Global Mean Sea Level is rising and it will continue to do so for centuries. Sustainable development aspirations are at risk because many people, assets and vital resources are concentrated along low-lying coasts around the world. Many coastal communities have started to consider the implications of sea level rise. Measures are being taken to address coastal hazards exacerbated by rising sea level, such as coastal flooding due to extreme events (e.g. storm surges, tropical cyclones), coastal erosion and salinization. However, many coastal communities are still not sufficiently adapted to today’s extreme sea levels.
Scientific evidence about sea level rise is clear: Global mean sea level rose by 1.5 mm per year during the period 1901–1990, accelerating to 3.6 mm per year during the period 2005–2015. It is likely to rise 0.61-1.10 m by 2100 if global greenhouse gas emissions are not mitigated (RCP8.5). However, a rise of 2 or more meters cannot be ruled out. It could rise to more than 3 m by 2300, depending on the level of greenhouse gas emissions and the response of the Antarctic ice sheet, which are both highly uncertain. Even if efforts to mitigate emissions are very effective, extreme sea level events that were rare over the last century will become common before 2100, and even by 2050 in many locations. Without ambitious adaptation, the combined impact of hazards like coastal storms and very high tides will drastically increase the frequency and severity of flooding on low-lying coasts.
Sea level rise, as well as the context for adaptation, will vary regionally and locally, thus action to reduce risks related to sea level rise takes different forms depending on the local circumstances. ‘Hard protection’, like dikes and seawalls, can effectively reduce risk under two or more meters of sea-level rise but it is inevitable that limits will be reached. Such protection produces benefits that exceed its costs in low-lying coastal areas that are densely populated, as is the case for many coastal cities and some small islands, but in general, poorer regions will not be able to afford hard protection. Maintaining healthy coastal ecosystems, like mangroves, seagrass beds or coral reefs, can provide ‘soft protection’ and other benefits. Sea-level rise can also be ‘accommodated’ by raising buildings on the shoreline, for example. Land can be reclaimed from the sea by building outwards and upwards. In coastal locations where the risk is very high and cannot be effectively reduced, ‘retreat’ from the shoreline is the only way to eliminate such risk. Avoiding new development commitments in areas exposed to coastal hazards and SLR also avoids additional risk.
For those unable to afford protection, accommodation or advance measures, or when such measures are no longer viable or effective, retreat becomes inevitable. Millions of people living on low-lying islands face this prospect, including inhabitants of Small Island Developing States, of some densely populated but less intensively developed deltas, of rural coastal villages and towns, and of Arctic communities who already face melting sea ice and unprecedented changes in weather. The resultant impacts on distinctive cultures and ways of life could be devastating. Difficult trade-offs are therefore inevitable when making social choices about rising sea level. Institutionalising processes that lead to fair and just outcomes is challenging, but vitally important.
Choices being made now about how to respond to sea-level rise profoundly influence the trajectory of future exposure and vulnerability to sea-level rise. If concerted emissions mitigation is delayed, risks will progressively increase as sea-level rise accelerates. Prospects for global climate-resilience and sustainable development therefore depend in large part on coastal nations, cities and communities taking urgent and sustained locally-appropriate action to mitigate greenhouse gas emissions and adapt to sea level rise.