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

This week an expert elicitation by Bamber and Aspinall was published in Nature Climate Change, which confirms that the body of expert opinion expects much higher sea level rise than the 4th IPCC report. The median contribution from ice sheets alone by 2100 was estimated as 29 cm, with a 95th percentile value of 84 cm. The paper compares a range for total sea-level rise for the RCP4.5 scenario of 33-132 cm based on their expert elicitation to our recent semi-empirical range (Schaeffer et al., Nature Climate Change 2012) of 64-121 cm.

Recent progress in understanding sea-level rise

Just before Christmas an overview of process-based sea-level estimates for the 20th Century was published by Gregory et al in Journal of Climate, a paper with many authors that presents a whole suite of estimates for individual sea-level contributions, partly data-based and partly model-based. The paper then looks at the sum of all these components, see Fig. 2.

Fig. 2: Comparison of timeseries of annual-mean global-mean sea-level rise from four analyses of tide-gauge data (lines) with the range of 144 synthetic timeseries (grey shading). Each of the synthetic timeseries is the sum of a different combination of thermal expansion, glacier, Greenland ice-sheet, groundwater and reservoir timeseries. Source: Gregory et al in Journal of Climate.

This diagram shows the range of sea-level histories that is obtained by combining all the single estimates in various combinations. It shows that the observed sea-level history can be obtained when combining the individual components in all possible combinations (144 in all), but the observations lie at the very edge of the range. The authors write:

We would judge that a given synthetic timeseries gave a satisfactory account of observed global mean sea-level rise if it lay within the uncertainty envelope for 90% of the time. Very few of the synthetic timeseries pass this test.

If we take the mid-point of the grey range, this shows that the central estimate of 20th Century sea-level rise, based on adding up all processes, is ~11 cm. The central estimate of observed rise is ~16 cm and ~40% larger.

The authors conclude that a residual trend is needed to make up for the discrepancy and argue that this must come from a long-term ice loss in Antarctica. They write:

If we interpret the residual trend as a long-term Antarctic contribution, an ongoing response to climate change over previous millennia, we may conclude that the budget can be satisfactorily closed.

I guess it depends on how easily one is satisfied. The estimated required residual trend is given as 0 – 0.2 mm/year, so it explains at most 2 cm of rise over the 20th Century and does not make up for the shortfall mentioned; as I understand it, it just increases the synthetic range enough to bring most observations into its 90% confidence interval, though still near its edge.

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.

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  1. J.L. Bamber, and W.P. Aspinall, "An expert judgement assessment of future sea level rise from the ice sheets", Nature Climate Change, vol. 3, pp. 424-427, 2013.
  2. M. Schaeffer, W. Hare, S. Rahmstorf, and M. Vermeer, "Long-term sea-level rise implied by 1.5 °C and 2 °C warming levels", Nature Climate Change, vol. 2, pp. 867-870, 2012.
  3. 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.
  4. S. Rahmstorf, "A Semi-Empirical Approach to Projecting Future Sea-Level Rise", Science, vol. 315, pp. 368-370, 2007.