EGU2020-5974
https://doi.org/10.5194/egusphere-egu2020-5974
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Concurrent Miocene Antarctic ice sheet growth and CO2 increase caused by disequilibrium

Lennert Stap, Gregor Knorr, and Gerrit Lohmann
Lennert Stap et al.
  • Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung, Bremerhaven, Germany (lennert.stap@awi.de)

Geological evidence indicates considerable Antarctic ice volume variations during the early to mid-Miocene. Hitherto, ice modelling studies have mostly used equilibrium simulations to explain this variability. In these simulations, the gain in precipitation due to increased temperatures has to outweigh the loss caused by increased ice melt, to obtain simultaneous ice sheet growth and CO2 level rise. Here, conceptualising ice dynamical model results, we find that this is not a necessary condition for the transiently evolving Miocene Antarctic ice sheet. Instead, ice volume increase when CO2 levels are rising can also be explained as a consequence of disequilibrium between the transiently changing ice volume and forcing climate. This disequilibrium permits a continuation of ice sheet growth after a gradual CO2 decline. When the CO2 level is increased again, the ice sheet is still adapting to a relatively large equilibrium volume. Lowering the periodicity of the forcing leads to a larger disequilibrium, and consequently larger CO2-ice volume phase differences. Furthermore, amplified forcing variability increases ice volume variations, because the growth and decay rates depend on the forcing. It also leads to a reduced average ice volume, which is induced by the growth rate generally being smaller than the decay rate. We therefore submit that retrieval of high resolution proxy-CO2 records covering the Miocene would be very beneficial to constrain ice modelling studies.

How to cite: Stap, L., Knorr, G., and Lohmann, G.: Concurrent Miocene Antarctic ice sheet growth and CO2 increase caused by disequilibrium, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5974, https://doi.org/10.5194/egusphere-egu2020-5974, 2020

Comments on the presentation

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Presentation version 1 – uploaded on 30 Apr 2020
  • CC1: Comment on EGU2020-5974, Bas de Boer, 04 May 2020

    Hi Lennert,

    Nice (follow up) work on the Miocene. Just wondering how you derived the CO2-Veq relation you mention in Slide 9. Is this using PISM simulations and CO2 reconstructions (for the Pleistocene??). Might be also that the AIS varriations are more symmetrical (meaning g=d if I'm correct), did you investigated that? And possible to determine a conceptual multi-variable model?

    • AC1: Reply to CC1, Lennert Stap, 04 May 2020

      Hi Bas,

      Thanks for your interest in our study. See my reply below.

      Just wondering how you derived the CO2-Veq relation you mention in Slide 9. Is this using PISM simulations and CO2 reconstructions (for the Pleistocene??).

      The CO2-Veq relation in slide 9 (Figure) is indeed constructed from steady-state PISM results (Stap et al., 2019). The mass balance was forced using atmosphere and ocean tempeatures, and precipitation output from COSMOS simulations. COSMOS was run using an early to mid-Miocene topography, interactive vegetation, and different CO2 levels. In all 9 simulations, PISM was run for 200kyr into equilibrium. The lower (glacial) branch simulations were started from no ice and a rebounded PD Antarctic bedrock topography. The upper (deglacial) branch simulations were started from the results of the low-CO2 PISM simulation. To construct a continuous relation, we assume the equilibrium ice volume varies linearly between these equilibrated PISM states.

      Might be also that the AIS varriations are more symmetrical (meaning g=d if I'm correct), did you investigated that?

      The growth and decay rates (g, d) are derived through finding a reasonably close match between transient results of the coupled model and of PISM simulations (Stap et al., 2019). The growth rates are generally higher than the decay rates.

      And possible to determine a conceptual multi-variable model? 

      I think that should be possible. For instance, a logical extension would be to include solar insolation variability in the future.


      Stap, L. B., Sutter, J., Knorr, G., Stärz, M., & Lohmann, G. (2019). Transient variability of the Miocene Antarctic ice sheet smaller than equilibrium differences. Geophysical Research Letters, 46(8), 4288-4298.