1School of Engineering and Applied Sciences & Department of Earth and Planetary Sciences, Harvard University, Cambridge MA 02138, USA
2NASA Langley Research Center, Hampton, Virginia, USA
3Environment Canada, Toronto, Ontario, Canada
4Environmental Engineering Institute, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
5Center for Climate Systems Modeling (C2SM), ETH Zürich, Universitätstrasse 16, Zürich, Switzerland
6Laboratoire de Modélisation de Chimie Atmosphérique, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
*current address: CSIRO – Marine and Atmospheric Research, GPO Box 3023, Canberra, ACT, 2601, Australia
Abstract. The Hudson Bay Lowlands (HBL) is the second largest boreal wetland ecosystem in the world and an important natural source of global atmospheric methane. We quantify the HBL methane emissions by using the GEOS-Chem chemical transport model to simulate aircraft measurements over the HBL from the ARCTAS and pre-HIPPO campaigns in May–July 2008, together with continuous 2004–2008 surface observations at Fraserdale (southern edge of HBL) and Alert (Arctic background). The difference in methane concentrations between Fraserdale and Alert is shown to be a good indicator of HBL emissions, and implies a sharp seasonal onset of emissions in late May (consistent with the aircraft data), a peak in July–August, and a seasonal shut-off in September. The model, in which seasonal variation of emission is mainly driven by surface temperature, reproduces well the observations in summer but its seasonal shoulders are too broad. We suggest that this reflects the suppression of emissions by snow cover and greatly improve the model simulation by accounting for this effect. Our resulting best estimate for HBL methane emissions is 2.3 Tg a−1, several-fold higher than previous estimates (Roulet et al., 1994; Worthy et al., 2000).