Contribution of Wave Setup and Swash to Projected Coastal Sea Level Changes

Along open coasts, wind-waves are a key driver of coastal changes and can be major contributors to coastal hazards. Wind-wave characteristics are projected to change in response to climate change, notably due to changes in atmospheric circulation patterns and the associated surface winds. Here, a first-order estimate of projected 20-yr mean wave setup and swash changes is provided along most of the world’s coastline over the middle and end of the 21^st century. Calculations are based on an ensemble of wave model projections under the representative concentration pathways (RCP) 8.5, on empirical formulations for wave setup and swash, and on different assumptions for the beach slope. Projected wave setup and swash changes are compared to other contributors currently accounted for in regional sea-level projections to extend existing projections of 21^st century coastal sea level changes.

Projected wave contribution changes exhibit a clear spatial heterogeneity with regional-scale consistency, and mostly average out at global scale. However, at regional or local scale, wave setup changes are a small yet non-negligible contributor to total coastal sea level 20-yr mean changes (which include global mean sea level rise-GMSLR) over the middle and end of the 21^st century. Wave setup can be a substantial contributor to local departures of coastal sea level changes from GMSLR. The contribution of waves to total water levels at the coast is much larger when swash is included. Wave contribution changes should therefore be included in projections of regional patterns of coastal sea level changes. The reported long-term changes in wave setup and swash also advocate for the inclusion of non-stationary wave contributions to projected regional patterns of coastal sea level changes, including for extreme events.