labs_title

Observational constraints on the distribution, seasonality, and environmental predictors of North American boreal methane emissions

S.M. Miller, D.E.J. Worthy, A.M. Michalak, S.C. Wofsy, E.A. Kort, T.C. Havice, A.E. Andrews, E.J. Dlugokencky, J.O. Kaplan, P.J. Levi, H. Tian and B. Zhang

Wetlands comprise the single largest global source of atmospheric methane, but current flux estimates disagree in both magnitude and distribution at the continental scale. In this study, we show that a relatively simple model of wetland distribution, soil moisture, and soil temperature can represent methane sources on regional scales observable by atmospheric measurements; that wetland methane fluxes have a broader spatial distribution across western Canada and into the northern U.S. than represented in existing models; and that western Canada, and especially Alberta, exhibits large anthropogenic methane emissions substantially above current inventory estimates, likely associated with energy production in the region.


Figure: The 24 month (2 year) mean estimated methane flux from (top) the deterministic model and (bottom) the final posterior estimate.

Abstract

Wetlands comprise the single largest global source of atmospheric methane, but current flux estimates disagree in both magnitude and distribution at the continental scale. This study uses atmospheric methane observations over North America from 2007 to 2008 and a geostatistical inverse model to improve understanding of Canadian methane fluxes and associated biogeochemical models. The results bridge an existing gap between traditional top-down, inversion studies, which typically emphasize total emission budgets, and biogeochemical models, which usually emphasize environmental processes. The conclusions of this study are threefold. First, the most complete process-based methane models do not always describe available atmospheric methane observations better than simple models. In this study, a relatively simple model of wetland distribution, soil moisture, and soil temperature outperformed more complex model formulations. Second, we find that wetland methane fluxes have a broader spatial distribution across western Canada and into the northern U.S. than represented in existing flux models. Finally, we calculate total methane budgets for Canada and for the Hudson Bay Lowlands, a large wetland region (50–60N, 75–96W). Over these lowlands, we find total methane fluxes of 1.8 ± 0.24 Tg C yr−1, a number in the midrange of previous estimates. Our total Canadian methane budget of 16.0 ± 1.2 Tg C yr−1 is larger than existing inventories, primarily due to high anthropogenic emissions in Alberta. However, methane observations are sparse in western Canada, and additional measurements over Alberta will constrain anthropogenic sources in that province with greater confidence.

Miller, S.M., D.E.J. Worthy, A.M. Michalak, S.C. Wofsy, E.A. Kort, T.C. Havice, A.E. Andrews, E.J. Dlugokencky, J.O. Kaplan, P.J. Levi, H. Tian, B. Zhang (2014) "Observational constraints on the distribution, seasonality, and environmental predictors of North American boreal methane emissions", Global Biogeochemical Cycles, 28, doi:10.1002/2013GB004580.