Rethinking Sea-Level Projections Using Families and Timing Differences

In the recent decade, many global and regional sea-level rise (SLR) projections have been published, which raises questions for users. Here, we present a series of strategies to help users to see the forest for the trees, by reducing the number of choices to be made. First, we use the similarities in the methodologies and contributing sources of the projections to group 82 projections from 29 publications into only 8 families. Second, we focus on the timing of reaching several global mean SLR thresholds, and the uncertainty therein, rather than the projected value in the year 2100. Finally, we combine the information on timing and families to define three categories which can support decision making. For global mean SLR up to 0.50 m the differences in timing are small, regardless of climate scenario or family, clearly indicating a timing window for adaptation decisions. For larger global mean SLR (0.75-1.00 m), the climate scenario becomes more important for the uncertainties in timing, but the SLR threshold will be crossed within a limited window of time, supporting adaptation decision making on the medium to long term. Beyond 1.00 m the differences between the families and climate scenario strongly determine the uncertainties in timing, and more information is needed, for instance using early warning signals, before decisions for adaptation can be made. We make recommendations on how each of these three categories, combined with the lead time and lifespan of adaptation options, can inform decisions on adaptation strategies using adaptation pathways planning.