Economically robust protection against 21st century sea-level rise

While sea-level rise could flood and displace hundreds of millions of people, a range of studies have shown that adaptation in the form of coastal protection can be very effective and cost-efficient in reducing coastal impacts. All of these studies have, however, investigated adaptation decisions within one given sea-level rise and one given socio-economic scenario, which is at odds with the actual decision problem coastal planers are facing in that they need to decide across scenarios. This study takes first steps at filling this gap by assessing for which parts of the global coastline coastal protection is economically robust across 21st century global mean sea-level rise from 0.3 m to 2.0 m, the five Shared Socio-economic Pathways and discount rates from 0 to 6%. We find that for 92,500 km of the global coastline it is economically robust to invest in protection (i.e. protection is cheaper than not protecting under every scenario considered). While these 92,500 km cover only 13% of the global coastline, they account for 90% of global coastal floodplain population and for 96% of assets in the global coastal floodplain. For 451,000 km of the global coastline (65%) covering 0.2% of coastal floodplain population and 0.2% of coastal floodplain assets globally, it is economically robust not to invest in protection. Our results suggest bifurcating coastal futures. On the one hand, richer, densely populated and urban areas are likely to be protected through higher and higher dikes. On the other hand, poorer rural areas will struggle to maintain safe human settlements and are likely to eventual retreat from the coast, raising difficult policy questions concerning transfer payments, compensation and liability for loss and damage.