on the astronomical forcing of simple conceptual ice age ... · on the astronomical forcing of...
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- V,normalized ice volume(arbitray units)
- I,normalized summer insolationat65°N(arbitrary units)
- 2statesmodel:glaciationanddeglaciation
- 𝜏i,𝜏g, 𝜏d:timeconstants(kyr)
- Glaciationstate :dV/dt =-I/𝜏i +1/𝜏g- Deglaciation state:dV/dt =-I/ 𝜏i – V/ 𝜏d
• Glaciationstart :when insolationis lowenough (I<I0).
• Deglaciation start :when combination ofinsolationandice volumereaches thedeglaciation threshold V0 (I+V>V0).
• Ice volumemodel,adapted from [1] with summer insolationat65°NasinputØWhat is theinfluenceofthesummer insolationchoice onthemodelresults ?
Several summer insolationdefinitions :
- Insolationatthesummer solstice(blue)
- Caloric season insolation:energy received duringthehalf oftheyear with thegreatest insolation
intensity (brown)
- Integratedsummer insolation(ISI)above agiventhreshold [2]:
- Above 300W/m2 (violet)- Above 400W/m2 (orange)
On the astronomical forcing of simple conceptual ice age modelsGaëlleLeloup1,2 ([email protected]),DidierPaillard1
1LaboratoiredesSciencesduClimatetdel’Environnement(CEA-CNRS-UVSQ),2ANDRA
Fig 1b):SpectralanalysisFig 1a):Normalized summer insolation
Obliquity Precession
« 100kyr world »Larger ice sheets
« 41kyr world »Smaller ice sheets1000kyr BP 1800kyr BP Time(kyr BP)
5 :Optimalthreshold V0
Only thesolsticeinsolationmodelworks forthewhole Myr with aconstantV0.Forthe3other summer insolation,thelastdeglaciation occurs oneinsolationpeak too early (atMIS3,around 50kyr BP)butwould occur attherighttimewith anincreased deglaciation threshold V0 forthelasttransition.
TheoptimalV0 is lower than forthe0– 1Myr BPperiod foreach insolation.Model/dataagreementis notasgoodasforthe0– 1Myr BPperiod.Noclear trend(increase ordecrease)oftheoptimaldeglaciation threshold V0between the1– 1.8and1.8– 2.6Myr BPperiods.
Ice age conceptual model Summer insolation
ModelresultsDatacomparison
Conclusions References
Power
[1]Amplitudeandphaseofglacialcyclesfromaconceptualmodel,Parrenin Paillard,2003,EPSL.[2]EarlyPleistoceneGlacialCyclesandtheIntegratedSummerInsolationForcing,Huybers etal.,2006,Science[3]Asimpleruletodeterminewhichinsolationcyclesleadtointerglacials,Tzedakis etal.,2017,Nature
Fig 2c):Modelresults for1.8– 2.6Myr BP
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Similar curves butdifferent contributionsfromobliquity andprecession
Thedeglaciationthreshold V0 is heldconstantovereachtimeperiod andchoosen tofitbestthedata(optimalthresholdV0).Modelresults anddataarenormalized oneachtimeperiod.
Ice ages can be well represented bya2state- glaciationanddeglaciation -conceptual model.
Mostofthetransitionsaremodelled correctly,buttheice volumearound1180,1830and2030kyr BPperiods is not,irrespective oftheinsolationused.
Some transitionsaremoresensitivetotheinputinsolation(I)andthedeglaciation threshold V0 than others.
Larger glaciations(« 100kyr world »)correspondtohigher deglaciationthreshold V0 than smaller glaciations(« 41kyr world »). Fig 3)optimaldeglaciationthresholdV0
foreach timeperiod
PerspectivesandopenquestionsØ Which physical mechanisms could explain thechangesover
timeofthedeglaciation threshold V0 ?
Ø Howsensitivetoparameter changesis thetimingofdeglaciations ?Andwhat arethemost frequentalternativedeglaciations with different parameters ?
Results suggest that thelastdeglaciation correspondstoafurther increase inV0,toavoid atoo early deglaciation atMIS3.
Asfound in[3],adeglaciation atMIS3(around 50kyr BP)instead ofMIS1is afrequent alternative.
Fig 2a) :Modelresults for0-1MyrBP Fig 2b) :Modelresults for1- 1.8Myr BP
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dicevolume
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dicevolume
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𝜏i, 𝜏g, 𝜏daresetto9,30,12kyrForeach insolationandtimeperiod thedeglaciation threshold V0 is varried.Results fortheV0 valuewhich bestfits thedataaredisplayed (optimalV0).
V0 V0 V01180kyr BP 1830kyr BP 2030kyr BP
MIS3