TransCom model simulations of CH<sub>4</sub> and related species: linking transport, surface flux and chemical loss with CH<sub>4</sub> variability in the troposphere and lower stratosphere

Abstract

A transport model intercomparison experiment (TransCom-CH₄) has been designed to investigate the roles of surface emissions, transport and chemical loss in simulating the global methane distribution. Model simulations were conducted using twelve models and four model variants and results were archived for the period of 1990–2007. The transport and removal of six CH₄ tracers with different emission scenarios were simulated, with net global emissions of 513 ± 9 and 514 ± 14 Tg CH₄ yr⁻¹ for the 1990s and 2000s, respectively. Additionally, sulfur hexafluoride (SF₆) was simulated to check the interhemispheric transport, radon (²²²Rn) to check the subgrid scale transport, and methyl chloroform (CH₃CCl₃) to check the chemical removal by the tropospheric hydroxyl radical (OH). The results are compared to monthly or annual mean time series of CH₄, SF₆ and CH₃CCl₃ measurements from 8 selected background sites, and to satellite observations of CH₄ in the upper troposphere and stratosphere. Most models adequately capture the vertical gradients in the stratosphere, the average long-term trends, seasonal cycles, interannual variations and interhemispheric gradients at the surface sites for SF₆, CH₃CCl₃ and CH₄. The vertical gradients of all tracers between the surface and the upper troposphere are consistent within the models, revealing vertical transport differences between models. We find that the interhemispheric exchange rate (1.39 ± 0.18 yr) derived from SF₆ is faster by about 11 % in the 2000s compared to the 1990s. Up to 60 % of the interannual variations in the forward CH₄ simulations can be explained by accounting for the interannual variations in emissions from biomass burning and wetlands. We also show that the decadal average growth rate likely reached equilibrium in the early 2000s due to the flattening of anthropogenic emission growth since the late 1990s. The modeled CH₄ budget is shown to depend strongly on the troposphere-stratosphere exchange rate and thus to the model's vertical grid structure and circulation in the lower stratosphere. The 15-model median CH₄ and CH₃CCl₃ atmospheric lifetimes are estimated to be 9.99 ± 0.08 and 4.61 ± 0.13 yr, respectively, with little interannual variability due to transport and temperature as noted by the ± 1 σ

Accelerating carbon uptake in the Northern Hemisphere: evidence from the interhemispheric difference of atmospheric CO₂ concentrations

“…The difference of simulated t ex between the two periods is 0.04 yr in the 'All grids' case. A CTM intercomparison experiment, TransCom-CH 4 (Patra et al, 2011), reported an average t ex of 1.3 yr during 1999Á2007 for the GEOS-Chem model, which is comparable to that of 1.29 yr derived in this study. Furthermore, the GEOS-Chem model was found to fall in the middle range of t ex among all the models participating in TransCom-CH4 and agree well with observed t ex (Patra et al, 2011).…”

“…The global total emissions of SF 6 from 1982 to 2008 were scaled to the values derived by Levin et al (2010), which were constrained by SF 6 observations. The emission estimates by Levin et al (2010) were verified by Rigby et al (2010) and also demonstrated by Patra et al (2011) to be adequate for use in independent transport models.…”

“…The simulation was initiated from January 1979 with a global uniform SF 6 mixing ratio of 0.75 ppt, corresponding to the observed global average mixing ratio reported by Rigby et al (2010). The model spin up was for 3 yr to get a reasonable latitudinal and vertical distribution of SF 6 (Patra et al, 2011). The distribution of SF 6 emissions in the simulation was taken from the EDGAR 4.0 (2009) (Fig.…”

MERLIN: A French-German Space Lidar Mission Dedicated to Atmospheric Methane

“…The inversion systems inferring CH 4 emissions from CH 4 mole fractions are limited by the quality of the underlying atmospheric transport model used (e.g., [22,23]) and by the uneven distribution of surface observations in space and time (e.g., [24]). Some key regions for the biogeochemical cycles of GHGs remain extremely difficult to access for long-term scientific activities because of their remote location or their political situation (e.g., the Arctic, tropical forests, some African countries).…”

Use of a process-based model for assessing the methane budgets of global terrestrial ecosystems and evaluation of uncertainty

“…Recent progress in measurement of the stable isotope ratios in atmospheric CH 4 (e.g., Lassey et al, 2011) and data-model fusion with the goal of optimizing estimates (e.g., Neef et al, 2010;Riley et al, 2011) may allow more precise assessments of each flux. Also, crosscomparison of model-estimated surface fluxes with atmospheric transport models (e.g., TransCom-CH 4 by Patra et al, 2011) would effectively reveal the accuracy and uncertainty in the present global CH 4 budget. As more and better data become available, model studies will become an increasingly important tool for analyzing the mechanisms underlying the global budget and for predicting future budgets under climate change.…”