Eight scenarios for anticipating sea-level rise through 2100

An important physical response to anthropogenic climate change is the long term rise in global mean sea-level (GMSL). Given this context the French National Alliance for Environmental Research, which brings together the main French research institutions concerned with environment, tasked its foresight group with shedding light on sea-level rise by 2100, estimating the possible impacts and consequences for the environment and societies, and recommending future research priorities.We employ a foresight approach based on the scenario method with 23 driving variables for the coastal system and four physical contexts for the sea-level rise (moderate, serious, severe and extreme) leading to the development of eight contrasted and unconnected scenarios starting in 2020 and extending to 2100. Each scenario is defined by a storyline. Storylines are determined mainly by coastal adaptation efforts and the political will to mitigate climate change. These are structured in three families: (1) Priority given to adaptation (four scenarios: Climate control, Frugality and anticipation, Resilient cities, Early adaptation and late mitigation), (2) Denial of the phenomenon (three scenarios: Passivity, Gradual coastal abandonment, From denial to reaction) and (3) Fragmented world (one scenario: Persistent fragmentation).Scenario trajectories without vigorous and widespread mitigation policies before 2040 show that coastal States will have to deal with ‘severe’ or ‘extreme’ sea level rise by the end of the century. ’Severe’ or ‘Extreme’ sea level rise outcomes are envisaged in five of the eight scenarios in the range of possible futures. Two scenarios lead to ‘serious’ sea level rise and one ideal scenario leads to a ‘moderate’ state. Adaptation efforts will be less cumbersome and costly when mitigation efforts have been early, widespread and sustained. The optimal scenario considers raising awareness of all actors in societies, leading to the introduction of fast and radical mitigation and adaptation efforts. All other scenarios mean that future generations may have to face major or even irreversible upheavals, at increasing costs