempera-tures of 470°C. You’ll model the greenhouse

Scientists theorize that it is possible toincrease or decrease the greenhouse effectby changing the amount of atmosphericgreenhouse gases, particularly carbon diox-ide (COide (CO2). Any increase in the amount of). Any increase in the amount ofthese gases would theoretically result in theincreased absorption of radiation. Levels ofatmospheric carbon dioxide are increasing.This in turn can lead to a rise in global tem-peratures, known as global warming.

Temperatures worldwide have indeed shownan upward trend over the past 200 years,with several of the warmest years on recordhaving occurred within the last two decades.If the trend continues, polar ice caps mightmelt, sea level might rise and flood coastalcities, deserts could spread into fertileregions, and the frequency and severity ofstorms could increase.

Based on available evidence, many scien-tists agree that global warming is occurring.They disagree, however, about what is caus-ing this warming. As you’ve learned, naturalcycles of Earth and the Sun can affect cli-mate. Some scientists hypothesize that thesenatural changes adequately explain theincreased temperatures. Mounting evidenceindicates, however, that the warming trendis a result of increases in atmospheric car-bon dioxide. Global warming remains a

ntroversial issue. Neither viewpoint canbe proven or disproven conclusively; itmight very well be that there are several fac-tors involved. However, if increased carbondioxide is responsible, two logical questionsfollow: What is causing the increase? Cananything be done to stop it?

ddiscoveryiscovery llabab model cloud cover - earth...

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