雅思阅读模拟题Suns fickle heart may leave us cold

雅思阅读模拟题：Sunsfickleheartmayleaveus

cold

Sun'sfickleheartmayleaveuscold

□25January2007

□FromNewScientistPrintEdition.

□StuartClark

1There'sadimmerswitchinsidethesunthatcausesitsbrightnesstorise

andfallontimescalesofaround100,000years-exactlythesameperiodas

betweeniceagesonEarth.Sosaysaphysicistwhohascreatedacomputermodel

ofourstar'score.

2RobertEhrlichofGeorgeMasonUniversityinFairfax,Virginia,modelled

theeffectoftemperaturefluctuationsinthesun'sinterior.Accordingtothe

standardview,thetemperatureofthesun'scoreisheldconstantbythe

opposingpressuresofgravityandnuclearfusion.However,Ehrlichbelievedthat

slightvariationsshouldbepossible.

3HetookashisstartingpointtheworkofAttilaGrandpierreofthe

KonkolyObservatoryoftheHungarianAcademyofSciences.In2005,Grandpierre

andacollaborator,Gáborágoston,calculatedthatmagneticfieldsinthesun's

corecouldproducesmallinstabilitiesinthesolarplasma.Theseinstabilities

wouldinducelocalisedoscillationsintemperature.

4Ehrlich'smodelshowsthatwhilstmostoftheseoscillationscanceleach

otherout,somereinforceoneanotherandbecomelong-livedtemperature

variations.Thefavouredfrequenciesallowthesun'scoretemperatureto

oscillatearounditsaveragetemperatureof13.6millionkelvinincycles

lastingeither100,000or41,000years.Ehrlichsaysthatrandominteractions

withinthesun'smagneticfieldcouldflipthefluctuationsfromonecycle

lengthtotheother.

5Thesetwotimescalesareinstantlyrecognisabletoanyonefamiliarwith

Earth'siceages：forthepastmillionyears,iceageshaveoccurredroughly

every100,000years.Beforethat,theyoccurredroughlyevery41,000years.

6Mostscientistsbelievethattheiceagesaretheresultofsubtle

changesinEarth'sorbit,knownastheMilankovitchcycles.Onesuchcycle

describesthewayEarth'sorbitgraduallychangesshapefromacircletoa

slightellipseandbackagainroughlyevery100,000years.Thetheorysaysthis

alterstheamountofsolarradiationthatEarthreceives,triggeringtheice

ages.However,apersistentproblemwiththistheoryhasbeenitsinabilityto

explainwhytheiceageschangedfrequencyamillionyearsago.

7"InMilankovitch,thereiscertainlynogoodideawhythefrequency

shouldchangefromonetoanother,"saysNeilEdwards,aclimatologistatthe

OpenUniversityinMiltonKeynes,UK.Noristhetransitionproblemtheonlyone

theMilankovitchtheoryfaces.Ehrlichandothercriticsclaimthatthe

temperaturevariationscausedbyMilankovitchcyclesaresimplynotbigenough

todriveiceages.

8However,Edwardsbelievesthesmallchangesinsolarheatingproducedby

MilankovitchcyclesarethenamplifiedbyfeedbackmechanismsonEarth.For

example,ifseaicebeginstoformbecauseofaslightcooling,carbondioxide

thatwouldotherwisehavefounditswayintotheatmosphereaspartofthe

carboncycleislockedintotheice.Thatweakensthegreenhouseeffectand

Earthgrowsevencolder.

9AccordingtoEdwards,thereisnolackofsuchmechanisms."Ifyouadd

theireffectstogether,thereismorethanenoughfeedbacktomakeMilankovitch

work,"hesays."Theproblemnowisidentifyingwhichmechanismsareatwork."

ThisiswhyscientistslikeEdwardsarenotyetreadytogiveuponthecurrent

theory."Milankovitchcyclesgiveusiceagesroughlywhenweobservethemto

happen.Wecancalculatewhereweareinthecycleandcompareitwith

observation,"hesays."Ican'tseeanywayoftesting[Ehrlich's]ideatosee

whereweareinthetemperatureoscillation."

10Ehrlichconcedesthis."Ifthereisawaytotestthistheoryonthe

sun,Ican'tthinkofonethatispractical,"hesays.That'sbecausevariation

over41,000to100,000yearsistoogradualtobeobserved.However,theremay

beawaytotestitinotherstars：reddwarfs.Theircoresaremuchsmaller

thanthatofthesun,andsoEhrlichbelievesthattheoscillationperiodscould

beshortenoughtobeobserved.Hehasyettocalculatethepreciseperiodor

theextentofvariationinbrightnesstobeexpected.

11NigelWeiss,asolarphysicistattheUniversityofCambridge,isfar

fromconvinced.HedescribesEhrlic