A first order assessment of long-term sea-level rise impacts beyond 2100 and the global and regional urgency for adaptation

Adaptation to sea-level rise (SLR) will be necessary to protect people at risk from flooding due to a combination of tide, surge and SLR in the future. This adaptation commitment requires adequate time and resources to prepare for the impacts of SLR that are expected within this century and beyond (here until 2150). In this study, we address the question of “when” in addition to “how much” adaptation to SLR is needed. We use a scenario-neutral approach to assess the amount of people at risk from flooding under different SLR magnitudes. We combine this scenario neutral approach with SLR projections for the SSPs in AR6 and population growth scenarios in the coastal zone to identify the timing, in which the number of additional people affected by SLR alone or in combination with a 100-year storm event will exceed a set of thresholds. The comparison of the timing for different SSPs demonstrates that it is rather a question of “when” than “if” these thresholds will be exceeded. Some countries will need to adapt to SLR within the next few decades to prevent an additional 1–5 million people from becoming affected by flooding. Other countries have more time for adaption but will face a rapid increase in the number of people at risk from flooding beyond 2100. Combining SLR impacts with projected population change further increases the number of people at risk in the middle of this century for most SSPs. Considering low-confidence high-end SLR scenarios that include the possibility for a more rapid melting of the ice sheets may shift expected impacts approximately 50 years forward. This means that adaptation needs to be implemented faster and sooner than previously anticipated, which may have consequences for the available adaptation options. Ignoring the potential and long-term (including beyond 2100) commitment for adaptation may lead to an adaptation gap and subsequently expensive retrofitting of infrastructure, creation of stranded assets, and less time to adapt at greater cost. In contrast, acknowledging and acting upon the long-term adaptation commitment can encourage timely adaptation and its alignment with other societal ambitions.