Spatial response of coastal marshes to increased atmospheric CO 2

• Published in: Proceedings of the National Academy of Sciences - PNAS 2015-12, Vol.112 (51), p.15580-15584
• Main Authors: Ratliff, Katherine M., Braswell, Anna E., Marani, Marco
• Format: Article
• Language: English
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1516286112

Coastal marshes provide numerous ecosystem services, are an important carbon sink, and are exposed to drowning as sea-level rise accelerates. Using a meta-analysis of the available observational data, we model the coupled marsh vegetation and morphological dynamics. We find that the fertilization effect of elevated atmospheric CO 2 significantly increases marsh resilience to drowning and decreases the spatial extent of marsh retreat under high rates of sea-level rise. While this direct CO 2 fertilization effect has so far been neglected in marsh modeling, we find it is central in determining marsh survival under the foreseeable range of climatic changes. The elevation and extent of coastal marshes are dictated by the interplay between the rate of relative sea-level rise (RRSLR), surface accretion by inorganic sediment deposition, and organic soil production by plants. These accretion processes respond to changes in local and global forcings, such as sediment delivery to the coast, nutrient concentrations, and atmospheric CO 2 , but their relative importance for marsh resilience to increasing RRSLR remains unclear. In particular, marshes up-take atmospheric CO 2 at high rates, thereby playing a major role in the global carbon cycle, but the morphologic expression of increasing atmospheric CO 2 concentration, an imminent aspect of climate change, has not yet been isolated and quantified. Using the available observational literature and a spatially explicit ecomorphodynamic model, we explore marsh responses to increased atmospheric CO 2 , relative to changes in inorganic sediment availability and elevated nitrogen levels. We find that marsh vegetation response to foreseen elevated atmospheric CO 2 is similar in magnitude to the response induced by a varying inorganic sediment concentration, and that it increases the threshold RRSLR initiating marsh submergence by up to 60% in the range of forcings explored. Furthermore, we find that marsh responses are inherently spatially dependent, and cannot be adequately captured through 0-dimensional representations of marsh dynamics. Our results imply that coastal marshes, and the major carbon sink they represent, are significantly more resilient to foreseen climatic changes than previously thought.