Methane flux from northern wetlands and tundra - An ecosystem source modelling approach

The magnitude and geographical distribution of natural sources and sinks of atmospheric CH4 in the biosphere are still poorly known. Estimates of the net contribution from northern wetlands have been lowered during recent years. According to current consensus, about 35 Tg CH4/yr originates from northern wetlands and tundra. A process-oriented ecosystem source model for CH4 is used here to obtain an independent estimate for this flux. The model estimates steady-state seasonal cycles of NPP and heterotrophic respiration (HR). It accounts for peatland carbon storage and then obtains CH4 emission as a proportion of HR with the constant of proportionality (as a range) estimated from observations. The model was shown consistent with seasonal data (including winter) on NPP, soil respiration and CH4 emission at sites spanning a range of latitudes and climates. Applied on a 1 degrees grid basis using standard climatological and wetland distribution data sets, this approach yields a total non-forested wetland and tundra emission (>50 degrees N) of 8.7 +/- 5.8 Tg CH4/yr. After inclusion of forested wetlands, we estimate a total emission from northern wetlands and tundra of 20 +/- 13 Tg CH4/yr. This is somewhat lower than current atmospherically based estimates. The difference may be due to localized high emissions, which have been reported, e.g., for West Siberian wetlands but which are not well understood and not included in current models.