The Response of Coastal Marshes to Global Warming

According to Kirwan et al. (2009), "when subjected to future [projected] rates of sea level rise, numerical models and statistical projections predict that marshland worldwide will decline on the order of 10-50% during the next 50-100 years," and they say that "such a decline could represent a catastrophic loss of ecosystem services by one of Earth's most valuable coastal environments." In an effort to explore this subject empirically, Kirwan et al. "compiled 56 measurements of aboveground annual productivity for Spartina alterniflora, the dominant macrophyte in North American coastal wetlands," along the Gulf Coast and Eastern Seaboard of the United States, as well as the east coast of Canada. So what did they find?

The three researchers report that "despite local and temporal variability, a significant (r = 0.83; P < 0.000001) latitudinal gradient of 25 g m^-2/year per degree of latitude exists across the entire geographic range of the compilation." In addition, they state that "the latitudinal gradient in productivity appears to be driven by temperature," noting that "annual productivity most significantly correlates with mean annual temperature (r = 0.58; P = 0.000005) and the annual number of growing degree days (r = 0.58; P = 0.000005)." Consequently, based on this information, they estimated the response of S. alterniflora productivity to future increases in global temperature, noting that their results suggest that "an increase in global temperature of 2-4°C by 2100 (IPCC, 2007) would cause productivity to increase by about 50-100 g m^-2/year."

For mid-Atlantic and northern marshes with current productivities ranging from 450 to 250 g m^-2/year, they say that the result they obtained "represents approximately a 10-40% increase in annual productivity," which they describe as being of "a magnitude similar to that of marsh lost due to sea level change (10-50%)," as calculated by numerical models for the same time period. Hence, they conclude that "increased growth under a warming climate may compensate for the amount of productivity lost by eroding marshland."

This conclusion is further strengthened by the fact that the increase in atmospheric CO₂ concentration expected to be experienced over the current century will likely boost marsh productivity even more. In addition, it is widely recognized, as Kirwan et al. describe it, that "increased vegetation growth will tend to promote higher marsh accretion rates, stabilize channel expansion, and decrease the ability for waves to erode the marsh platform." As a result, their final conclusion is that "the combined impacts of future global change (e.g. sea level, temperature, CO₂) could actually increase [italics added] the total productivity of marshland."