The economically optimal mix and timing of coastal adaptation in Europe to 2150.

Coastal adaptation to sea level rise involves combining and sequencing adaptation options into adaptation pathways that include adaptation tipping points, which are moments requiring a switch between options, often due to economic limits. While local case studies show costs depend on the economically optimal timing of actions and adaptation tipping points, this remains unexplored on global and continental scales. We present economically optimal adaptation pathways for Europe's coastal floodplains through 2150. For 95% of the coastline requiring adaptation, the optimal timing for initial actions is immediate due to current economic under-adaptation, a condition in which the costs of additional adaptation are outweighed by the reduction in expected flood damages. By 2150, retreat is economically optimal for 22% of the coastline, incurring lower total costs than protection, which is optimal for 9%. Adaptation tipping points are primarily driven by sea level rise and necessitate a switch from accommodation to either protection or retreat, or from retreat to inland protection combined with retreat in the foreland. Their timing depends on climate change, occurring 32 years earlier under higher climate change scenarios compared to lower ones.