EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Transient Pleistocene simulations with a new coupled climate-ice-sheet model

Dipayan Choudhury1,2, Axel Timmermann1,2, Fabian Schloesser3, and David Pollard4
Dipayan Choudhury et al.
  • 1IBS Center for Climate Physics, Busan, Korea, Republic of (
  • 2Pusan National University, Busan 46241, South Korea
  • 3International Pacific Research Center, University of Hawaii at Manoa, Honolulu, HI 96822, USA
  • 4Earth and Environmental Systems Institute, Pennsylvania State University, Pennsylvania 16802, USA

Orbital and COvariations over glacial timescales are widely held responsible as drivers of the ice-age cycles of the Pleistocene. Alongside these glacial cycles, our paleoclimate history is marked with abrupt changes and millennium scale variabilities. However, the relative contributions of these forcings over glacial transitions and mechanisms of abrupt changes are not very well understood. Here, using the recently developed three-dimensional coupled climate – ice-sheet model (LOVECLIM – Penn State University ice-sheet model), we simulate the glacial inception over the period of MIS7 to MIS6 (240-170ka). This period is the coldest interglacial post the Mid-Brunhes Event and includes one of the fastest glaciation/deglaciation events of the Late Pleistocene, over MIS7e-7d-7c (236-218ka); which we use here to benchmark our transient coupled model runs. Our results suggest that glacial inceptions are more sensitive to orbital variations, whereas terminations need both forcings to work in tandem over a tiny ablation zone at the southern margins of ice sheets. And abrupt changes may result from a critical interplay between the climate and the cryosphere systems. Using multiple ensembles in combination with conceptual dynamical systems’ models, we test the sensitivity of ice-sheets to various physical factors and discuss the presence of multiple equilibrium states and runaway effects. Additionally, our simulations show that regional scale variations at the southern end of Laurentide can lead to a bifurcation of the system and play a role even in orbital-scale ice-sheet growth/decay.

How to cite: Choudhury, D., Timmermann, A., Schloesser, F., and Pollard, D.: Transient Pleistocene simulations with a new coupled climate-ice-sheet model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2272,, 2020

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Display material version 1 – uploaded on 29 Apr 2020
  • CC1: Comment on EGU2020-2272, User deleted account, 06 May 2020

    Hi. Just wondering about some parameter settings. This 'a' parameter for CO2 seems like a climate senstivity parameter. So what is ECS default and is it then that much higher when a=2.5 (causing runaway effect)? You referend to Tobais his paper in 2016, and I remember the discussion on climate sensitivity there. But in the coupling sense it seems to be more sensitive, since there's also a runaway for a=1.8 (slide 6)?

    • AC1: Reply to CC1, Dipayan Choudhury, 06 May 2020

      Hey Bas, Friedrich et al reported sensitivity of avg 3.2K (1.78-4.88K). As far as I recall, our sensitivity was between 2-4K for the alpha range we tried. Its true that we are having a runaway at the MIS6 inception for alpha=1.8, although we tuned our parameters over the MIS7e-7d-7c period (240-215ka). One reason could be that small imbalances are getting integrated over 70k years and then they reach a point of no comeback. But adjusting the 'm' parameter in the energy balance equation did help in stabilizing the runaways towards the end of the simulation. We are currently trying to figure out the optimum parameter sets for this later period.



  • CC2: Comment on EGU2020-2272, Lennert Stap, 06 May 2020

    Hello Dipayan,

    Great study, thanks for showing! Regarding your results in slide #5: in Stap et al. (2014) (, we looked at this from the other side. We found that a termination was impossible without changing insolation (Figs. 11,13). However, opposite to your results, we couldn't get a glaciation going at steady 280 ppm CO2 (Fig. 12). I wonder where this CO2 threshold lies with your model.

    Regards, Lennert

    • AC2: Reply to CC2, Dipayan Choudhury, 07 May 2020

      Hi Lennert,

      In our current setup, keeping insolation fixed at 240ka value and using only transient GHG forcing leads to no noticeable changes in the ice volume. Insolation forcing is crucial both for inception and termination at MIS 5d. For terminations, CO2 > 220ppm is enough for the insolation forcing to cause a termination. But for inceptions, orbital only runs can still cause ~40m SLE inceptions for CO2 = 280ppm at MIS 5d. However, we did not get an inception for CO2 =350ppm and 400ppm. So compared to Stap et al. (2014) threshold of 280ppm, our threshold would lie somewhere between 280ppm and 350ppm. 



      • CC3: Reply to AC2, Lennert Stap, 07 May 2020

        Thanks, interesting stuff! I'll be sure to give your discussion paper a read.