Systematic biases in large‐scale estimates of wetland methane emissions arising from water table formulations

Most earth system models that simulate wetland methane emissions ignore spatial variability of water table depth. We compared methane emissions estimated using two commonly‐used water table approximations ( wet ‐ dry and uniform ) to a scheme that accounts for spatial variability, for several combinations of methane model parameter values and a range of water table depth spatial variability ( σ zwt ). The approximations' biases were consistent across methane model parameter sets. The wet‐dry scheme's biases were smallest in absolute value (+/−5–10%) when σ zwt was large (38 cm) but increased to between −85% and −95% as σ zwt approached 0. The uniform scheme's biases were largest (+/−100% or more) when σ zwt was large and vanished as σ zwt approached 0. For σ zwt values typical of boreal wetlands, these schemes' previous predictions that global wetland methane emissions will double by 2100 are in error by factors ranging from 0.7 to 1.3. Similar biases may affect paleoclimate studies.