Deglaciation of the Northern Hemisphere :
Contributions of Orbital, Greenhouse and Freshwater Forcings
Lauren J. Gregoire, Paul J. Valdes, Tony J. Payne
School of Geographical Sciences, University of Bristol, UK - [email protected]
2. Methodology
1.Introduction
We model the evolution of the Northern
Hemisphere (NH) ice sheets through the
last deglaciation.
We force an ice sheet model with
temperatures and precipitations from a
transient simulation of 21-9 ka (thousand
years BP) run with a fast General
Circulation Model (GCM).
We investigate :
• The evolution of the geometry and mass
balance of the North American and
Eurasian ice sheets.
• The contribution of orbital, greenhouse
gases (GHG) and freshwater forcings to
the deglaciation of the NH ice sheets
Climate forcing: FAMOUS fast GCM [1]
Ice sheet model: Glimmer-CISM [7]
3D thermo-mechanical model with shallow
Transient simulation of 21-9 ka [2]
ice
approximation.
Includes:
Evolving boundary
• Positive Degree Day (PDD) mass balance scheme
conditions:
•
Isostatic
rebound
 ICE: Orography, ice
• Basal sliding
sheet extent and land-sea
Model
initialised
with
ice
sheets
built
up
through
the
monthly
mask from Ice-5G [3],
last
glacial-interglacial
cycle
[8].
Temp.
updated every 1kyr
Parameters adjusted to optimise ice volume and
&
isostatic
rebound
through
the
deglaciation.
 GHG: Trace gases [4], Precip.
N.
continuously changing
Age (ka)
Model parameters
America
Atmospheric lapse rate
 ORB: Orbital forcing [5],
continuously changing
 FW: Freshwater forcing
[6], continuously changing
Eurasia
Unit
oC/km
5
PDD snow
3
4
mm/day/oC
PDD ice
8
10
mm/day/oC
Flow enhancement factor
5
-
Geothermal heat flux
50
mW/m2
Basal traction constant*
Mantle relaxation time
10 (till) & 0.5
1000
mmyr-1Pa-1
yr
* The Basal traction depends on the presence of sediment (till) on the bed following [9]
3. Evolution of the Laurentide and Fennoscandian ice sheets
N. America
Eurasia
♦ Ice-5G [3]
Jul.
Separation of Laurentide
and Cordilleran ice sheets
Age (ka)
North America:
• Ice extent closely follows Ice-5G
except that the separation of the
Laurentide and Cordilleran ice
sheets is delayed and ice extent is
reduced at 9 ka.
• A Peak in mass balance at 11.7 ka
when Laurentide and Cordilleran
ice sheets separate produces a
meltwater flux up to 0.38 Sv and
12 m of sea level rise in 500 yr.
Eurasia:
• Ice thickness overestimated at the
beginning but the deglaciation is
quicker than field evidence
suggests [10].
 Likely due to a bias in the
climate forcing.
• Peak in mass balance when
Scandinavian and Barents Sea ice
sheets separate.
Evolution of the North American and Eurasian ice sheets through the deglaciation.
Temperature is the July potential temperature over the ice sheet domains.
Model
Ice elevation (m)
Ice-5G
21 ka
Age (ka)
Eurasia
References
9 ka
Orography (m)
Model
Ice-5G
21 ka
• ORB: the orbital forcing starts the deglaciation of
North America at 19 ka and Eurasia at 20 ka.
• GHG: the effect of GHG starts around 17 ka.
• ICE: changes in surface albedo due to Ice-5G are
significant enough to produce half of the deglaciation.
• FW: Meltwater pulse 1a, at 14 ka, noticeably slows
down the deglaciation of the Laurentide ice sheet. The
effect is insignificant in Eurasia because little ice is left
then.
• When ½ of the initial volume has been lost, the
contributions to the volume lost is :
N. America (12 ka) Eurasia (15 ka)
GHG
18 %
7%
ORB
35 %
30 %
ICE
46 %
61 %
Age (ka)
12 ka
Ice elevation (m)
4. Contributions from the different forcings
North America
15 ka
Calculated contribution of ice albedo (ICE), orbit (ORB) and
GHG forcings (excluding freshwater) to the decrease in volume
of the NH ice sheets since 21ka.
[6] Freshwater flux reconstructed as part of the ORMEN project.
[7] Rutt et al., 2009. J. Geophys. Res. 114.
[1] Smith et al., 2008. Geosci. Model Dev. 1, 53-68.
[2] Kahana et al. (in prep.) Transient simulation of the deglaciation. [8] Gregoire et al., 2010. Geophys. Res. Abs. Vol. 12, EGU-9492
[9] Laske and Masters, 1997. EOS Trans. AGU 78.
[3] Peltier, 2004. Rev. of Earth and Planet. Sci. 32, 111-149.
[10] Svendsen et al., 2004. Quat. Sci. Rev., 23(11-13), 1229-1271.
[4] Spahni et al., 2005. Science 310, 1317-1321.
[5] Berger, A., Loutre, M., 1992. Earth Planet. Sc. Lett. 111, 369-382. [11] Dyke et al., 2003. Geol. Survey of Canada Open File 1547.
17 ka
14 ka
12 ka
Orography (m)
5. Conclusions
We forced a model of the NH ice sheets with a
temporally and spatially detailed modelled climate of
the last deglaciation.
• The separation of the Laurentide and Cordilleran ice
sheets (~14 ka [11]) creates a meltwater flux equivalent
in terms of timing and volume to Meltwater Pulse 1a.
• The separation of the Scandinavian and Barents Sea
ice sheets also lead to an acceleration of the
deglaciation.
• The changes in surface albedo due to Ice-5G
boundary conditions contribute to ½ of the
deglaciation of the NH in terms of volume.
• If we consider the changes in surface albedo to be a
feedback in the coupled climate-ice sheet system, then
in North America:
 Orbit contributes to 2/3 of the deglaciation
 GHGs contribute to 1/3 of the deglaciation
FUTURE WORK: Perform a fully coupled climate
ice-sheet simulation of the last deglaciation
Acknowledgments
This work is part of the Marie Curie RTN Network for Ice
sheets and Climate Evolution (NICE). It was carried out using
the computational facilities of the Advanced Computing
Research Centre, Bristol University.