Download The LGM

Part III: Deglacial and Millennial
Climate Changes
Lecture 14: The LGM
(Last Glacial Maximum)
(Chapter 12)
Last Glacial Maximum
Big signal
21,000
Much more data, with 14C dating
Mammals of the Last Glacial Maximum
(21,000 years ago)
Cave painting of LGM era in Southern France
Obital similar
CO2 sensitivity
Climate forcing since LGM
Ice Sheet
(NH)
CLIMAP: Reconstruction of LGM Climate
Colder !
LGM
August SST
LGM- present
August SST
LGM SST Changes, Data uncertainty
CLIMAP 81:
El Nino-like
Prell 85:
La Nina-like
Broccoli, A., 2000
How large is the Ice Sheet at LGM?
More Ice!
Why the greatest ice sheet not over Siberia?
LGM reconstruction of
North America Ice Sheet
East-west
asymmetry, why?
Sea Level Change
LGM
IPCC/AR4 2007
LGM Coastlines
Ice sheet contribution to sea level
Which value
should be used
for isotope
balance?
LGM desert and sand
(expanded)
Sand dunes today
Sand dunes LGM
LGM desert and dust
Drier and windier! 10 times more dust
LGM desert dust
LGM loess deposits
Projected Precipitation Changes (%) in response to Future CO2 (AR4
Rich get richer and poor get poorer!
Obital similar
CO2 sensitivity
Climate forcing at LGM
Ice Sheet
(NH)
Implication to future projectioin?
Sand dunes
today
LGM
Ice Age climate deposits
Winds lift and transport dust
downstream and around the
world
Colder/drier
loss of vegetation
cover, stronger wind.
weight
grinding
Freezing-thaw
Water/rain rework
Ice sheet
moraines boulder, cobble
sand
loess
(silt-size) dust
(clay-size)
LGM ice rafting
Freshwater melting..
Model/Data Comparison
How to compare model climate with data (vegetation)?
Biome models
Now:
Interactive climateecosystem model
Vegetation and Climate
Pollen distributions and climate
Wetness
indicator
Cold spiece
Warm spiece
Pollen percentage and climate
Pollen history in a lake core
Wetness minimum in mid-Holocene?
Iincreasing warming
Decrease cooling
Modern/LGM spruce pollen
Data/model LGM spruce pollen
Why wetter in the southwest at LGM ?
Simulated LGM Climate
Alaska, warmer
Cold, drier
Split of jet, snow
storm, cold, wetter
Model/data mismatch
Too much warm-adapted trees,
Cooler Mississippi outflow at LGM may
help the cooling in the observation.
Vegetation in Europe
Modern
LGM
Cold and dry, out flow from
Scandinavia ice sheet and
North Atlantic
LGM northern Asia
Colder and drier
Stronger Aleutian High, stronger
winter monsoon flow, reduced
moisture source from N. Atlantic
Closing of Bering Strait….
LGM climate in the Southern Hemisphere
Why global synchronous cooling?
CO2
LGM sea ice expansion
around Antarctic
Why synchronous global cooling?
CO2
LGM tropical cooling: Important for Testing Climate Sensitivity
Evidence for small cooling: Ocean, 1-2oC
Not much difference of plankton
assemblages (Pacific and Indian Oceans)
Similar estimates:
Alkenone method
and CLIMAP modern analogue
The problem of tropical cooling:
mismatch between SST and mountain snowline
Evidence of large cooling: Land, DT > 4 - 6 C
Mountain snowline
NCEP Reanal. Mean Annual Temp - Cordillera Transect (1973-2002)
200
300
400
mb
500
Freezing level
600
700
800
900
1000
-50 -40 -30 -20 -10
0
10
20
30
40
0
10
20
latitude
-50 -40 -30 -20 -10
°C
50
60
70
1910
Recession of
the Grinnell
Glacier
“Glacier
National Park”
1997
Source: D. Fagre, USGS, 2004
Tibetan Plateau:
Trends in surface air temperature with elevation: 1961-90
3rd polar amplification?
Source: Liu and Chen, 2000
Biosphere “hotspots” identified by Conservation International
Much of the
American
Cordillera
has high
biodiversity
but is under
threat from
climate
change and
land use
pressures
Projected change in annual mean temperature with 2x CO2
9000
3.50
8000
3.00
7000
C2.75
m
6000
5000
2.50
°C
2.25
4000
2.00
3000
1.75
2000
1.50
1.25
1000
1.00
0
-50 -40 -30 -20 -10
0
10
20
30
40
50
60
70
Latitude
Source: Bradley et al., 2004: GRL
Clues to Tropical cooling discrepancy?
• High altitude more sensitive to climate
change (lapse rate change)
• Plant physiology effect (trees are less
efficient in photosynthesis under a lower
CO2 environment)
Response of North Atlantic thermohaline
present
LGM
LGM model-data comparison: SST
21 ka – 0ka
AMOC model-data comparison
Proxy
Δ 13C
0ka
21ka
CCSM3
Salinity
AMOC
AMOC model-model comparison??
Observation
Δ
13C
AMOC in Models (PMIP2)
CCSM
0k
a
HadCM
MIROC
ECBILT_
CLIO
21k
a
Otto-Bliesner et al., 2007, GRL
Future Projection of AMOC in IPCC
IPCC, 2001, TAR
IPCC, 2007, AR4
Is it consistent with LGM evidence?
Water Age
LGM
Wind, or Buoyance?
Holocene
SO brine injection  AABW
Obs: Δ
13C
CCSM: Salinity AMOC
Ideal Age
LGM
~1000
yrs
21k
a
~500
yrs
Reference for reading
• Shin S., Z. Liu, B. Otto-Bliesner, E. Brady, J. Kutzbach, 2002:
Southern Ocean sea-ice control of the glacial North Atlantic
thermohaline circulation. Geophys. Res. Lett., 30,
DOI:10.1029/2002GL015513
• Liu, Z., S. Shin, R. Webb, W. Lewis and B. Otto-Bliesner, 2005:
Atmospheric CO2 forcing on glacial thermohaline and climate.
Geophys. Re. Lett, 32, L02706, doi:10.1029/2004GL021929
End of Lecture 13