Expert judgements of sea-level rise at the local scale

Whilst local projections of sea-level rise (SLR) are necessary to facilitate targeted climate change adaptation and communication strategies, downscaling from global climate models can be problematic. Here, we use expert probability judgement to elicit a suite of local projections, and associated uncertainties, for future SLR on the Severn Estuary in the south-west of the UK. Eleven experts from a range of policy and academic backgrounds took part in a structured probability elicitation exercise for the years 2050, 2100 and 2200. In addition to the quantitative elicitation, the experts’ reasoning during the task was qualitatively analysed. Quantitative analyses show that although there is consensus that sea levels will rise on the Estuary in future, there is wide variation between judgements and much uncertainty regarding the magnitude of future rise. For example, median estimates of SLR (compared to the 2011 level) range from 9.6 to 40 cm in the year 2050; 20 to 100 cm in 2100; and 35 to 300 cm in 2200. Fifty per cent confidence intervals and ninety per cent confidence intervals vary even more. Qualitative analyses indicate that experts’ judgements may have been influenced by their choice of methods and information sources, the ways in which they thought about the future, and heuristics. The study shows the merits of integrating qualitative and quantitative methods to explore the reasoning behind uncertainty judgements. We conclude that where expert probability judgements are to be used to characterise uncertainty such reasoning should be made explicit.