1. O. Elison Timm 1 A. Timmermann 1,4 T. Friedrich 1 A. Abe-Ouchi 2,3 J. Knies 5 Forced response of a Northern Hemisphere ice-sheet model to climate changes during the last 130,000 years 1. International Pacific Research Center, University of Hawaii at Manoa, Honolulu, HI, United States. 2. Center for Climate System Research, University of Tokyo, Kashiwa, Japan. 3. Research Institute for Global Change, JAMSTEC, Yokohama, Japan. 4. Dept. of Oceanography, University of Hawaii at Manoa, Honolulu, HI, United States. 5 Geological Survey of Norway, Trondheim, Norway

2. Introduction Glacial-Interglacial Cycles Glacial-Interglacial Cycles Orbital forcing: precession and/or obliquity. (Berger in the 1970s, P. Huybers, 2005, 2006, 2010) Prolonged synchronous increase in NH and SH summer insolation (Schulz and Zeebe (2006)) What are the driving forces? What are the driving forces? Changes in land albedo, sea-ice, ocean circulation, vegetation, carbon cycle. We use LOVECLIM (V1.0) an ’Earth System Model of Intermediate Complexity’ to obtain atmospheric forcing fields for the community Ice-sheet model GLIMMER How important are internal feedbacks? How important are internal feedbacks? Methods:Methods: Transient paleoclimate simulationsTransient paleoclimate simulations Time-slice sensitivity experimentsTime-slice sensitivity experiments

3. Hypothesis: Increasing summer insolation triggers ice- sheet discharge, reduction in Atlantic Meridional Overturning Circulation, NH cooling, and thus negative, stabilizing feedback. IRD events during times of increasing summer insolation IRD events during times of increasing summer insolation (Timmermann et al., Paleoceanography, 2010) time H6 C21 C24

4. LOVECLIM Ice-sheet forcing from IcIES ECBilt – atmosphere T21, L3 t2m precip t2m precip CLIO – ocean sea-ice 3x3, L20 aiaaia aiaaia air-sea fluxes VECODE – vegetation t2m,precip albedo t2m,precip albedo LOCH – Marine carbon cycle aiaaia aiaaia CO 2 fluxes Transient external forcing Freshwater Forcing albedo + orography albedo + orography

5. External forcing Sources: Daily mean irradiance for true longitude λ=90 o (mid-June) were calculated with the routines provided by Dr. Laskar (Laskar 2004). Atmospheric CO 2 derived from Antarctic ice-cores (as described in Timm et al, Paleoceanography, 2008) time

6. Modeling Approach: Modeling Approach: For our transient simulations and sensitivity studies, we use LOVECLIM (V1.0) an Earth System Model Of Intermediate Complexity Northern Hemisphere Ice-sheet model (IcIES) with time dependent atm. forcing Obtain atmospheric response patterns to CO 2 and orbital forcing with GCM model Proxy records with time series of atmospheric CO 2 and orbital forcing (130,000 BP – 0 BP) Proxy records with time series of atmospheric CO 2 and orbital forcing (130,000 BP – 0 BP) NH ice thickness, orography. (130,000 BP – 0 BP) NH ice thickness, orography. (130,000 BP – 0 BP) LOVECLIM: Transient simulation with forcing from NH ice- sheets, orbital changes, and atm. CO 2 LOVECLIM: Transient simulation with forcing from NH ice- sheets, orbital changes, and atm. CO 2 Northern Hemisphere Ice-Sheet model (GLIMMER*) with time dependent atm. forcing. (Abe-Ouchi et al, Clim. Past., 2007) * Rutt, I. C., M. Hagdorn, N. R. J. Hulton, and A. J. Payne, J. Geophys. Res., 114, F02004, 2009

7. Shutdown of the Atlantic Meridional Overturning Circulation (AMOC) Shutdown of the Atlantic Meridional Overturning Circulation (AMOC) Difference in summer air temperatures 102ka BP minus 114ka BP. (high minuxs low NH summer insolation) Difference in summer air temperatures at 108ka BP with/ without freshwater input into North Atlantic (AMOC shutdown – active AMOC) Why does precessional forcing not lead to glacial terminations before 19 ka BP?

8. Shutdown of the Atlantic Meridional Overturning Circulation (AMOC) Shutdown of the Atlantic Meridional Overturning Circulation (AMOC) LOVECLIM simulation: Diagnosed mass balance over the ice-sheets of North America and Greenland LOVECLIM simulation: Diagnosed mass balance over the ice-sheets of Eurasia time Why does precessional forcing not lead to glacial terminations before 19 ka BP?

9. Shutdown of the Atlantic Meridional Overturning Circulation (AMOC) Shutdown of the Atlantic Meridional Overturning Circulation (AMOC) Difference in annual mean air temperatures between 18 and 17ka BP Difference in annual mean air temperatures at 18-17 ka BP with 17ka BP shutdown of AMOC. Last glacial termination and Heinrich 1 event

10. GLIMMER* forced with climatological annual cycle from transient simulation with LOVECLIM and freshwater forcing in North Atlantic. Climatology at 18 and 17ka BP (with bias correction) Effect of AMOC shutdown onto NH ice-sheets Effect of AMOC shutdown onto NH ice-sheets 18 ka BP (active AMOC) equilibrium17 ka BP (AMOC shutdown) equilibrium * Rutt, I. C., M. Hagdorn, N. R. J. Hulton, and A. J. Payne, J. Geophys. Res., 114, F02004, 2009

11. Effect of incremental CO 2 increase and insolation increase onto NH ice- sheets Effect of incremental CO 2 increase and insolation increase onto NH ice- sheets 18 ka BP (active AMOC) equilibrium17 ka BP (AMOC active) equilibrium GLIMMER * forced with climatological annual cycle from transient simulation with LOVECLIM without freshwater input. Climatology at 18 and 17ka BP (with bias correction) images created with IDV (Integrated Data Viewer) * Rutt, I. C., M. Hagdorn, N. R. J. Hulton, and A. J. Payne, J. Geophys. Res., 114, F02004, 2009

12. GLIMMER forced with climatological annual cycle from transient simulation LOVECLIM at 18 an 17ka BP Effect of AMOC shutdown onto NH ice-sheets Effect of AMOC shutdown onto NH ice-sheets Adjustment to THC shutdown 17 ka AMOC shutdown 17 ka BP active AMOC Adjustment over 15000 model years Difference after 15000 model years +2000m -1000m time 246 8 10 12 14 6.3 6.9 7.5

13. Transient Ice-Sheet Modeling Approach: Transient Ice-Sheet Modeling Approach: For our transient simulations and sensitivity studies, we use LOVECLIM (V1.0) an Earth System Model Of Intermediate and the community ice-sheet model GLIMMER* Northern Hemisphere Ice-sheet model (IcIES) with time dependent atm. forcing Obtain atmospheric response patterns to CO 2 and orbital forcing with GCM model Proxy records with time series of atmospheric CO 2 and orbital forcing (130,000 BP – 0 BP) Proxy records with time series of atmospheric CO 2 and orbital forcing (130,000 BP – 0 BP) NH ice thickness, orography. (130,000 BP – 0 BP) NH ice thickness, orography. (130,000 BP – 0 BP) LOVECLIM: Transient simulation with forcing from NH ice- sheets, orbital changes, and atm. CO 2 LOVECLIM: Transient simulation with forcing from NH ice- sheets, orbital changes, and atm. CO 2 Northern Hemisphere Ice-Sheet model (GLIMMER*) with time dependent atm. forcing. (Abe-Ouchi et al, Clim. Past., 2007) * Rutt, I. C., M. Hagdorn, N. R. J. Hulton, and A. J. Payne, J. Geophys. Res., 114, F02004, 2009

14. Comparison between the ice-sheet simulations obtained with IcIES, LOVECLIM-GLIMMER and ICE5G (Peltier, 2004) IcIES, GLIMMER simulation IcIES, GLIMMER simulation Termination I simulated with IcIES and GLIMMER lags the ICE5G reconstructed timing of the deglaciation. LOVECLIM-GLIMMER underestimates the early buildup of the Eurasian ice-sheet. time

15. Effect of global temperature offset on ice-volume GLIMMER simulation GLIMMER simulation LOVECLIM-GLIMMER: problem with the early buildup of the Eurasian ice-sheet: Sensitivity runs with temperature offsets (-8, -2, -1, 1, 2, 8 K ) suggest negative precipitation bias is the likely cause for the failure in ice-sheet buildup over Eurasia between MIS5a-d (115- 80ka B.P.) With cooler global temperatures (larger ice-volumes) the response to precessional forcing increases. t2m offset 8 2 1 0 -2 -8 time

16. Summary Forced response of a global ice-sheet model to climate changes during the last 130,000 years What are the driving forces? What are the driving forces? How important are internal feedbacks? How important are internal feedbacks? Ice-sheet simulations indicate strong sensitivity to precessionally driven summer insolation changes. We discussed an additional feedback: AMOC shutdown provide stabilizing, negative feedback mechanism for ice-sheet growth. => The sequence of freshwater input into the North Atlantic between 115ka and 80ka BP could have helped to sustain glacial conditions during periods of high summer insolation.