Advances in representing interactive methane in ModelE2-YIBs (version 1.1)
Authors: Harper Kandice L. , Zheng Yiqi , Unger Nadine 
Published: November 2018

Abstract. Methane (CH4) is both a greenhouse gas and a
precursor of tropospheric ozone, making it an important focus of
chemistry–climate interactions. Methane has both anthropogenic and natural
emission sources, and reaction with the atmosphere's principal oxidizing
agent, the hydroxyl radical (OH), is the dominant tropospheric loss process
of methane. The tight coupling between methane and OH abundances drives
indirect linkages between methane and other short-lived air pollutants and
prompts the use of interactive methane chemistry in global
chemistry–climate modeling. In this study, an updated contemporary
inventory of natural methane emissions and the soil sink is developed using
an optimization procedure that applies published emissions data to the NASA
GISS ModelE2-Yale Interactive terrestrial Biosphere (ModelE2-YIBs) global
chemistry–climate model. Methane observations from the global surface
air-sampling network of the Earth System Research Laboratory (ESRL) of the
US National Oceanic and Atmospheric Administration (NOAA) are used to
guide refinement of the natural methane inventory. The wetland methane flux
is calculated as a best fit; thus, the accuracy of this derived flux assumes
accurate simulation of methane chemical loss in the atmosphere and accurate
prescription of the other methane fluxes (anthropogenic and natural). The
optimization process indicates global annual wetland methane emissions of
140 Tg CH4 yr−1. The updated inventory includes total global annual
methane emissions from natural sources of 181 Tg CH4 yr−1 and a
global annual methane soil sink of 60 Tg CH4 yr−1. An
interactive methane simulation is run using ModelE2-YIBs, applying dynamic
methane emissions and the updated natural methane emissions inventory that
results from the optimization process. The simulated methane chemical
lifetime of 10.4±0.1 years corresponds well to observed lifetimes.
The simulated year 2005 global-mean surface methane concentration is 1.1 %
higher than the observed value from the NOAA ESRL measurements.
Comparison of the simulated atmospheric methane distribution with the NOAA
ESRL surface observations at 50 measurement locations finds that the
simulated annual methane mixing ratio is within 1 % (i.e., +1 % to
−1 %) of the observed value at 76 % of locations. Considering the 50
stations, the mean relative difference between the simulated and observed
annual methane mixing ratio is a model overestimate of only 0.5 %.
Comparison of simulated annual column-averaged methane concentrations with
SCIAMACHY satellite retrievals provides an independent post-optimization
evaluation of modeled methane. The comparison finds a slight model
underestimate in 95 % of grid cells, suggesting that the applied methane
source in the model is slightly underestimated or the model's methane sink
strength is slightly too strong outside of the surface layer. Overall, the
strong agreement between simulated and observed methane lifetimes and
concentrations indicates that the ModelE2-YIBs chemistry–climate model is
able to capture the principal processes that control atmospheric methane.