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Post date: Feb 9, 2015 9:02:21 PM
Sea level rise occurs in most seas and oceans around the world. Also in front of the Netherlands, where it was roughly 20cm over the last century. It receives contributions from a number of different sources, such as the expansion of the oceans (together with possible circulation changes), and ice melt coming from the worlds glaciers and the large ice sheets of Greenland and Antarctica. As many people know, sea-ice does not contribute to sea-level rise, as it floats on the water. However, indirectly it may still influence the rate of sea-level rise indirectly because of its high reflectivity. If the much darker water underneath sea-ice is exposed to direct sunlight, it will quickly heat up in summer. You see, a complex subject with complex feedbacks.
Modelling and understanding variability of sea-level rise is a challenge. To date, it is not possible to model all contributions simultaneously within a single model. This is partly because it is computationally not feasible. For example, the ice melt in Greenland depends substantially on the very complex details of the geometry and bathymetry of the fjords. In addition, there is the general believe that we (I mean "we" scientists) have also not (yet) completely mastered the full details of complex processes like iceberg calving, nor of ice-sheet "lubrication". To get an idea, look in the most recent IPCC report (AR5), where details are discussed. It then becomes clear that one needs to accommodate sea-level scenarios with a substantial uncertainty, and probabilistic approaches come in naturally.
Recently, studies have appeared where estimates of sea-level rise are obtained in a way that is very different from the conventional approach based on modelling using physically based models. The method that is used is to conduct a so-called expert judgment assessment. Several questions related to sea-level rise are asked to as many as possible experts in the field. These experts have particular views, that may differ substantially from the modelling community. One of these expert-judgement studies (Bamber and Aspinall, 2013) has appeared in Nature Climate Change in 2013. After reading the paper I had the impression that at least one of the key messages present in the data, was left almost entirely undiscussed. This message was that the experts themselves came up with rather different estimates for future sea level rise. In other words, there was rather low consensus among the experts, especially in the high-end estimates. And what do you do then? How are you going to interpret the results? Well, yes, probabilistically. But there you have to be careful not to influence the outcome by the methodology you use. The shading in the top figure gives an idea of the "consensus" distribution for various quantiles of the sea-level rise pdf, as obtained from the expert opinions. This figure is more fully discussed in a paper that has just been accepted to be published in Climatic Change. Among the topics discussed is this "expert consensus", and also some further sensitivities in the approach of Bamber and Aspinall (2013), which you might want to take care of if you consider to conduct an expert assessment yourself. As always, please contact me if you want a copy but have no access.
Abstract: "In a recent paper Bamber and Aspinall (2013) (BA13) investigated the sea-level rise that may result from the Greenland and Antarctic ice sheets during the 21st century. Using data from an expert judgment elicitation, they obtained a final high-end (P95) value of +84 cm integrated sea-level change from the ice sheets for the 2010--2100 period. However, one key message was left largely undiscussed: The experts had strongly diverging opinions about the ice-sheet contributions to sea-level rise. We argue that such (lack of) consensus should form an essential and integral part of the subsequent analysis of the data. By employing a method that keeps the level of consensus included, and that is also more robust to outliers and less dependent on the choice of the underlying distributions, we obtain on the basis of the same data a considerably lower high-end estimate for the ice-sheet contribution, +53 cm (+38-77 cm interquartile range of 'expert consensus'). The method compares favourably with another recent study on expert judgement derived sea-level rise by Horton et al. (2014). Furthermore we show that the BA13 results are sensitive to a number of assumptions, such as the shape and minimum of the underlying distribution that were not part of the expert elicitation itself. Our analysis therefore demonstrates that one should be careful in considering high-end sea-level rise estimates as being well-determined and fixed numbers."
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