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Faced with sea level rise and the intensification of extreme events, human populations living on the coasts are developing responses to address local situations. A synthesis of the literature on responses to coastal adaptation allows us to highlight different adaptation strategies. Here, we analyze these strategies according to the complexity of their implementation, both institutionally and technically. First, we distinguish two opposing paradigms – fighting against rising sea levels or adapting to new climatic conditions; and second, we observe the level of integrated management of the strategies. This typology allows a distinction between four archetypes with the most commonly associated governance modalities for each. We then underline the need for hybrid approaches and adaptation trajectories over time to take into account local socio-cultural, geographical, and climatic conditions as well as to integrate stakeholders in the design and implementation of responses. We show that dynamic and participatory policies can foster collective learning processes and enable the evolution of social values and behaviors. Finally, adaptation policies rely on knowledge and participatory engagement, multi-scalar governance, policy monitoring, and territorial solidarity. These conditions are especially relevant for densely populated areas that will be confronted with sea level rise, thus for coastal cities in particular.

AbstractCoastal zones are fragile and complex dynamical systems that are increasingly under threat from the combined effects of anthropogenic pressure and climate change. Using global satellite derived shoreline positions from 1993 to 2019 and a variety of reanalysis products, here we show that shorelines are under the influence of three main drivers: sea-level, ocean waves and river discharge. While sea level directly affects coastal mobility, waves affect both erosion/accretion and total water levels, and rivers affect coastal sediment budgets and salinity-induced water levels. By deriving a conceptual global model that accounts for the influence of dominant modes of climate variability on these drivers, we show that interannual shoreline changes are largely driven by different ENSO regimes and their complex inter-basin teleconnections. Our results provide a new framework for understanding and predicting climate-induced coastal hazards.

The proliferation of holopelagic Sargassum spp. (Sargassum) in the tropical North Atlantic Ocean is of concern for populations and coastal ecosystems in the Caribbean Sea, Gulf of Mexico, and West Africa. Satellite detections have enabled rough assessments of the quantity of algae that drifts seasonally in the open ocean, with seasonal peaks of Sargassum biomass reaching 10 to 20 million tons since 2018. Although the impacts on the coast have been widely publicized, there are no estimates of the quantities of Sargassum that accumulate on the coasts. This study proposes novel vulnerability indicators that combine information on Sargassum stranded biomass, ecosystem and socioeconomic factors to assess risks posed by Sargassum on coastal regions. Quantities of Sargassum that accumulate in the coastal strip at the regional scale were derived by combining the satellite detections in the open ocean with an algal growth-transport-stranding model. It shows that the amount of Sargassum accumulating on the Atlantic coast is of the order of 10% of the biomass estimated offshore and has accumulated between 2 and 10 million tons per year over the last five years. Vulnerability indices identify the Mexican Caribbean, northern Lesser Antilles, and eastern Great Antilles as the most vulnerable regions, facing significant ecosystemic and socioeconomic pressures from Sargassum influxes.

Abstract Coastal zones are fragile and complex dynamical systems that are increasingly under threat from the combined effects of anthropogenic pressure and climate change. Yet, the key environmental factors that drive regional coastline changes remain poorly quantified. Here, using global satellite derived shoreline positions from 2000 to 2017 and a variety of reanalysis products, we demonstrate that coastlines are under the influence of 3 main drivers: the sea-level, and also ocean waves and fluvial inputs. The relative contribution of each of the drivers vary across the global coastline, with about a third exhibiting a clear dominance of one of the drivers (60% for sea level, 30% for rivers and 10% for waves). Furthermore, by establishing that these environmental forcing are all substantially constrained by El Niño Southern Oscillation (ENSO) at interannual time scales, we derive a conceptual global model of yearly shoreline changes that integrates the complex and diverse planetary climate influence of ENSO on each of these 3 drivers. This model reproduces well the observed shoreline changes with a global correlation of 0.4 and up to 0.6 in the tropical belt. We believe it represents a new solid physical and mathematical framework for understanding ENSO-driven littoral hazards as well as an efficient yet simple approach for their prediction.

Surface gravity waves generated by winds are ubiquitous on our oceans and play a primordial role in the dynamics of the ocean–land–atmosphere interfaces. In particular, wind-generated waves cause fluctuations of the sea level at the coast over timescales from a few seconds (individual wave runup) to a few hours (wave-induced setup). These wave-induced processes are of major importance for coastal management as they add up to tides and atmospheric surges during storm events and enhance coastal flooding and erosion. Changes in the atmospheric circulation associated with natural climate cycles or caused by increasing greenhouse gas emissions affect the wave conditions worldwide, which may drive significant changes in the wave-induced coastal hydrodynamics. Since sea-level rise represents a major challenge for sustainable coastal management, particularly in low-lying coastal areas and/or along densely urbanized coastlines, understanding the contribution of wind-generated waves to the long-term budget of coastal sea-level changes is therefore of major importance. In this review, we describe the physical processes by which sea states may affect coastal sea level at several timescales, we present the methods currently used to estimate the wave contribution to coastal sea-level changes, we describe past and future wave climate variability, we discuss the contribution of wave to coastal sea-level changes, and we discuss the limitations and perspectives of this research field.