the co-evolution of continental ice cover and 3d ground temperature over the last glacial cycle lev...
TRANSCRIPT
The Co-evolution of continental ice cover and 3D ground temperature
overthe last glacial cycle
Lev Tarasov Memorial University of Newfoundland
(Thanks to A.S. Dyke, Richard Gyllencreutz, Oystein Lohne, Jan Mangerud, W. R. Peltier, and John Inge
Svendsen)
Outline
MOTIVATION: Ice-sheet evolution What makes the model results meaningful?
PHYSICS: Glacial Systems Model (GSM) DATA BRIDGING DATA and PHYSICS: model
calibration SOME RESULTS
Questions
What did the the northern Hemispheric ice-sheets do?
How do we find out? How did the changing ice-
sheets affect the ground temperature field?
How close to equilibrium is the present-day 3D ground temperature field?
Lot’s of poorly constrained components in the glacial system
Poorly constrained system; model pre(retro)dictions have generally
lacked meaningful error bars => no meaningful interpretation.Solution: 3 components:
#1: Glacial Systems Model (GSM) 3D thermo-
mechanically coupled ice-sheet model, 0.5 * 1.0 (lat/long) model resolution
VM2 viscosity model detailed surface
mass-balance and ice-calving modules
global gravitationally self-consistent RSL solver
fully coupled surface drainage solver
Model thermodynamics
3D ice thermodynamics 3D heat advection and vertical diffusion
Bed-thermal model Vertical diffusion only Non-uniform 20 layer grid spans 3 km Deep geothermal heat flux field from Pollack et al (1993) Simple: scalar thermal conductivity and heat capacity Standard permafrost treatment using effective heat capacity Parametrized TTOP model (based on Smith and
Riseborough, 2002) for near surface ground temperature as a function of air temperature
Large-scale and Long-time scale Horizontal resolution: 1 by 0.5 degrees
Climate forcing
Last Glacial Maximum (LGM) precipitation and temperature from 4 (6 for N. A.) highest resolution Paleo Model Intercomparison Project GCM runs
Mean and EOF fields
Present day observed fields
120 kyr climate forcing(based ss09 chronology)
Embrace uncertainty: Lots of ensemble parameters
3(5 for North America) ice dynamical 13(16) regional precipitation
LGM precipitation EOFs most significant for North America 4(4) temperature 4(4) ice calving 4(2) ice margins 1 model version = 29 (32)
Need constraints -> #2: DATA
Oct/06 margin chronology
Deglacial margin chronology
(Dyke, 2004) 36 time-slices +/- 50 km uncertainty Margin buffer Margin forcing:
2 ensemble parameters control extent of adjustments to surface mass-balance
Calibration minimizes amount of margin forcing
RSL data; site weighting (A. S. Dyke and U. of Toronto RSL database)
Noisy data and non-linear system => need #3: calibration and error bars
Bayesian calibration
Sample over posterior probability distribution for the ensemble parameters given fits to observational data using Markov Chain Monte Carlo (MCMC) methods
Sampling also subject to additional volume and ice thickness constraints
North American calibration also uses strand-line observations
Large ensemble Bayesian calibration
Bayesian neural network integrates over weight space
effectively generate millions of model runs
LGM characteristics
Dynamic ice-sheet streaming
constrained by sub-glacial till availability and thermodynamics
Large Keewatin ice-dome
Validation of glaciological model
RSL fits are overall as good as that of the non-glaciological ICE5-G model
North American model also validated against GRACE observations for present-day rate of mass-change
Surface topopgraphy similar to that infered on the basis of striations and landform orientation
Thermodynamic results (North American are published, Eurasian
are preliminary)
Borehole comparison to observations (93.72W, 51.03N, with error bars !!!)
Caveat, dependence on thermal conductivity (66.64W, 50.21N)
Present-day permafrost distribution
Permafrost depth evolution: impact of surface ice
Present day proximity to equilibrium
European LGM ice-sheet (Problem: less ice-cover)
Kola Borehole Temperature profile
Kola latitudinal transect: Present Proximity to Equilibrium
Kola
Ice area evolution comparison : North America versus Europe
Summary
Glaciological results North America: Large Keewatin ice dome Dynamic ice-sheets
Permafrost/ground temperature: Large variations in permafrost depth over glacial cycle Results suggest southern half of the Laurentide ice-sheet
was often extensively warm-based Significant present-day dis-equilibrium
Impact of ice on present-day ground temperature field: Is small outside of permafrost zone
Bayesian calibration method links data and physics (model) -> rational error bars
Calibration selects a climate forcing that produces a significantly different present day ground thermal field than that from a simple average of PMIP 1 and 2 General Circulation model results
For more results, refer to Tarasov and Peltier, JGR 2007
Request/Plea
Need an online database of deep borehole observations (especially for
Eurasia)!!!!
Probability of warm based ice over last
20kyr for N.A.
Present-day permafrost distribution
Kola Transect: Present-day ice versus no-ice models
Kola borehole time transect
Present ensemble mean permafrost depth for Europe