EGU2020-7084
https://doi.org/10.5194/egusphere-egu2020-7084
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

The transient sensitivity of sea level rise

Aslak Grinsted and Jens Hesselbjerg Christensen
Aslak Grinsted and Jens Hesselbjerg Christensen
  • Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark (ag@glaciology.net)

We are warming our planet, and sea levels are rising as oceans expand and ice on land melts. This instigates a threat to coastal communities and ecosystems, and there is an urgent need for sea level predictions encompassing all known uncertainties to plan for it. Comprehensive assessments have concluded that sea level is unlikely to rise by more than about 1.1m this century but with further increase beyond 2100. However, some studies conclude that considerably greater sea level rise could be realised and an expert elicitation assign a substantially higher likelihood to this scenario. Here, we show that models used to assess future sea level in AR5 & SROCC have a lower sea level sensitivity than inferred from observations. By analyzing mean rate of change in sea level (not sea level itself), we identify a near linear relationship with global mean surface temperature in both model projections, and in observations. The model projections fall below expectations from the more recent observational period. This comparison suggests that the likely range of sea level projections in IPCC AR5 and SROCC would be too low.

How to cite: Grinsted, A. and Christensen, J. H.: The transient sensitivity of sea level rise, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7084, https://doi.org/10.5194/egusphere-egu2020-7084, 2020

Comments on the presentation

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Presentation version 1 – uploaded on 02 May 2020
  • CC1: Comment on EGU2020-7084, Michael Wolovick, 04 May 2020

    What is the refernce period for the temperature anomalies?

    • AC1: Reply to CC1, Aslak Grinsted, 04 May 2020

      I have used the same as AR5: 1986-2005.  

  • CC2: Comment on EGU2020-7084, Irina Melnikova, 06 May 2020

    Could you please explain more about your proposed conclusion on spin up issues.  Are they too short for sea level projections?

    • AC2: Reply to CC2, Aslak Grinsted, 06 May 2020

      I dont go much into detail on that in the presentation. 

      The slope of the line is the sensitivity, but the intercept between the line and the x-axis is the temperature for which dS/dt = 0 - We call that "the balance temperature".  To me it looks like SROCC actually has a reasonable slope - It is almost parallel to the historical slope (ignoring the non-linearity). However, if you look at the balance temperature, then that seems way too warm compared to historical data. This means that present-day temperatures are not as far from equilibrium for present temperatures as it should be. 

      This suggests to me that atleast one of the contributor models have been run forwards in time from a state that is too close to equilibrium. This is related to spin-up and due to the long inertia/memory of the individual contributors.

      Take-away 1: model runs used for projections are start all the way back in 1850 so that we can check the performance is against historical data.

      Take away 2: It is important to validate the combined GMSL model and not just the individual components. Some of the contributors (e.g. antarctic) do not have long data records and are difficult to validate.

      (Note: there is a table in the supplement where i do some stats on balance temperature)

      • CC3: Reply to AC2, Irina Melnikova, 08 May 2020

        Thank you for explanation! Would be great to see whether AR6 does better!