Sea-level rise: Where we stand at the start of 2013 — Part 2

My short and clear answer to this question is: No. I say this as someone who is a physicist and great believer in process-based modelling. And as someone who heads a department of about fifty scientists, almost all of whom are engaged in process-based modelling of various aspects of the climate system, many in ice sheet and ocean modelling with direct application to sea level. Have a look at our sea level pages to get an impression. In contrast, the semi-empirical sea-level modelling in my department is pursued only by one PhD student – it is a small sideline of our research.

Ample reasons for my answer are found by looking at the large uncertainties and differences in process-based estimates of past contributions to sea level, which exceed a factor of three for glacier melt rates and even involve disagreements about the sign of Greenland mass changes (see Figs. 2 and 3 in Gregory et al. 2012).

Two more reasons: only last year the effect of increasing snow fall on ice discharge from Antarctica was included in a model, and it was found that this cancels out about half of the additional snow fall and is the dominant reason for increased ice discharge (Winkelmann et al., Nature 2012). So a first-order, even dominant, mechanism is only now being implemented in ice sheet models. And likewise last year, it was shown (I think convincingly) that the temperature threshold for a complete loss of the Greenland ice sheet is probably far lower (by about a factor of two) than previously estimated (Robinson et al., Nature Climate Change 2012). A factor-two revision and a newly discovered first-order mechanism are both not exactly signs of a mature stage of modelling having been reached. I know enough about modelling the Greenland and Antarctic ice sheets and mountain glaciers to be certain that a lot of work remains to be done before these models can be called mature.

Some conclusions, also concerning IPCC

Here I have put forth a different viewpoint to that of Gregory et al. 2012: process models have improved and now underpredict the 20th Century sea-level rise to a lesser extent than they used to. As a consequence they also predict a much greater sea-level rise for the future, vindicating the semi-empirical models which suggested that earlier projections were too low. However, process models are still not mature, and they still fall short of predicting the full extent of 20th Century sea-level rise, so the budget is not yet satisfactorily closed. I think the expert elicitation or the NOAA scenario document discussed in Part 1 of this post suggest that many experts share this viewpoint.

Such differing viewpoints are a normal and important part of the scientific discourse – this is all a good and healthy discussion that I enjoy and that will continue to improve our understanding. Despite the large overlap between the authors of Gregory et al and the IPCC sea-level chapter, I trust that after the review process the IPCC will provide a balanced account.

The recent improvements in understanding have confirmed the concerns of many sea-level experts, namely that the 4th IPCC report has understated the risks of future sea-level rise because the projection models used were not mature. In the meta-literature about scientific assessments, this IPCC sea-level problem has become a poster child for a poor handling of structural uncertainty, see e.g. Oppenheimer et al. (2007), O'Reilly et al. (2012) or Brysse et al. (2012), also when compared to assessments of other problems like ozone depletion O'Reilly et al. (2011).

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  1. J.M. Gregory, N.J. White, J.A. Church, M.F.P. Bierkens, J.E. Box, M.R. van den Broeke, J.G. Cogley, X. Fettweis, E. Hanna, P. Huybrechts, L.F. Konikow, P.W. Leclercq, B. Marzeion, J. Oerlemans, M.E. Tamisiea, Y. Wada, L.M. Wake, and R.S.W. van de Wal, "Twentieth-Century Global-Mean Sea Level Rise: Is the Whole Greater than the Sum of the Parts?", Journal of Climate, vol. 26, pp. 4476-4499, 2013.
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