A Model Ensemble for Projecting Multidecadal Coastal Cliff Retreat During the 21st Century

Abstract

AbstractSea cliff retreat rates are expected to accelerate with rising sea levels during the 21st century. Here we develop an approach for a multimodel ensemble that efficiently projects time‐averaged sea cliff retreat over multidecadal time scales and large (>50 km) spatial scales. The ensemble consists of five simple 1‐D models adapted from the literature that relate sea cliff retreat to wave impacts, sea level rise (SLR), historical cliff behavior, and cross‐shore profile geometry. Ensemble predictions are based on Monte Carlo simulations of each individual model, which account for the uncertainty of model parameters. The consensus of the individual models also weights uncertainty, such that uncertainty is greater when predictions from different models do not agree. A calibrated, but unvalidated, ensemble was applied to the 475‐km‐long coastline of Southern California (USA), with four SLR scenarios of 0.5, 0.93, 1.5, and 2 m by 2100. Results suggest that future retreat rates could increase relative to mean historical rates by more than twofold for the higher SLR scenarios, causing an average total land loss of 19–41 m by 2100. However, model uncertainty ranges from ±5 to 15 m, reflecting the inherent difficulties of projecting cliff retreat over multiple decades. To enhance ensemble performance, future work could include weighting each model by its skill in matching observations in different morphological settings.