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

In any case the now higher sea-level estimates from process models for the 20th Century naturally also imply higher projections for the 21st. Here it is important to compare like with like – same emissions scenario, same time interval. E.g. for the A1B scenario over the interval 1990-2095, the 4th IPCC report’s central estimate is 34 cm while the semi-empirical estimate of Rahmstorf (2007) is 78 cm. While we do not want to enter discussions about the draft 5th IPCC report (this would be premature, given there will still be numerous changes), given that it is in the public domain now it is no secret that for this scenario it projects 59 cm – a whopping 73% increase over the 4th report and much closer to my 2007 semi-empirical estimate. However, the more recent semi-empirical models have tended to give higher projections, so there remains a substantial gap between these two modelling approaches.

Personally, for various reasons I expect that process-based estimates may well keep edging up in future as the models are improved, considering e.g. the recent Nature-Paper by Winkelmann et al. and the remaining tendency to underestimate past rise. I dearly hope, however, that the truth will turn out to be a lower rise than suggested by semi-empirical models, for the sake of all people who live near the sea or love the coast.

Is 20th C sea-level rise related to global warming?

The Gregory et al. paper was greeted with enthusiasm in “climate skeptics” circles, since it includes the peculiar sentence:

The implication of our closure of the budget is that a relationship between global climate change and the rate of global-mean sea-level rise is weak or absent in the past.

The abstract culminates in a similar phrase, which can easily be misunderstood as meaning that global warming has not contributed to sea-level rise. That is wrong of course, and the claimed closure of the sea-level budget in this paper is only possible because increasing temperatures are taken into account as the prime driver of 20th Century sea-level rise.

When read in full context, the true meaning of the statement becomes clear: it is intended to discredit semi-empirical sea-level modelling. That is both fallacious and odd, given that the paper does not even contain any examination of the link between global temperature and the rate of global sea-level rise which is at the core of semi-empirical models, and which has been thoroughly examined in a whole suite of papers (e.g. Rahmstorf et al. 2011). Instead, it dismisses semi-empirical models offhand based on two arguments.

The first is that individual contributions to the sea-level budget do not show a clear link to global temperature. That is simply a fundamental misunderstanding of the semi-empirical approach: its principal idea is to cut through the uncertainty and complexity surrounding the time evolution of the individual components by considering only the overall sea-level rise and its link to global temperature. The usefulness of this idea is based on the following factors:

1. More accurate data. We may assume that the observational data for the time evolution of global sea-level rise are much more accurate than those for any individual component. A particular irony is that the glacier melt component of “process models” is in fact estimated by a semi-empirical equation quite similar to the one we use for sea level, but poorly validated since data are available only for ~350 of the world’s ~ 200,000 glaciers. Thus results from questionable semi-empirical modelling are used to dismiss rather better-validated semi-empirical modelling.

2. Partial cancellation of regional climate variability. It is obvious that e.g. the Greenland ice sheet responds to local and not global temperature, so it is not surprising that the Greenland component alone shows little relation to global temperature in the past. However, the ice sheet contributions come from both polar areas, the mountain glacier components from global land masses across a range of latitudes (with a mid- to high-latitude bias), while thermal expansion is particularly sensitive to warming over low-to mid-latitude oceans since the thermal expansion coefficient is much larger there than in colder waters. This broad mix of different regions contributing to sea-level rise makes it likely that the total rise is more clearly linked to global-mean temperature than any single component.

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  1. S. Rahmstorf, "A Semi-Empirical Approach to Projecting Future Sea-Level Rise", Science, vol. 315, pp. 368-370, 2007.
  2. R. Winkelmann, A. Levermann, M.A. Martin, and K. Frieler, "Increased future ice discharge from Antarctica owing to higher snowfall", Nature, vol. 492, pp. 239-242, 2012.
  3. S. Rahmstorf, M. Perrette, and M. Vermeer, "Testing the robustness of semi-empirical sea level projections", Clim Dyn, vol. 39, pp. 861-875, 2012.