The future of the Portuguese (SW Europe) most vulnerable coastal areas under climate change – Part I: Performance evaluation and shoreline evolution from a downscaled bias corrected wave climate ensemble

Some of the most disruptive effects of climate change are projected to be felt along the coastlines. The combined effects of future changes in water levels and wave climate along coastal areas constitute one of the most serious threats to their sustainable evolution, compromising critical infrastructures, resources, ecosystems, and communities. Understanding long-term changes in coastal areas remains challenging, however, due to their multivariate and multi-time-and-space-scale nature. In this study, we propose an innovative methodology for a complete vulnerability assessment of sandy low-lying coastal areas, based on dynamic, ensemble-based projections from the Coupled Model Intercomparison Project phase 5 (CMIP5). In the current Part I study, the effects of sea level rise (SLR) and nearshore wave climate changes on future shoreline evolution are assessed at five key-locations along the Portuguese coastline. Longshore sediment transport (LST) projections are computed, and sedimentary imbalances are quantified. Overall, robust shoreline retreat of up to 300 m is projected, especially along the Portuguese northern and central coastal areas, with continued erosion driven mainly by sediment imbalance and SLR. The projected decrease in future nearshore wave energy is responsible for a slight alleviation in erosion trends, up to 6.33%, whereas the increase of northerly incoming waves is expected to lead to northward beach rotations along western Mainland Portugal. The resulting shoreline evolution, responsible for the loss of up to 0.786 km2 of dry land by 2100 along the 14 km of analysed coastline, is projected to threaten several Portuguese urban areas, calling for the implementation of adequate coastal management and adaptation plans, to reduce the impacts of climate change on population, infrastructures, livelihood, and ecosystems.