Carboncyclereservoirsandfluxes
Learningobjective:Studentswillquantifyandcomparethesizeofvariouscarbonreservoirs(sinks)and
fluxes(transfersbetweenreservoirs).
Materialsneeded:
•
•
•
•
largesheetsofpaper(atleast2mindiameter)
pencilsormarkers,metricruler(meterstickwithmmscalegradations)
string
thumbtack(orafinger)
BackgroundinformationforPartI:Wearegoingtomakeagraphicrepresentationofthesizeofthe
differentcarbonreservoirs(sometimescalled“sinks”)onEarth.Theestimatedsizeofeachreservoiris
giveningigatonnes(billionsofmetrictons)ofcarbon(ametrictonis1000kilograms).Wewillrepresent
thedifferentreservoirswithcirclesofdifferentsizes.Wewanttheareaofthecircletocorrespondto
theamountofcarboninthedifferentreservoirsintheEarthsystem,sowecalculatetheradiusofthe
circlebyreconfiguringtheformula:
A=πr2
WhereAistheareaofthecircleandristheradiusofthecircle.πistheconstant3.14159...
Inthisactivity,wearestartingwiththecircle’sarea(equivalenttothecarbon“load”ofthatreservoir,in
gigatonnes),andwewanttofigureouthowtodrawacircleoftheappropriatesize.Tofindtheradius,
wetakethesquarerootoftheproductofthearea(totalgigatonnesofcarbon)andπ.Oncetheradius
hasbeendetermined,wecanmeasureoffthatdistanceonastringwithapencilormarkerattheend.
Wepinorholdoneendofthestringdown(atourcircle’scenter),andthenusethestring’sfulllengthto
traceoutthecircumferenceofthecircle,likethis:
Reservoir
Areaofcircle(each
mm2equivalentto2
Radiusof
gigatonnesofcarbon) circle(mm)
Atmosphere
426
11.6
Vegetation+soils
1,900
24.6
Fossilfuelsstillunburned
2,150
26.2
Oceans
19,000
77.8
Crust
2,850,000
952.5
Mantle
5,000,000
1261.6
(Datasource:RalphKeeling,ScrippsInstitutionofOceanography)
Convertingmetricdistances:1000mm=100cm=1m
Usingthematerialsinyourclassroom,youshouldcreatesixcircles,oneforeachofthemajorreservoirs
ofcarbon.Eachcircleshouldhavearadiusmatchingthenumberinthefinalcolumnabove.Labeleach
circlewiththenameofthereservoiritrepresents.
Discussion:
1. WhichofEarth’scarbonreservoirsisthesmallest?Doesthis“smallest”rankindicateitis
thereforeunimportant?Explain.
2. Howmuchlargerwouldthe“atmosphere”carbonreservoirgrowiftheentire“fossilfuel”
reservoirwereminedandthenburned?
ProceedtopartII,nextpage.
PartII:Carbondoesnotjustsitstillinagivenreservoir.Certaintransfers(called“fluxes”)movecarbon
fromonereservoirtoanother.Forinstance,whenplantsengageinphotosynthesis,theypullcarbon
dioxidefromtheatmosphereandlockitupinthevegetation+soilsreservoir.Whenanimalseatplants
andthenmetabolizethecarbonintheirfood,theyexhalecarbondioxide,afluxthatreturnsthecarbon
totheatmosphereagain.
Thefollowingtablelistssomeofthenaturalfluxesbetweencarbonreservoirs,ingigatonnesofcarbon
peryear,withhuman-inducedchangesshowninboldtype:
Howmuchcarbonisremoved
fromtheatmosphere?
Howmuchcarbonisputout
intotheatmosphere?
(none)
↑9gtC/yr
Photosynthesis
↓120gtC/yrnatural
andanother3gtC/yrfrom
peopleburningfossilfuels
(none)
Plantrespiration
(none)
↑60gtC/yr
(none)
↑60gtC/yr
↓90gtC/yrnatural
andanother2gtC/yrfrom
peopleburningfossilfuels
(none)
(none)
↑90gtC/yr
(none)
↑0.15gtC/yr
↓0.15gtC/yr
(none)
Fossilfuelburningby
people
Microbialandfungal
respirationinsoil
Absorptionintothe
oceansacrossthe
ocean/atmosphere
interface
Degassingfrom
oceansacrossthe
ocean/atmosphere
interface
Volcaniceruptions
Weatheringof
continentalcrust
(Datasource:NASA)
Discussion:
3. Overthecourseofayear,whichreservoirsareleastaffectedbycarbonfluxes?Youcanevaluate
theanswertothisquestionbycalculatingwhatpercentage“turnsover”eachyear.(Toanswer
thisquestion,divideeachreservoir’sannualfluxbythesizeofthetotalreservoir.Whichonehas
thesmallestnumber?)
4. Overthecourseofayear,whichreservoirsaremostaffectedbycarbonfluxes?Aswiththe
previousquestion,youcanevaluatetheanswertothisquestionbycalculatingwhatpercentage
“turnsover”eachyear.(Toanswerthisquestion,divideeachreservoir’sannualfluxbythesizeof
thetotalreservoir.Whichonehasthebiggestnumber?)
5. Nowthathumansareburningfossilfuels,whatisthenetchangeinthesizeoftheatmospheric
reservoirfromoneyeartothenext?(Expressyouransweringigatonnes.)
6. Eachgigatonofcarbonthatendsupbeingshiftedfromthe“fossilfuel”reservoirintothe
atmosphereincreasestheproportionoftheatmospherethatiscarbondioxide.Typically,we
measuretheCO2proportionoftheatmosphereinpartspermillionbyvolume(ppm).Eachppm
isequivalenttoabout2gigatonnesofcarbon.Basedonthisconversionfactor,byhowmany
ppmwouldyouexpecttheatmosphericconcentrationofCO2toincreaseeachyear?
7. LookupthecurrentconcentrationofCO2intheatmosphereatthiswebsite:
http://www.esrl.noaa.gov/gmd/ccgg/trends/
Ifyouranswerto#6aboveisaccurateandcontinuesunchangedintothefuture,whatwould
youexpecttheconcentrationofCO2tobeinEarth’satmosphere10yearsfromnow?
…100yearsfromnow?
…1000yearsfromnow?
8. Whatpercentagechangeintheoverall“carbonload”oftheatmosphericreservoirdoesthis
represent,ascomparedtopre-IndustrialRevolutionlevelsof~280ppm?
…Today’sconcentration?_____________________________
…yourprojectionfor10yearsfromnow?____________________________
…yourprojectionfor100yearsfromnow?___________________________
…yourprojectionfor1000yearsfromnow?___________________________
9. Thissimplelinearpredictionisunlikelytocometrue,however.Whatcomplicationsmakeit
difficulttoaccuratelypredictthefuturesizeofthe“atmosphere”reservoir?