Sea-level scenarios for coastal adaptation: the example of France

Climate change scenarios are a typical request of adaptation practioners. Within its third national adaptation plan, France is developing a consistent set of climate scenarios based on global warming levels. The scenario currently under consideration would lead to a global mean temperature increase of 3°C with respect to the preindustrial period, which is consistent with the current climate policies to 2100. Later on, these scenarios would be integrated in the regulation, for example in order to update risk assessment guidance.

Here, we present how sea-level rise scenarios aligned with this global warming level were produced. We selected emulated simulations for each component of future sea-level rise consistently, including ocean and ice components, following a method similar to that of the 6^th assessment report of the IPCC, yet with specific attention to the consistency of uncertainty treatment before and after 2100. This responds to the needs to consider impacts of sea-level rise over hundred years, that is, to 2125 within coastal risk prevention plans. Furthermore, we added simulations considering a potential collapse of ice-sheets at 3°C of global warming levels in 2100. We consider only vertical land motions related to the Glacial Isostatic Adjustment as new observations from the Copernicus Land Motion service are now available for local stakeholder’s use.

The results show that the 87^th percentile of projections is close to 80cm in 2100 with respect to 1995-2014 for the majority of mainland and overseas French regions, whether ice-sheets collapse is considered or not. Conversely, median values display differences of about 10cm depending whether ice sheet collapse is hypothesized or not. In the context of the development of these new scenarios, simplicity was considered a key criterion of success to ensure that all users - and not only those with high climate literacy - can effectively use scenarios. Hence, we propose to use one single scenario corresponding to the 87^th percentile of the projections. This corresponds to a cautious approach consistent with the risk prevention policy in France. This does not preclude advanced users considering additional scenarios such as low-likelihood/high-impact scenarios voluntarily.

This work was performed within a project supported by the ministry in charge of Environment. We thank the steering and scientific committees of this project for useful comments and inputs.