Geomorphic and ecological constraints on the coastal carbon sink

Climate change is driving a fundamental reorganization of the coastal zone, in which carbon-rich ecosystems are simultaneously migrating, accreting and submerging. In this Review, we discuss the geomorphic processes constraining soil carbon accumulation and the ecological processes constraining greenhouse gas emissions that together determine the net radiative forcing of the coastal landscape and the size of the coastal carbon sink. Sea level rise creates vertical and lateral accommodation space that potentially enhances soil carbon accumulation in marshes and mangroves. However, sea level rise and saltwater intrusion lead to ecosystem transitions that result in loss of existing carbon pools, altering the balance between carbon sequestration and greenhouse gas emissions. We propose that ecosystem reorganization results in large, but mostly offsetting, changes in radiative forcing. At the continental scale, losses in soil carbon sequestration and wood production are offset by reduced methane emissions associated with salinization of freshwater ecosystems. Nevertheless, climate effects on the coastal carbon sink remain largely studied within individual ecosystems. Future work is needed to determine the connectivity of carbon between ecosystems, changes in carbon accumulation and greenhouse gas emissions as ecosystem transitions occur, and the fate of carbon as it moves through the coastal landscape.