Decreasing anthropogenic methane emissions in Europe and Siberia inferred from continuous carbon dioxide and methane observations at Alert, Canada

The rate of increase in global atmospheric methane (CH4) abundance has steadily declined since the late 1980s with near zero increase from 1999 through 2006. At the Canadian Baseline Observatory at Alert, Canada (82 degrees 28'N, 62 degrees 30'W), continuous measurements of methane (CH4) and carbon dioxide (CO2) have been made since 1987. During winter, both gases are frequently highly correlated during well-defined episodes lasting from 2 to 5 days. We observe a gradual decrease in the ratios of CH4/CO2 during these episodes from similar to 16 ppb CH4 (ppm CO2)(-1) to similar to 12 ppb CH4 (ppm of CO2)(-1) over the entire record. An atmospheric transport model with prescribed CO2 and CH4 source distributions is used to partition simulated CH4 events into contributions by region. We show that anthropogenic emissions from Europe and Siberia account for more than 85% of the CO2 and CH4 enhancements simulated at Alert, but without a change in CH4 emissions, modeled CH4/CO2 ratios remain constant. To reproduce the observed trend in the ratio of CH4/CO2, the model requires a reduction in emissions of CH4 on the order of 30 Tg (13.6 to 33.4 Tg) in Europe and Siberia over the observational period. This is about twice the drop reported by the Emission Database for Global Atmospheric Research (EDGAR) emissions inventory and large enough to account for the leveling off of the global atmospheric CH4 burden observed over the past 20 years.