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[1] 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°28′N, 62°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 ∼16 ppb CH4 (ppm CO2)−1 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.

[1] In atmospheric transport models, the covariation of the atmospheric transport and annually neutral biospheric CO2 flux is usually evident as the annual zonal mean surface CO2 concentration gradient. Using the NIES transport model and CO2 flux from the Biome-BGC model, the covariations of different transport mechanisms and CO2 flux were examined and quantified. Including the covariation of the total transport (processes included in the NIES model) and CO2 flux, the annual average pole to pole CO2 concentration gradient is 3.5 ppm and interhemispheric difference of the average extratropical surface concentration is 2.5 ppm. The conventional covariation mechanism of the seasonal variation of planetary boundary layer mixing height and CO2 flux accounts for approximately 45% of the CO2 concentration gradient. Another important contribution to the CO2 concentration gradient in this model is the covariation of the extratropical anomaly transport (mainly by cyclones and anticyclones) and the biospheric flux, which accounts for about 55%. This alternate physical mechanism is the association of stronger meridional (north–south) anomaly transport (under strong baroclinic instability condition) with higher CO2 concentration from soil respiration in the winter and weaker anomaly transport (weak baroclinic instability condition) with lower CO2 concentration from photosynthetic uptake in the summer. The net result of the meridional transport and flux covariation is a north–south annual zonal mean CO2 concentration gradient.