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We describe an assimilation system for atmospheric methane (CH4),CarbonTracker-CH4, and demonstrate the diagnostic value of global orzonally averaged CH4 abundances for evaluating the results. We showthat CarbonTracker-CH4 is able to simulate the observed zonal averagemole fractions and capture inter-annual variability in emissions quite wellat high northern latitudes (53–90° N). In contrast, CarbonTracker-CH4 isless successful in the tropics where there are few observations andtherefore misses significant variability and is more influenced by priorflux estimates. CarbonTracker-CH4 estimates of total fluxes at highnorthern latitudes are about 81 ± 7 Tg CH4 yr−1, about12 Tg CH4 yr−1 (13%) lower than prior estimates, a result that isconsistent with other atmospheric inversions. Emissions from Europeanwetlands are decreased by 30%, a result consistent with previous work byBergamaschi et al. (2005); however, unlike their results, emissions fromwetlands in boreal Eurasia are increased relative to the prior estimate.Although CarbonTracker-CH4 does not estimate an increasing trend inemissions from high northern latitudes for 2000 through 2010, significantinter-annual variability in high northern latitude fluxes is recovered.Exceptionally warm growing season temperatures in the Arctic occurred in2007, a year that was also anonymously wet. Estimated emissions from naturalsources were greater than the decadal average by 4.4 ± 3.8 Tg CH4 yr−1 in 2007. CarbonTracker-CH4 estimates for temperate latitudes are only slightlyincreased over prior estimates, but about 10 Tg CH4 yr−1 isredistributed from Asia to North America. This difference exceeds theestimated uncertainty for North America (±3.5 Tg CH4 yr−1).We used time invariant prior flux estimates, so for the period from 2000 to2006, when the growth rate of global atmospheric CH4 was very small,the assimilation does not produce increases in natural or anthropogenicemissions in contrast to bottom-up emission data sets. After 2006, whenatmospheric CH4 began its recent increases, CarbonTracker-CH4allocates some of the increases to anthropogenic emissions at temperatelatitudes, and some to tropical wetland emissions. For temperate NorthAmerica the prior flux increases by about 4 Tg CH4 yr−1 duringwinter when biogenic emissions are small. Examination of the residuals atsome North American observation sites suggests that increased gas and oilexploration may play a role since sites near fossil fuel production areparticularly hard for the inversion to fit and the prior flux estimates atthese sites are apparently lower and lower over time than what theatmospheric measurements imply. The tropics are not currently well resolved by CarbonTracker-CH4 due tosparse observational coverage and a short assimilation window. However,there is a small uncertainty reduction and posterior emissions are about18% higher than prior estimates. Most of this increase is allocated totropical South America rather than being distributed among the globaltropics. Our estimates for this source region are about 32 ± 4 Tg CH4 yr−1,in good agreement with the analysis of Melack et al. (2004) who obtained 29 Tg CH4 yr−1 for the most productive region,the Amazon Basin

CarbonTracker-CH&lt;sub&gt;4&lt;/sub&gt;: an assimilation system for estimating emissions of atmospheric methane

CarbonTracker-CH<sub>4</sub>: an assimilation system for estimating emissions of atmospheric methane