Large-scale loss of Mediterranean coastal marshes under rising sea levels

Over the past centuries, coastal marsh areas have been declining, mostly as a consequence of human impacts, including direct wetland conversion and land reclamation. More recently, accelerated global sea-level rise poses an additional challenge for the longevity of existing coastal marshes. This risk is further compounded by densely populated coastal zones, where coastal infrastructure inhibits the capacity of coastal marshes to migrate inland in response to rising sea levels (coastal squeeze). In the Mediterranean, coastal wetlands, incl. saltmarshes, are important contributors to the region’s high biodiversity, and provide a set of invaluable ecosystem services. Here, we present a study on the modelling of the future development of Mediterranean coastal marshes, taking into account both their ability to vertically adjust to increasing sea levels through sediment accumulation and their capacity to migrate inland in response to rising sea levels where sufficient inland migration space is available. In contrast to previous global studies, our preliminary results indicate an overall loss of coastal marshes by 2100 for all climate and management scenarios, even under low sea-level rise scenarios and when abundant inland migration space is available. Total losses are projected between 17% and 94% for RCP 2.6 with maximum available space for inland space; and RCP 8.5 with minimum space, respectively. A total loss of coastal marshes is projected for some Mediterranean countries by 2100. Nevertheless, the implementation of coastal management strategies facilitating the inland migration of coastal marshes as well restoration of catchment-to-coast sediment connectivity and enhancement of sediment trapping capacities can, to some degree, mitigate future coastal marsh losses.