Perspectives on sea-level rise and coastal impacts beyond SROCC

Impacts of sea-level rise play an important role in the sea-level chapter and summary for policymakers of SROCC. Emphasized content includes the uncertainty in regional sea-level rise and the amplification of extreme sea level events. Here, we propose two advances regarding uncertainties and extremes beyond SROCC. First, we present an explicit quantification of the co-dependence between the different components of sea-level rise, derived from the CMIP5 ensemble. This co-dependence is essential for a correct calculation of regional uncertainties. We show that the ad-hoc co-dependences used in both the fifth assessment report (AR5) and SROCC lead to an overestimation of regional uncertainties and demonstrate how these estimates can be improved. Second, as impacts are not only ruled by regional mean sea-level rise but also by extreme events, we propose a method to quantify the impact of waves on the amplification of extreme events. As waves are typically ignored in extreme sea level studies, they are also not incorporated in SROCC. Waves introduce a large dependency on local shoreline properties. In general, the amplification of extreme events appears to be overestimated when waves are neglected, implying that more time may be available to mitigate or adapt to increased coastal hazards than concluded in SROCC. Overall, we conclude that two significant advances can be made in future IPCC reports with respect to the physical characterization of coastal impacts, namely the explicit co-dependence between components of sea-level rise, and the inclusion of waves in estimates of future changes in extreme events. These advances will improve the usefulness of sea-level rise information for policymakers and stakeholders as their interest is usually on the local scale.