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Climate variability impacts CH 4 wetland sources as changes in flux density per unit area and via expansion or contraction of wetland areas in response to surface hydrological processes. This paper is a first attempt to isolate the role of varying wetland area on the seasonal and interannual variability of CH 4 wetland emissions over the past decade. Wetland area extent at monthly intervals was provided over the period 1993–2000 by a suite of satellite observations from multiple sensors. The regionally variable fraction of wetland area was optimized using satellite observations of flooded area as a first estimate and further adjusted to match the seasonal cycle of CH 4 fluxes retrieved from a global atmospheric inversion. Wetland flux densities of CH 4 were calculated by coupling the ORCHIDEE global vegetation model with a process‐based wetland CH 4 emission model, calibrated by optimizing its parameters at the site level against representative CH 4 flux time series. For boreal bogs north of 50°N, we found that variations in area contributed about 30% to the annual flux. For temperate and tropical wetlands, the variations in area has almost no influence on the annual CH 4 emissions but contributes significantly to the seasonal behavior, accounting for 40% and 66% of the seasonal amplitude of fluxes, respectively. In contrast, the interannual variability of wetland area appears to be the dominant cause of interannual variations in regional CH 4 emissions from wetlands at all latitudes (largest in the tropics), with up to 90% of annual flux anomalies explained by wetland area anomalies in some years. For example, in 1998, boreal wetlands north of 50°N contributed to approximately 80% of the positive anomaly according to our calculations. We also found that climate anomalies can lead to both increased emitting areas and decreased flux densities at the same time, with opposite effects on the total CH 4 flux entering the atmosphere. With a view to forecasting the future trajectory of atmospheric methane content, our results point to the absolute necessity to be able to predict the variations in wetland extent, a hydrological problem, in order to affirm the reliability of simulations of changing methane emissions perturbed by climate.
Ringeval et al. (Fri,) studied this question.