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Analysis of recent atmospheric methane trends using models and observations

McNorton, Joe Ramu (2016) Analysis of recent atmospheric methane trends using models and observations. PhD thesis, University of Leeds.

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Over the past two decades the growth rate of methane has shown large variability on multi-year timescales, the reasons for which are not well understood. The JULES land surface model, TOMCAT 3-D chemical transport model and observations have been used to investigate causes for these variations, with a specific focus on wetland emissions and atmospheric loss. The role of atmospheric variability in the recent methane trends was investigated using TOMCAT, driven by variations in global mean hydroxyl concentrations derived from methyl chloroform observations. Results show that between 1999 and 2006, a stall in the atmospheric methane growth rate was, in part, caused by changes in the atmospheric loss. This was due largely to relatively small changes in global mean hydroxyl concentrations over time, with minor contributions from variations in atmospheric transport and temperature. Methane emissions from various wetland inventories were evaluated using TOMCAT and observations, and recent trends in emissions were investigated. Emissions calculated by JULES were spatially and temporally similar to a top-down emission inventory and produced good agreement with satellite observations when used in TOMCAT (R = 0.84). Emissions derived for the period 1993 – 2012 show a statistically significant (95%-level) positive trend of 0.43 Tg/yr. This suggests a long-term positive trend in wetland emissions that may continue. During the stall in methane growth (1999-2006) modelled wetland emissions were 0.4 Tg/yr lower than average. This suggests that a decrease in wetland emissions contributed to the observed stall in methane growth. The wetland methane processes within JULES were developed to include transport, oxidation, sulphate suppression, unsaturated production and methane storage pools. The parameters required for the additional processes were derived using a perturbed parameter ensemble to optimise the fit with observed fluxes. This slightly increased model performance at flux sites from R = 0.32 in the standard model to R = 0.34 in the updated model. The new version of JULES was tested using TOMCAT and satellite observations, and model agreement improved from R = 0.84 to R = 0.87, additionally the root-mean-squared-error reduced from 17.17 ppb to 15.09 ppb. This suggests the optimised additional model processes slightly improved model performance.

Item Type: Thesis (PhD)
Keywords: Methane, Wetlands, Hydroxyl, Atmosphere, Climate
Academic Units: The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds) > Institute for Atmospheric Science (Leeds)
Identification Number/EthosID: uk.bl.ethos.689258
Depositing User: Dr Joe R McNorton
Date Deposited: 27 Jun 2016 12:23
Last Modified: 06 Oct 2016 14:42
URI: http://etheses.whiterose.ac.uk/id/eprint/13294

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