Deep uncertainties in shoreline change projections: an extra-probabilistic approach applied to sandy beaches

Rémi Thiéblemont,Moisés Álvarez-Cuesta,Alexandra Toimil,Gonéri Le Cozannet,Jérémy Rohmer,Iñigo J Losada

doi:10.5194/nhess-21-2257-2021

Abstract

Abstract. Global mean sea level rise and its acceleration are projected to aggravate coastal erosion over the 21st century, which constitutes a major challenge for coastal adaptation. Projections of shoreline retreat are highly uncertain, however, namely due to deeply uncertain mean sea level projections and the absence of consensus on a coastal impact model. An improved understanding and a better quantification of these sources of deep uncertainty are hence required to improve coastal risk management and inform adaptation decisions. In this work we present and apply a new extra-probabilistic framework to develop shoreline change projections of sandy coasts that allows consideration of intrinsic (or aleatory) and knowledge-based (or epistemic) uncertainties exhaustively and transparently. This framework builds upon an empirical shoreline change model to which we ascribe possibility functions to represent deeply uncertain variables. The model is applied to two local sites in Aquitaine (France) and Castellón (Spain). First, we validate the framework against historical shoreline observations and then develop shoreline change projections that account for possible (although unlikely) low-end and high-end mean sea level scenarios. Our high-end projections show for instance that shoreline retreats of up to 200 m in Aquitaine and 130 m in Castellón are plausible by 2100, while low-end projections revealed that 58 and 37 m modest shoreline retreats, respectively, are also plausible. Such extended intervals of possible future shoreline changes reflect an ambiguity in the probabilistic description of shoreline change projections, which could be substantially reduced by better constraining sea level rise (SLR) projections and improving coastal impact models. We found for instance that if mean sea level by 2100 does not exceed 1 m, the ambiguity can be reduced by more than 50 %. This could be achieved through an ambitious climate mitigation policy and improved knowledge on ice sheets.

Highlights

Global mean sea level rose over the period 2006–2015 at a rate more than 2 times larger than over the whole 20th century and is projected to continue rising for the centuries to come (Oppenheimer et al, 2019)
Global mean sea level rise and its acceleration are projected to aggravate coastal erosion over the 21st century, which constitutes a major challenge for coastal adaptation
We validate the framework against historical shoreline observations and develop shoreline change projections that account for possible low-end and high-end mean sea level scenarios

Summary

Introduction

Global mean sea level rose over the period 2006–2015 at a rate more than 2 times larger than over the whole 20th century and is projected to continue rising for the centuries to come (Oppenheimer et al, 2019). Recent analysis of satellite-derived shoreline changes has revealed that a quarter of the world’s sandy beaches are eroding (Luijendijk et al, 2018) and that the overall surface of eroded land recorded over the period 1984– 2015 (about 28 000 km2) is 2 times larger than the surface of gained land (Mentaschi et al, 2018) This situation is projected to worsen with climate change (Ranasinghe, 2016; Vousdoukas et al, 2020). An improved understanding and a better quantification of these sources of uncertainty are required to improve coastal risk

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Conclusion