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

Sea 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 (>50km) 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 2m 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 19-41m by 2100. However, model uncertainty ranges from 5 to 15m, 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. Plain Language Summary Sea cliffs are common geological featuresfound on nearly 75% of the world's open ocean coastlinesyet relatively few studies have considered how sea cliffs will respond to sea level rise during the 21st century. In this paper, we present five different numerical models that work cooperatively to project future cliff response to sea level rise, with application to the highly populated coast of Southern California. Although models of sea cliff retreat are difficult to test, results suggest that sea cliffs will retreat on average between 19 and 41m by the year 2100, depending on factors like the amount of sea level rise, historical sea cliff retreat rates, surf zone slope, and beach elevation. The U.S. Geological Survey provides model results to coastal managers and the general public for use as a hazard assessment tool.