Long-term sea-level rise necessitates a commitment to adaptation: A first order assessment

Without adaptation, sea-level rise (SLR) will put more people at risk of flooding. This requires a timely and adequate commitment to adaptation. In this paper, we show how adaptation needs to unfold over time to manage climate-induced SLR. We use a novel scenario-neutral approach, applied globally and subsequently combined with SLR and population scenarios, to assess when, where, and how fast to adapt up to 2150. As rates of SLR accelerate, adaptation needs to occur at an increasing pace or at a larger scale. While it is certain that adaptation will be necessary, it is uncertain when and how fast. After only similar to 0.15 m SLR relative to 2020, 1 million people need to adapt to permanent submergence and the amount of people at risk of a 100-year flood increases with 21% to 83 million people. This would occur in the next 30 (20-45) years for RCP4.5 and within 25 (18-36) years under RCP8.5, assuming no change in protection or population. The uncertainty in timing increases with higher SLR, albeit for some impacts it can still a matter of time. Population at risk of a 100-year flood doubles after 0.75 m SLR which could occur by similar to 2080 (2068-2088), 2100 (2085-2130), or 2150 (2115-beyond 2150) under a high-end, RCP8.5, or RCP4.5 scenario respectively. The rate, at which the risk increases, differs strongly per country. In some countries an additional 1-5 million people of the present population will be at risk of a 100-year flood within the next two decades, while others have more time to adapt but will see rapid growth of risk past 2100. Combining SLR impacts with projected population change further increases the number of people at risk of a 100-year flood by similar to 13% between 2040-2060 (under both RCP8.5-SSP5 and RCP4.5-SSP2). This can be managed through protecting, floodproofing or limiting developments in high-risk areas. A commitment to adaptation is inevitable to maintain risk at present levels. With increasing warnings of the potential for accelerated SLR due to rapid ice sheet melt, adaptation may need to happen faster and sooner than previously anticipated which can have consequences for how to adapt. Failure to acknowledge the potential and long-term (including beyond 2100) adaptation commitment in development and adaptation planning may lead to a commitment gap and subsequently expensive retrofitting of infrastructure, creation of stranded assets, and less time to adapt at greater cost. In contrast, considering the long-term adaptation commitment can support timely adaptation and alignment with other societal goals.