Local Sea Level Changes: Assessing and Accounting for the Risk Associated With the Low-Probability, High-Risk Tail of the Risk Spectrum

Abstract:

Stakeholders in the coastal zone, particularly the urban coasts, are turning to science to get information on future Local Sea Level (LSL) rise. Many scientists and scientific committees respond to this request with a range of plausible trajectories (RPT) defined by a number of possible trajectories each corresponding to a certain scenario. Often these assessments take a starting point in the small number of global sea level trajectories provided by the IPCC. This approach is inherently deterministic. The resulting RPT, which can be quite large, is considered as reflecting “uncertainty in LSL projections.” Non-scientists often use the RPT to select a preferred and much narrower sub-RPT, for which they plan, or they use the “large uncertainty” to justify not taking any measures. In response to societal needs, science focuses on a reduction of the uncertainties through improved deterministic models. This approach has a number of problems: (1) The complexity of LSL as the outcome of many local, regional and global earth system processes, including anthropogenic processes, renders a deterministic approach to prediction invalid. (2) Most assessments of the RPT account for an incomplete set of relevant earth system processes, and for each processes make assumptions that (often arbitrarily) constrain the contribution from this process. (3) LSL is an inherently probabilistic variable that has a broad probability density function (PDF), with a complex dependency of this PDF on the PDFs of the many contributing processes. In particular, the contribution from the large ice sheets has a PDF with low-probability high-impact tails that are generally neglected in deterministic LSL projections and in the sub-RPT used for coastal planning. A fully probabilistic assessment of the risk associated with LSL rise indicates that the standard deterministic assessment not only neglect most of the low-probability, high-risk tail of the PDF but also medium-probability, high-risk parts. This creates a high potential for low or medium-probability events that could challenge a global civilization, which is depending on crucial coastal infrastructure with growing urban coasts built under the principle assumption that future LSL will not change much more than what civilization experienced during the (geologically short) past.