Deglaciation LGM climate controlled byLGM climate controlled by

Ice sheets and Ice sheets and atmospheric COatmospheric CO22

Deglacial world shiftDeglacial world shift Higher insolation and COHigher insolation and CO22

Smaller ice sheetsSmaller ice sheets As insolation increasedAs insolation increased

Ice sheets meltedIce sheets melted• Influenced climate Influenced climate

much lessmuch lessCOCO22 had a largely had a largely

secondary role in secondary role in climateclimate

Timing of Ice Sheet Melting Determined by dating Determined by dating

organic remains organic remains formed during ice formed during ice sheet retreatsheet retreat Although scarce, Although scarce,

suitable samples suitable samples existexist

N. American ice N. American ice sheets began sheets began retreating 14,000 retreating 14,000 1414C C years agoyears ago Gone by 6,000 Gone by 6,000 1414C C

years agoyears ago Area does not yield Area does not yield

ice volumeice volume Thickness of ice Thickness of ice

debatabledebatable

Sea Level from Coral Reefs One meter of sea level rise = 0.4 million kmOne meter of sea level rise = 0.4 million km33 of ice of ice

Total global ice volume can be compared with Total global ice volume can be compared with insolation recordinsolation record

Coral reefs on Barbados gave sea level historyCoral reefs on Barbados gave sea level history Know Barbados had minor subsidenceKnow Barbados had minor subsidence 1414C dated sea level curve supports expectationsC dated sea level curve supports expectations

Rate of sea level rise maximized during maximum Rate of sea level rise maximized during maximum summer insolationsummer insolation Insolation record well knownInsolation record well known

Summer insolation maximum 10,000 years agoSummer insolation maximum 10,000 years ago Barbados corals gave a Barbados corals gave a 1414C dated record of sea C dated record of sea

level riselevel rise

14C Age Not True Age When When 1414C dated corals C dated corals

dated by Th/Udated by Th/U 1414C ages were too youngC ages were too young

Implication was that Implication was that rate of rate of 1414C production C production from from 1414NN

• Greater during Greater during LGMLGM

More More 1414C present in C present in samplesample

• Gives an age that Gives an age that is too youngis too young

Young age confirmed Young age confirmed by tree ring studiesby tree ring studies

Implications for Ice Volume The Th/U chronology The Th/U chronology

more accuratemore accurate Highest rates of sea Highest rates of sea

level riselevel rise Before maximum Before maximum

summer insolationsummer insolation The bigger they are, the The bigger they are, the

quicker they meltquicker they melt Generally consistent Generally consistent

with Milankovitch with Milankovitch theorytheory

Response time curve Response time curve predict lagpredict lag Must be other Must be other

feedbacks at workfeedbacks at work

Rise in Sea Level Not Smooth Record of sea level Record of sea level

rise is not smoothrise is not smooth Rapid rise from Rapid rise from

20K to 14K years 20K to 14K years agoago

Slow from 14-12K Slow from 14-12K yearsyears

More rapid rise More rapid rise after 12K yearsafter 12K years

Rates of sea level Rates of sea level rise changed rise changed dramaticallydramatically

Rate of Sea Level Rise Rate of sea level rise Rate of sea level rise

slowed significantly slowed significantly between 14K and 12K between 14K and 12K years agoyears ago

Two major pulses of Two major pulses of freshwater influx to freshwater influx to oceansoceans Melting glaciersMelting glaciers

Glacier melting Glacier melting episodicepisodic

Flow of meltwater Flow of meltwater to oceans episodicto oceans episodic

Meltwater Pulses Oxygen isotopic Oxygen isotopic

composition of planktic composition of planktic foraminiferaforaminifera Monitor freshwater Monitor freshwater

influx to oceaninflux to ocean Anomalously low Anomalously low 1818O O

measured in measured in foraminiferaforaminifera Norwegian SeaNorwegian Sea

Barents Sea ice Barents Sea ice sheetsheet

Gulf of MexicoGulf of MexicoLaurentide ice sheet Laurentide ice sheet

via the Mississippi via the Mississippi RiverRiver

Meltwater Pulses – Additional Evidence

Ice-rafted debris in non-fossiliferous Ice-rafted debris in non-fossiliferous sediments west of Irelandsediments west of Ireland Suggest large influx of fresh water into Suggest large influx of fresh water into

North SeaNorth SeaSourced by massive release of ice bergsSourced by massive release of ice bergs

Sediments deposited between 17K and Sediments deposited between 17K and 14.5K years ago14.5K years agoCoincide with first major meltwater Coincide with first major meltwater pulsepulse•Calving ice bergs would accelerate ice Calving ice bergs would accelerate ice sheet melting sheet melting

Younger Dryas Mid-deglacial pause in ice meltingMid-deglacial pause in ice melting

Accompanied by brief climate coolingAccompanied by brief climate coolingParticularly in subpolar N. Atlantic Particularly in subpolar N. Atlantic OceanOcean

Pollen records in Europe and Scotland Pollen records in Europe and Scotland indicateindicateCold-tolerant tundra (including the Cold-tolerant tundra (including the Arctic plant Arctic plant DryasDryas))•Displaced early growth of forestsDisplaced early growth of forests

Evidence of Younger Dryas also found Evidence of Younger Dryas also found in N. Atlantic sedimentsin N. Atlantic sediments

Younger Dryas Southward re-Southward re-

advance of advance of polar water in polar water in the N. Atlantic the N. Atlantic evident in evident in faunal faunal assemblagesassemblages

Reversal Reversal towards Artic towards Artic vegetation in vegetation in EuropeEurope

Cold-tolerant Cold-tolerant insects in insects in England (~7°C)England (~7°C)

Younger Dryas Recorded in Greenland Recorded in Greenland

ice coreice core Ice sheet Ice sheet

accumulation rates accumulation rates changed abruptlychanged abruptly

Ice accumulation Ice accumulation slow during LGM slow during LGM and Younger Dryasand Younger Dryas

Large changes in Large changes in windblown dustwindblown dust

As indicated by Ca As indicated by Ca content in corescontent in cores

Younger Dryas was Younger Dryas was cold, dry and windy cold, dry and windy climateclimate

Causes of Younger Dryas Broecker called upon Broecker called upon

change in NADW change in NADW formationformation Meltwater diverted Meltwater diverted

from Gulf of Mexico from Gulf of Mexico to N. Atlanticto N. Atlantic

Pulse of low-salinity Pulse of low-salinity meltwater cut off meltwater cut off NADW formationNADW formationCut off heat Cut off heat

transfer to transfer to subpolar Atlantic subpolar Atlantic from tropicsfrom tropics

Critics of Broecker Meltwater pulses to N. AtlanticMeltwater pulses to N. Atlantic

Occurred when global rates of ice melting Occurred when global rates of ice melting were a factor of 5 lowerwere a factor of 5 lowerWith such low rates of meltwater influxWith such low rates of meltwater influx

•How could such a small diversion How could such a small diversion cause such a big change in climate?cause such a big change in climate?

Mechanisms causing cooling hotly debatedMechanisms causing cooling hotly debated Cooling appears global (Cooling appears global (e.g.e.g., greenhouse , greenhouse

gases)gases)Signal could be transferred quickly from Signal could be transferred quickly from N. hemisphere ice sheetsN. hemisphere ice sheets

Testing Broecker’s Model If thermohaline overturn in N. Atlantic slowedIf thermohaline overturn in N. Atlantic slowed

Decrease northward heat transportDecrease northward heat transportWarm tropical AtlanticWarm tropical Atlantic

If greenhouse gas reductionIf greenhouse gas reduction Produced Younger Dryas coolingProduced Younger Dryas cooling

Expect synchronous global coolingExpect synchronous global cooling SST measurements in tropical AtlanticSST measurements in tropical Atlantic

Help sort out mechanismHelp sort out mechanismGreenland ice core records clearly Greenland ice core records clearly

document N. hemisphere, high latitude document N. hemisphere, high latitude coolingcooling

Due to heat released from N. AtlanticDue to heat released from N. Atlantic•N. Atlantic cooledN. Atlantic cooled

Synchronous or Asynchronous Cooling?

Oxygen isotope records from Oxygen isotope records from GRIP and Byrd ice coresGRIP and Byrd ice cores Temperature differences Temperature differences

between N. and S. between N. and S. hemisphereshemispheres

Suggest asynchronous Suggest asynchronous coolingcooling

•Changes in rates of Changes in rates of NADW formationNADW formation

Yet terrestrial climate records Yet terrestrial climate records suggest synchronous coolingsuggest synchronous cooling Changes in greenhouse Changes in greenhouse

gas concentrationsgas concentrations Oceanic or atmospheric Oceanic or atmospheric

controlcontrol

Temperature Records SST based on alkenone SST based on alkenone

unsaturation in core unsaturation in core taken near Grenadataken near Grenada Tropical western N. Tropical western N.

Atlantic (12°N)Atlantic (12°N) 1818O from coexisting O from coexisting

planktic foraminiferplanktic foraminifer During Younger DryasDuring Younger Dryas

Alkenone SST increaseAlkenone SST increase GRIP temperature GRIP temperature

decreasesdecreasesAsynchronous Asynchronous coolingcooling

Mechanisms of Change Compare SST with benthic Cd/CaCompare SST with benthic Cd/Ca

Cd/Ca record from Bermuda RiseCd/Ca record from Bermuda Rise Cd indicator of phosphateCd indicator of phosphate Barometer of changes in the Barometer of changes in the

source of deep watersource of deep water• Low nutrient N. Atlantic Low nutrient N. Atlantic

deep waterdeep water• High nutrient Antarctic High nutrient Antarctic

sourcessources Cd/Ca maximum in younger Dryas Cd/Ca maximum in younger Dryas

indicates less NADWindicates less NADW Slowdown in NADW formation Slowdown in NADW formation

from injection of freshwaterfrom injection of freshwater Cooled N. Atlantic and Cooled N. Atlantic and

GreenlandGreenland Less heat transferred N. from Less heat transferred N. from

tropics so tropics warmedtropics so tropics warmed

Support from 10Be Variations in production rates of atmospheric Variations in production rates of atmospheric

1010Be and Be and 1414CC Linked to solar activity and Earth’s magnetic Linked to solar activity and Earth’s magnetic

fieldfield Concentration of atmospheric Concentration of atmospheric 1414C also C also

affected by removal of radiocarbonaffected by removal of radiocarbonChanges in global carbon cycleChanges in global carbon cycle

Muscheler Muscheler et alet al. (2000 . (2000 NatureNature, 408:567-570) , 408:567-570) used used 1O1OBe to constrain production rates of Be to constrain production rates of 1414C C during Younger Dryasduring Younger Dryas Residual variation due to carbon cycleResidual variation due to carbon cycle

Consistent with lower ventilation ratesConsistent with lower ventilation rates•Therefore a reduction in deep water Therefore a reduction in deep water

formation during Younger Dryasformation during Younger Dryas

Warm Eastern & Cold Western Atlantic

SST records off northwest Africa show cooling SST records off northwest Africa show cooling during younger Dryasduring younger Dryas Implies southward advection of cold waterImplies southward advection of cold water

Along Canary CurrentAlong Canary Current Meltwater shut down NADW formationMeltwater shut down NADW formation

Reduced NADW formation causedReduced NADW formation causedTropical western and southern Atlantic Tropical western and southern Atlantic

warmedwarmedEastern and northern Atlantic cooledEastern and northern Atlantic cooled

Results predict asynchronous N. and S. Results predict asynchronous N. and S. hemisphere temperatures in ice coreshemisphere temperatures in ice cores On short times scalesOn short times scales

Consistent with Cuffy and Vineux (2001)Consistent with Cuffy and Vineux (2001)

Paradox: Freshwater Influx? If rate of NADW formation is reduced by If rate of NADW formation is reduced by

meltwater pulse during Younger Dryasmeltwater pulse during Younger Dryas Why was the pre-Younger Dryas climate and Why was the pre-Younger Dryas climate and

presumably NADW formation seemingly presumably NADW formation seemingly unaffectedunaffectedLarge documented meltwater pulses?Large documented meltwater pulses?

•Paradox seemingly resolved if large Paradox seemingly resolved if large pulses originated from Antarctic ice sheetpulses originated from Antarctic ice sheet

Recent modeling (Clark Recent modeling (Clark et alet al. 2002 . 2002 NatureNature, , 415:863-869)415:863-869) Thermohaline circulation sensitive to small Thermohaline circulation sensitive to small

changes in hydrologic cycle (~0.1 Sv)changes in hydrologic cycle (~0.1 Sv) Why no thermohaline circulation in Pacific?Why no thermohaline circulation in Pacific?

Surface waters are too fresh to sinkSurface waters are too fresh to sink

Huh? Antarctic Ice Sheet Melting Large ice sheets melted early Large ice sheets melted early

and melted fastand melted fast 1818O data from Norwegian O data from Norwegian

sea imply early melting of sea imply early melting of Barents ice sheetBarents ice sheet

Sediments in N. Sea and Sediments in N. Sea and 1818O O data in Gulf of Mexico imply data in Gulf of Mexico imply melting of Laurentide ice melting of Laurentide ice sheetsheet

Indicate N. hemisphere ice Indicate N. hemisphere ice sheet meltingsheet melting

Some evidence exists for early Some evidence exists for early deglacial warming in Antarcticadeglacial warming in Antarctica Suggest that this acted as a Suggest that this acted as a

trigger for melting ice sheets trigger for melting ice sheets in north polar regionsin north polar regions

Abrupt Melting Events Suggest feed backs in climate system Suggest feed backs in climate system

accelerated ice sheet meltingaccelerated ice sheet melting Iceberg calving would increase rate of meltingIceberg calving would increase rate of melting

Moves ice quickly into relatively warm watersMoves ice quickly into relatively warm waters Ice sheets in marginal marine environmentsIce sheets in marginal marine environments

Susceptible to rapid meltingSusceptible to rapid melting Internal flow of ice sheets increasedInternal flow of ice sheets increased

Ice fluxed to margins along ice streamsIce fluxed to margins along ice streams•Effectively thinning the ice sheetEffectively thinning the ice sheet•Lowering volume but not aerial extent of Lowering volume but not aerial extent of

iceice

Changes in Landscapes Morphological changes accompanied Morphological changes accompanied

deglaciationsdeglaciations Proglacial lakesProglacial lakes FloodingFlooding

When impounded water in proglacial When impounded water in proglacial lakes was suddenly releasedlakes was suddenly released

Rise in sea levelRise in sea levelInundation of coastal regionsInundation of coastal regionsSubmerged land connections Submerged land connections between continents exposed during between continents exposed during LGMLGM

Proglacial Lakes Proglacial lakes develop Proglacial lakes develop

in bedrock depressions in bedrock depressions left by melting ice left by melting ice sheetssheets Lake Agassiz, largest Lake Agassiz, largest

proglacial lake N. proglacial lake N. AmericaAmerica200,000 km200,000 km22, 100 m , 100 m deep (20,000 kmdeep (20,000 km33))

Sudden release of Sudden release of large proglacial lakes large proglacial lakes caused massive floodscaused massive floods

Increased Insolation Produced Monsoons

Earth’s orbital configuration Earth’s orbital configuration 10K years ago10K years ago Summer insolation 8% higher Summer insolation 8% higher

than todaythan today Conducive to summer Conducive to summer

monsoon developmentmonsoon development Model simulations supported by Model simulations supported by

geologic observationsgeologic observations Lake levels higher inLake levels higher in

ArabiaArabiaNorth AfricaNorth AfricaSoutheastern AsiaSoutheastern Asia

Enhanced Upwelling in Arabian Sea

Strong monsoon winds blowing across Somalia Strong monsoon winds blowing across Somalia and eastern Arabiaand eastern Arabia Enhanced coastal upwellingEnhanced coastal upwelling Altering the planktic foraminifera speciesAltering the planktic foraminifera species

Climate Evidence Evidence for wet climate Evidence for wet climate

range fromrange from Large dry river valleys Large dry river valleys

in desertsin deserts Fossil evidence Fossil evidence

includesincludesGrass pollen in lake Grass pollen in lake depositsdeposits

Variety of water-Variety of water-loving animals loving animals (hippopotamuses, (hippopotamuses, crocodiles, turtles, crocodiles, turtles, rhinoceroses, etc)rhinoceroses, etc)

Timing 1414C dates for lake C dates for lake

deposits in N. deposits in N. AfricaAfrica Match the 10K Match the 10K

insolation insolation maximummaximum

When corrected When corrected for greater for greater 1414C C productionproduction

Intensity Summer insolation 8% higher but lakes 24% larger in Summer insolation 8% higher but lakes 24% larger in

volumevolume Relationship not necessarily linearRelationship not necessarily linear

Mismatch between models and observationsMismatch between models and observations Required addition of vegetation-moisture feedbackRequired addition of vegetation-moisture feedback

Insolation Reduced Monsoons Decreased summer insolation expected to weakened Decreased summer insolation expected to weakened

summer monsoonssummer monsoons Lake levels in N. Africa match well expected patternsLake levels in N. Africa match well expected patterns

Most lakes today much lower or dried outMost lakes today much lower or dried out

Climate Change Over Last 10K Years

Ice sheets melting (reduced influence)Ice sheets melting (reduced influence) Atmospheric COAtmospheric CO22 levels stable and high levels stable and high Summer insolation gradually decreasingSummer insolation gradually decreasing

Expect warmer and then cooler climateExpect warmer and then cooler climate

Vegetation General gradual General gradual

movement of warm-movement of warm-adapted biomes northadapted biomes north Pollen records Pollen records

indicate spruce and indicate spruce and oak moved northoak moved north

Mid-glacial produced Mid-glacial produced no-analog vegetationno-analog vegetation Mixtures that do Mixtures that do

not exist todaynot exist today Different response Different response

of a particular type of a particular type of plant to changing of plant to changing climateclimate

Peak Deglacial Warmth With atmospheric COWith atmospheric CO22 levels levels

steady and highsteady and high Glacial ice largely meltedGlacial ice largely melted

Summer insolation and Summer insolation and vegetation changes vegetation changes affected temperaturesaffected temperatures

Insolation 5% higher Insolation 5% higher warmed high latitudeswarmed high latitudes

Displacement of high-Displacement of high-albedo tundra by low-albedo albedo tundra by low-albedo spruce caused positive spruce caused positive feedbackfeedback Greater warmingGreater warming

Cooling Followed Deglacial Warming

Ample evidence for Ample evidence for gradual coolinggradual cooling Summer insolation Summer insolation

dropped over last 10K dropped over last 10K yearsyears

Less frequent melting Less frequent melting of ice capsof ice caps

More frequent sea ice More frequent sea ice off Greenland off Greenland indicated by drop in indicated by drop in diatomsdiatoms

Advances in ice caps Advances in ice caps on Arctic islandson Arctic islands

Lower Atlantic SSTLower Atlantic SST Southward shift in the Southward shift in the

boundary between boundary between spruce and tundraspruce and tundra

Future Climate Over the next 10K years precession maximize at low latitudeOver the next 10K years precession maximize at low latitude

Intensify summer monsoonsIntensify summer monsoons Tilt should minimize at high N. latitudesTilt should minimize at high N. latitudes

Help promote further glaciationsHelp promote further glaciations Pattern consistent with glaciations in next few thousand yearsPattern consistent with glaciations in next few thousand years

Predictions complicated by millennial-scale oscillations and Predictions complicated by millennial-scale oscillations and anthropogenic greenhouse gasesanthropogenic greenhouse gases