27 ICE TIME
Ihadadream,whichwasnotalladream.
Thebrightsuinguish’d,aarsDidwander...
—Byron,“Darkness”
IN1815ontheislandofSumbawainIndonesia,ahandsomeandlong-quiestmountaiamboraexplodedspectacularly,killingahuhousandpeoplewithitsblastandassociatedtsunamis.Itwasthebiggestvolicexplosiohousandyears—150timesthesizeofMountSt.Helens,equivalenttosixtythousandHiroshima-sizedatombombs.
Newsdidn’ttravelterriblyfastinthosedays.InLondoimesranasmallstory—actuallyaletterfromamert—sevenmonthsaftertheevent.ButbythistimeTambora’seffectswerealreadybeihirty-sixcubicmilesofsmokyash,dust,andgrithaddiffusedthroughtheatmosphere,obsgtheSun’sraysandcausihtocool.
Suswereunusuallybutblearilycolorful,amemorablycapturedbytheartistJ.M.
W.Turner,whocouldnothavebeenhappier,butmostlytheworldexistedunderanoppressive,duskypall.ItwasthisdeathlydimhatinspiredtheByronlinesabove.
Springnevercameandsummerneverwarmed:1816becameknownastheyearwithoutsummer.Cropseverywherefailedtogrow.InIrelandafamineandassociatedtyphoidepidemickilledsixty-fivethousandpeople.InNewEngland,theyearbecamepopularlyknoweenHundredandFrozetoDeath.MfroststiilJuneandalmostnoplantedseedwouldgrow.Shortoffodder,livestockdiedorhadtobeprematurelyslaughtered.Ineverywayitwasadreadfulyear—almostcertainlytheworstforfarmersiimes.Yetgloballythetemperaturefellbyonlyabout1.5degreesFahre.Earth’snaturalthermostat,asstistswouldlearn,isanexceedinglydelicateinstrument.
Theeeurywasalreadyachillytime.FortwohundredyearsEuropeandNorthAmeriparticularhadexperiencedaLittleIceAge,asithasbeeknown,whichpermittedallkindsofwis—frostfairsohames,ice-skatingracesalongDutals—thataremostlyimpossiblenow.Iteriod,inotherwords,whenfrigiditywasmupeople’sminds.Soerhapsexih-turygeologistsforbeingslowtorealizethattheworldtheylivedinwasinfactbalmyparedwithformerepochs,andthatmuchofthelandaroundthemhadbeenshapedbycrushingglaciersandcoldthatwouldwreckevenafrostfair.
Theykherewassomethingoddaboutthepast.TheEuropeanlandscapewaslitteredwithinexplicableanomalies—thebonesofarctideerinthewarmsouthofFrance,hugerocksstrandedinimprobableplaces—aencameupwithiivebutnotterriblyplausibleexplanations.OneFrenaturalistnameddeLuc,tryingtoexplainhowgranitebouldershadetoresthighuponthelimestoneflanksoftheJuraMountains,suggestedthatperhapstheyhadbeenshottherebypressedairincaverns,likecorksoutofapopguermforadisplacedboulderisaic,butintheeeurytheexpressiooapplymoreoftentothetheoriesthantotherocks.
ThegreatBritishgeologistArthurHallamhassuggestedthatifJamesHuttoherofgeology,hadvisitedSwitzerland,hewouldhaveseenatohesignificeofthecarvedvalleys,thepolishedstriations,thetelltalestrandlineswhererockshadbeendumped,aherabundantcluesthatpointtopassingicesheets.Unfortunately,Huttonwasnotatraveler.
Butevenwithnothierathisdisposalthansedhandats,Huttoedoutofhandtheideathathugebouldershadbeencarriedthreethousaupmountainsidesbyfloods—allthewaterintheworldwon’tmakeaboulderfloat,hepoi—andbecameohefirsttueforwidespreadglaciation.Unfortunatelyhisideasescapednotidforanotherhalfturymostnaturaliststioinsistthatthegougesonrockscouldbeattributedtopassingcartsoreventhescrapeofhobnailedboots.
Localpeasants,uninatedbystificorthodoxy,ker,however.ThenaturalistJeandeCharpeoldthestoryofhowin1834hewaswalkingalongatrylahaSwisswoodcutterwhetotalkingabouttherocksalongtheroadside.Thewoodcuttermatter-of-factlytoldhimthatthebouldershadefromtheGrimsel,azoneofgranitesomedistanceaway.“WhenIaskedhimhowhethoughtthatthesestoneshadreachedtheirlocation,heansweredwithouthesitation:‘TheGrimselglaciertransportedthemonbothsidesofthevalley,becausethatglacierextehepastasfarasthetownofBern.’”
Charpentierwasdelighted.Hehadetosuchaviewhimself,butwhenheraisedthenotionatstificgatherings,itwasdismissed.OneofCharpentier’sclosestfriendswasanotherSwissnaturalist,LouisAgassiz,whoaftersomeinitialskepticismcametoembradeventuallyallbutappropriate,thetheory.
AgassizhadstudiedunderCuvierinParisandhepostofProfessorofNaturalHistoryattheCollegeofNeuchatelinSwitzerland.AnotherfriendofAgassiz’s,abotanistnamedKarlSchimper,wasactuallythefirsttothetermiceage(inGermanEiszeit),in1837,andtoproposethattherewasgoodevideoshowthaticehadoncelainheavilyacrossnotjusttheSs,butovermuchofEurope,Asia,andNorthAmerica.ItwasaradiotioAgassizhishencameverymuchtretitasAgassizincreasinglygotthecreditforwhatSchimperfelt,withsomelegitimacy,washistheory.
Charpentierlikewiseendedupabitterenemyofhisoldfriend.AlexandervonHumboldt,yetanotherfriend,mayhavehadAgassizatleastpartlyinmindwhenheobservedthattherearethreestagesiificdiscovery:first,peopledenythatitistrue;thentheydenythatitisimportant;finallytheycreditthewrongperson.
Atallevents,Agassizmadethefieldhisown.Inhisquesttouandthedynamicsofglaciatioeverywhere—deepintodangerouscrevassesanduptothesummitsofthecraggiestAlpinepeaks,oftenapparentlyunawarethatheandhisteamwerethefirsttoclimbthem.NearlyeverywhereAgassizenteredanunyieldiaoaccepthistheories.
Humboldturgedhimtoreturntohisareaofrealexpertise,fossilfish,andgiveupthismadobsessionwithice,butAgassizossessedbyanidea.
Agassiz’stheoryfoundevenlesssupportinBritain,wheremostnaturalistshadneverseenaglacierandoftencouldn’tgraspthecrushingforcesthatibulkexerts.“Couldscratchesandpolishjustbeduetoice?”askedRoderickMurchisoninamogtooing,evidentlyimaginingtherocksascoveredinakindoflightandglassyrime.Tohisdyingday,heexpressedthefraincredulityatthose“ice-mad”geologistswhobelievedthatglacierscouldatforsomuch.WilliamHopkins,aCambridgeprofessorandleadingmemberoftheGeologicalSociety,endorsedthisview,arguingthatthenotionthaticecouldtransportboulderspresented“suchobviousmeicalabsurdities”astomakeitunworthyofthesociety’sattention.
Undaunted,Agassiztraveledtirelesslytopromotehistheory.In1840hereadapapertoameetingoftheBritishAssociationfortheAdvaofSGlasgowatwhichheenlycriticizedbythegreatCharlesLyell.ThefollowiheGeologicalSocietyofEdinburghpassedaresolutiongthattheremightbesomegeneralmeritiheorybutthatcertainlyappliedtoScotland.
Lyelldideventuallyeround.Hismomentofepiphanycamewhenherealizedthatamoraine,orlineofroearhisfamilyestateinScotland,whichhehadpassedhundredsoftimes,couldonlybeuoodifoneacceptedthataglacierhaddroppedthemthere.Buthavingbeeverted,LyellthenlosthisnerveandbackedofffrompublicsupportoftheIceAgeidea.Itwasafrustratingtimefassiz.Hismarriagewasbreakingup,Schimperwashotlyacghimofthetheftofhisideas,Charpentierwouldn’tspeaktohim,andthegreatestlivinggeologistofferedsupportofonlythemosttepidandvacillatingkind.
In1846,AgassiztraveledtoAmericatogiveaseriesoflecturesalastfoueemhecraved.Harvardgavehimaprofessorshipandbuilthimafirst-ratemuseum,theMuseumofparativeZoology.DoubtlessithelpedthathehadsettledinNewEngland,wherethelongwintersencedacertainsympathyfortheideaofinterminableperiodsofcold.ItalsohelpedthatsixyearsafterhisarrivalthefirststificexpeditiontoGreenlaedthatnearlythewholeofthatsemitiwascoveredinanicesheetjustliketheaoneimaginedinAgassiz’stheory.Atlonglast,hisideasbegantofindarealfollowing.TheoraldefectofAgassiz’stheorywasthathisiceageshadnocause.Butassistancewasabouttoefromanunlikelyquarter.
Inthe1860s,journalsandotherlearnedpublicationsinBritaioreceivepapersonhydrostatics,electricity,andotherstificsubjectsfromaJamesCrollofAnderson’sUyinGlasgow.Ohepapers,onhowvariatioh’sorbitmighthaveprecipitatediceages,ublishedinthePhilosophicalMagazinein1864andwasreizedatonceasaworkofthehigheststandard.Sotherewassomesurprise,andperhapsjustatoubarrassment,whenitturthatCrollwasnotanacademicattheuy,butajanitor.
Bornin1821,Crollgrewuppoor,andhisformaleducationlastedonlytotheageofthirteen.Heworkedatavarietyofjobs—asacarpenter,insurancesalesman,keeperofatemperael—beforetakingapositionasajanitoratAnderson’s(nowtheUyofStrathclyde)inGlasgow.Bysomehowindughisbrothertodomuchofhiswork,hewasabletopassmanyquieteveningsintheuylibraryteaghimselfphysics,meics,astronomy,hydrostatidtheotherfashionablescesoftheday,andgraduallybegantoproduceastringofpapers,articularemphasisoiohandtheireffeclimate.
CrollwasthefirsttosuggestthatcyclicalgesintheshapeofEarth’sorbit,fromelliptical(whichistosayslightlyoval)tonearlycirculartoellipticalagain,mightexplaintheoareatoficeages.Noonehadeverthoughtbeforetosideranastronomicalexplanationforvariatioh’sweather.ThanksalmosteoCroll’spersuasivetheory,peopleinBritaiobeorerespohenotionthatatsomeformertimepartsoftheEarthhadbeeninthegripofice.Whenhisiyandaptitudewerereized,CrollwasgivenajobattheGeologicalSurveyofScotlandandwidelyhonored:
hewasmadeafellowoftheRoyalSocietyinLondonandoftheNewYorkAcademyofSdgivenanhonreefromtheUyofSt.Andrews,amongmuchelse.
Unfortunately,justasAgassiz’stheorywasatlastbeginningtofindvertsinEurope,hewasbusytakingitintoevermoreexoticterritoryinAmerica.Hebegantofindevidenceflacierspracticallyeverywherehelooked,includiheequator.EventuallyhebecamevihaticehadoncecoveredthewholeEarth,extinguishingalllife,whichGodhadthenre-created.heevidenceAgassizcitedsupportedsuchaview.heless,inhisadoptedtryhisstaturegrewailhewasregardedasonlyslightlybelowadeity.Whenhediedin1873Harvardfeltitnecessarytoappointthreeprofessorstotakehisplace.
Yet,assometimeshappens,histheoriesfellswiftlyoutoffashiohanadecadeafterhisdeathhissuccessorinthechairofgeologyatHarvardwrotethatthe“so-calledglacialepoch...sopopularafewyearsagoamongglacialgeologistsmaynowberejectedwithouthesitation.”
PartoftheproblemwasthatCroll’sputationssuggestedthatthemostreticeageoccurredeightythousandyearsago,whereasthegeologicalevidencreasinglyindicatedthatEarthhadundergonesomesortofdramaticperturbationmuchmoreretlythanthat.
Withoutaplausibleexplanationforwhatmighthaveprovokedaniceage,thewholetheoryfellintoabeyahereitmighthaveremainedforsometimeexceptthatintheearly1900saSerbianacademiamedMilutinMilankovitch,whohadnobackgroundiialmotionsatall—hewasameigiraining—developedanuediier.MilankovitchrealizedthattheproblemwithCroll’stheorywasnotthatitwasincorrectbutthatitwastoosimple.
AsEarthmovesthroughspace,itissubjeotjusttovariationsinthelengthandshapeofitsorbit,butalsotorhythmicshiftsinitsangleoforientationtotheSun—itstiltandpitdwobble—allaffegthelengthandiyofsunlightfallingonanypatchofland.Inparticularitissubjecttothreegesinposition,knownformallyasitsobliquity,precession,andetricity,periodsoftime.Milankovitderediftheremightbearelationshipbetweentheseplexcyclesandtheingsandgoingsoficeages.Thedifficultywasthatthecycleswereofwidelydifferehs—ofapproximately20,000,40,000,and100,000years,butvaryingineachcasebyuptoafewthousandyears—whichmeantthatdeterminingtheirpointsofintersespansoftimeinvolvedanearlyendlessamountofdevotedputation.EssentiallyMilankovitchhadtoworkouttheangleanddurationofiningsolarradiationateverylatitudeoh,ineveryseason,foramillionyears,adjustedforthreeever-gingvariables.
HappilythisreciselythesortofrepetitivetoilthatsuitedMilankovitch’stemperament.Forthewentyyears,evenwhileonvacation,heworkedceaselesslywithpendslideruleputiablesofhiscycles—workthatnowcouldbepletedinadayortwouter.Thecalculationsallhadtobemadeinhissparetime,butin1914MilankovitchsuddenlygotagreatdealofthatwhenWorldWarIbrokeoutandhewasarrestedowingtohispositionasareservistintheSerbianarmy.HespentmostofthefouryearsunderloosehousearrestinBudapest,requiredonlytoreporttothepoliceaweek.TherestofhistimeentwinthelibraryoftheHungarianAcademyofSces.Heossiblythehappiestprisonerofwarinhistory.
Theeventualouteofhisdiligentscribblingswasthe1930bookMathematicalClimatologyaronomicalTheoryofClimatiges.Milankovitchwasrightthattherewasarelationshipbetweeniceagesandplaarywobble,thoughlikemostpeopleheassumedthatitwasagradualincreaseinharshwihatledtotheselongspellsofess.ItwasaRussian-Germaeist,WladimirK?ppen—father-in-lawofourteicfriendAlfredWegener—whosawthattheprocesswasmoresubtle,andrathermoreunnerving,thanthat.
Thecauseoficeages,K?ppendecided,istobefoundinmers,notbrutalwinters.
Ifsummersaretooeltallthesnowthatfallsonagivenarea,moreiningsunlightisbouncedbackbythereflectivesurface,exacerbatingthecoolingeffedencimoresnowtofall.Thesequencewouldtendtobeself-perpetuating.Assnowaccumulatedintoanicesheet,theregionwouldgrowcooler,promptingmoreicetoaccumulate.AstheglaciologistGwenSchultzhasnoted:“Itisnotnecessarilytheamountofsnowthatcausesicesheetsbutthefactthatsnow,howeverlittle,lasts.”Itisthoughtthataniceagecouldstartfromasingleunseasonalsummer.Theleftoversnowreflectsheatandexacerbatesthechillingeffect.“Theprocessisself-enlarging,unstoppable,andoheiceisreallygrowingitmoves,”saysMcPhee.Youhaveadvangglaciersandaniceage.
Inthe1950s,becauseofimperfectdatingteology,stistswereuocorrelateMilankovitch’scarefullyworked-outcycleswiththesupposeddatesoficeagesasthenperceived,andsoMilankovitdhiscalculationsincreasinglyfelloutoffavor.Hediedin1958,uoprovethathiscycleswerecorrect.Bythistime,writeJohnandMaryGribbin,“youwouldhavebeenhardpressedtofindageologistormeteistwhardedthemodelasbeinganythingmorethananhistoricalcuriosity.”Notuntilthe1970sandtherefiofapotassium-argohodfordatingaseafloorsedimentswerehistheoriesfinallyvindicated.
TheMilankovitchcyclesaloenoughtoexplaincyclesoficeages.Manyotherfactorsareinvolved—hedispositionofthetis,inparticularthepresenceoflandmassesoverthepoles—butthespecificsoftheseareimperfectlyuood.Ithasbeensuggested,however,thatifyouhauledNorthAmerica,Eurasia,andGreenlandjustthreehundredmilesnorthwewouldhavepermaandinescapableiceages.Weareverylucky,itappears,togetanygoodweatheratall.Evenlesswelluoodarethecyparativebalminesswithiniceages,knownasinterglacials.Itismildlyunnervingtoreflectthatthewholeofmeaningfulhumanhistory—thedevelopment,thecreationoftowns,theriseofmathematidwritingandsdalltherest—hastakenplacewithinanatypicalpatchoffairweather.Previousinterglacialshavelastedaslittleaseightthousandyears.Ourownhasalreadypasseditstenthousandthanniversary.
Thefactis,wearestillverymuaniceage;it’sjustasomewhatshrunkehoughlessshruhanmanypeoplerealize.Attheheightofthelastperiodofglaciation,arouhousandyearsago,about30pertoftheEarth’slandsurfacewasunderice.Tenpertstillis—andafurther14pertisinastateofpermafrost.Three-quartersofallthefreshwaterohislockedupiniceevennow,andwehaveicecapsatbothpoles—asituationthatmaybeuniqueih’shistory.ThattherearesnowywihroughmuchoftheworldandpermaglaciersevenintemperateplacessuewZealandmayseemquitenatural,butinfactitisamostunusualsituationforthepla.
Formostofitshistoryuntilfairlyrettimesthegeneralpatterhwastobehotwithnopermaiywhere.Thecurrenticeage—iceepochreally—startedaboutfortymillionyearsago,andhasrangedfrommurderouslybadtonotbadatall.Iceagestendtowipeoutevidenceofearliericeages,sothefurtherbackyougothemoresketchythepicturegrows,butitappearsthatwehavehadatleastseventeensevereglacialepisodesi2.5millionyearsorso—theperiodthatcideswiththeriseofHomoerectusinAfricafollowedbymodernhumans.TwoonlycitedculpritsforthepresentepocharetheriseoftheHimalayasandtheformationoftheIsthmusofPanama,thefirstdisruptingairflows,thesedocurrents.India,onisland,haspushedtwothousandkilometersintotheAsianlandmassoverthelastforty-fivemillionyears,raisingnotonlytheHimalayas,butalsothevastTibetanplateaubehihehypothesisisthatthehigherlandscapewasnotonlycooler,butdivertedwindsinawaythatmadethemflownorthandtowardNorthAmerica,makingitmoresusceptibletolong-termchills.Then,beginningaboutfivemillionyearsago,Panamarosefromthesea,closingthegapbetweenNorthandSouthAmerica,disruptingtheflowscurreweenthePacifidAtlantidgingpatternsofprecipitationacrossatleasthalftheworld.OnesequencewasadryingoutofAfrica,whichcausedapestoclimbdownoutoftreesandgolookingforanewwayoflivingontheemergingsavannas.
Atallevents,withtheosandtisarraheyarenoearsthaticewillbealong-termpartofourfuture.AcctoJohnMcPhee,aboutfiftymlacialepisodesbeexpected,eachlastingahuhousandyearsorso,beforeeforareallylongthaw.
Beforefiftymillionyearsago,Earthhadnulariceages,butwhenwedidhavethemtheyteobecolossal.Amassivefreezingoccurredabout2.2billionyearsago,followedbyabillionyearsorsoofwarmth.Thentherewasanothericeageevenlargerthanthefirst—sethatsomestistsarenowreferringtotheageinwhichitoccurredastheCryogenian,orsupericeage.TheditionismorepopularlyknownasSnowballEarth.
“Snowball,”however,barelycapturesthemurderousnessofditions.Thetheoryisthatbecauseofafallinsolarradiationofabout6pertandadropoffintheprodu(orretention)ofgreenhousegases,Earthessentiallylostitsabilitytoholdontoitsheat.Itbecameakindofall-overAntarctica.Temperaturesplungedbyasmuchas80degreesFahreheentiresurfaceoftheplamayhavefrozensolid,withoiceuptohalfamilethickathigherlatitudesandtensofyardsthickeveniropics.
Thereisaseriousprobleminallthisinthatthegeologicalevidendicatesiceeverywhere,includingaroundtheequator,whilethebiologicalevidencesuggestsjustasfirmlythattheremusthavebeeersomewhere.Forohing,obacteriasurvivedtheexperiendtheyphotosynthesize.Forthattheyneededsunlight,butasyouwillknowifyouhaveevertriedtopeerthroughit,icequicklybeesopaqueandafteronlyafewyardswouldpassonnolightatall.Twopossibilitieshavebeensuggested.Ohatalittleowaterdidremainexposed(perhapsbecauseofsomekindoflocalizedwarmingatahotspot);theotheristhatmaybetheiedinsuchawaythatitremairanslut—aditionthatdoessometimeshappeninnature.
IfEarthdidfreezeover,thenthereistheverydifficultquestionofhowitevergain.Anicyplashouldrefleuchheatthatitwouldstayfrozenforever.Itappearsthatrescuemayhaveefromourmolteninterior.Onceagain,wemaybeiedtoteicsforallowingustobehere.Theideaisthatweweresavedbyvoloes,whichpushedthroughtheburiedsurface,pumpingoutlotsofheatandgasesthatmeltedthesnowsandre-formedtheatmosphere.Iingly,theendofthishyper-frigidepisodeismarkedbytheCambrianoutburst—thespringtimeeventoflife’shistory.Infact,itmaynothavebeenastranquilasallthat.AsEarthwarmed,itprobablyhadthewildestweatherithaseverexperienced,withhurriespowerfulenoughtoraisewavestotheheightsofskyscrapersandrainfallsofindescribableiy.
Throughoutallthisthetubewormsandclamsandotherlifeformsadheringtodeepoventsundoubtedlywentonasifnothingwereamiss,butallotherlifeohprobablycameascloseasiteverhastochegoutentirely.Itwasallalongtimeagoandatthisstagewejustdon’tknow.
paredwithaCryogenianoutburst,theiceagesofmorerettimesseemprettysmallscale,butofcoursetheywereimmenselygrandbythestandardsofanythingtobefouhtoday.TheWissianicesheet,whichcoveredmuchofEuropeandNorthAmerica,wastwomilesthipladmarchedforwardatarateofaboutfourhundredfeetayear.
Whatathingitmusthavebeentobehold.Evenattheirleadingedge,theicesheetscouldbenearlyhalfamilethick.Imagiandingatthebaseofawalloficetwothousahigh.
Behindthisedge,overanareameasuringinthemillionsofsquaremiles,wouldbenothingbutmoreice,withonlyafewofthetallestmountainsummitspokingthrough.Wholetissaggeduheweightofsomuchidevennow,twelvethousandyearsaftertheglaciers’withdrawal,arestillrisingbatoplace.Theicesheetsdidn’tjustdribbleoutbouldersandlonglinesofgravellymoraines,butdumpedentirelandmasses—LongIslandandCapedNantucket,amongothers—astheyslowlysweptalong.It’slittlewohatgeologistsbefassizhadtroublegraspingtheirmoalcapacitytoreworklandscapes.
Ificesheetsadvancedagain,wehavenothinginourarmorythatcoulddeflectthem.In1964,atPrinceWilliamSoundinAlaska,ohelargestglacialfieldsinNorthAmericawashitbythestroearthquakeeverrecordedoi.Itmeasured9.2ontheRichterscale.Alongthefaultlihelandrosebyasmuchastwehequakewassoviolent,infact,thatitmadewatersloshoutofpoolsinTexas.AndwhateffectdidthisunparalleledoutbursthaveontheglaciersofPrinceWilliamSound?all.Theyjustsoakeditupaonmoving.
Foralongtimeitwasthoughtthatwemovedintoandoutoficeagesgradually,overhundredsofthousandsofyears,butthatthathashecase.ThankstoicecoresfromGreenlandwehaveadetailedrecordofclimateforsomethingoverahuhousandyears,andwhatisfoundthereisnotf.ItshowsthatformostofitsrethistoryEarthhasbeennothinglikethestableandtranquilplacethatcivilizationhasknown,butratherhaslurchedviolentlybetweenperiodsofwarmthandbrutalchill.
Towardtheendofthelastbigglaciation,sometwelvethousandyearsago,Earthbegantowarm,andquiterapidly,butthenabruptlyplungedbatobittercoldforathousandyearsorsoiknowntosceastheYoungerDryas.(Thenameesfromthearctitthedryas,whichisohefirsttoreizelandafteranicesheetwithdraws.TherewasalsoanOlderDryasperiod,butitwasn’tsosharp.)Attheendofthisthousand-yearonslaughtaveragetemperaturesleaptagain,byasmuchassevendegreesiyyears,whichdoesn’tsoundterriblydramaticbutisequivalenttoexgingtheclimateofSdinaviaforthatoftheMediterraneaninjusttwodecades.Locally,geshavebeenevenmoredramatic.Greenlandicecoresshowthetemperaturestheregingbyasmuchasfifteendegreesintenyears,drasticallyalteringrainfallpatternsandgrowingditions.Thismusthavebeelingenoughonathinlypopulatedplaodaythesequenceswouldbeprettywellunimaginable.
Whatismostalarmingisthatwehavenoidea—none—whatnaturalphenomenacouldsoswiftlyrattleEarth’sthermometer.AsElizabethKolbert,writingintheNewYorker,hasobserved:“Nokernalforce,orevenanythathasbeenhypothesized,seemscapableofyankiemperaturebadforthasviolently,andasoften,asthesecoreshaveshowhecase.”Thereseemstobe,sheadds,“somevastandterriblefeedbackloop,”
probablyinvolvingtheosanddisruptionsofthenormalpatternsofocirculation,butallthisisalongwayfrombeinguood.
OheoryisthattheheavyinflowofmeltwatertotheseasatthebeginningoftheYoungerDryasreducedthesaltiness(andthusdensity)ofnorthernos,causingtheGulfStreamtoswervetothesouth,likeadrivertryingtoavoidacollision.DeprivedoftheGulfStream’swarmth,thenorthernlatitudesreturochillyditions.ButthisdoesoexplainwhyathousandyearslaterwhehwarmedonceagaintheGulfStreamdidn’tveerasbefore.Instead,weweregiventheperiodofunusualtranquilityknownastheHoloe,thetimeinwhichwelivenow.
Thereisnoreasontosupposethatthisstretchofclimaticstabilityshouldlastmuger.
Infaeauthoritiesbelievethatweareinforevenworsethanwhatwentbefore.ItisnaturaltosupposethatglobalwarmingwouldactasausefulterweighttotheEarth’steoplungebatoglacialditions.However,asKolberthaspoi,whenyouarefrohafluctuatingandunpredic松语文学www.16sy.coM免费小说阅读
Thebrightsuinguish’d,aarsDidwander...
—Byron,“Darkness”
IN1815ontheislandofSumbawainIndonesia,ahandsomeandlong-quiestmountaiamboraexplodedspectacularly,killingahuhousandpeoplewithitsblastandassociatedtsunamis.Itwasthebiggestvolicexplosiohousandyears—150timesthesizeofMountSt.Helens,equivalenttosixtythousandHiroshima-sizedatombombs.
Newsdidn’ttravelterriblyfastinthosedays.InLondoimesranasmallstory—actuallyaletterfromamert—sevenmonthsaftertheevent.ButbythistimeTambora’seffectswerealreadybeihirty-sixcubicmilesofsmokyash,dust,andgrithaddiffusedthroughtheatmosphere,obsgtheSun’sraysandcausihtocool.
Suswereunusuallybutblearilycolorful,amemorablycapturedbytheartistJ.M.
W.Turner,whocouldnothavebeenhappier,butmostlytheworldexistedunderanoppressive,duskypall.ItwasthisdeathlydimhatinspiredtheByronlinesabove.
Springnevercameandsummerneverwarmed:1816becameknownastheyearwithoutsummer.Cropseverywherefailedtogrow.InIrelandafamineandassociatedtyphoidepidemickilledsixty-fivethousandpeople.InNewEngland,theyearbecamepopularlyknoweenHundredandFrozetoDeath.MfroststiilJuneandalmostnoplantedseedwouldgrow.Shortoffodder,livestockdiedorhadtobeprematurelyslaughtered.Ineverywayitwasadreadfulyear—almostcertainlytheworstforfarmersiimes.Yetgloballythetemperaturefellbyonlyabout1.5degreesFahre.Earth’snaturalthermostat,asstistswouldlearn,isanexceedinglydelicateinstrument.
Theeeurywasalreadyachillytime.FortwohundredyearsEuropeandNorthAmeriparticularhadexperiencedaLittleIceAge,asithasbeeknown,whichpermittedallkindsofwis—frostfairsohames,ice-skatingracesalongDutals—thataremostlyimpossiblenow.Iteriod,inotherwords,whenfrigiditywasmupeople’sminds.Soerhapsexih-turygeologistsforbeingslowtorealizethattheworldtheylivedinwasinfactbalmyparedwithformerepochs,andthatmuchofthelandaroundthemhadbeenshapedbycrushingglaciersandcoldthatwouldwreckevenafrostfair.
Theykherewassomethingoddaboutthepast.TheEuropeanlandscapewaslitteredwithinexplicableanomalies—thebonesofarctideerinthewarmsouthofFrance,hugerocksstrandedinimprobableplaces—aencameupwithiivebutnotterriblyplausibleexplanations.OneFrenaturalistnameddeLuc,tryingtoexplainhowgranitebouldershadetoresthighuponthelimestoneflanksoftheJuraMountains,suggestedthatperhapstheyhadbeenshottherebypressedairincaverns,likecorksoutofapopguermforadisplacedboulderisaic,butintheeeurytheexpressiooapplymoreoftentothetheoriesthantotherocks.
ThegreatBritishgeologistArthurHallamhassuggestedthatifJamesHuttoherofgeology,hadvisitedSwitzerland,hewouldhaveseenatohesignificeofthecarvedvalleys,thepolishedstriations,thetelltalestrandlineswhererockshadbeendumped,aherabundantcluesthatpointtopassingicesheets.Unfortunately,Huttonwasnotatraveler.
Butevenwithnothierathisdisposalthansedhandats,Huttoedoutofhandtheideathathugebouldershadbeencarriedthreethousaupmountainsidesbyfloods—allthewaterintheworldwon’tmakeaboulderfloat,hepoi—andbecameohefirsttueforwidespreadglaciation.Unfortunatelyhisideasescapednotidforanotherhalfturymostnaturaliststioinsistthatthegougesonrockscouldbeattributedtopassingcartsoreventhescrapeofhobnailedboots.
Localpeasants,uninatedbystificorthodoxy,ker,however.ThenaturalistJeandeCharpeoldthestoryofhowin1834hewaswalkingalongatrylahaSwisswoodcutterwhetotalkingabouttherocksalongtheroadside.Thewoodcuttermatter-of-factlytoldhimthatthebouldershadefromtheGrimsel,azoneofgranitesomedistanceaway.“WhenIaskedhimhowhethoughtthatthesestoneshadreachedtheirlocation,heansweredwithouthesitation:‘TheGrimselglaciertransportedthemonbothsidesofthevalley,becausethatglacierextehepastasfarasthetownofBern.’”
Charpentierwasdelighted.Hehadetosuchaviewhimself,butwhenheraisedthenotionatstificgatherings,itwasdismissed.OneofCharpentier’sclosestfriendswasanotherSwissnaturalist,LouisAgassiz,whoaftersomeinitialskepticismcametoembradeventuallyallbutappropriate,thetheory.
AgassizhadstudiedunderCuvierinParisandhepostofProfessorofNaturalHistoryattheCollegeofNeuchatelinSwitzerland.AnotherfriendofAgassiz’s,abotanistnamedKarlSchimper,wasactuallythefirsttothetermiceage(inGermanEiszeit),in1837,andtoproposethattherewasgoodevideoshowthaticehadoncelainheavilyacrossnotjusttheSs,butovermuchofEurope,Asia,andNorthAmerica.ItwasaradiotioAgassizhishencameverymuchtretitasAgassizincreasinglygotthecreditforwhatSchimperfelt,withsomelegitimacy,washistheory.
Charpentierlikewiseendedupabitterenemyofhisoldfriend.AlexandervonHumboldt,yetanotherfriend,mayhavehadAgassizatleastpartlyinmindwhenheobservedthattherearethreestagesiificdiscovery:first,peopledenythatitistrue;thentheydenythatitisimportant;finallytheycreditthewrongperson.
Atallevents,Agassizmadethefieldhisown.Inhisquesttouandthedynamicsofglaciatioeverywhere—deepintodangerouscrevassesanduptothesummitsofthecraggiestAlpinepeaks,oftenapparentlyunawarethatheandhisteamwerethefirsttoclimbthem.NearlyeverywhereAgassizenteredanunyieldiaoaccepthistheories.
Humboldturgedhimtoreturntohisareaofrealexpertise,fossilfish,andgiveupthismadobsessionwithice,butAgassizossessedbyanidea.
Agassiz’stheoryfoundevenlesssupportinBritain,wheremostnaturalistshadneverseenaglacierandoftencouldn’tgraspthecrushingforcesthatibulkexerts.“Couldscratchesandpolishjustbeduetoice?”askedRoderickMurchisoninamogtooing,evidentlyimaginingtherocksascoveredinakindoflightandglassyrime.Tohisdyingday,heexpressedthefraincredulityatthose“ice-mad”geologistswhobelievedthatglacierscouldatforsomuch.WilliamHopkins,aCambridgeprofessorandleadingmemberoftheGeologicalSociety,endorsedthisview,arguingthatthenotionthaticecouldtransportboulderspresented“suchobviousmeicalabsurdities”astomakeitunworthyofthesociety’sattention.
Undaunted,Agassiztraveledtirelesslytopromotehistheory.In1840hereadapapertoameetingoftheBritishAssociationfortheAdvaofSGlasgowatwhichheenlycriticizedbythegreatCharlesLyell.ThefollowiheGeologicalSocietyofEdinburghpassedaresolutiongthattheremightbesomegeneralmeritiheorybutthatcertainlyappliedtoScotland.
Lyelldideventuallyeround.Hismomentofepiphanycamewhenherealizedthatamoraine,orlineofroearhisfamilyestateinScotland,whichhehadpassedhundredsoftimes,couldonlybeuoodifoneacceptedthataglacierhaddroppedthemthere.Buthavingbeeverted,LyellthenlosthisnerveandbackedofffrompublicsupportoftheIceAgeidea.Itwasafrustratingtimefassiz.Hismarriagewasbreakingup,Schimperwashotlyacghimofthetheftofhisideas,Charpentierwouldn’tspeaktohim,andthegreatestlivinggeologistofferedsupportofonlythemosttepidandvacillatingkind.
In1846,AgassiztraveledtoAmericatogiveaseriesoflecturesalastfoueemhecraved.Harvardgavehimaprofessorshipandbuilthimafirst-ratemuseum,theMuseumofparativeZoology.DoubtlessithelpedthathehadsettledinNewEngland,wherethelongwintersencedacertainsympathyfortheideaofinterminableperiodsofcold.ItalsohelpedthatsixyearsafterhisarrivalthefirststificexpeditiontoGreenlaedthatnearlythewholeofthatsemitiwascoveredinanicesheetjustliketheaoneimaginedinAgassiz’stheory.Atlonglast,hisideasbegantofindarealfollowing.TheoraldefectofAgassiz’stheorywasthathisiceageshadnocause.Butassistancewasabouttoefromanunlikelyquarter.
Inthe1860s,journalsandotherlearnedpublicationsinBritaioreceivepapersonhydrostatics,electricity,andotherstificsubjectsfromaJamesCrollofAnderson’sUyinGlasgow.Ohepapers,onhowvariatioh’sorbitmighthaveprecipitatediceages,ublishedinthePhilosophicalMagazinein1864andwasreizedatonceasaworkofthehigheststandard.Sotherewassomesurprise,andperhapsjustatoubarrassment,whenitturthatCrollwasnotanacademicattheuy,butajanitor.
Bornin1821,Crollgrewuppoor,andhisformaleducationlastedonlytotheageofthirteen.Heworkedatavarietyofjobs—asacarpenter,insurancesalesman,keeperofatemperael—beforetakingapositionasajanitoratAnderson’s(nowtheUyofStrathclyde)inGlasgow.Bysomehowindughisbrothertodomuchofhiswork,hewasabletopassmanyquieteveningsintheuylibraryteaghimselfphysics,meics,astronomy,hydrostatidtheotherfashionablescesoftheday,andgraduallybegantoproduceastringofpapers,articularemphasisoiohandtheireffeclimate.
CrollwasthefirsttosuggestthatcyclicalgesintheshapeofEarth’sorbit,fromelliptical(whichistosayslightlyoval)tonearlycirculartoellipticalagain,mightexplaintheoareatoficeages.Noonehadeverthoughtbeforetosideranastronomicalexplanationforvariatioh’sweather.ThanksalmosteoCroll’spersuasivetheory,peopleinBritaiobeorerespohenotionthatatsomeformertimepartsoftheEarthhadbeeninthegripofice.Whenhisiyandaptitudewerereized,CrollwasgivenajobattheGeologicalSurveyofScotlandandwidelyhonored:
hewasmadeafellowoftheRoyalSocietyinLondonandoftheNewYorkAcademyofSdgivenanhonreefromtheUyofSt.Andrews,amongmuchelse.
Unfortunately,justasAgassiz’stheorywasatlastbeginningtofindvertsinEurope,hewasbusytakingitintoevermoreexoticterritoryinAmerica.Hebegantofindevidenceflacierspracticallyeverywherehelooked,includiheequator.EventuallyhebecamevihaticehadoncecoveredthewholeEarth,extinguishingalllife,whichGodhadthenre-created.heevidenceAgassizcitedsupportedsuchaview.heless,inhisadoptedtryhisstaturegrewailhewasregardedasonlyslightlybelowadeity.Whenhediedin1873Harvardfeltitnecessarytoappointthreeprofessorstotakehisplace.
Yet,assometimeshappens,histheoriesfellswiftlyoutoffashiohanadecadeafterhisdeathhissuccessorinthechairofgeologyatHarvardwrotethatthe“so-calledglacialepoch...sopopularafewyearsagoamongglacialgeologistsmaynowberejectedwithouthesitation.”
PartoftheproblemwasthatCroll’sputationssuggestedthatthemostreticeageoccurredeightythousandyearsago,whereasthegeologicalevidencreasinglyindicatedthatEarthhadundergonesomesortofdramaticperturbationmuchmoreretlythanthat.
Withoutaplausibleexplanationforwhatmighthaveprovokedaniceage,thewholetheoryfellintoabeyahereitmighthaveremainedforsometimeexceptthatintheearly1900saSerbianacademiamedMilutinMilankovitch,whohadnobackgroundiialmotionsatall—hewasameigiraining—developedanuediier.MilankovitchrealizedthattheproblemwithCroll’stheorywasnotthatitwasincorrectbutthatitwastoosimple.
AsEarthmovesthroughspace,itissubjeotjusttovariationsinthelengthandshapeofitsorbit,butalsotorhythmicshiftsinitsangleoforientationtotheSun—itstiltandpitdwobble—allaffegthelengthandiyofsunlightfallingonanypatchofland.Inparticularitissubjecttothreegesinposition,knownformallyasitsobliquity,precession,andetricity,periodsoftime.Milankovitderediftheremightbearelationshipbetweentheseplexcyclesandtheingsandgoingsoficeages.Thedifficultywasthatthecycleswereofwidelydifferehs—ofapproximately20,000,40,000,and100,000years,butvaryingineachcasebyuptoafewthousandyears—whichmeantthatdeterminingtheirpointsofintersespansoftimeinvolvedanearlyendlessamountofdevotedputation.EssentiallyMilankovitchhadtoworkouttheangleanddurationofiningsolarradiationateverylatitudeoh,ineveryseason,foramillionyears,adjustedforthreeever-gingvariables.
HappilythisreciselythesortofrepetitivetoilthatsuitedMilankovitch’stemperament.Forthewentyyears,evenwhileonvacation,heworkedceaselesslywithpendslideruleputiablesofhiscycles—workthatnowcouldbepletedinadayortwouter.Thecalculationsallhadtobemadeinhissparetime,butin1914MilankovitchsuddenlygotagreatdealofthatwhenWorldWarIbrokeoutandhewasarrestedowingtohispositionasareservistintheSerbianarmy.HespentmostofthefouryearsunderloosehousearrestinBudapest,requiredonlytoreporttothepoliceaweek.TherestofhistimeentwinthelibraryoftheHungarianAcademyofSces.Heossiblythehappiestprisonerofwarinhistory.
Theeventualouteofhisdiligentscribblingswasthe1930bookMathematicalClimatologyaronomicalTheoryofClimatiges.Milankovitchwasrightthattherewasarelationshipbetweeniceagesandplaarywobble,thoughlikemostpeopleheassumedthatitwasagradualincreaseinharshwihatledtotheselongspellsofess.ItwasaRussian-Germaeist,WladimirK?ppen—father-in-lawofourteicfriendAlfredWegener—whosawthattheprocesswasmoresubtle,andrathermoreunnerving,thanthat.
Thecauseoficeages,K?ppendecided,istobefoundinmers,notbrutalwinters.
Ifsummersaretooeltallthesnowthatfallsonagivenarea,moreiningsunlightisbouncedbackbythereflectivesurface,exacerbatingthecoolingeffedencimoresnowtofall.Thesequencewouldtendtobeself-perpetuating.Assnowaccumulatedintoanicesheet,theregionwouldgrowcooler,promptingmoreicetoaccumulate.AstheglaciologistGwenSchultzhasnoted:“Itisnotnecessarilytheamountofsnowthatcausesicesheetsbutthefactthatsnow,howeverlittle,lasts.”Itisthoughtthataniceagecouldstartfromasingleunseasonalsummer.Theleftoversnowreflectsheatandexacerbatesthechillingeffect.“Theprocessisself-enlarging,unstoppable,andoheiceisreallygrowingitmoves,”saysMcPhee.Youhaveadvangglaciersandaniceage.
Inthe1950s,becauseofimperfectdatingteology,stistswereuocorrelateMilankovitch’scarefullyworked-outcycleswiththesupposeddatesoficeagesasthenperceived,andsoMilankovitdhiscalculationsincreasinglyfelloutoffavor.Hediedin1958,uoprovethathiscycleswerecorrect.Bythistime,writeJohnandMaryGribbin,“youwouldhavebeenhardpressedtofindageologistormeteistwhardedthemodelasbeinganythingmorethananhistoricalcuriosity.”Notuntilthe1970sandtherefiofapotassium-argohodfordatingaseafloorsedimentswerehistheoriesfinallyvindicated.
TheMilankovitchcyclesaloenoughtoexplaincyclesoficeages.Manyotherfactorsareinvolved—hedispositionofthetis,inparticularthepresenceoflandmassesoverthepoles—butthespecificsoftheseareimperfectlyuood.Ithasbeensuggested,however,thatifyouhauledNorthAmerica,Eurasia,andGreenlandjustthreehundredmilesnorthwewouldhavepermaandinescapableiceages.Weareverylucky,itappears,togetanygoodweatheratall.Evenlesswelluoodarethecyparativebalminesswithiniceages,knownasinterglacials.Itismildlyunnervingtoreflectthatthewholeofmeaningfulhumanhistory—thedevelopment,thecreationoftowns,theriseofmathematidwritingandsdalltherest—hastakenplacewithinanatypicalpatchoffairweather.Previousinterglacialshavelastedaslittleaseightthousandyears.Ourownhasalreadypasseditstenthousandthanniversary.
Thefactis,wearestillverymuaniceage;it’sjustasomewhatshrunkehoughlessshruhanmanypeoplerealize.Attheheightofthelastperiodofglaciation,arouhousandyearsago,about30pertoftheEarth’slandsurfacewasunderice.Tenpertstillis—andafurther14pertisinastateofpermafrost.Three-quartersofallthefreshwaterohislockedupiniceevennow,andwehaveicecapsatbothpoles—asituationthatmaybeuniqueih’shistory.ThattherearesnowywihroughmuchoftheworldandpermaglaciersevenintemperateplacessuewZealandmayseemquitenatural,butinfactitisamostunusualsituationforthepla.
Formostofitshistoryuntilfairlyrettimesthegeneralpatterhwastobehotwithnopermaiywhere.Thecurrenticeage—iceepochreally—startedaboutfortymillionyearsago,andhasrangedfrommurderouslybadtonotbadatall.Iceagestendtowipeoutevidenceofearliericeages,sothefurtherbackyougothemoresketchythepicturegrows,butitappearsthatwehavehadatleastseventeensevereglacialepisodesi2.5millionyearsorso—theperiodthatcideswiththeriseofHomoerectusinAfricafollowedbymodernhumans.TwoonlycitedculpritsforthepresentepocharetheriseoftheHimalayasandtheformationoftheIsthmusofPanama,thefirstdisruptingairflows,thesedocurrents.India,onisland,haspushedtwothousandkilometersintotheAsianlandmassoverthelastforty-fivemillionyears,raisingnotonlytheHimalayas,butalsothevastTibetanplateaubehihehypothesisisthatthehigherlandscapewasnotonlycooler,butdivertedwindsinawaythatmadethemflownorthandtowardNorthAmerica,makingitmoresusceptibletolong-termchills.Then,beginningaboutfivemillionyearsago,Panamarosefromthesea,closingthegapbetweenNorthandSouthAmerica,disruptingtheflowscurreweenthePacifidAtlantidgingpatternsofprecipitationacrossatleasthalftheworld.OnesequencewasadryingoutofAfrica,whichcausedapestoclimbdownoutoftreesandgolookingforanewwayoflivingontheemergingsavannas.
Atallevents,withtheosandtisarraheyarenoearsthaticewillbealong-termpartofourfuture.AcctoJohnMcPhee,aboutfiftymlacialepisodesbeexpected,eachlastingahuhousandyearsorso,beforeeforareallylongthaw.
Beforefiftymillionyearsago,Earthhadnulariceages,butwhenwedidhavethemtheyteobecolossal.Amassivefreezingoccurredabout2.2billionyearsago,followedbyabillionyearsorsoofwarmth.Thentherewasanothericeageevenlargerthanthefirst—sethatsomestistsarenowreferringtotheageinwhichitoccurredastheCryogenian,orsupericeage.TheditionismorepopularlyknownasSnowballEarth.
“Snowball,”however,barelycapturesthemurderousnessofditions.Thetheoryisthatbecauseofafallinsolarradiationofabout6pertandadropoffintheprodu(orretention)ofgreenhousegases,Earthessentiallylostitsabilitytoholdontoitsheat.Itbecameakindofall-overAntarctica.Temperaturesplungedbyasmuchas80degreesFahreheentiresurfaceoftheplamayhavefrozensolid,withoiceuptohalfamilethickathigherlatitudesandtensofyardsthickeveniropics.
Thereisaseriousprobleminallthisinthatthegeologicalevidendicatesiceeverywhere,includingaroundtheequator,whilethebiologicalevidencesuggestsjustasfirmlythattheremusthavebeeersomewhere.Forohing,obacteriasurvivedtheexperiendtheyphotosynthesize.Forthattheyneededsunlight,butasyouwillknowifyouhaveevertriedtopeerthroughit,icequicklybeesopaqueandafteronlyafewyardswouldpassonnolightatall.Twopossibilitieshavebeensuggested.Ohatalittleowaterdidremainexposed(perhapsbecauseofsomekindoflocalizedwarmingatahotspot);theotheristhatmaybetheiedinsuchawaythatitremairanslut—aditionthatdoessometimeshappeninnature.
IfEarthdidfreezeover,thenthereistheverydifficultquestionofhowitevergain.Anicyplashouldrefleuchheatthatitwouldstayfrozenforever.Itappearsthatrescuemayhaveefromourmolteninterior.Onceagain,wemaybeiedtoteicsforallowingustobehere.Theideaisthatweweresavedbyvoloes,whichpushedthroughtheburiedsurface,pumpingoutlotsofheatandgasesthatmeltedthesnowsandre-formedtheatmosphere.Iingly,theendofthishyper-frigidepisodeismarkedbytheCambrianoutburst—thespringtimeeventoflife’shistory.Infact,itmaynothavebeenastranquilasallthat.AsEarthwarmed,itprobablyhadthewildestweatherithaseverexperienced,withhurriespowerfulenoughtoraisewavestotheheightsofskyscrapersandrainfallsofindescribableiy.
Throughoutallthisthetubewormsandclamsandotherlifeformsadheringtodeepoventsundoubtedlywentonasifnothingwereamiss,butallotherlifeohprobablycameascloseasiteverhastochegoutentirely.Itwasallalongtimeagoandatthisstagewejustdon’tknow.
paredwithaCryogenianoutburst,theiceagesofmorerettimesseemprettysmallscale,butofcoursetheywereimmenselygrandbythestandardsofanythingtobefouhtoday.TheWissianicesheet,whichcoveredmuchofEuropeandNorthAmerica,wastwomilesthipladmarchedforwardatarateofaboutfourhundredfeetayear.
Whatathingitmusthavebeentobehold.Evenattheirleadingedge,theicesheetscouldbenearlyhalfamilethick.Imagiandingatthebaseofawalloficetwothousahigh.
Behindthisedge,overanareameasuringinthemillionsofsquaremiles,wouldbenothingbutmoreice,withonlyafewofthetallestmountainsummitspokingthrough.Wholetissaggeduheweightofsomuchidevennow,twelvethousandyearsaftertheglaciers’withdrawal,arestillrisingbatoplace.Theicesheetsdidn’tjustdribbleoutbouldersandlonglinesofgravellymoraines,butdumpedentirelandmasses—LongIslandandCapedNantucket,amongothers—astheyslowlysweptalong.It’slittlewohatgeologistsbefassizhadtroublegraspingtheirmoalcapacitytoreworklandscapes.
Ificesheetsadvancedagain,wehavenothinginourarmorythatcoulddeflectthem.In1964,atPrinceWilliamSoundinAlaska,ohelargestglacialfieldsinNorthAmericawashitbythestroearthquakeeverrecordedoi.Itmeasured9.2ontheRichterscale.Alongthefaultlihelandrosebyasmuchastwehequakewassoviolent,infact,thatitmadewatersloshoutofpoolsinTexas.AndwhateffectdidthisunparalleledoutbursthaveontheglaciersofPrinceWilliamSound?all.Theyjustsoakeditupaonmoving.
Foralongtimeitwasthoughtthatwemovedintoandoutoficeagesgradually,overhundredsofthousandsofyears,butthatthathashecase.ThankstoicecoresfromGreenlandwehaveadetailedrecordofclimateforsomethingoverahuhousandyears,andwhatisfoundthereisnotf.ItshowsthatformostofitsrethistoryEarthhasbeennothinglikethestableandtranquilplacethatcivilizationhasknown,butratherhaslurchedviolentlybetweenperiodsofwarmthandbrutalchill.
Towardtheendofthelastbigglaciation,sometwelvethousandyearsago,Earthbegantowarm,andquiterapidly,butthenabruptlyplungedbatobittercoldforathousandyearsorsoiknowntosceastheYoungerDryas.(Thenameesfromthearctitthedryas,whichisohefirsttoreizelandafteranicesheetwithdraws.TherewasalsoanOlderDryasperiod,butitwasn’tsosharp.)Attheendofthisthousand-yearonslaughtaveragetemperaturesleaptagain,byasmuchassevendegreesiyyears,whichdoesn’tsoundterriblydramaticbutisequivalenttoexgingtheclimateofSdinaviaforthatoftheMediterraneaninjusttwodecades.Locally,geshavebeenevenmoredramatic.Greenlandicecoresshowthetemperaturestheregingbyasmuchasfifteendegreesintenyears,drasticallyalteringrainfallpatternsandgrowingditions.Thismusthavebeelingenoughonathinlypopulatedplaodaythesequenceswouldbeprettywellunimaginable.
Whatismostalarmingisthatwehavenoidea—none—whatnaturalphenomenacouldsoswiftlyrattleEarth’sthermometer.AsElizabethKolbert,writingintheNewYorker,hasobserved:“Nokernalforce,orevenanythathasbeenhypothesized,seemscapableofyankiemperaturebadforthasviolently,andasoften,asthesecoreshaveshowhecase.”Thereseemstobe,sheadds,“somevastandterriblefeedbackloop,”
probablyinvolvingtheosanddisruptionsofthenormalpatternsofocirculation,butallthisisalongwayfrombeinguood.
OheoryisthattheheavyinflowofmeltwatertotheseasatthebeginningoftheYoungerDryasreducedthesaltiness(andthusdensity)ofnorthernos,causingtheGulfStreamtoswervetothesouth,likeadrivertryingtoavoidacollision.DeprivedoftheGulfStream’swarmth,thenorthernlatitudesreturochillyditions.ButthisdoesoexplainwhyathousandyearslaterwhehwarmedonceagaintheGulfStreamdidn’tveerasbefore.Instead,weweregiventheperiodofunusualtranquilityknownastheHoloe,thetimeinwhichwelivenow.
Thereisnoreasontosupposethatthisstretchofclimaticstabilityshouldlastmuger.
Infaeauthoritiesbelievethatweareinforevenworsethanwhatwentbefore.ItisnaturaltosupposethatglobalwarmingwouldactasausefulterweighttotheEarth’steoplungebatoglacialditions.However,asKolberthaspoi,whenyouarefrohafluctuatingandunpredic松语文学www.16sy.coM免费小说阅读
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