CarbonTracker-CH 4 : an assimilation system for estimating emissions of atmospheric methane

• Published in: Atmospheric Chemistry and Physics
• Main Authors: L. Bruhwiler, E. Dlugokencky, K. Masarie, M. Ishizawa, A. Andrews, J. Miller, C. Sweeney, P. Tans, D. Worthy
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2014
• Subjects: Physics; QC1-999; Chemistry; QD1-999; Arctic
• Online Access: https://doi.org/10.5194/acp-14-8269-2014; https://doaj.org/article/73302c2117cf4f9a8f13054180c2a74c

We describe an assimilation system for atmospheric methane (CH 4 ), CarbonTracker-CH 4 , and demonstrate the diagnostic value of global or zonally averaged CH 4 abundances for evaluating the results. We show that CarbonTracker-CH 4 is able to simulate the observed zonal average mole fractions and capture inter-annual variability in emissions quite well at high northern latitudes (53–90° N). In contrast, CarbonTracker-CH 4 is less successful in the tropics where there are few observations and therefore misses significant variability and is more influenced by prior flux estimates. CarbonTracker-CH 4 estimates of total fluxes at high northern latitudes are about 81 ± 7 Tg CH 4 yr −1 , about 12 Tg CH 4 yr −1 (13%) lower than prior estimates, a result that is consistent with other atmospheric inversions. Emissions from European wetlands are decreased by 30%, a result consistent with previous work by Bergamaschi et al. (2005); however, unlike their results, emissions from wetlands in boreal Eurasia are increased relative to the prior estimate. Although CarbonTracker-CH 4 does not estimate an increasing trend in emissions from high northern latitudes for 2000 through 2010, significant inter-annual variability in high northern latitude fluxes is recovered. Exceptionally warm growing season temperatures in the Arctic occurred in 2007, a year that was also anonymously wet. Estimated emissions from natural sources were greater than the decadal average by 4.4 ± 3.8 Tg CH 4 yr −1 in 2007. CarbonTracker-CH 4 estimates for temperate latitudes are only slightly increased over prior estimates, but about 10 Tg CH 4 yr −1 is redistributed from Asia to North America. This difference exceeds the estimated uncertainty for North America (±3.5 Tg CH 4 yr −1 ). We used time invariant prior flux estimates, so for the period from 2000 to 2006, when the growth rate of global atmospheric CH 4 was very small, the assimilation does not produce increases in natural or anthropogenic emissions in contrast to bottom-up emission data sets. After ...