Earth Science Introduction - SERCserc.carleton.edu/files/eet/globe/EarthSysInt.pdfGLOBE® 2003...

GLOBE® 2003 Introduction - 1 Earth System Science

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Introduction

Why Study Earth SystemScience?Perceiving Earth as a system begins when we firstfeel warmth from sunshine or get wet standing inthe rain. Understanding Earth as a system – EarthSystem Science – requires a quantitative explora-tion of the connections among all parts (atmo-sphere, hydrosphere, lithosphere, and biosphere)of the system. The measurements of the GLOBEProgram provide students with the means to be-gin this exploration for themselves.

The processes comprising the global environmentare interconnected. Many of the major environ-mental issues of our time have driven scientiststo study how these connections operate on a glo-bal basis – to understand the Earth as a system.

Studies of the stratospheric ozone layer involvequestions about the processes which create anddestroy ozone. Scientists have learned that ozone,a chemical primarily found in a layer centeredabout 25 km above Earth’s surface, is connectedto biological activity happening below Earth’ssurface. Different chemicals, present in the air intrace amounts, control the abundance of ozonein the atmosphere. The sources of these trace con-stituents include microorganisms in the soil andwater, land plants, and even some animals.

Scientists studying climate changes are also in-terested in the connections between the differentEarth processes. Some of the trace gases in theatmosphere make it more difficult for heat (in-frared radiation) to escape from the Earth’s sur-face to space. The amounts of these greenhousegases found in the atmosphere are tied to thephysical, chemical, and biological processes tak-ing place in soil and water and on land. They arealso influenced by the circulation of the oceansand atmosphere. To predict the future course ofthe climate we need to understand this detailedfabric of connections.

Ecologists study the way in which the living andnon-living components of an ecosystem interact.

Individual organisms and species compete and co-operate with one another. In some cases, interde-pendence is so strong that different plants andanimals cannot reproduce or even exist withouteach other. There is a web of life with extensiverecycling of nutrients, and each organism plays arole. If one component of the ecosystem is changedthe effects ripple through the system.

Scientists do not know all the Earth system con-nections yet, but they keep working to gain a morecomplete understanding. GLOBE students canhelp through data collection and student research.GLOBE students and scientists working togetherwill improve our understanding of the Earth sys-tem. As students conduct the full range of GLOBEmeasurements (perhaps spread over several schoolyears in multiple grades), they should gain a per-ception that the environment is the result of aninterplay among many processes that take placelocally, regionally, and globally on time scales rang-ing from seconds to centuries. This is a key GLOBElesson. The learning activities in this chapter helpstudents learn this as they study annual variationsin environmental parameters (the Seasons and Phe-nology section) and examine the connectionsamong the various phenomena measured inGLOBE on local, regional, and global spatial scales(the Exploring the Connections section).

In addition to learning activities, there are phe-nology protocols within the Seasons and Phenol-ogy section. Phenology is the study of livingorganisms’ response to seasonal changes in theirenvironment. Change in the period betweengreen-up and senescence, often synonymous withthe growing season, may be an indication of glo-bal climate change. Broad-area estimates of thelengths of growing seasons are primarily basedon satellite data. However, remote sensing esti-mates from satellites are not exact because theactual behavior of the plants must be inferred fromthe collective appearance of their foliage. GLOBEstudent observations, the only global network ofground-based plant phenology observations, willhelp scientists validate their estimates of global

Earth Science


greenness values that they derive using satellitedata. Monitoring the length of the growing sea-son is important for society so that it can betteradapt to variations in the length of the growingseason and to other impacts of climate change,which may affect food production, economicgrowth, and human health.

The Big PictureThe planet we call Earth is made up of five‘spheres’, the atmosphere, hydrosphere, lithos-phere, cryosphere, and biosphere, connected toeach other in a complex web of processes. SeeFigure EA-I-1. The atmosphere consists of thegases and particles suspended in the air. Theoceans, inland water bodies, ground water, andice sheets (cryosphere), comprise the hydrosphere.The lithosphere refers to the solid earth; the core,mantle, crust, and soil layers (pedosphere). Theplaces on Earth where organisms live are collec-tively known as the biosphere. Instead of focus-ing on the individual parts of the Earth, Earthsystem scientists use chemistry, biology, and phys-ics to study the cycles that connect these sphereswith each other and with the energy from the sun,which ultimately drives almost all of theseprocesses.

The major cycles that connect the different partsof the Earth are the energy cycle (see Figure EA-I-2), the water cycle (hydrologic cycle, see FigureEA-I-3), and the cycles of important individualelements (e.g., carbon, nitrogen, see Figure EA-I-4). Each cycle is made up of reservoirs, placeswhere energy, water, and elements are stored for aperiod of time (e.g., chemical energy, sea ice,oceans, carbon dioxide), fluxes, the movement ofmatter from one reservoir to another (e.g., pre-cipitation, transpiration, ocean currents, wind,river flow) and processes that change the form ofenergy, water, and elements (e.g., photosynthesis,condensation, fire). Every GLOBE measurementis designed to help Earth System scientists in theirgoal of determining the sizes of Earth’s reservoirsand the rate of fluxes into and out of thesereservoirs.

Energy from the sun flows through the environ-ment, heating the atmosphere, the oceans, andthe land surface, and fueling most of the bio-sphere. See Figure EA-I-2. Differences in theamount of energy absorbed in different places setthe atmosphere and oceans in motion and helpdetermine their overall temperature and chemi-cal structure. These motions, such as wind pat-terns and ocean currents redistribute energythroughout the environment. Eventually the en-ergy that began as sunshine (short-wave radia-tion) leaves the planet as Earth shine (lightreflected by the atmosphere and surface back intospace) and infrared radiation (heat, also calledlong wave radiation) emitted by all parts of theplanet which reaches the top of the atmosphere.This flow of energy from the sun, through theenvironment, and back into space is a major con-nection in the Earth system; it defines Earth’sclimate.

Water and chemical elements are cycled throughthe environment. Water melts, evaporates, con-denses, and freezes, and is moved from place toplace in the atmosphere, the oceans, across theland surface, and through soil and rocks. See Fig-ure EA-I-3. Each of the chemical elements un-dergoes chemical reactions, but the total amountof each on Earth remains essentially fixed. In thisway, the environment consists of a set of cyclesfor water, carbon, nitrogen, phosphorous, etc.Since the cycles of the elements involve life,chemicals, and the solid Earth, they are collec-tively known as biogeochemical cycles. Figure EA-I-4 shows one of these, the carbon cycle.


1216km

2270km

2885km

Crust8 - 40 km

Crust

Ocean(Hydrosphere)

4 km

Atmosphere

Soil (Pedosphere)

2 m

Ice 1 km

Atmosphere480 km

InnerCore

OuterCore

Mantle

Lithosphere

Thermosphere

Mesosphere

Stratosphere

Troposphere

Atm

osph

ere

480

km

8 - 1

5 km

50 k

m80 k

m

Figure EA-I-1: Schematic Diagram of the Earth System from the Center of the Earth to 480 km up into the Atmosphere


Absorbed by clouds

Solar Energy absorbed at surface

51%Total energy lost by surface

51%

Absorbed by atmosphere

Infrared radiation

from surface

Reflected fromEarth’s surface

4%

Earth’s Energy Budget

Reflected by clouds

20%

Reflected by atmosphere

6%

Incomingsolar energy

100%

Radiated to space from clouds and atmosphere

64%

Surface to air and clouds

through conduction

and convection7%

Infrared radiation

direct from Earth’s surface

6%

3%

16%

Infrared radiation

from surface

absorbed by atmosphere

15%

Surface to air and clouds by latent heat in

water vapor through

evaporation23%

Air

Land and Oceans

Figure EA-I-2: Schematic Diagram of the Earth’s Energy Budget


Gla

cier

LandPercolation

Ground Water

Snow and Ice

Rain

Surface Runoff Precipitation Evaporation

Evaporation

Transpiration

RiverSoils

Ocean

Surface Water

Sun

Cloud

Atmospheric Water

Precipitation

Lake

Figure EA-I-3: The Hydrologic Cycle

Air

Volcanic Eruption

Volcano

LandOcean

Life on Surface

and in SoilLand Use Changes

Fossil Fuel andCement Production

Respiration

Respiration

Fossil Fuels (coal & oil)

Photosynthesis

Photosynthesis

Sediments

SurfaceExchange

Plant Lifein Ocean

Animal Lifein Ocean

Organic and Inorganic Soils

Ground Water

Gases

Figure EA-I-4: The Carbon Cycle


Components of the Earth SystemThe GLOBE program has students take measure-ments of many parts of the Earth’s systems. Thetable below indicates where the GLOBE investiga-tions lie with the components of the Earth system.

Components ofthe Earth System GLOBE Investigations

Atmosphere (Air) Atmosphere Investigation

Oceans and Fresh Hydrology Investigationwater bodies

Cryosphere (ice) Atmosphere Investigation(solid precipitation)

Hydrology Investigation(frozen water sites)

Soil Soil Investigation

Terrestrial (land) Land Cover Investigationvegetation Earth as a System

Phenology Investigation

Cycles of the Earth SystemIn the environment, energy can be in the form ofradiation (solar or short-wave radiation and in-frared or long-wave radiation), sensible heat (ther-mal energy), latent heat (heat released when watergoes from the gas to the liquid or solid state), ki-netic energy (energy of motion including winds,tides, and ocean currents), potential energy (storedenergy), and chemical energy (energy absorbedor released during chemical reactions). Scientistswant to know, model and predict the amount ofenergy in all of its forms in each component ofthe Earth system, how it is exchanged among thecomponents, and how it is moved from place toplace within each of the components.

The energy cycle is intertwined with the hydro-logic cycle. Some of the energy in the sunlightreaching Earth’s surface causes evaporation fromsurface water and soils. The atmosphere trans-ports the resulting water vapor until it condensesin clouds, releasing the latent energy that evapo-rated the water. Water droplets and ice particlesin clouds grow in size until they form precipita-tion, falling to the surface as rain, snow, sleet, or

hail. Once the precipitation falls, the water canremain frozen on the surface to melt at a latertime, evaporate again into the atmosphere, fillspaces in the soil, be taken up by plants, be con-sumed by animals, leach through the soil intogroundwater, run off the land surface into rivers,streams, lakes and ultimately into the oceans orbecome part of a surface water body. Snow andice reflect more sunlight back to space than oceanwater or most other types of land cover, so theamount of snow or ice covering Earth’s surfaceaffects the energy cycle.

Together, the combined energy and hydrologiccycles affect the biogeochemical cycles. In the at-mosphere, chemical reactions driven by sunlightcreate and destroy a rich mixture of chemicalsincluding ozone. Some of these chemicals com-bine with water to form aerosols –liquid and solidparticles suspended in the air. Atmosphericchemicals and aerosols become incorporated inwater droplets and ice crystals and are carriedfrom the atmosphere to the surface by precipita-tion. Microorganisms in the soil and surface wa-ters, plants, and animals all take in chemicals fromthe air and water around them and release otherchemicals into the atmosphere, fresh water bod-ies, and oceans. Winds enhance evaporation ofwater from the surface and blow fine grain par-ticles into the air where they are suspended asaerosols. Agricultural and industrial activities alsoinput and remove energy, water, gases, and par-ticles from surface waters, soil, rocks, and air. Thequantity and distribution of gases such as watervapor, carbon dioxide, nitrous oxide (N

2O), and

methane in the atmosphere determine how in-frared radiation is absorbed and transmitted be-tween Earth’s surface and space. This in turnaffects the temperature at the surface and through-out the atmosphere. There are many other waysin which the energy, water, and biogeochemicalcycles interact and influence our environment,far more than can be described here.

How GLOBE Measurements Contributeto Earth System StudiesGLOBE measurements of the temperature of air,water bodies, and soil help track the energy cycle.GLOBE students also measure cloud cover, cloud


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type, aerosols, water transparency, and land cover.Each of these observations helps scientists deter-mine what happens to the solar radiation (sun-light) and the thermal infrared radiationoriginating on Earth (heat). How much sunlightis reflected or absorbed by clouds or Earth’s sur-face? How much out-going infrared radiation isabsorbed by the atmosphere and how much is re-radiated back downward?

GLOBE measurements of liquid and solid precipi-tation, relative humidity, soil moisture, land cover,and canopy and ground cover and the identifica-tion of the dominant and codominant species oftrees help track the hydrologic cycle. Knowing thecharacteristics of the top meter of soil and its in-filtration properties enables scientists to calculatehow water will pass into and through the soil; soilbulk density and particle density determine howmuch water can be stored in the soil. Measure-ments of the surface temperature of a water bodyand of soil moisture and temperature enable esti-mation of evaporation rates. How much rain fallson Earth? Is the hydrologic cycle becoming moreintense? Are the various fluxes in the hydrologiccycle increasing?

GLOBE observations contribute to the study ofthe biogeochemical cycles. Measurements of thepH of precipitation, soil horizons, and surfacewaters are fundamental because pH influenceshow different chemical elements interact withwater flowing through the environment. Lower-ing pH can mobilize different chemicals from thesurfaces of rocks and soil particles. Living plantsare a significant reservoir in the carbon cycle.Measurements of the mass of dried grasses andthe circumference and height of trees enable esti-mation of how much carbon is stored in the liv-ing biomass of a forest or grassland. As carbon isadded to the atmosphere, how much is taken outby terrestrial vegetation?

Open versus Closed SystemsIf you look at Earth from outer space, the Earth isan almost closed system. A closed system is one inwhich no matter enters or leaves. (An isolated sys-tem is one in which no matter or energy enters orleaves.) Other than the transfer of some gases and

particles entering Earth’s atmosphere, the com-ponents remain on Earth without new additions.When studying Earth as a whole, you usually donot need to consider the effects of inputs and out-puts to the Earth system except for the energyfrom the sun.

Smaller systems can be nested within larger sys-tems. For instance, you can study a watershed —the land area which all drains into a commonwater body. Watersheds come in a variety of sizeswith smaller ones combining to form larger ones.For example, you could study the entire areawhich drains into the Arctic Ocean, or focus onlyon the MacKenzie River basin in Canada, or onjust the Liard River, a tributary of the MacKenzie.Where you define the boundaries of your system,as a watershed, depends on the questions beingasked. These concepts will be developed more inExploring the Connections.

Any system within the Earth system, such as awatershed, is considered an open system. Waterand chemicals as well as energy enter and leavethe boundaries of the system. Still, the compo-nents of this open system may be more closelyconnected to one another than they are to ex-changes between the system and its surroundings.The inputs and outputs may be important forunderstanding the dynamics of the system youare studying.

Scales of Space and TimeAll the processes of the Earth system occur onspecific space and time scales. Some occur on ascale so small that our eyes cannot see them, whileother phenomena cover an entire continent or thewhole planet. The time scales for different phe-nomena vary tremendously as well. Some atmo-spheric chemical reactions happen in fractions ofa second. The formation of soil with its interplayof physical, chemical, and biological characteris-tics happens locally over many years (generallyat a rate of 1 cm of depth per century). Majorweather systems including hurricanes usuallydevelop and dissipate on time scales of one totwo weeks and cover hundreds of kilometers.

Parts of the various cycles of the Earth system canbe measured and understood locally on relatively


short time scales, seconds to days; in other cases,one must try to characterize the whole globe fordecades to test theories, understand processes, andgain overall knowledge. Let’s consider one ex-ample of each situation:

1. The balance in the amount and flow ofwater in a small watershed.We can sample the input of water to thesurface by measuring precipitation at oneor more sites (the more sites, the better theestimate will be). The evaporation of watercan be calculated from temperaturemeasurements of the surface soil and waterand knowledge of the surface soil moistureand particle size distribution or texture.The transpiration of water by trees andother plants can be estimated by mappingthe land cover, measuring canopy andground cover at a number of sites, andidentifying the dominant species of treesin the forests and woodlands.Measurements of soil moisture and thelevels of streams, lakes, and rivers tell howmuch water is stored in the watershed(discounting aquifers or other majorunderground water bodies). The level ofthe stream or river through which waterflows out of the watershed is an indicationof how fast this flow is. The inputs andoutputs must balance with the change inthe amount of water stored. Most of theneeded measurements are included in theGLOBE protocols and the others can oftenbe obtained from other sources ormeasured with help from local scientists.

2. Understanding the El Niño/SouthernOscillation (ENSO)The warm episodes of the ENSO occur atirregular intervals of two to seven years.Changes develop across the entireequatorial Pacific basin and effects havebeen observed developing as much as sixmonths later throughout the temperatezones of both hemispheres. Small remnantphenomena from warm events have beenobserved by satellites as much as ten yearslater. To thoroughly characterize thisphenomenon and its effects we must take

data for many years on a global scale andlook for connections, causes, andconsequences. Predictions based on anoverall understanding of the ENSO can beexamined locally using data recordscovering many months including the datasets collected and reported as part ofGLOBE. GLOBE student data of airtemperature and precipitation can becompared with model predictions ofENSO effects to help determine theadequacy of our current understandingand modeling abilities.

Key ConceptsAs discussed in the previous pages, when study-ing Earth as a system, there are a few key con-cepts to understand. These are:

• The Earth is a system made up ofcomponents.

• Energy, water, and the chemical elementsare stored in various places and forms andare transported and transformed byvarious processes and cycles.

• Connections among phenomena can betraced through the energy, hydrologic andbiogeochemical cycles.

• Phenomena happen on a range of timeand spatial scales.

Atmosphere

Lithosphere/Pedosphere

Hydrosphere

Biosphere

Four Major Components of the Earth System

Note: See Diagramming Earth as a System in Ex-ploring the Connections Introduction.


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The Earth as a SystemThe Seasonal Cycle

The Seasonal Picture: Why are thereseasons?Earth’s seasons change in a regular fashion andbring a rhythm to our lives. Whether it is the ar-rival of winter snows, monsoon rains, or summerheat, our environment changes constantly, andthese profound changes occur over relatively shorttime periods. That they recur in predictable wayshelps make such huge, complex changes compre-hensible. Many ancient civilizations observed thatthe Sun’s position in the sky changed throughoutthe year and were able to construct calendars andmake predictions based on their observations,which they used for agricultural and religiouspurposes.

All seasonal changes are driven by shifts in theintensity of sunlight reaching Earth’s surface (in-solation). More energy per unit area leads to highertemperatures, which results in more evaporation,which produces more rain, which starts plantsgrowing. This sequence describes Spring for manymid-latitude climates. Since visible light is themain form of solar energy reaching Earth, daylength is a reasonably accurate way to gauge thelevel of insolation and has long been used as away to understand when one season ends and thenext one begins. The first day of summer, (sum-mer solstice) is the longest day of the year. Winterstarts on the shortest day of the year, (winter sol-stice). The first days of spring and fall are whenthe day and night are of equal length — roughly12 hours each. These days are named vernal andautumnal equinoxes.

The changing day length results from the Earth’saxis of rotation being inclined 23.5˚ with respectto the plane of its orbit around the sun. FigureEA-I-5 shows the inclined Earth at different posi-tions in its orbit. Notice how at the solstice posi-tions, each pole is tilted either toward or away

from the Sun. The pole inclined toward the Sunreceives 24 hours of sunlight, and the one inclinedaway is in Earth’s shadow and experiences 24hours of darkness. At the equinox positions, Earthis inclined in a way so that each pole receives equalamounts of insolation. This discussion focuses onthe poles because they experience the greatestextremes of insolation. Because of the inclinationof Earth’s axis, insolation levels at every point onEarth change constantly. We call the aggregateeffects of these changing levels seasons.

Figure EA-I-5: Tilt of the Earth’s Axis

The tilt in Earth’s axis of rotation has an addi-tional effect, which amplifies the length of dayeffect. At every latitude, the Earth’s surface is at adifferent angle with respect to the incoming sun-light. Look at Figure EA-I-6. When the surface isperpendicular to the sunlight, the sun is straightoverhead, and the amount of sunlight striking afixed area is at its maximum. As the sun moveslower in the sky and the angle at which sunlightstrikes the ground decreases, the intensity of sun-light striking the same area gets smaller. In thesummer, the sun is closer to being straight over-head at local solar noon than in the winter exceptclose to the equator. So, not only is the day longerin summer than in winter, but the sun deliversmore energy to each unit of area of Earth’s surfacein the hemisphere where it is summer.

23.5o

JuneSolstice

SeptemberEquinox

Sun

DecemberSolstice

MarchEquinox


How Latitude Influences the Amountof Energy per Unit of Surface Area

RadiantEnergyfrom the Sun

a.

b.

c.

Factors Influencing Local SeasonalPatternsLatitude

Figure EA-I-7 shows how insolation levels varywith latitude throughout the year. Because of thisvariation, latitude has a powerful influence indetermining seasonal conditions and the annualpatterns of environmental and climatic parameterssuch as precipitation and temperature. Becauseof the differences in the duration and directnessof insolation, the world can be divided into thezones shown in Figure EA-I-8. The same seasoncan be quite different in the Tropical, Temperateand Polar zones.

Continental and Marine Climates

Climate also varies dramatically depending on theamount of water in the environment. When sun-light strikes the surface of water, four things keepthe water surface from warming as much as theland surface. First, the specific heat or the energy

Figure EA-I-6: How Latitude Affects the Amount ofIncoming Energy from the Sun

Figure EA-I-8: Approximate Global Climate Zones

Global Climate Zones

Equator 0˚

60˚ S

60˚ N

30˚ N

30˚ S TemperateZone

TropicalZone

TemperateZone

Polar Zone

Polar Zone

it takes to heat one gram of water one K is 1 cal g-1

K-1 compared to 0.4 cal g-1 K-1 for soil. It thereforetakes 2.5 times the energy to heat water by 1K thanit takes to heat soil 1K. Second, some of the sun-light penetrates many meters into the water col-umn. This spreads the incoming energy down intothe water body and the surface is less warmed.Also, colder water from lower depths mixes tosome extent with the surface water and moder-ates its temperature changes. Third, winds pro-duce movement in the surface waters which causesa mixing of heat throughout the surface layer.Fourth, as surface water warms, evaporation in-creases. Evaporation cools the surface and so thetemperature of the water surface responds less tosolar heating than the land surface. Land whichis near large bodies of water that do not freeze inwinter has a marine climate. This features largeramounts of moisture and smaller temperaturechanges from summer to winter than a continentalclimate. The size of a continent affects both thetemperature range and the amount of moisturein the interior – the larger the continent, the fur-ther away the ocean and the larger the differencebetween summer and winter.

Wind Direction

The direction of the prevailing winds also affectslocal climate. If an area is downwind of the ocean(the west coasts of continents in mid-latitudes)the climate is strongly affected by the presence ofthe ocean as described above. If the winds areblowing from the interior of the continent, thenthey tend to be dry and to bring with them thelarger contrasts in summer and winter tempera-

Continental Marine

Mar

ine

Marine

Examples of Continental and MarineClimates

Figure EA-I-9: Continental and Marine Climates


Figure EA-I-7: Incoming Solar Radiation Throughout the Year

January Solar Radiation July Solar Radiation

March Solar Radiation

May Solar Radiation

September Solar Radiation

November Solar Radiation


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tures. Areas in the high latitude parts of the tem-perate zones and downwind of lakes receive largeamounts of lake-effect snow while the lakes areunfrozen. Generally, prevailing winds connect thelocal climate with that upwind. Seasonal changesin prevailing wind direction can make seasonalcontrasts greater or smaller.

Geographical Features

Geographical features have profound impacts onthe climate of nearby regions. For example, moun-tains can cause moist air to rise and precipitateout almost all of its moisture. When dry air de-scends behind the mountain, it lacks enoughmoisture to provide much precipitation. Themountains create a rain shadow. See Figure EA-I-10. Many deserts are found in such rain shad-

ows. In addition to arid land, typical desert re-gions lack the atmospheric moisture that acts asinsulation between the Earth’s surface and space(water is the major greenhouse gas on Earth). Con-sequently, desert areas easily radiate their heat en-ergy out to space, and day and night temperaturedifferences are considerable.

Elevation also influences seasonal patterns.Changes in elevation can affect the environmentas much as changes in latitude. Average air tem-perature falls approximately 1˚ C for every 150meter increase in elevation, and, in terms of grow-ing season, every 300 m increase in elevation isroughly equivalent to moving poleward by 400-500 km (roughly four to five degrees of latitude).Mountain tops can be thought of as climatic is-

Figure EA-I-10: Mountain Producing a Rain Shadow Effect

Rain Shadow

Rainy

win

dwar

d slo

pe Dry lee slope

Prevailingwind

Moist, warmair rises

Ocean

Moisturecondensesas air cools

Air depleted of moisture sinksand warms, becoming dryer

Rain Shadow

Elevation(m)

Temperature (°C)11 am May 23, 1997

Biome

The Impact of Elevation

500

1,000

1,500

1,935

8.0

3.5

-1.5

- 6.0

DeciduousForest

Mt. WashingtonNew Hampshire, USALat. 44° N Long. 71° W

ConiferousForest

Alpine

Arctic Tundra

Lowest Latitude Where Biome Typically

Occurs at Sea Level

29° N

44° N

52° N

55° N

Figure EA-I-11: Impact of Elevation on Climate Zone


lands where, in the Northern Hemisphere, north-ern species extend their ranges southward onmountains where conditions resemble those ofmore northern latitudes. Plants growing on thetop of New Hampshire’s Mt. Washington (1,935m) would feel right at home growing at sea levelin the Arctic tundra, 2,400 km to the north inCanada. See Figure EA-I-11.

Calcutta, India

Temperature

J F M A M J J A S O N D

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(C

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J F M A M J J A S O N D

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Berkeley, California

Temperature

Figure EA-I-12: Climatograms for Calcutta, India and Berkeley, California

Students can study each of these effects by look-ing at GLOBE school data. A climatogram showsthe monthly mean temperature and monthly to-tal water equivalent of the precipitation for thewhole year. Comparing these diagrams for schoolsin different areas (see Figure EA-I-12) makes thesedifferences clear and prompts questions about thereasons for these differences.


Figure EA-I-13: Global Surface Air Temperature in January and July, 1988.

January Air Temperature

July Air Temperature


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The Earth System throughthe Seasonal CycleIn GLOBE, the seasonal cycle plays a role in thetiming of some measurements. Examining GLOBEdata through the seasonal cycle can give you someunderstanding of how Earth works as a system.We can see this by examining some examples ofhow the seasonal cycle affects different compo-nents of the Earth system. The examples here mayprovide some background material to better un-derstand and interpret GLOBE data. These ex-amples indicate our current understanding andare based on previous studies. Many of the GLOBEdata will reveal some of these seasonal patterns.As well, GLOBE data will expand and refine ourunderstanding of seasonal patterns by examiningmany sites over a long period of time.

The Atmosphere through the SeasonalCycleTemperature

The relationship between air temperature and thenumber of daylight hours is a familiar seasonalchange to people in mid and high latitudes. The

35˚ North

35˚ South

Absorbed by land and oceans

51%

Carried to clouds and atmosphere by latent heat

and water vapor

Solar Radiation at the Top of the Atmosphere

J F M A M J

Month

J A S O N D

Wat

ts p

er m

eter

squ

ared

600

500

400

300

200

100

0

Figure EA-I-14: Seasonal Cycle of Solar Radiation at 35˚ N and 35˚ S

air in the lowest layer of the atmosphere iswarmed through its contact with Earth’s surface.During the summer (July in the northern hemi-sphere and January in the southern hemisphere),when the elevation of the sun is high, the moreconcentrated input of energy from the sun andthe increase in daylight hours warm the surfacewhich in turn warms the air. During the winter(January in the northern hemisphere and July inthe southern hemisphere), when the amount ofsolar radiation is spread over more surface areabecause the elevation of the sun is low and thereare fewer daylight hours, the sun warms the sur-face less, resulting in less heating of the air. Com-pare the distribution of solar radiation in Januaryand July (Figure EA-I-7) with the temperaturedistribution in January and July (Figure EA-I-13)respectively.

It takes time for Earth’s surface to warm and forthe atmosphere to fully respond to these changesin surface warmth. The time when the solar ra-diation is the strongest outside the tropics is inJune in the northern hemisphere and Decemberin the southern hemisphere. See Figure EA-I-14.This is when the solstices occur. However, gen-


erally temperatures are warmest about twomonths later, in August in the northern hemi-sphere and February in the southern hemisphere.See Figure EA-I-15. This is due to the amount oftime required to heat the upper layer of the oceansand the lower layer of the atmosphere.

Figure EA-I-15: Seasonal cycle of maximum surface air temperature at Kingsburg High School in the United States(located at about 35˚ N) and Shepparton High School in Australia (located at about 35˚ S)

Precipitation

At low latitudes, seasonal temperature changesare not as dramatic as in middle and high lati-tudes, but there is usually a definite seasonalchange in precipitation patterns. Equatorial re-gions often experience “wet” and “dry” seasons.


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Figure EA-I-16: Seasonal cycle of precipitation through the year at Kingburg High School in California USA, ReynoldsJr. Sr. High School in Pennsylvania USA, and Boa Amponsem Secondary School, Dunkwa, Ghana


The time of year at which these occur is depen-dent on many factors such as regional topogra-phy and proximity to large bodies of water.

Other localities show seasonal patterns in precipi-tation as well. See Figure EA-I-16. Some regionsreceive no precipitation for months at a time. Inother locations precipitation is evenly distributedthroughout the year. Some places have one rainyseason and one dry season, while others have twoof each during the year. The timing of rains withinthe year has a major effect on agriculture. Medi-terranean climates are characterized by winterrains while other regions experience only sum-mer rains.

Water Vapor and Relative Humidity

Since the saturation value for atmospheric watervapor is strongly influenced by temperature, boththe absolute concentration of water vapor and thedew point temperature have a strong seasonalcycle. The highest concentrations of water vaporand the highest dew points occur during summerand the lowest in winter. Relative humidity tendsto be highest during the rainy season. However, itcan be high even in the winter when the air isrelatively cold.

Clouds

In the tropics, a band of low pressure and cloudi-ness known as the Intertropical Convergence Zone(ITCZ) extends across the oceans. Global satelliteimagery shows clouds that extend across oceanicregions, where thunderstorms are active. The av-erage position of the ITCZ varies with the season,moving north in northern hemisphere summerand south in southern hemisphere summer. SeeFigure EA-I-17.

There are seasonal variations in clouds in otherregions. Generally, there is greater cloud cover dur-ing the rainy season when observed cloud typesare mostly nimbostratus and cumulonimbus. Dur-ing warmer months, cumulus type clouds are mostlikely to be observed in most locations due to theheating of Earth’s surface. During winter months,because there is less heating, stratus type cloudsare more often observed. Vigorous frontal systemsthat occur during the spring and summer months

at mid latitudes can, and often do, cause largethunderstorm clouds (cumulonimbus). Near theeastern coastlines, cooler water can bring stratustype clouds to the region year-round.

Aerosols

Aerosols are colloids consisting of liquid dropletsor solid particles dispersed throughout a gas. Fogand mist are examples of liquids dispersed in agas and smoke is an example of solid particlesdispersed in a gas. Aerosols affect the optical thick-ness of the atmosphere being greatest during sum-mer and least in winter. Other seasonal events canalso influence the amount of haze, especially duststorms, forest fires and agricultural activities.

Atmospheric Composition

Atmospheric trace gas concentrations also exhibitdistinct seasonal cycles. The longest record of atrace gas measurement is for carbon dioxide (CO

2)

and its seasonal cycle reflects the seasonality offorest growth. Lowest concentrations occur in thenorthern hemisphere spring and summer as thebiosphere uses CO

2 for photosynthesis. Concen-

trations increase during northern hemisphere au-tumn and winter as CO

2 is no longer taken up by

vegetation growth, and decay of leaves puts CO2

back into the atmosphere. This cycle is dominatedby the larger extent of terrestrial vegetation in thenorthern hemisphere. See Figure EA-I-18.

Another important trace gas is ozone, which ex-ists in the lower atmosphere as both a naturalcomponent, where its primary source is the strato-sphere, and as a pollutant, where it is formed as aresult of emissions from combustion sources. Atnorthern middle latitudes, surface ozone peaksin the summer when sunlight is most intense andphotochemical reactions happen most quickly,converting hydrocarbons and nitrogen oxides intoozone. At southern mid-latitudes, on the otherhand, summer concentrations of surface ozoneare lower because there are less emissions fromcombustion than in the Northern Hemisphere.In the tropics, surface ozone concentrations aregenerally highest in September and October be-cause this is the time when widespread biomassburning occurs and gases from these fires gener-ate ozone through photochemistry. Thus, the sea-


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Figure EA-I-19: Surface water temperature and dissolved O2 at Reynolds Jr. Sr. High School in 1998

Atmospheric Carbon Dioxide Concentration, Mauna Loa Observatory, Hawaii

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Figure EA-I-18: The seasonal variation of carbon dioxide (CO2) in the atmosphere from 1986 through 1988 measuredat Mauna Loa Hawaii


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sonal cycle of surface ozone concentrations is af-fected by human activity and is quite variable de-pending on where observations are made.

Surface Water through the SeasonalCycleThe physical and chemical characteristics of abody of water are influenced by the seasonal cyclethrough changes in solar radiation, precipitation,air temperature, wind patterns and snow and icemelting. Figure EA-I-19 shows how temperatureand dissolved oxygen (DO) varies throughout theyear. The saturation level of DO is inversely re-lated to temperature (i.e. as temperature increasesthe amount of DO that can be dissolved in waterdecreases). The observed pattern in any given wa-ter body depends on the amount of biological ac-tivity.

Seasonal Turnover in Lakes

Many lakes show seasonal patterns of verticalmixing. Lakes in either warm temperate or coldtemperate zones show one mixing event (or turn-over) in the year. In other temperate regions thatbridge temperatures of cold and warm temperatezones or at high elevations in subtropical regions,there are two turnovers. The spring turnover oc-curs after ice melts. Ice floats because it is lessdense than water, which is most dense at 4˚C. Aswater warms to near 4˚C, the surface water maybecome more dense than bottom water and sink.Relatively little wind energy is required to mixthe whole lake (spring turnover). As springprogresses, the top layers of the lake becomewarmer and thus less dense. The colder, moredense water remains on the bottom, and a zoneof rapid temperature change occurs between thewarmer layer on the top and the colder layer onthe bottom. This is known as thermal stratifica-tion. In the fall, with less solar radiation reachingthe water and greater heat loss from the surfaceat night, the temperature stratification breaksdown. Eventually the mixed layer extends down-ward, until the temperature and density differ-ences between the mixed and bottom waterbecome so slight that a strong wind in autumncan overcome any resistance to mixing and thelake undergoes a turnover.

Plant Growth in Lakes, Estuaries, and Oceans

Seasonal changes in water temperature, sunlight,and nutrient availability affect plant life in waterbodies.

Nutrients tend to fall through the water column,and vertical mixing usually returns nutrients tonear the surface and may promote rapid growthin phytoplankton. Increases in plant growth trig-ger changes in the entire food chain and can re-sult in increased animal growth and reproduction,as well as increased bacterial decomposition. Intemperate areas, increases in water temperatureand sunlight availability in the spring combinewith seasonal increases in nutrients mixed up fromdeeper water to promote rapid growth. In tropi-cal areas, where sunlight amount and tempera-ture change little throughout the year, changes inwind patterns can result in vertical mixing inoceans, seas and large lakes.

Most of the plant production takes place in sur-face and near surface waters where light is avail-able for photosynthesis. During the summermonths there is little vertical mixing in some lakesand estuaries. Organic matter falls from the sur-face to deeper waters and is eaten by animals ordecomposed by bacteria. These organisms requireoxygen. Respiration, lack of vertical mixing andwarm temperatures can lead to low oxygen lev-els. In some places the summer can become a criti-cal period for fish and other creatures that live inbottom waters.

Streams and Rivers

Streams and rivers can show seasonal changes inthe amount and composition of water resultingfrom changes in precipitation, evaporation, snow-melt, and run-off. How these factors affect thebiota are areas of active research. Soluble chemi-cals which have accumulated in the winter snowpack tend to be concentrated in the first melt waterand can cause rapid changes (usually decreases)in the pH of streams. The first big rain storm fol-lowing a prolonged dry period also washes chemi-cals that have accumulated on roads and otherland surfaces into water bodies. The volume ofwater flowing in a stream or river often affects itswater quality. Low flow conditions can permit thebuildup of nitrates or the depletion of dissolved


oxygen. Floods and major rain storms wash largeamounts of debris into waterways and can reshapethe entire flood plain of a river or stream whiletransporting soil particles to new locations.

Soil through the Seasonal CycleSoil Temperature

As with the atmosphere and water bodies, themost obvious seasonal change in soils is in theirtemperature. As the sun gets higher in the sky inthe spring the increase in solar radiation warmsthe surface, increasing the soil temperature.

The soil undergoes a strong daily (diurnal) as wellas seasonal cycle in temperature, especially at midlatitudes. See Figure EA-I-20. The soil cycle lagsslightly behind the air temperature cycle so that,in general, the soil temperature is slightly warmerthan air at night, and is slightly cooler than air inthe morning. The lag time will depend on theparticle size distribution, the amount of organicmatter, and the amount of moisture in the soil.The cycle is most evident at the surface of the soiland decreases with depth. Soil scientists use the

Figure EA-I-20: Seasonal cycle of the 5 cm soil temperature at Vang barne-og ungdomsskule in Valdres, Norwayfrom January 1, 2000 to January 1, 2002.

temperature at 50 cm to define the Mean AnnualSoil Temperature (MAST) which stays relativelyconstant from year to year. This temperature cyclein soils is important in that it has a strong effecton phenology, influencing when plants will “greenup” in the spring, or “die back” in the fall. It alsoaffects the insulation needed for pipes that areburied in the soil to prevent freezing in the win-ter, and is used to control temperatures in base-ments and storage areas which are below ground.

Soil Moisture

Another characteristic of soil that changes throughthe seasonal cycle is the soil moisture. The mainsource of soil moisture is precipitation. The sea-sonal variation in soil moisture is controlled byseasonal variations in precipitation and snow meltand by the effect of seasonal variations in tem-perature on evaporation. See Figure EA-I-21. Forexample, if the rainy season occurs during thewinter, the soil water content will be high, whilethe summer will be a time of increasing tempera-


Figure EA-I-21: Maximum air temperature, precipitation, and soil moisture at 10 and 90 cm at Reynolds Jr. Sr. High School inPennsylvania USA from April 1, 1998 to October 1, 1998.


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ture leading to higher evaporation and dryer con-ditions in the soil.

Decomposition

The decomposition of organic material is also af-fected by seasonal changes. The microorganismsthat perform the decomposition process requiremoisture and heat in order to thrive. Thus, therate of decomposition of organic material is de-pendent on the soil temperature and moisture.All of these vary through the seasonal cycle, andso there is a seasonal cycle in the rate of decom-position of organic material. This seasonal cyclemay not be as simple as that exhibited by tem-perature and moisture. This is because the soilmicroorganisms may die or become inactive whenconditions are too hot, too cold, too dry, or com-pletely saturated. In general, the more decompo-sition, the more CO

2 and N

2O are produced and

exchanged into the atmosphere.

Land Cover and Phenology through theSeasonal CyclePhenology is the study of living organisms’ responseto seasonal and climatic changes in the environ-ment in which they live. The GLOBE measure-ments in the Phenology protocols (this chapter)focus on plant phenology. Seasonal changes in-clude variations in day length or duration of sun-light, precipitation, temperature, and otherlife-controlling factors. The plant growing seasonis the period between green-up and green-down(senescence). See Figure EA-I-22. Green-up andsenescence can be used to examine regional and

Figure EA-I-22: The length of the growing season defines what kind of plants can grow at a particular location.

global vegetation patterns, interannual variation,and vegetation responses to climate change. Achange in the period between green-up and se-nescence may be an indication of global climatechange.

Plant green-up is initiated when dormancy (a stateof suspended growth and metabolism) is brokenby environmental conditions such as longer hoursof sunlight and higher temperatures in temper-ate regions, or rains and cooler temperatures indesert areas. As plants begin green-up, leaf chlo-rophyll absorbs sunlight for photosynthesis. Pho-tosynthesis fixes carbon dioxide from theatmosphere.

With the start of green-up, plants also begin totranspire water from the soil to the atmosphere.This affects atmospheric temperature, humidity,and soil moisture. During green-down, throughleaf fall, plants reduce water loss when water sup-ply is greatly limited during winters for temper-ate plants, and during dry spells for desert plants.

Monitoring the length of the growing season isimportant for society because the length of thegrowing season has a direct effect on food andfiber production and thus on society’s ability tosupport itself. Therefore, in investigating this sea-sonal variation, GLOBE schools are providinginformation to scientists so that they can betterunderstand the Earth system and how it respondsto various influences and to society so that it canbe better prepared to adapt to variations in thelength of the growing season.

Growing Season4-6 Monthsfor Apples

Growing Season10-15 Monthsfor Bananas


The Earth System onDifferent Spatial Scales

The Earth as a System at the Local ScaleComponents

Each of the GLOBE investigations requires stu-dents to choose a study site or a set of samplesites where they will take their measurements. Ateach of these sites many of the components of theEarth system investigated by GLOBE students arepresent. At the hydrology study site, for example,air, soil and a body of water are all present. Ter-restrial vegetation is often present as well, and fora number of sites, snow or ice – elements of thecryosphere – are present at least some of the year.Figure EA-I-23 is a photograph of the hydrologystudy site at Reynolds Jr. Sr. High School inGreenville, Pennsylvania, USA where students canidentify each of these components and can exam-ine where interactions between the componentstake place.

Some examples of these interactions are:

• Evaporation and exchange of heat betweenair and water.

• Exchanges of water and gases between theair and vegetation.

• Exchanges of water and nutrients betweensoil and the root systems of grasses andtrees.

• Evaporation and exchange of heat andgases between air and soil.

• Exchanges of water, chemicals, andsediments between soil and water at thesides and bottom of a water body.

• All of the Earth system components areexposed to the sunlight. This exposure tosunlight affects the temperatures of thevarious components, the photosynthesis inplants, rates of decomposition in soils, andchemical cycles.

Cycles: Energy, Hydrologic, andBiogeochemical

The exchanges among the air, water, soil, and ter-restrial vegetation are parts of the energy cycle,the hydrologic cycle, and the various bio-geochemical cycles. As an example, let’s considerhow energy and water are cycling through thissite (Reynolds Jr. Sr. High School) and discusspH, which influences the biogeochemical cycles.

Sunlight strikes the surface of the river as well asthe trees, grass, and pavement on the bank. Someof the energy in the sunlight heats the water andthe land surface, raising the temperature of thesurface soil and water. The remaining energy isreflected back up into the atmosphere. Depend-ing on the cloud cover, some of this energy maybe reflected again toward the surface. Water fromthe river and the soil evaporates, cooling the sur-face and taking energy into the atmosphere. Whenthe temperature of the air is lower than that ofthe surface, the air is warmed through contactwith the land and water. When the reverse is true,the land and water are warmed through contactwith the air. As the soil warms, energy is storedin it. As the river flows, it carries away any energystored through the warming of the water. Simi-larly, the air brings energy with it or carries en-ergy away. Precipitation may be warmer or colderthan the surface, and the exchange of energy be-tween the rain or snow and the surface will alsoprovide heating or cooling.

GLOBE measurements allow you to track someof the flow and storage of energy. The key mea-surements are those of the air, surface water, andsoil temperatures. With these you can calculatethe direct energy exchange between the atmo-sphere and the surface. Temperature, soil mois-ture, and relative humidity measurements enablethe calculation of evaporation rates from the landand water surfaces. You can compare the amountof energy lost from the surface through evapora-tion to the direct heat exchange with the atmo-sphere and determine at what times one is moresignificant than the other.

In the hydrologic cycle, water is exchanged amongthe air, river, soil, and land vegetation. Precipita-tion forms in the atmosphere and then falls onto


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nFigure EA-I-23: Photograph of the hydrology study site at Reynolds Jr. Sr. High School in Greenville PennsylvaniaUSA annotated with various interactions between components of the Earth system

the surface – the water, soil, plants, and pave-ment. Water flows off the pavement and into thesoil. Some flows across the surface or through thesoil into the river. The various grasses and treestake in water through their roots and lose thiswater to the atmosphere through their leaves.Some water evaporates from the soil and fromthe surface of the river. If the surface is colderthan the dew point of the air, moisture in the at-mosphere will directly condense on the surface.Water also flows into the site from upstream andup hill and flows downstream, out of the site, inthe river.

GLOBE measurements of precipitation capturemost of the inputs of water from the atmosphere.The flow of water in the river can be calculated ifyou know the slope of the river bed, the depthprofile across the river, and the level of the water.Some hydrology study sites are located on riverswhere flow is monitored by government agen-

cies, and these discharge data can be obtainedfrom public databases. Storage of water in the soilcan be calculated by measuring soil porosity andsoil moisture. Evaporation rates can be calculatedby measuring relative humidity and air and sur-face temperatures. You can see how the soil mois-ture responds to precipitation and to dry periodsas well. You can study whether the river level isinfluenced by local inputs or primarily controlledby what happens upstream.

The chemical composition of the precipitation canalter the composition of the river water and ofthe soil, and affect plant and animal life. It canalso impact the rate of decomposition of organicmaterial in the soil and of rocks and minerals inthe river bed. The pH of precipitation is deter-mined by the gases and particles which dissolvein rain drops and snow flakes. Carbon dioxide inthe air tends to give precipitation a pH of about5.6, while other constituents move this figure up


or down. Most combustion-related gases lowerpH, while alkaline airborne soil particles raise pH.Chemistry is happening in the soil and the riverwater as well. If the alkalinity of either is high,the pH will not respond significantly to the dif-ferent pH of precipitation, but if it is low, the pHwill change. Over time, the pH of the soil maychange due to the cumulative effects of precipita-tion. Ultimately the pH of the river reflects thepH of the surrounding soil, of precipitation, andof the water upstream.

GLOBE measurements of the pH of the precipi-tation, soil horizons, and surface water, and thealkalinity of the surface water enable you to ex-amine the question of how the river pH respondsto precipitation events and floods. Over time, aschool’s dataset may show changes in soil pH. pHvariations through the soil profile may also illus-trate how pH is changing.

Biogeochemical cycles also promote exchangesbetween the different components of the Earthsystem. Examples of these exchanges include:

Exchanges between air and water:• transfer of oxygen, carbon dioxide,

nitrogen, water vapor (throughevaporation) and other gases

Exchanges between water and soil:• storage of water in the soil• percolation of water through soil into the

water bodies or ground water carryingchemicals and particles

• runoff processes.Exchanges between the soil and land cover:

• use of water stored in soil by the roots ofthe land cover

• use of nutrients stored in soil• substrate for plants• heat storage for plants and

microorganisms• air spaces for exchange of oxygen and

carbon dioxide during respiration andphotosynthesis

Exchanges between air and land cover:• evapotranspiration process.

Exchanges between air and soil:• precipitation and evaporation processes• heat and energy transfer• exchanges of gases produced in the

process of decomposition of organicmaterial and microbial respiration.

The rates of the exchanges of chemicals betweenthe different components of the Earth system de-pend on a number of factors. These factors in-clude the type of chemical reactions occurringwithin the different components, the temperatureof the components, the concentrations of the vari-ous gases in each of the components and themotion of the components at the interface whichpromotes exchange.


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Earth as a System at the Regional ScaleThe processes that allow the components of theEarth system to interact on a local scale, such as ahydrology study site, may also act at the regionalscale. See Figure EA-I-24.

What Defines a Region?

The regional scale is larger than the local scale andis generally characterized by some common fea-ture or features that differentiate it from neigh-boring regions. Regions can be defined in differentways. They can have natural boundaries, human-made boundaries, or political/social boundaries.Some examples of regions are:

Natural• a watershed• a mountain range• a river basin• a desert• a plain• a peninsula

Figure EA-I-24: Diagram of Earth System at the Regional Scale IndicatingInteractions Among the Different Components

Human-made boundaries• a watershed in which a boundary is a

dam• an area larger than a local study site

bounded by highways, railroads, andbridges

• a natural area surrounded by populatedregions or a populated region surroundedby a natural area

• a park or game preservePolitical/social boundaries

• a state or province• a country

Many of the processes that cause the interactionsbetween the different components of the Earthsystem at the regional scale are the same as thoseat the local scale. However, to quantify the mag-nitude of the processes, measurements generallymust be taken at numerous locations through-out the region. For example, if one wants to studythe urban heat island effect, temperature mea-


surements are required within the urban area aswell as in the surrounding countryside. Further-more, temperatures will differ between areas withlawns, plants, and trees, and those which are al-most completely covered by buildings and pave-ment; what is observed in an area that is primarilyresidential may differ from that in a commercialor industrial area. So in order to get a better rep-resentation of the entire urban area, measure-ments from multiple sites are needed fromdifferent sections within the urban environment.

Likewise, suppose you want to develop a hydro-logic model for a watershed of a river that flowsinto an estuary along the coast and the onlyGLOBE schools in the watershed are near themouth of the river (where it enters the estuary).Using only these data for the entire watershedmay lead to inaccuracies because temperature,precipitation, soil types and textures, and landcover, among other things, may differ greatlythroughout the watershed. Measurements mustcover more of the watershed to give an accuratemodel. The lack of spatial coverage for many datais a problem scientists frequently face. Sometimesa gross approximation is the best that a scientistcan do with limited data. Hence, the more GLOBEschools taking data, the better!

Inputs and Outputs

In order to understand the Earth system at theregional scale you must consider the inputs andoutputs to the region, in addition to the interac-tions among the components within the region.See Figure EA-I-25 The region may be somewhatclosed in the sense that liquid water may not leaveit, or it may be open with rivers flowing throughit. The atmosphere will always be bringing in-puts from outside and carrying outputs away;these include energy, water vapor, trace chemi-cals, and aerosols. The moving air also bringsweather systems into and out of your region,which will affect air temperature, cloud cover,and precipitation.

Atmospheric inputs and outputs can greatly af-fect a region. The air entering your region willbring with it characteristics from upwind. Thesecharacteristics can include smoke from an indus-trial plant or agricultural burning, seeds from a

forest or grassland, or moisture evaporated fromlakes or rivers. The impact of these characteris-tics on your region must be considered. Likewise,what leaves your region in the atmosphere willinfluence other regions. As the atmosphere movesit carries trace gases from a region where they areproduced to places where there are no localsources of these chemicals. The worst examplesof air pollution happen where air is trapped, usu-ally by mountains or by an inversion layer (a layerof air in which the temperature increases as youmove from bottom to top) in the atmosphere. Thewinds also can carry away significant amounts ofmoisture and dust from a region. Plumes of Sa-haran dust are so prominent at times that theycan be seen on satellite cloud images and the dustis blown all the way across the Atlantic Ocean.

GLOBE schools across a region can cooperate togain a comprehensive picture of the energy andwater cycles within the region and to trace someparts of the biogeochemical cycles. In a water-shed, the characteristics measured in the surfacewater of streams, lakes, and rivers can be mea-sured at a variety of sites. These characteristicsare strongly influenced by the microclimate of theregion which is quantified by measurements ofair temperature and precipitation, the soil char-acter which may vary across the watershed andneed to be measured in a number of places, andthe land cover. Schools may combine their Landsatimages to gain a complete satellite picture of theregion and this can become the basis for a com-prehensive regional land cover map. The dynam-ics of the watershed can be studied using GLOBEmeasurements of specific weather events, soilmoisture and infiltration rates, and whatever dataare available on the flow rates of the streams andrivers.

Earth as a System at the Continental/Global ScaleThe learning activities in this chapter that aredesigned to help your students understand thelargest spatial scales of the Earth system focus onthe continental scale. This is the largest practicalscale for meaningful examination of GLOBE data,although it could be considered the largest re-gional scale. The global scale encompasses the


Figure EA-I-25: Photograph of the Earth System on the Regional Scale with Inputs and Outputs


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whole Earth, all of the atmosphere, hydrosphere,pedosphere, cryosphere, and biosphere. If oneincludes the interior of the planet as well, at thisscale, Earth is an almost closed system - one inwhich almost no matter enters or leaves. Note:An isolated system is one in which no energy ormatter enters or leaves. See Figure EA-I-26. Infact, the Earth system is closed except for the in-put of energy from the sun, the balancing loss ofenergy to space, the extremely small loss of hy-drogen from the top of the atmosphere, and thecontinuous input of gases, dust, and meteoritesfrom space, and the few satellites which we havesent beyond Earth’s orbit. Studies of Earth systemscience also treat the inputs of gases, energy, dust,and lava from Earth’s interior and the recycling ofmaterial into the crust and upper mantle as exter-nal inputs to and outputs from an almost closedsystem. These exchanges with the interior of theplanet tend either to happen on long time scalesof tens of thousands to millions of years (geologictime) or to happen almost instantaneously andunpredictably. These latter phenomena, particu-larly large volcanic eruptions, play havoc withshort-term climate predictions.

Earth

Atmosphere

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Small exchange of dust and

gas and imput of meteorites

0 25 50 75

Percent Cloud Cover

How Do the Local, Regional, and GlobalScales Interact?

Within the global Earth system the local and re-gional scales all contribute to how each of thecomponents (the atmosphere, open waters,cryosphere, soil and terrestrial vegetation) inter-act with each other as a whole at the global scale.These interactions occur on many different timescales – the characteristic times over which pro-cesses or events occur.

All of the GLOBE measurements are taken at thelocal scale but they sample phenomena with vari-ous time scales. The maximum and minimum airtemperatures address the daily time scale, whiletree height and circumference indicate growthover an annual cycle, and characterization of asoil profile may document the results of thousandsof years. Most of the learning activities also in-volve the local scale and shorter time scales. How-ever, some of the learning activities, such as thosein this chapter, broaden your perspective to theregional and global scales to help you understandhow local scale environments fit into the regionaland global scale contexts. These large scales in-volve changes over long and short periods. TodayGLOBE measurements only cover a few years andprimarily contribute to studies of current pro-cesses and phenomena. Eventually, as the GLOBEdatabase extends further in time, the measure-ments will contribute to scientific studies onlonger time scales of decades to centuries wherethere are currently major concerns about globalclimate change.

The following sections describe the various com-ponents of the Earth system in the context of theglobal scale. Understanding these largest spatial-scale processes will help you more fully under-stand the context for your local study sites, andhow the Earth system connects us all.

The Earth System Components at the GlobalScale: The Atmosphere (Air)

The atmosphere is the gaseous envelope of theEarth. The local properties of the lower atmo-sphere vary on time scales of minutes to seasonsand years. Winds change speed and direction,clouds form and dissipate, precipitation falls,humidity comes and goes, some trace gases such

Figure EA-I-26: Diagram of the Earth as an AlmostClosed System


as ozone build up and then go away, and air tem-perature rises and falls. These local variations arecaused by the daily and annual cycles in sunlightand some shifts in ocean circulation such as theEl Niño/Southern Oscillation. The overall struc-ture and composition of the atmosphere and theclimate change more slowly, on time scales rang-ing from a decade to millions of years.

As illustrated in Figure EA-I-6. the tropics receivemore energy from the sun per unit of surface areathan the temperate or polar zones. In fact, eventhough the warmer tropics radiate more heat tospace than high latitude regions, the tropics re-ceive more energy from the sun than they radiateaway! Where does this excess energy go? The cir-culation of the atmosphere and the oceans car-ries this energy, in the form of heat, to higherlatitudes.

If we consider the average north-south motion ofthe atmosphere, warm air from near the equatorrises and moves toward the poles. At roughly 30˚latitude, the air cools, falls, and moves equator-ward near the surface. A similar pattern exists inthe polar zones, with air rising at roughly 60˚ lati-tude and falling at the poles. The tropical andpolar zones bracket the temperate zones and drivetheir circulation patterns. As a result, the air intemperate zones moves poleward at low altitudes,rises at roughly 60˚, returns equatorward aloft andfalls at roughly 30˚. The interaction of warm andcold air masses between 30˚ and 60˚ latitude pro-duces the succession of low (storm) and high (fairweather) pressure systems that move from westto east in mid-latitudes. See Figure EA-I-27.

The Earth System Components at the GlobalScale: The Hydrosphere (Bodies of Water)

The hydrosphere encompasses all the bodies ofwater on Earth including groundwater. At the glo-bal scale, it is the oceans and the larger seas thatare important. The time scales on which theoceans vary range from a month near the surface,to over a thousand years for deep ocean circula-tion.

The ocean receives energy from sunlight trans-mitted through the atmosphere. The albedo(reflectivity) of the oceans is relatively low, about0.1, which means that 90% of the solar radiationfalling on the ocean surface is absorbed. Theoceans also exchange long wave (thermal infra-red) radiation with the atmosphere.

Ocean CirculationCirculation within the oceans occurs through twobasic processes. The first is the horizontal circu-lation of the upper ocean that is driven by forcesinduced by surface winds. This surface circula-tion is coupled to deep ocean circulation (ther-mohaline) that is driven by differences in thedensity of seawater due to changes in tempera-ture and salinity. During winter in the polar re-gions, the ocean surface cools and sea ice forms.As the water freezes, most of the salt is left dis-solved in the liquid water. This increase in salin-ity, particularly in the north Atlantic, causes thesurface water to become dense enough to sinkand to become bottom water. This bottom waterflows toward the equator and eventually returnsto the surface. Scientists call this global circula-tion of ocean waters a conveyor belt which con-nects the surface and deep waters of the Atlantic,Pacific, and Indian Oceans. See Figure EA-I-28.

The ocean surface is in direct contact with theatmosphere. Large exchanges of aerosols and gasestake place at this boundary. Gases that are moreabundant in the atmosphere, such as carbon di-oxide, are taken up in the ocean water while gasesformed in the oceans, such as methyl bromide,are released into the air and are the largest natu-ral sources of some atmospheric trace gases. Theseprocesses happen much faster than the thermo-haline circulation of the oceans. Today’s surfaceseawater is in equilibrium with the present com-

General AtmosphericCirculation Patterns

RadiantEnergyfrom the Sun

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Figure EA-I-27 General Atmospheric Circulation Patterns


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position of the atmosphere, but gases dissolvedin bottom water reflect atmospheric conditionsfrom roughly 1500 years ago. Through thisgradual overturning of ocean water, gases, suchas carbon dioxide, whose atmospheric concen-tration have increased over the last 1500 years,are gradually taken up by the ocean, lesseningtheir abundance in the air.

Biological ActivityBiological activity is also affected by circulationpatterns around the globe. There are areas, forinstance, where upwelling occurs. Upwelling is theprocess by which deep, cold, nutrient-rich wa-ters rise to the surface. Phytoplankton, microscopicplants floating in the water, form the base of theocean food chain, and their abundance limits thepopulations of most other ocean creatures. Whereocean surface waters lack nutrients, growth andreproduction of phytoplankton are limited. Ar-eas where upwelling occurs are generally nutri-ent-rich and highly productive and have largecommercial fisheries.

Biological activity in the oceans plays a major rolein the global carbon cycle. Phytoplankton in nearsurface waters take up carbon through photosyn-thesis. Some dead organic matter such as shellsof microscopic organisms or fecal pellets fromanimals fall through the water column to the oceanbottom and become buried in sediments. Here inthe deep ocean, the carbon in the organic matteris essentially removed from the atmosphere.

The Earth System Components at the GlobalScale: The Cryosphere (Ice)

The Role of the Cryosphere in EnergyTransferThe cryosphere is the solid water component ofthe Earth system. The two main forms of ice aresea ice and continental ice. Either can be coveredwith snow. Ice has an albedo (reflectivity) thatranges from about 0.5 to 0.8. This is generallyhigher than what’s underneath it. The albedo ofnewly fallen snow ranges even higher, up to 0.9.So, where covered by ice, Earth’s surface reflectsmore than half the solar radiation falling on it backto space. Ice and snow also insulate Earth’s sur-face, cutting off evaporation which removes amajor source of heat to the atmosphere above.

Sea IceSea ice is frozen seawater. If the water is salty, as itis in the ocean and the seas, during the freezingprocess the salt is left in the water, making thewater saltier and denser, and the sea ice less salty.Sea ice floats on the ocean/sea surface and rangesfrom thin frazzle ice which has just formed andbarely coats the surface, to thick ice, which haslasted through many years and may be up to 10m thick. However the average ice thickness is 3meters in the Arctic and 1.5 meters around Ant-arctica. Under the stress of wind and ocean cur-rents, sea ice cracks and moves around. The cracksexpose areas of relatively warm ocean water tothe cold atmosphere during winter. In winter, thispermits a large exchange of energy from high lati-tude oceans where the water temperature is justabout freezing to the atmosphere where air tem-peratures are well below zero.

Sea ice has a large seasonal cycle and changes ontime scales of a few weeks to a few months. Themagnitude of these seasonal changes is very sen-sitive to climate conditions in the atmosphere andoceans, extending the time scales associated withsea ice variations from months to tens of thou-sands of years–the time scale for ice ages.

Land IceContinental ice includes ice sheets such as thosein Antarctica (up to 4 km thick) and Greenland(up to 3 km thick), and valley glaciers (generally10-100 m thick). Most of the fresh water on Earthis frozen in these ice sheets. Continental ice isformed from snow accumulating at the surfaceand compressing over time into ice. This processis very slow compared to the changes in sea ice.Ice sheets change on time scales ranging frommonths (for rapidly moving valley glaciers) to tensof thousands of years. These longer changes areassociated with ice ages.

Even when frozen, water still flows from themountains to the oceans. When snow falls in win-ter, melts in the spring, trickles into a mountainbrook, flows into a stream and then a river, andfinally into the ocean, the water’s journey is com-pleted in a year or less. When the snow falls on aglacier, the journey becomes much longer andlasts for many years. The deep layers of the


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Greenland ice sheet which have been sampledwith ice cores record conditions when snow fellover 250,000 years ago and are a major source ofinformation about longer-term changes in climate.

The Earth System Components at the GlobalScale: The Pedosphere (Soil)

The pedosphere is the portion of Earth’s land sur-face covered by layers of organic matter and ofweathered rocks and minerals which are less than2.0 mm in size together with the organisms thatlive in these layers. The surface temperature ofthe pedosphere responds quickly to the daily andseasonal cycles in air temperature, changing ontime scales ranging from hours to months. Thealbedo of bare soil averages about 0.3, meaningthat 70% of the solar radiation falling on it is ab-sorbed. However, there are many different soiltypes, so this number varies from place to placeand from season to season. The land surface isoften covered by vegetation which intercepts thesunlight before it reaches the soil.

Just like the atmosphere and the ocean, there aremovements within the pedosphere and lithos-phere that act to redistribute the energy receivedfrom the sun. Conduction, convection, and ra-diation processes all operate within the soil toredistribute energy within the soil profile. The rateand amount of distribution depends on soil prop-erties such as the particle size distribution, bulkdensity, water content, and organic matter con-tent.

The pedosphere forms as a result of the interac-tion of the five soil forming factors: parent mate-rial (the mineral or formerly living material fromwhich the soil is derived), climate (both macro-and micro-climate), topography (including slope,position, and aspect), biota (plants, animals in-cluding humans, and all other organisms), andthe amount of time for which each of the otherfactors has interacted. Four major processes oc-cur in response to the soil forming factors: addi-tions, losses, transfers, and transformations. Theprocesses of addition include inputs such as heatand energy, water, nutrients, organic matter, ordeposits of materials. Losses of energy and heat,water, nutrients from plant uptake or leaching,and erosion of soil material also take place. Trans-

fers occur when materials within the soil, such aswater, clay, iron, plant nutrients, or organic mat-ter are moved from one horizon to another. Lastly,transformations include the change of soil con-stituents from one form to another within the soil,such as liquid water to ice, large particles to smallerparticles, organic matter to humus, and oxidizediron to reduced iron. Each of the five factors andthe corresponding four processes produce a lo-calized soil profile with specific characteristics andhorizon attributes.

Under well drained conditions, when respirationof organisms and roots in the soil is at its opti-mum, a great deal of CO

2 is produced. The per-

centage of CO2 in the soil can be 10 to over 100

times greater than in the atmosphere above thesoil. This soil CO

2 becomes a source to the atmo-

sphere as it diffuses upward to the surface, or isreleased when the soil is disturbed from plowingor other turnover processes. Respiration is onlyone source of soil CO

2 to the atmosphere. Soil

organic matter decomposition provides anothervery large pool of CO

2 and CH

4 to the atmosphere.

Nitrogen is the most abundant element in the at-mosphere, yet it is not in a form that is availableto plants, and is often the most limiting nutrientfor plant growth. Soil organisms and certain pro-cesses help to convert atmospheric N

2 into a form

plants can use. These forms are nitrate (NO3-) or

ammonium (NH4+). Other organisms convert or-

ganic forms of nitrogen from plant and animalremains into plant-usable forms. Nitrogen can alsobe removed from the soil and become a source ofnitrogen to the atmosphere and to ground or sur-face water.

The Earth System Components at the GlobalScale: Terrestrial Vegetation (land plants)

Land plants connect the soil and atmosphere. In-dividual plants form this connection on time scalesranging from a few weeks to over 1000 years.However, land vegetation collectively affects theEarth system on time scales of seasons to thou-sands of years and longer. As land plants growthey reshape the environment around them. Theyshade the surface, block the wind, intercept pre-cipitation, pump water from the ground into theair, remove nutrients from soil and some trace


gases from air, hold soil against erosion, and lit-ter the ground with leaves and twigs which even-tually increase the organic content of the soil. Inthese ways, terrestrial vegetation plays a signifi-cant role in the energy, water, and biogeochemi-cal cycles. The expansion and growth of forestsin particular removes carbon dioxide from theatmosphere in significant amounts.

Educational ObjectivesStudents participating in the activities presentedin this chapter should gain scientific inquiry abili-ties and understanding of a number of scientificconcepts. These abilities include the use of a va-riety of specific instruments and techniques totake measurements and analyze the resulting dataalong with general approaches to inquiry. TheScientific Inquiry Abilities listed in the grey boxare based on the assumption that the teacher hascompleted the protocol including the Looking Atthe Data section. If this section is not used, not allof the Inquiry Abilities will be covered. The Sci-ence Concepts included are outlined in the UnitedStates National Science Education Standards asrecommended by the US National Research Coun-cil and include those for Earth and Space Scienceand Physical Science. The Geography Conceptsare taken from the National Geography Standardsprepared by the National Education StandardsProject. Additional Enrichment Concepts specificto the atmosphere measurements have been in-cluded as well. The gray box at the beginning ofeach protocol or learning activity gives the keyscientific concepts and scientific inquiry abilitiescovered. The following tables provide a summaryindicating which concepts and abilities are cov-ered in which protocols or learning activities.

Earth Science Introduction - SERCserc.carleton.edu/files/eet/globe/EarthSysInt.pdfGLOBE® 2003...

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Transcript of Earth Science Introduction - SERCserc.carleton.edu/files/eet/globe/EarthSysInt.pdfGLOBE® 2003...