On the challenges of modeling the net radiative forcing of wetlands: reconsidering Mitsch et al. 2013

Abstract

Wetlands play a role in regulating global climate by removing carbon dioxide (CO₂) from the atmosphere and sequestering it as soil carbon, and by emitting methane (CH₄) and nitrous oxide (N₂O) to the atmosphere. In a recent article in this journal (Mitsch et al. Landscape Ecol 28:583–597, 2013), CO₂ sequestration and CH₄ emissions were modeled for several freshwater wetlands that vary in vegetation type, climate, and hydrology. The authors of that study made significant errors that caused them to underestimate the importance of wetland CH₄ emissions on climate dynamics. Here, I reanalyze the Mitsch et al. dataset and show that all of their wetlands had an initial warming effect but eventually caused negative net radiative forcing within ~60–14,000 years, depending on the ratio of CO₂ sequestration to CH₄ emissions. The addition of a N₂O component to the model suggested that typical wetland N₂O emission rates would contribute only a minor burden to wetland radiative forcing, although specific application of this three-gas model is limited by the paucity of sites where CO₂ sequestration, CH₄ emission, and N₂O exchange rates have all been measured. Across the landscape, many natural wetlands may already cause negative net radiative forcing when integrated over their lifetime. However, caution should be applied when using carbon sequestration as a rationale for designing wetland construction and restoration projects since freshwater wetlands may have a net positive (warming) effect on climate for decades to centuries or longer.