Deglaciation LGM climate controlled by LGM climate controlled by  Ice sheets and atmospheric CO 2 Deglacial world shift Deglacial world shift  Higher insolation and CO 2  Smaller ice sheets  As insolation increased  Ice sheets melted Influenced climate much lessInfluenced climate much less  CO 2 had a largely secondary role in climate

Timing of Ice Sheet Melting Determined by dating organic remains formed during ice sheet retreat Determined by dating organic remains formed during ice sheet retreat  Although scarce, suitable samples exist N. American ice sheets began retreating 14, C years ago N. American ice sheets began retreating 14, C years ago  Gone by 6, C years ago Area does not yield ice volume Area does not yield ice volume  Thickness of ice debatable

Sea Level from Coral Reefs One meter of sea level rise = 0.4 million km 3 of ice One meter of sea level rise = 0.4 million km 3 of ice  Total global ice volume can be compared with insolation record Coral reefs on Barbados gave sea level history Coral reefs on Barbados gave sea level history  Know Barbados had minor subsidence  14 C dated sea level curve supports expectations Rate of sea level rise maximized during maximum summer insolation Rate of sea level rise maximized during maximum summer insolation  Insolation record well known  Summer insolation maximum 10,000 years ago  Barbados corals gave a 14 C dated record of sea level rise

14 C Age Not True Age When 14 C dated corals dated by Th/U When 14 C dated corals dated by Th/U  14 C ages were too young  Implication was that rate of 14 C production from 14 N Greater during LGMGreater during LGM  More 14 C present in sample Gives an age that is too youngGives an age that is too young  Young age confirmed by tree ring studies

Implications for Ice Volume The Th/U chronology more accurate The Th/U chronology more accurate  Highest rates of sea level rise  Before maximum summer insolation The bigger they are, the quicker they melt The bigger they are, the quicker they melt  Generally consistent with Milankovitch theory Response time curve predict lag Response time curve predict lag  Must be other feedbacks at work

Rise in Sea Level Not Smooth Record of sea level rise is not smooth Record of sea level rise is not smooth  Rapid rise from 20K to 14K years ago  Slow from 14-12K years  More rapid rise after 12K years Rates of sea level rise changed dramatically Rates of sea level rise changed dramatically

Rate of Sea Level Rise Rate of sea level rise slowed significantly between 14K and 12K years ago Rate of sea level rise slowed significantly between 14K and 12K years ago Two major pulses of freshwater influx to oceans Two major pulses of freshwater influx to oceans  Melting glaciers  Glacier melting episodic  Flow of meltwater to oceans episodic

Meltwater Pulses Oxygen isotopic composition of planktic foraminifera Oxygen isotopic composition of planktic foraminifera  Monitor freshwater influx to ocean Anomalously low  18 O measured in foraminifera Anomalously low  18 O measured in foraminifera  Norwegian Sea  Barents Sea ice sheet  Gulf of Mexico  Laurentide ice sheet via the Mississippi River

Meltwater Pulses – Additional Evidence Ice-rafted debris in non-fossiliferous sediments west of Ireland Ice-rafted debris in non-fossiliferous sediments west of Ireland  Suggest large influx of fresh water into North Sea  Sourced by massive release of ice bergs  Sediments deposited between 17K and 14.5K years ago  Coincide with first major meltwater pulse Calving ice bergs would accelerate ice sheet meltingCalving ice bergs would accelerate ice sheet melting

Younger Dryas Mid-deglacial pause in ice melting Mid-deglacial pause in ice melting  Accompanied by brief climate cooling  Particularly in subpolar N. Atlantic Ocean  Pollen records in Europe and Scotland indicate  Cold-tolerant tundra (including the Arctic plant Dryas) Displaced early growth of forestsDisplaced early growth of forests  Evidence of Younger Dryas also found in N. Atlantic sediments

Younger Dryas Southward re- advance of polar water in the N. Atlantic evident in faunal assemblages Southward re- advance of polar water in the N. Atlantic evident in faunal assemblages Reversal towards Artic vegetation in Europe Reversal towards Artic vegetation in Europe Cold-tolerant insects in England (~7°C) Cold-tolerant insects in England (~7°C)

Younger Dryas Recorded in Greenland ice core Recorded in Greenland ice core  Ice sheet accumulation rates changed abruptly  Ice accumulation slow during LGM and Younger Dryas  Large changes in windblown dust  As indicated by Ca content in cores  Younger Dryas was cold, dry and windy climate

Causes of Younger Dryas Broecker called upon change in NADW formation Broecker called upon change in NADW formation  Meltwater diverted from Gulf of Mexico to N. Atlantic  Pulse of low-salinity meltwater cut off NADW formation  Cut off heat transfer to subpolar Atlantic from tropics

Critics of Broecker Meltwater pulses to N. Atlantic Meltwater pulses to N. Atlantic  Occurred when global rates of ice melting were a factor of 5 lower  With such low rates of meltwater influx How could such a small diversion cause such a big change in climate?How could such a small diversion cause such a big change in climate? Mechanisms causing cooling hotly debated Mechanisms causing cooling hotly debated  Cooling appears global (e.g., greenhouse gases)  Signal could be transferred quickly from N. hemisphere ice sheets

Testing Broecker’s Model If thermohaline overturn in N. Atlantic slowed If thermohaline overturn in N. Atlantic slowed  Decrease northward heat transport  Warm tropical Atlantic If greenhouse gas reduction If greenhouse gas reduction  Produced Younger Dryas cooling  Expect synchronous global cooling SST measurements in tropical Atlantic SST measurements in tropical Atlantic  Help sort out mechanism  Greenland ice core records clearly document N. hemisphere, high latitude cooling  Due to heat released from N. Atlantic N. Atlantic cooledN. Atlantic cooled

Synchronous or Asynchronous Cooling? Oxygen isotope records from GRIP and Byrd ice cores Oxygen isotope records from GRIP and Byrd ice cores  Temperature differences between N. and S. hemispheres  Suggest asynchronous cooling Changes in rates of NADW formationChanges in rates of NADW formation Yet terrestrial climate records suggest synchronous cooling Yet terrestrial climate records suggest synchronous cooling  Changes in greenhouse gas concentrations  Oceanic or atmospheric control

Temperature Records SST based on alkenone unsaturation in core taken near Grenada SST based on alkenone unsaturation in core taken near Grenada  Tropical western N. Atlantic (12°N)  18 O from coexisting planktic foraminifer  18 O from coexisting planktic foraminifer During Younger Dryas During Younger Dryas  Alkenone SST increase  GRIP temperature decreases  Asynchronous cooling

Mechanisms of Change Compare SST with benthic Cd/Ca Compare SST with benthic Cd/Ca  Cd/Ca record from Bermuda Rise  Cd indicator of phosphate  Barometer of changes in the source of deep water Low nutrient N. Atlantic deep waterLow nutrient N. Atlantic deep water High nutrient Antarctic sourcesHigh nutrient Antarctic sources Cd/Ca maximum in younger Dryas indicates less NADW Cd/Ca maximum in younger Dryas indicates less NADW  Slowdown in NADW formation from injection of freshwater  Cooled N. Atlantic and Greenland  Less heat transferred N. from tropics so tropics warmed

Support from 10 Be Variations in production rates of atmospheric 10 Be and 14 C Variations in production rates of atmospheric 10 Be and 14 C  Linked to solar activity and Earth’s magnetic field  Concentration of atmospheric 14 C also affected by removal of radiocarbon  Changes in global carbon cycle Muscheler et al. (2000 Nature, 408: ) used 1O Be to constrain production rates of 14 C during Younger Dryas Muscheler et al. (2000 Nature, 408: ) used 1O Be to constrain production rates of 14 C during Younger Dryas  Residual variation due to carbon cycle  Consistent with lower ventilation rates Therefore a reduction in deep water formation during Younger DryasTherefore a reduction in deep water formation during Younger Dryas

Warm Eastern & Cold Western Atlantic SST records off northwest Africa show cooling during younger Dryas SST records off northwest Africa show cooling during younger Dryas  Implies southward advection of cold water  Along Canary Current Meltwater shut down NADW formation Meltwater shut down NADW formation  Reduced NADW formation caused  Tropical western and southern Atlantic warmed  Eastern and northern Atlantic cooled Results predict asynchronous N. and S. hemisphere temperatures in ice cores Results predict asynchronous N. and S. hemisphere temperatures in ice cores  On short times scales  Consistent with Cuffy and Vineux (2001)

Paradox: Freshwater Influx? If rate of NADW formation is reduced by meltwater pulse during Younger Dryas If rate of NADW formation is reduced by meltwater pulse during Younger Dryas  Why was the pre-Younger Dryas climate and presumably NADW formation seemingly unaffected  Large documented meltwater pulses? Paradox seemingly resolved if large pulses originated from Antarctic ice sheetParadox seemingly resolved if large pulses originated from Antarctic ice sheet Recent modeling (Clark et al Nature, 415: ) Recent modeling (Clark et al Nature, 415: )  Thermohaline circulation sensitive to small 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 sink

Huh? Antarctic Ice Sheet Melting Large ice sheets melted early and melted fast Large ice sheets melted early and melted fast   18 O data from Norwegian sea imply early melting of Barents ice sheet  Sediments in N. Sea and  18 O data in Gulf of Mexico imply melting of Laurentide ice sheet Indicate N. hemisphere ice sheet melting Indicate N. hemisphere ice sheet melting Some evidence exists for early deglacial warming in Antarctica Some evidence exists for early deglacial warming in Antarctica  Suggest that this acted as a trigger for melting ice sheets in north polar regions

Abrupt Melting Events Suggest feed backs in climate system accelerated ice sheet melting Suggest feed backs in climate system accelerated ice sheet melting  Iceberg calving would increase rate of melting  Moves ice quickly into relatively warm waters  Ice sheets in marginal marine environments  Susceptible to rapid melting  Internal flow of ice sheets increased  Ice fluxed to margins along ice streams Effectively thinning the ice sheetEffectively thinning the ice sheet Lowering volume but not aerial extent of iceLowering volume but not aerial extent of ice

Changes in Landscapes Morphological changes accompanied deglaciations Morphological changes accompanied deglaciations  Proglacial lakes  Flooding  When impounded water in proglacial lakes was suddenly released  Rise in sea level  Inundation of coastal regions  Submerged land connections between continents exposed during LGM

Proglacial Lakes Proglacial lakes develop in bedrock depressions left by melting ice sheets Proglacial lakes develop in bedrock depressions left by melting ice sheets  Lake Agassiz, largest proglacial lake N. America  200,000 km 2, 100 m deep (20,000 km 3 )  Sudden release of large proglacial lakes caused massive floods

Increased Insolation Produced Monsoons Earth’s orbital configuration 10K years ago Earth’s orbital configuration 10K years ago  Summer insolation 8% higher than today  Conducive to summer monsoon development Model simulations supported by geologic observations Model simulations supported by geologic observations  Lake levels higher in  Arabia  North Africa  Southeastern Asia

Enhanced Upwelling in Arabian Sea Strong monsoon winds blowing across Somalia and eastern Arabia Strong monsoon winds blowing across Somalia and eastern Arabia  Enhanced coastal upwelling  Altering the planktic foraminifera species

Climate Evidence Evidence for wet climate range from Evidence for wet climate range from  Large dry river valleys in deserts  Fossil evidence includes  Grass pollen in lake deposits  Variety of water- loving animals (hippopotamuses, crocodiles, turtles, rhinoceroses, etc)

Timing 14 C dates for lake deposits in N. Africa 14 C dates for lake deposits in N. Africa  Match the 10K insolation maximum  When corrected for greater 14 C production

Intensity Summer insolation 8% higher but lakes 24% larger in volume Summer insolation 8% higher but lakes 24% larger in volume  Relationship not necessarily linear Mismatch between models and observations Mismatch between models and observations  Required addition of vegetation-moisture feedback

Insolation Reduced Monsoons Decreased summer insolation expected to weakened summer monsoons Decreased summer insolation expected to weakened summer monsoons Lake levels in N. Africa match well expected patterns Lake levels in N. Africa match well expected patterns  Most lakes today much lower or dried out

Climate Change Over Last 10K Years  Ice sheets melting (reduced influence)  Atmospheric CO 2 levels stable and high  Summer insolation gradually decreasing  Expect warmer and then cooler climate

Vegetation General gradual movement of warm- adapted biomes north General gradual movement of warm- adapted biomes north  Pollen records indicate spruce and oak moved north Mid-glacial produced no-analog vegetation Mid-glacial produced no-analog vegetation  Mixtures that do not exist today  Different response of a particular type of plant to changing climate

Peak Deglacial Warmth With atmospheric CO 2 levels steady and high With atmospheric CO 2 levels steady and high Glacial ice largely melted Glacial ice largely melted  Summer insolation and vegetation changes affected temperatures Insolation 5% higher warmed high latitudes Insolation 5% higher warmed high latitudes Displacement of high-albedo tundra by low-albedo spruce caused positive feedback Displacement of high-albedo tundra by low-albedo spruce caused positive feedback  Greater warming

Cooling Followed Deglacial Warming Ample evidence for gradual cooling Ample evidence for gradual cooling  Summer insolation dropped over last 10K years  Less frequent melting of ice caps  More frequent sea ice off Greenland indicated by drop in diatoms  Advances in ice caps on Arctic islands  Lower Atlantic SST  Southward shift in the boundary between spruce and tundra

Future Climate Over the next 10K years precession maximize at low latitude Over the next 10K years precession maximize at low latitude  Intensify summer monsoons Tilt should minimize at high N. latitudes Tilt should minimize at high N. latitudes  Help promote further glaciations Pattern consistent with glaciations in next few thousand years Pattern consistent with glaciations in next few thousand years  Predictions complicated by millennial-scale oscillations and anthropogenic greenhouse gases