Evidence for a link between climate and northern wetland methane emissions

Wetlands are an important source of atmospheric methane (CH4), but the strength of this source and its sensitivity to potential changes in climate are still uncertain. In this study, continuous measurements from 1990 to 1998 of atmospheric CH4 from the Canadian observational sites at Fraserdale (49°53′N 81°34′W) and Alert (82°27′N 62°31′W) are used to estimate CH4 emissions from the Hudson Bay Lowland (HBL), a 320,000 km2 semicontinuous wetland region in central Canada. The HBL comprises ∼10% of the total area of northern wetlands. A conceptually simple approach was used to calculate the methane emission flux using the CH4 concentration difference between Alert and Fraserdale, the residence time of the air mass over the HBL, and the mixing height of the convective boundary layer. Emission rates estimated using this approach for 1990 compare well with empirical aircraft and tower flux measurements made within the HBL during the same time period, thus indicating that the methodology used is reasonable. Annual CH4 emission rates range from 0.23 to 0.50 Tg CH4 yr−1 and are much lower than many empirical flux measurements observed at other northern wetland sites. A seasonal temperature sensitivity with a Q10 of about 4 was found. Moreover, the observed interannual variations in emissions are well correlated to variations in annual air temperatures corresponding to a sensitivity of Q10 ≈ 7. That is, a 10°C change in annual temperature would result in a sevenfold change in wetland emissions which is much larger than Q10 values used in current global CH4 models (typically Q10 ≈ 1.5). Our findings suggest that northern wetland emissions are probably overestimated to date but may increase significantly due to predicted global warming.