Evidence for a link between climate and northern wetland methane emissions

Wetlands are an important source of atmospheric methane (CH4), but the stre ngth 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 CH, from the Canadian observational sites at Fraserdale (49 degrees 53'N 81 degrees 34'W) and Alert (82 degrees 27'N 62 degrees 31'W) a re used to estimate CH, emissions from the Hudson Bay Lowland (HBL), a 320, 000 km(2) semicontinuous wetland region in central Canada. The HBL comprise s similar to 10% of the total area of northern wetlands. A conceptually sim ple approach was used to calculate the methane emission flux using the CH4 concentration difference between Alert and Fraserdale, the residence time o f the air mass over the HBL, and the mixing height of the convective bounda ry layer. Emission rates estimated using this approach for 1990 compare wel l with empirical aircraft and tower flux measurements made within the HBL d uring the same time period, thus indicating that the methodology used is re asonable. Annual CH4 emission rates range from 0.23 to 0.50 Tg CH4 yr(-1) a nd are much lower than many empirical flux measurements observed at other n orthern wetland sites. A seasonal temperature sensitivity with a Q(10) of a bout 4 was found. Moreover, the observed interannual variations in emission s are well correlated to variations in annual air temperatures correspondin g to a sensitivity of Q(10) approximate to 7. That is, a 10 degrees C chang e in annual temperature would result in a sevenfold change in wetland emiss ions which is much larger than Q(10) values used in current global CH4 mode ls (typically Q(10) approximate to 1.5). Our findings suggest that northern wetland emissions are probably overestimated to date but may increase sign ificantly due to predicted global warming.