Climate in the Last 20,000 years

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Climate in the Last 20,000 years

• Last Glacial Maximum
• Low temperatures and expansive ice sheets
• Low atmospheric CO2 levels (190 ppmv)
• Dry, windy and sparsely vegetated landscapes
• Quite different mammalian fauna

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Why the Cooling?

• Seasonal insolation similar to today
• Expected if glaciers reaching maximum extent
• Still insolation cannot explain climate 20K years
  ago
• Cooling caused by
• Presence of ice sheets
• Lower atmospheric greenhouse gas levels

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CLIMAP Reconstructions

• Continental glaciations (25 land mass 10
  today)
• Earth 4C colder

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CLIMAP Reconstructions

• N. Atlantic 8C colder and sea ice more extensive
• Less cooling away from northern ice sheets
• Expansion of Southern Ocean sea ice
• Equatorial temperatures a little colder

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Extent of Ice Sheets

• Ice sheets were at maximum extent 21,000 years
  ago
• Major ice sheets covered shelf regions
• Barents and part of Kara Seas

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Ice Sheet Thickness

• CLIMAP assumed thick, mature ice sheets
• Sea level did not drop enough to accommodate
  thick ice sheets
• N. American ice sheets rested on sediments
• Slid more readily
• Bedrock rebound rates support thin ice sheets

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Dry Glacial Environments

• Evidence for glacial aridity is bountiful
• Glacial loess deposits widespread

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Dry Glacial Environments

• Desert environments source of much dust today
• At LGM, environments produced more dust

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Dry Glacial Environments

• At LGM
• Sand dune deposition more extensive
• Drier climate
• Stronger winds
• Virtually every region on Earth
• Shows LGM drier and dustier
• Unfortunately, model simulations poor

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COHMAP

• Combined terrestrial ecology modeling approach
  to understand Holocene
• Assemble records of boundary conditions
• Climate drivers
• Ice sheet size and greenhouse gases
• Model simulations using changing boundary
  conditions
• Compared with climate records from marine and
  lacustrine environments

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Pollen Analysis

• Rainfall and temperature control distribution of
  plants
• Percentages of pollen allow climate
  reconstruction and can be 14C dated
• Many lakes in northern N. America formed from
  glacial retreat
• Contain pollen from LGM to present

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Pollen in Northern Lakes

• Spruce pollen indicates cold conditions
• Oak pollen indicates warmer climates
• Prairie grass pollen indicate a drier climate
• Transitions follow deglaciation of region

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Pollen Distributions

• Pollen analyzed in lake cores worldwide
• Provide geographic distribution of vegetation
• LGM
• Deglaciation
• Geographic distribution of vegetation
• Compared with output model simulations

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Spruce Distributions

• Spruce found today in northeastern Canada
• During LGM, found only in northern US lakes
• Distribution agrees well with model simulations

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GCM and High-Level Winds

• Presence of ice sheets can redirect jet stream
• Low-level atmospheric flow blocked by ice
• LGM flow causes clockwise spiral of cold air
• Southwestward flow over N. Atlantic
• Flow west across US
• Clockwise spiral over Scandinavia cooled Europe
• Upper level jet crossed Atlantic and flowed into
  Europe at 45-50N

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Cold Glacial North Atlantic

• Largest ocean cooling in N. Atlantic
• Pushed the Gulf Stream and N. Atlantic Drift
  towards Portugal
• Highest rates of deposition of ice rafted debris
• North of 50N
• Southward flowing icebergs encountered warm water
  and melted

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Southwestern United States

• Southward displacement of jet stream
• Brought increased moisture to SW
• Increased winter storm
• Cloudiness inhibited evaporation
• Most extensive lake deposits glacial Lake
  Bonneville
• Pacific NW dry
• Southward displacement of jet stream
• Cool, dry west winds from mid-continent

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Eastern United States

• Extensive lake deposits allow rigorous model
  tests
• Mismatch for elm pollen (warm-adapted diciduous)
• Model simulations underestimate actual cooling
• Cooling may be due to Mississippi River inflow
• Gulf of Mexico, cooling much of SE United States

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LGM in Europe

• Modern vegetation dominated by forests
• Conifers in north
• Deciduous in south
• At LGM
• Arctic tundra large area south of ice sheets
• Mostly prairie and grassy steppe
• Forests only on southern margin

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Cooling in Europe

• Clockwise flow of cold low-level winds
• Across Scandinavian ice sheets
• Cold North Atlantic sea surface temperatures
• Mediterranean temperatures moderated
• Winter storms
• Brought to region by southward-shifted jet stream
  across the Atlantic

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LGM in Asia

• Modern Asia covered by forests
• LGM grass steppe
• Forests largely absent
• Very harsh climate
• Probably produced deep permafrost
• Growth of grasses during summer thaw
• Model simulations suggest strong high-pressure
  cell in Siberia during winter
• Moisture to region from N. Atlantic nonexistent
• Cold N. Pacific and Bering Sea from Siberian winds

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LGM in Antarctica

• Winter sea ice extended further north
• Shift in the Polar Front (region of high
  productivity)
• Not simple driven by bathymetry
• Simple models suggest
• Lower CO2 levels
• Decreased flow of warm saline NADW

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LGM in Australia

• Showed expanded development of dune deposits
• Arid climate and intensification of CCW winds
• Large regions north of Australia exposed with
  drop in sea level
• Probably resulted from lower CO2 levels
• Shift of storms to south in Southern Ocean

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LGM in South America

• Suggestion of drier conditions in Amazon
• Cooler ocean SST, lower sea level and low CO2
• Know Amazon was wetter from Paul Bakers work
• Wetter from south shift in westerly winds
• Even if Amazon wetter
• Could not offset loss of vegetation in N.
  hemisphere

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Cold LGM Tropics Conundrum

• CLIMAP studies indicate tropical cooling
• 1-2C compared with today in most regions
• Southern tropical Pacific 1C warmer
• Terrestrial indicators suggest 4-6C cooling
• What caused the cooling?
• Was it glaciers?
• Too far away
• Was it insolation?
• Similar to today
• Was it greenhouse gases?
• Perhaps, as a result of lower trapping of
  background radiation
• Was it the data?

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LGM a Model for Greenhouse World?

• If LGM tropical cooling from CO2
• Could be a model for future climate change
• Response of the Earth climate system
• Atmospheric CO2 change of 90 ppmv
• What will be the future warming of the tropical
  oceans?
• Warming of the Planet?

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CLIMAP Cooling Pros

• Temperature estimates from faunal assemblages
• Species that existed in tropics at LGM
• Not much different from modern
• Conclude that cooling was slight (average 1.5C)

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CLIMAP Cooling Pros

• Independent agreement from alkenones
• Alkenone paleotemperatures agree with those from
  faunal assemblages
• d18O from planktic foraminifera
• Also show general agreement
• When ice volume signal removed (1.1)

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CLIMAP Cooling Cons

• Faunal assemblage have low sensitivity to
  temperature change at 20C
• No change observed because no change expected
• Food, not temperature controlled species
• Pacific a poor place to examine calcareous shells
• Extensive dissolution of carbonate
• Preferential or no preservation
• Alkenone and d18O data suggest agreement in some
  parts of the world ocean
• Not all
• Ocean margins and enclosed seas
• Probably cooler no data or unique assemblages
  limit confirmation of speculation

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Large LGM Cooling Pros

• Margins of tropical and subtropical mountain
  glaciers
• Descended 600-1000 m
• Using modern lapse-rate cooling suggests 4-6C
  cooling
• Descent of tree line and other vegetation on
  tropical mountains
• Lapse-rate cooling again suggests 4-6C drop in
  temperature

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Large LGM Cooling Cons

• Terrestrial cooling uses modern lapse-rate (6.5C
  km-1)
• Change of only 1C km-1 would resolve
  discrepancies
• Consistent with models assuming steeper
  lapse-rate in drier glacial tropics
• Terrestrial environments difficult to date
• Suggestion that plants are sensitive to CO2
• Require growth was limited by CO2
• Could help explain lower vegetation lines

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Logical Conclusions

• Marginal basins colder than CLIMAP estimates
• Logical that shallow basins cool more than deep
  ocean
• Open ocean cooled a little
• Land cooled a lot
• Coastal regions cooled somewhat more than deep
  oceans