EGU2020-20029, updated on 12 Jun 2020
https://doi.org/10.5194/egusphere-egu2020-20029
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

The role of history and strength of the oceanic forcing in sea-level projections from Antarctica with the Parallel Ice Sheet Model

Ronja Reese1, Anders Levermann1,2,3, Torsten Albrecht1, Hélène Seroussi4, and Ricarda Winkelmann1,2
Ronja Reese et al.
  • 1Potsdam Institute for Climate Impact Research (PIK), Member of the Leibniz Association, Potsdam, Germany (ronja.reese@pik-potsdam.de)
  • 2University of Potsdam, Institute of Physics and Astronomy, Potsdam, Germany
  • 3LDEO, Columbia University, New York, USA
  • 4Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

Mass loss from the Antarctic Ice Sheet constitutes the largest uncertainty in projections of future sea-level rise. Ocean-driven melting underneath the floating ice shelves and subsequent acceleration of the inland ice streams is the major reason for currently observed mass loss from Antarctica and is expected to become more important in the future. Here we show that for projections of future mass loss from the Antarctic Ice Sheet, it is essential (1) to better constrain the sensitivity of sub-shelf melt rates to ocean warming, and (2) to include the historic trajectory of the ice sheet. In particular, we find that while the ice-sheet response in simulations using the Parallel Ice Sheet Model is comparable to the median response of models in three Antarctic Ice Sheet Intercomparison projects – initMIP, LARMIP-2 and ISMIP6 – conducted with a range of ice-sheet models, the projected 21st century sea-level contribution differs significantly depending on these two factors. For the highest emission scenario RCP8.5, this leads to projected ice loss ranging from 1.4 to 4.3 cm of sea-level equivalent in the ISMIP6 simulations where the sub-shelf melt sensitivity is comparably low, opposed to a likely range of 9.2 to 35.9 cm using the exact same initial setup, but emulated from the LARMIP-2 experiments with a higher melt sensitivity based on oceanographic studies. Furthermore, using two initial states, one with and one without a previous historic simulation from 1850 to 2014, we show that while differences between the ice-sheet configurations in 2015 are marginal, the historic simulation increases the susceptibility of the ice sheet to ocean warming, thereby increasing mass loss from 2015 to 2100 by about 50%. Our results emphasize that the uncertainty that arises from the forcing is of the same order of magnitude as the ice-dynamic response for future sea-level projections.

How to cite: Reese, R., Levermann, A., Albrecht, T., Seroussi, H., and Winkelmann, R.: The role of history and strength of the oceanic forcing in sea-level projections from Antarctica with the Parallel Ice Sheet Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20029, https://doi.org/10.5194/egusphere-egu2020-20029, 2020

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Display material version 1 – uploaded on 06 May 2020
  • CC1: Comment on EGU2020-20029, Thomas Kleiner, 07 May 2020

    Hi Ronja

    The total run-length of the model prior to the projections seams critical. What would happen, if the first spin-up (without historical) would run for another 164 years to account for the period 1850 - end of 2014, so that all simulations have the same run length?

    And how sensitive is your initial state to the choice of spin-up length? What happens if you run the spin-up for e.g. 11 kyrs, 13 kyrs, 15 kyrs?

    Thomas

    • AC2: Reply to CC1, Ronja Reese, 08 May 2020

      Hi Thomas,
      thanks for bringing up these two points. To test the effect of the simulation length, I continued the spin-up without historic run for 164 years and then started the experiment with NorESM rcp85 ocean forcing. The mass loss is, similar to the other experiments, compared with the corresponding control simulation:


      In numbers, mass loss increases by 41% when starting directly from the spin-up and by 37% when starting from the extended spin-up - so this seems to play a minor role. The spin-up length migth yield similar effects, but I will need to do further experiments to test for this.

      Best,
      Ronja

      • CC4: Reply to AC2, Thomas Kleiner, 08 May 2020

        Wow, this was fast. So this is really the historical forcing. Thank you!

  • CC2: Comment on EGU2020-20029, Helen Amanda Fricker, 07 May 2020

    Hi Ronja -- in your historic simulation, in what year does the ice sheet start to lose significant mass?

    • AC1: Reply to CC2, Ronja Reese, 07 May 2020

      Hi Helen,
      Thanks for your question! 
      Mass loss starts in about 1900 and continues until 2015. However, overall mass loss in the historic simulation is small (3.6mm of SLE from 1850 to 2015 versus control) and the observed acceleration not captured until 2015:

      Large variability in surface and basal mass balance from 1960 onward causes a slower increase in mass loss, here forced with NorESM historic forcing as provided by ISMIP6. But we see increasing mass losses and an acceleration when we continue the run after 2015 without additional forcing.

      For the experiments here we selected an initial state based on present-day geometry and ice velocities without considering past evolution. In teh future, we want to include this and test the historic forcing to hopefully be able to do `hindcasting’ experiments that better fit observations of mass loss and magnitudes of thinning patterns in present-day.
      Best,
      Ronja

      • CC3: Reply to AC1, Helen Amanda Fricker, 07 May 2020

        Thank you so much, Ronja this is great. Really nice work.