Sea-level rise and storm surges, relationship status: complicated!

Increasing coastal flood risk is one of the major consequences of mean sea-level rise (SLR) and will make costly adaptation, and in extreme cases migration to higher ground, inevitable. Extensive research has been directed toward reconstructing past SLR and developing global and regional SLR projections (Church et al 2013). These projections are used in impact and adaptation studies (e.g. Hinkel et al 2014) to facilitate risk-informed decision making. However, in order to understand coastal impacts under current and futureclimateandsocio-economicconditions,notonly robust SLRprojections are requiredbut also aprofound knowledge of the drivers and occurrence of present-day and future extreme sea levels (ESL), as ESL drive the impacts (Bindoff et al 2007). SLR already threatens several small island states in the Pacific and Indian Oceans and, depending on how much sea level will rise in the coming decades and centuries, other coastal areas will become uninhabitable. This will be a gradual process that can be anticipated. ESL on the other hand occur randomly and accurate forecasts become only available a few days or hours before the impacts occur. Recent events like Hurricanes Harvey and Irma have revealed how vulnerable and ill-prepared many coastal communities are for such extreme events. Therefore, adequate adaptation has to take into account SLR and ESL as well as the uncertainties inherent to both of them (see below for more details). ESL (excluding tsunamis) result from tropical or extra-tropical cyclones as a combination of high tides and storm surges, and in many cases a dynamic wave component. Tide gauges located along the world’s coastlines that have been recording water levels for many decades or even centuries provide the most accurate source of information on ESL, but they are unevenly distributed (with most of them located in North America and Europe) (Menéndez and Woodworth 2010) hampering broad-scale (up to global) assessments. Hydrodynamic numerical models, on the other hand, can simulate ESL data for all coastlines, but they are computationally very expensive, especially when applied for large areas, and they rely on accurate bathymetric data and high-resolution (in space and time) wind information, as the most important driver for storm surges; both are often unavailable. Hence, at continental and global scales there has been a limited number of studies of present-day and future ESL. This is also reflected in the assessments by the Intergovernmental Panel on Climate Change (IPCC) that include dedicated SLR chapters in all five reports, yet no, or only relatively short sub-chapters on ESL with limited information. In the Fifth Assessment Report (AR5) (Church et al 2013) multiplication factors were derived for many tide gauges around the globe, indicating the increase in frequency with which certain extreme levels will be exceeded under different SLR scenarios. This was an important improvement compared to the AR4 (Bindoff et al 2007), which did not provide such information on future ESL changes. However, the statistical approach that was used relies on the simplified assumption of a constant change in the frequency for all extreme events; i.e. the frequency of moderate events will change in the same way as the frequency of the most extreme events. The same statistical approach has been used in other global studies where present-day and future ESL and associated impacts were assessed (Hinkel et al 2014, Muis et al 2016). A recent publication by Buchanan et al (2017) addresses this issue. The authors employed a much more sophisticated and rigorous statistical methodology to assess the variation in multiplication factors (or amplification factors, as referred to in the study) across different levels of extremes, with a focus on the contiguous United States (US). Importantly, they found that SLR will result in stronger increases in the number