Coastal marshes are valuable ecosystems that provide important nutrients to coastal waters that help sustain local food webs. They are also increasingly recognized as valuable carbon sinks, sequestering significant quantities of carbon both above and below ground. In recent years, however, concerns have been expressed that these ecosystems are in danger of collapsing in response to rising sea levels that are projected to occur as a consequence of CO2-induced global warming. If such fears are correct, melting ice will increase the rate of sea level rise beyond which these ecosystems can keep up, essentially dooming them to a submerged death, which would have substantial repercussions on surrounding communities.
But how likely is it that this gloomy scenario will occur?
Investigating this very topic, the three-member research team of Ratliff et al. (2015) used a one-dimensional ecomorphodynamic model to “assess the direct impacts of elevated CO2 on marsh morphology, relating to ongoing and emerging environmental change.” According to the authors, previous works have revealed large increases in marsh plant biomass productivity in response to elevated concentrations of atmospheric CO2, yet “direct CO2 effects on vegetation and marsh accretion (as opposed to its indirect effects, e.g., via the increase in temperature) have not yet been incorporated into marsh models. As a result, they note the relative importance of CO2 effects on marsh dynamics “remains unknown” … until now, that is.
Using a meta-analysis of CO2 enrichment data, Ratliff et al. were able to model the impact of CO2 fertilization on coastal marsh vegetation and morphological dynamics for varying rates of relative sea level rise that are projected to occur under future global warming. Accordingly, the authors report “we found that the fertilization effect of elevated atmospheric CO2 significantly increases marsh resilience to drowning and decreases the spatial extent of marsh retreat under high rates of sea level rise.” In addition, they found that the more expansive marshes under elevated CO2 resulted in greater carbon sequestration such that “the fertilization effect may also contribute to a stabilizing feedback within the climate system, where increasing biomass production and organic deposition consequently sequester greater amounts of CO2.”
In light of the above findings, Ratliff et al. conclude that their results “imply that coastal marshes, and the major carbon sink they represent, are significantly more resilient to foreseen climate changes than previously thought.” And that good news should suppress fears of the untimely demise of coastal marsh vegetation due to possible climate change-related increases in sea level. Sadly, such fears could have been avoided altogether, if the models would have included this important direct benefit of atmospheric CO2 enrichment from the get-go.
Ratliff, K.M., Braswell, A.E. and Marani, M. 2015. Spatial response of coastal marshes to increased atmospheric CO2. Proceedings of the National Academy of Sciences, USA 112: 15,580-15,584.