The Uncertain Climate Footprint of Wetlands Under Human Pressure

Wicken flux tower

New research published in PNAS has attempted to untangle the differences in methane, carbon dioxide, and nitrous oxide fluxes from natural and managed wetlands. As MethaneNet readers will know, wetlands play an important role in global methane dynamics but many questions remain unanswered. The new study, which primarily used FLUXNET sites, analysed data collected using both static chamber and eddy covariance methods in temperate, boreal, and arctic locations. As such, the dataset is likely to be the first large-scale synthesis of wetland flux data generated by eddy covariance.

As is frequently observed for wetlands, most of the studied sites were net sources of methane, and net sinks of carbon dioxide. For temperate sites, it was found that drainage and conversion to agriculture reduced methane fluxes but increased carbon dioxide fluxes. Rewetting or restoring such sites then creates a methane source that is not offset by the associated carbon dioxide sink. To fully quantify the impacts of management, sites with annual greenhouse gas budgets (including nitrous oxide from agricultural sites) were used to calculate radiative forcing (RF) on a 100 year time scale. These calculations also accounted for carbon offtakes, such as forestry or crop harvesting. The calculations suggested that conversion of arctic and boreal wetlands to agriculture resulted in a positive RF. Temperate wetlands converted to agriculture generally had a positive RF, though there was variation due to the type and intensity of management.

In their conclusions, the international group of authors stress the importance of long-term monitoring to fully understand ecosystem responses to both natural and anthropogenic changes. They further add that such responses may not be adequately captured by manual static chamber sampling, and that focus should be directed towards eddy covariance and quasi-continuous chamber measurements. They conclude: “Our results prove that management intensity strongly influences the net climate footprint of wetlands and in particular the conversion of natural ecosystems to agricultural land ultimately leads to strong positive RF.” These results therefore have important policy implications regarding land management and climate.

Reference: Petrescu, A.M.R., Lohila, A., Tuovinen, J-P., et al. 2015. The uncertain climate footprint of wetlands under human pressure. PNAS, doi: 10.1073/pnas.1416267112.

Photo: Mike Peacock

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