24th November 2011.
The striking correlation between atmospheric concentrations of CH4 and CO2, as revealed in ice-core records, has long been a source of speculation as it is not immediately obvious why the two should be so closely linked. A paper by Boardman and co-authors1, published in December’s New Phytologist, has provided new information about the linkage, demonstrating experimentally for the first time that very low concentrations of atmospheric CO2 associated with glacial maxima can lead to lower CH4 emissions from certain wetlands.
Lead author Carl Boardman describes the two-year long experiment investigating the impact of CO2 starvation on wetland CH4 emissions as “an ambitious technical challenge”. The work involved exposing minerotrophic fen and ombrotrophic bog mesocosms to glacial maxima CO2 concentrations (200 ppm) and comparing the associated CH4 emissions to those from mesocosms exposed to ambient CO2 concentrations (400 ppm). In the fen mesocosms, CH4 emissions were suppressed by 29% in the low CO2 atmosphere, with the effect particularly pronounced during warmer summer temperatures. By contrast, the low CO2 had no effect on CH4 emissions from the ombrotrophic bog.
Wetlands are the largest natural source of atmospheric CH4. It is not difficult to think up reasons why CH4 emissions might decrease under low CO2, as was observed in this experiment – for example decreased plant productivity hence less root exudate and so a reduced supply of labile carbon for methanogens, or knock-on effects associated with changes in plant composition. It is far harder to pin down the precise process involved. The reasons why the bog and the fen responded differently to the simulated glacial maxima conditions are also far from clear. What is clear, as noted by Keller2 in a commentary on the Boardman et al paper, is that this study shows how anthropogenic activities leading to changes in amounts of atmospheric CO2, have “already impacted ecosystem processes and will continue to do so in the future”.
Boardman further notes that the results “reaffirm the biological link between atmospheric CO2 and CH4 ice core records” and also “highlight the requirement for better representation of the interaction between low levels of atmospheric CO2 and wetland CH4 emission processes in numerical models aiming to understand the glacial-interglacial variation in CH4 concentration in the atmosphere.”
Boardman CP, Gauci V, Watson JS, Blake S and Beerling DJ. 2011. Contrasting wetland CH4 emission responses to simulated glacial atmospheric CO2 in temperate bogs and fens. New Phytologist 192(4): 898-911.
Keller JK. 2011. Wetland and the global carbon cycles: what might the simulated past tell us about the future? New Phytologist 192(4): 789-792.