Download ATS150 Climate Science for Poets Spring 2016 Study Guide for

ATS150
ClimateScienceforPoets
StudyGuideforMidTermExam
Spring2016
We’llhavethefirstexaminATS150onFridayMarch11,from1PMto1:50,inthe
regularclassroom.Theexamwillcount1/3ofyourgradeforthecourse.Youwill
needtobringapencilorpentotheexam,butyouwon’tneeda“bluebook.”
I’mmainlytryingtoseethatyouunderstandconceptspresentedinthelectures.I
don’twantyoutomemorizeabunchofstufffromthenotes.Theexamwillbe
“opennotes,”meaningyoucanuseprintedcopiesofthelecturenotes,or
books,orstuffyou’vewrittendownduringtheexam.Youmaynotconsultone
another,orlookstuffupontheinternet.Theexamwillonlycoverstuffwe’vetalked
aboutinclass.Therewillbenocalculationsormathproblemsontheexam.All
questionsontheexamwillbechosenfromthesetof“candidate”questions
availableontheclasswebsite.
Theexamcoverseverythingwe’vetalkedaboutsofar,exceptforthatveryfirst
lecture(whereIexplainedthatclimatechangeisSimple,Serious,andSolvable).
Here’salistofstufftostudy:
I. EarthSystemOverview
a. Atmosphericcomposition
AirisalmostentirelyO2andN2,withalittleCO2,H2O,Argon,…
b. Originanddevelopmentoftheatmosphere
Airstartedoutasgasesfromvolcanoes(mostlyCO2,H2O)
Watercondensedtomakeoceans
CO2wastakenupbyplantsandburiedasfossilsovertime
O2was&isreleasedbyplantsaccumulatedintheair
O2getsblastedbyUVtomakestratosphericozone(O3)
OzoneprotectssurfacefromUV,allowedlifeonland
c. Platetectonicsandearlyclimates
Continentsmovearoundlikeraftsonsofterrocksunderneath
Changesinpositionsoflandandseaschangedclimate(always)
d. GlaciersandIceages
FormostofEarth’shistorytherehasbeennolandorseaice
Forthelastcoupleofmillionyears,Earthhashadiceages
Duringiceages,hugeicesheetscover1/3oftheland
Mostrecenticeagemaxwas18,000yearsago
IcecoveredN.AmericafromCanadatoNYC,Ohio,Nebraska
Mosticemeltedabout12,000yearsago
ContinentalicesheetsstillcoverGreenlandandAntarctica
Iceagescomeandgobecauseofperiodicchangesinourorbit
e. WateronEarth(theHydrosphere)
NearlyallthewateronEarthisintheoceans(97%)
Almostalltherestisincontinentalicesheets
Theoceansareabout4kmdeeponaverage
Deepoceansareverycoldeverywhere(about3C=37F)
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ClimateScienceforPoets
StudyGuideforMidTermExam
Spring2016
Seasurfacetemperaturenearfreezingatpoles,verywarmin
tropics…especiallyhuge“warmpool”inwesternPacific
Warmwaterisbuoyant…floatsontop.Coldwatersinks.
Oceancurrentscarrywarmwater->poles,cold->tropics
Arcticicecapisfloatingseaice(only1-3metersthick)
Antarcticiceisonland,thousandsofmetersthick!
Watervaporinairisalmostallnearthesurface(0–3km)
Mostwatervaporinthetropics(warmairholdmoreH2O)
Waterevaporatesfromsubtropicaloceans,condensesin
clouds,fallsasrainandsnow
f. AtmosphericCirculation
Moreenergyin(sunshine)thanout(thermalIR)intropics
Moreenergyout(thermalIR)thanin(sunshine)atpoles
“Job”ofairandoceansismovingenergyfromtropicstopoles!
Aconveyorbeltofwarmairrisingintropicsandsinkingin
polesmightwork,butEarth’srotationpreventsthis
RotatingEarthhas“CoriolisForce”thatpusheswindsaround
“TradeWinds”blowfromeasttowestatsurfaceintropics
“Jetstreams”blowfromwesttoeastoutsideoftropics
Forestsgrowwhereit’swet,desertswhereit’sdry,Grasslands
inbetween,andTundrawhereit’scold
II. EnergyandElectromagneticRadiation
a. Changesinenergymakethingshappen
Energycanbetransformedfromonekindtoanotherormoved
aroundfromplacetoplace,butnevercreatedordestroyed
Kindsofenergy:radiant(light),kinetic,potential,chemical
AllenergyonEarthcomesfromtheSun(nuclearfusion)
Internalenergyismolecularmotion(temperature)
Energycanbemovedaroundbyconduction,convection,or
radiation…Earthonlyexchangesbyradiationto/fromspace
b. Electromagneticradiationisenergymovingthroughemptyspace
Radiationisoscillationofenergybetweenelectric&magnetic
fields,movingat186,000milespersecond(speedoflight)
Wavelengthishowfarlightgoesinonecycle(elect-mag-elect)
Shortwavescarrymoreenergythanlongwaves
Visiblelightisjustpartofthe“electromagneticspectrum”
Bluelighthaswavelengthof0.4microns(µ,1/1,000,000m)
Redlighthaswavelengthofabout0.7µ
Bluelighthasmoreenergythanredlight
Lightwithwavesshorterthanvioletiscalledultraviolet(UV)
Lightwithwaveslongerthanrediscalledinfrared(IR)
Shorter(higherenergy)thanUVisXrays,gammarays
LongerthanIR(lessenergy)ismicrowaves,radioandTV
c. Solarradiationhaslotsofwavelengthsinaspectrum
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ClimateScienceforPoets
StudyGuideforMidTermExam
Spring2016
Mostlyinvisible(44%)andnearIR.Peakisinvisible(duh!)
Only7%ofsolarenergyisinUV,butitisdangerous
Earthemitsradiationtoo,inthermalIR(about10µ)
Sunlightcanbeabsorbed,reflected,ortransmitted
About51%ofincomingsunlightreachesEarth’ssurface
Scatteringofsunlightbyairisstrongestatshortwavelengths
(that’swhysunnyskyisblueandsunsetsarered)
Sunlightreachingsurfacecanbeabsorbedorreflected
Percentageofsunlightreflectediscalled“albedo”
Highalbedoareas(clouds,snow,sand)coolsurfacebutlow
albedoareas(forests,oceans,asphalt)warmsurfaceby
absorbing
III. LayerModelandtheGreenhouseEffect
a. Everythingemitsradiation.
Hotterobjectsemitmore,atshorterwavelengthsthancolderobjects
Idealobjectsthatabsorb(andemit)allradiationthathitsthemare
called“blackbodies”
BlackbodiesemitradiantenergyaccordingtoE=σT4
σ (pronounced"sigma") isjustanumber=5.67x10-8W/m2perK4
E=σT4meansenergyemissionisverysensitivetotemperature!
b. Givenbrightnessofsun,Earthmustemit342W/m2soT=255°K
(-18°C=0°F),buttruemeasuredsurfacetemperatureis288K
Differencebetweenequivalentemissiontempof0°Fandmeasured
surfacetempof59°Fisduetothermalemissiondownwardbyair!
ThisisknownastheGreenhouseeffect
c. Imagineasinglelayerof“glass”abovethesurfacethatistransparent
tosunlightbutbehavesasablackbodyintheinfrared
TheglassmustemittospaceatthesameT=255°Kasthebaresurface
intheprevious“barerock”model
Theglassalsoemitsdownward,atσTglass4sothatthesurfaceis
warmedbyboththeSunandtheinfraredemissionfrotheglass.This
makesthesurfacetolerablywarm!
Addingmorelayersofglassmakesthesurfaceevenwarmerbecause
eachglasslayeriswarmedbythelayeraboveit.
d. DownwardthermalIRfromatmosphereabsorbedatsurfaceisalmost
twiceasmuchenergyassolarradiationabsorbedatsurface(96vs51
unitswhere100unitsissunshineattopofatmosphere)!
IV. GreenhouseGases
a. Atomscanabsorbphotons&havetheirelectrons“jumpup”toa
higherenergylevel.Whentheelectron“fallsbackdown”aphotonof
thesameenergy(wavelength)isemitted
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Spring2016
b. Moleculescanalsoabsorb&emitphotonsbychangingenergystates,
buttransitionsareweaker(vibration&rotation)sophotonsareless
energetic(longerwavelengthIR)
c. 99%ofairmoleculesarediatomic,sohardlyanyvibration/rotation
transitions,almostnointeractionwithIRphotons
d. Thermalabsorptionandemissionbyatmosphereisalmostentirely
duetomolecularvibrationandbendingofCO2andH2Omolecules
e. CO2hasstrongtransitionsduetoasymmetricstretching(4.3µm),
symmetricstretching(7.2µm),andbending(14.3µm).
f. H2Ois“polar”withapositively-charged(H)endandanegativelycharged(O)end.Itcanthereforecombineflipswithothertransitions,
soitabsorbsmanydifferentwavelengthsofIRphotons
g. H2Oabsorptionismostpowerfulinwarmregionsbecausecoldair
(especiallyathighaltitude)can’tholdmuchwatervapor
h. VerystrongCO2absorptionatsomewavelengthsmeansthatIR
photonscanonlyescapetheatmospherefromverycoldupper
troposphere
i. EffectofaunitincreaseinCO2conecntrationonEarth’sradiation
diminisheswithconcentrationbecausethere’slessavailableto
absorb.EverydoublingofCO2reducesoutgoingemissionbyabout4
Wm-2,resultinginwarmingofabout3°Ctore-establishequilibrium
V. VerticalVariationsofTemperatureandVerticalMixing
a. Theatmospherelosesheatbyradiationtospace,butitiswarmed
frombelowbyverticalmixingofheatfromthesurface
b. ThestrengthoftheGreenhouseEffectisdirectlyrelatedtothefact
thatradiationcanonlyescapefromthecoldupperatmosphere,so
verticalmixingofheatfrombelowactstocooltheplanetbybringing
heatuptowhereitcanescapebyradiation
c. Atmosphericpressureisdirectlyrelatedtotemperatureanddensity.
Increasingpressurecausesincreasingtemperature&viceversa.
d. Atmosphericpressureistheweightoftheoverlyingair,sopressure
(andthereforetemperature)decreasesrapidlywithheight
e. Atagivenpressure(height),densityvariesinverselywith
temperature.Warmairislessdensethancoldair
f. Airthatiswarmer(lessdense)thanitssurroundingsfeelsanupward
forcecalledbuoyancywhichmakesitaccelerateupward(andvice
versa)
g. Risingair“tradesheightforheat”(heatenergyforpotentialenergy),
andcoolsatarateofabout10°Cperkmunlessheatisaddedor
subtracted.Thisrateofcoolingwithheightiscalledthedrylapse
rate
h. Iftheambientaircoolswithheightfasterthanthedrylapserate(10
°Cperkm),thenaliftedparcelwillaccelerateupward.Thisis
unstableair!
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i.
Spring2016
Iftheambientaircoolswithheightmoreslowlythanthedrylapse
rate(10°Cperkm),thenaliftedparcelwillfallbackdown.Thisis
stableair!
j. Risinghumidaircoolsatthedrylapserateuntilitreachesits
dewpointtemperature.Afterthattherisingaircoolsmuchmore
slowly(about4°Cperkm)becauseitiswarmedbythelatentheatof
condensation.
k. Theaverageobservedlapserateintherealatmosphereisabout6°C
perkm,stableforrisingdryairbutunstableforrisingmoistair
(clouds).
l. Liftingverymoistaircausesthunderstormsthatmixheatvigorously
throughoutthetroposphere.Thunderstormcloudsstoprisingwhen
theyhitthetropopause(baseofthestratosphere)whereozone
formationwarmstheatmospheresothecloudsarecoolerthantheir
surroundings.
m. AnatmospherewithmorewatervaporabsorbsmoreIRradiation
(strongerGreenhouseEffect),butalsomixesheatmorevigorouslyto
highaltitudes(weakerGreenhouseEffect).Thismakesglobal
warminghardtopredict.
VI. TheEarth’sEnergyBudget
a. ThesurfacegetsitswarmthpartlyfromtheSun,butmostlyfromthe
warmairradiatingdownduetogreenhousegasemissions
b. Theatmospheregetsitswarmthmostlyfromthesurface(warmed
frombelow!)byupwardthermalemission,condensationofwater,
anddirectconvectiveheating
c. Solarradiationabsorbedbythesurfaceisstrongestinthetropics,
weakestatthepoles,andalsodependsonalbedoandclouds
d. OutgoingthermalradiationfromtheEarthisalsostrongestinthe
tropics,butitdropsoffwithlatitudelesssteeplythantheincoming
solar.Italsodependsstronglyonclouds,withwaymorethermal
coolingfrom(hotsurface)clearareaslikedesertsandwaylessover
cloudy(highcold)areaslikethecloud-topsoverrainforests
e. ThenetheatingoftheEarthbyradiationresultsfromtheabsorbed
solarminustheoutgoingthermalradiation.There’sabignetsurplus
inthetropicsandbignetlossfromthepoles.
f. Energyistransportedbythecirculatingatmosphere(winds)and
oceans(currents)togetridofthetropicalsurplusandpolardeficitsin
radiativeheatingandcooling
g. Earth’ssurfacewarmstheatmospheremostlybyradiation,abitless
byevaporation(latentheating),andmuchlessbyrisingcurrentsof
warmair
h. SeasonsarenotcausedbychangesintheEarth-Sundistancebut
ratherbythetiltoftheEarth’saxiswhichmakestheSun’sraysstrike
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VII.
ClimateScienceforPoets
StudyGuideforMidTermExam
Spring2016
moredirectlyandforalongerdayinsummer(lowersunandshorter
daysinwinter)
i. Winter/summerandDay/nightcyclesintemperatureatthesurface
aredelayed(timelagged)comparedtochangesinsunshine,becauseit
takestimeforthesurfacetowarmandcool.Thisdelayismuch
strongeroveroceansthanland.
CirculationoftheAtmosphere:Wind
a. Windis“pushedaround”by5forces(3real,twoimaginary)
i. Realforces:Gravity,pressuredifferences(gradient),friction
ii. Imaginaryforces:Coriolisandcentripetal
iii. Heatingliftstheairagainstgravity
iv. Liftedairpushesagainstadjacentair(pressuregradients)
v. Combinationofliftingandpushingproducedbygeographic
variationsinheatingandcoolingcausesplanetary-scale
circulationsoftheatmosphere(wind)andoceans(currents)
vi. ThesecirculationsacttobalanceEarth’senergybudget,
movingheatfromhotplaces(tropicalsurface)tocoldplaces
(upperairandpoles)
b. Pressure-gradientforce
i. Causedbydifferentamountsofheating/coolingindifferent
places
ii. Airmoves(windblows)fromhighpressuretolowpressure
c. Coriolisforce
i. Notarealforce,buttheapparentdeflectionofwindsandocean
currentsthatresultsfromEarth’sspinunderneathus
ii. Alwayspushes90°totherightofmotionintheNorthern
Hemisphere(totheleftinSouthernHemisphere)
iii. Strengthofdeflectionisproportionaltospeedofmotion
d. Geographyofaircirculationandpatternsofwinds
i. “HadleyCell:”risinghotaircausedbytropicalconvergence
andrainforcesairtoblowtowardEquatornearsurface,
awayfromEquatoraloft(alsocausesrainforests!)
ii. “TradeWinds:”InflowingsurfaceairtowardEquatorial
convergenceisdeflectedtorightinNorthernHemisphere
(NETrades)andtoleftinSouthernHemisphere(SETrades)
iii. “Subtropicalsubsidence:”sinkingbranchofHadleyCellnear
30°latitudeinbothhemisphereassociatedwithdeserts
iv. “Midlatitudewesterlies:”warmedairflowingpolewardis
deflectedeastward(“westerlywind”)inbothhemispheres
e. Polarvortex,jetstreams,andwinterstorms
i. Polarairextremelycoldinpolarnight(becauseoutgoing
thermalradiationisunopposedbysolarheating)
ii. Thermalcontractionofairinpolarwintercausesverystrong
pressuregradientforcestotryto“fillitin”
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ClimateScienceforPoets
StudyGuideforMidTermExam
Spring2016
iii. “JetStreams:”Coriolisforcedeflectsair“falling”intopolar
nighttospinveryfastindirectionofEarth’sspin(toward
east,westerlyjetstreamwinds)
iv. Polarvortex/jetstreamblocksdirectpolewardheatflow
v. Wavesinjetstream(winterstorms,warm/coldfronts)arethe
mainmechanismformixingpolar&subtropicalairmasses
CirculationoftheOceans:
a. SubtropicalGyres,westernandeasternboundarycurrents
i. “EkmanTransport:”WaterispushedbyCoriolisforcetoright
ofwindinNorthernHemisphere(leftinSH)
ii. Ekmanflowmoveswatertowardtropicsinmidlats(becauseof
westerlies)andpolewardoutoftropics(becauseofeasterly
TradeWinds,causingitto“pileup”insubtropics
iii. Gyres:”Coriolisforcecauseselevatedwaterinsubtropical
oceanstorotateclockwiseinNH,counterclockwiseinSH.The
subtropicalgyrescarryhugeamountsofheatpoleward!
iv. “WesternBoundaryCurrents:”(partofgyres;e.g.Gulf
Stream)Fast-flowingcurrentsonwesternsidesofoceans
(eastcoasts)thatcarrywarmwaterpoleward
v. “EasternBoundaryCurrents:”(partofgyres;e.g.California
Current)slow-flowingcoldreturnflowoneasternsidesof
oceans(westcoasts)thatcarrycoldwatertowardtropics
vi. “Coastalupwelling:”equatorwardflowalongeastern
boundaries(westcoasts)causesEkmanflowoffshore,so
verycoldwaterisforcedtosurfacefromdepth.Causescold
deserts,lowcloudsandfog(e.g.,Baja,Namibia,Peru)
b. EquatorialOceansandElNino
i. NoCoriolisforceatEquator,sowaterTradeWindspushwarm
surfacewaterwestwardacrossequatorialoceans
ii. “Equatorialupwelling:”causedbydivergingsurfacewateras
NETradespushwaterNWandSETradespushwaterSW
iii. Warmwater“pilesup”andbecomesverydeep.“Warmpool”
(Sea-SurfaceTemp,SST>30C)thesizeofSiberiainWestern
PacificandIndianOcean,hundredsofmetersdeep!(source
ofenergyforIndianMonsoonandtorrentialrains)
iv. ColdwaterforcedtoriseinEasternPacificbecausewarm
waterpushedaway,veryproductivefisheryondesertcoast
v. “ElNino:”OccurswhenTradeWindsrelax,warmwater
slosheseastward,capsEQupwellingineasternPacific.
Monsoonrainsoftenfail,torrentialrainsineast(espPeru).
ChangesinrainfallheatingreduceEWpressuregradient,
reinforceweakeningofTradeWinds.Weatherensues!
c. Thermohaline(heat-salt)CirculationandConveyorBelt
i. HotdryNETradeWindsblowingoffofSaharaevaporatelots
offreshwaterfromsubtropicalAtlantic,leavingsaltbehind
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Spring2016
ii. SaltyNorthAtlanticwatergetsverycoldnearGreenland
iii. ColdsaltyN.Atlanticwaterverydense,sinkslikearock!Forms
“NorthAtlanticDeepWater”whichslowlyfillsoceanbottoms
iv. NADWflowssouthalongbottomofAtlanticallthewayto
Antarctic,thenaround&roundandbackupthroughPacific
andIndianOceans.SurfacewaterslowlyreturnsinAtlantic.
v. Thermohaline“conveyorbelt”takesabout1000yearstocycle
vi. Thisistheonlywaythedeepoceanever“sees”theatmosphere
vii. MovesaHUGEamountofheatpoleward,releases1/3asmuch
heattoNorthAtlanticregionasreceivedfromsunlightthere!
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