Sea-level rise risks to coastal cities: what are the limits to adaptation?

Understanding the consequence of sea-level rise for coastal cities has long lead times and huge political implications. Civilisation has emerged and developed during a period of several thousand years during which in geological terms sea level has been unusually stable. We have now moved out of this period raising important challenges for the future. In 2005 there were 136 coastal cities with a population exceeding one million people and a collective population of 400 million people. All these cities are threatened by flooding from the sea to varying degrees and these risks are increasing due to growing exposure (people and assets), rising sea levels due to climate change, and in some cities, significant coastal subsidence due to human agency (drainage and groundwater withdrawals from susceptible soils). City abandonment due to sea-level rise is widely discussed in the media, but most of the discussion is speculative. The limits to adaptation and abandonment of cities are not predictable in a formal sense - while the rise in mean sea level raises the likelihood of a catastrophic flood, extreme events are what cause damage and trigger a response, be it abandonment or a defence upgrade. Several types of potential adaptation limits can be recognised: (1) physical/engineering limits; (2) economic/financial limits; and (3) socio-political limits. The latter two types of limits are much less understood, and yet issues such as loss of confidence rather than a simple engineering failure may be instrumental in the future of a coastal city. There are few studies which quantify the sea-level rise threshold at which cities will be challenged, especially for large rises exceeding a metre or more. Exceptions include London and the Thames Estuary and the Amsterdam and Rotterdam (the Netherlands) where adaptation to a rise of sea level of up to 4 m or more appears feasible. This lack of knowledge on sea-level rise thresholds for coastal cities is of concern, and similar assessments could be considered more widely. By taking a proactive approach to adaptation assessment the full scale of the long term challenge can be illuminated and appropriate long term adaptive actions developed which work with the long-term development of the city.