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The coupled chemistry of methane, carbon monoxide (CO), and hydroxyl radical (OH) can modulate methane's 9-year lifetime. This is often ignored in methane flux inversions, and the impacts of neglecting interactive chemistry have not been quantified. Using a coupled-chemistry box model, we show that neglecting the effect of methane source perturbation on [OH] can lead to a 25% bias in estimating abrupt changes in methane sources after only 10 years. Further, large CO emissions, such as from biomass burning, can increase methane concentrations by extending the methane lifetime through impacts on [OH]. Finally, we quantify the biases of including (or excluding) coupled chemistry in the context of recent methane and CO trends. Decreasing CO concentrations, beginning in the 2000's, have notable impacts on methane flux inversions. Given these nonnegligible errors, decadal methane emissions inversions should incorporate chemical feedbacks for more robust methane trend analyses and source attributions.

To assess global carbon cycle variability, we decompose the net land carbon sink into the sum of gross primary productivity (GPP), terrestrial ecosystem respiration (TER), and fire emissions and apply a Bayesian framework to constrain these fluxes between 1980 and 2014. The constrained GPP and TER fluxes show an increasing trend of only half of the prior trend simulated by models. From the optimization, we infer that TER increased in parallel with GPP from 1980 to 1990, but then stalled during the cooler periods, in 1990–1994 coincident with the Pinatubo eruption, and during the recent warming hiatus period. After each of these TER stalling periods, TER is found to increase faster than GPP, explaining a relative reduction of the net land sink. These results shed light on decadal variations of GPP and TER and suggest that they exhibit different responses to temperature anomalies over the last 35 years.

We quantify the impact of atmospheric transport and limited marine boundary layer sampling on changes in global and regional methane burdens estimate using tracer transport model simulations with annually repeating methane emissions and sinks but varying atmospheric transport patterns. We find the 1σ error due to this transport and sampling effect on annual global methane increases to be 1.11 ppb/year and on zonal growth rates to be 3.8 ppb/year, indicating that it becomes more critical at smaller spatiotemporal scales. We also find that the trends in inter‐hemispheric and inter‐polar difference of methane are significantly influenced by the effect. Contrary to a negligible trend in the inter‐hemispheric difference of measurements, we find, after adjusting for the transport and sampling, a trend of 0.37 ± 0.06 ppb/year. This is consistent with the emission trend from a 3‐D inversion of the measurements, suggesting a faster increase in emissions in the Northern Hemisphere than in the Southern Hemisphere. Plain Language Summary: Changes in global and regional atmospheric burdens of methane are determined by the net effect of sources and sinks and atmospheric transport. Many studies approximate the burdens based on measurements from a network of globally distributed surface air sampling sites. Here we quantify the impact of atmospheric transport and limited marine boundary layer sampling (transport and sampling) on changes in global and regional methane burden estimates using tracer transport model simulation with annually repeating methane emissions and sinks but varying atmospheric transport patterns. We find that for assessing the annual global methane increases, a measurement‐only approach is fairly accurate. However, extending the measurement‐only analysis to hemispheric or latitudinal variations is more problematic as transport influences quickly become significant or even dominant. We find a large impact of transport and sampling effect on the inter‐hemispheric and inter‐polar difference. Key Points: Transport gives rise to CH4 variations in the spatiotemporal gradients, which are not caused by source or sink changesWe find the transport and sampling error on annual global CH4 increases to be 1.11 ppb/year and on zonal growth rates to be 3.8 ppb/yearThe transport‐adjusted inter‐hemispheric difference has a trend of 0.37 ppb/year, consistent with the emission trend from a 3‐D inversion [ABSTRACT FROM AUTHOR]