| Past records of atmospheric methane (CH4) abundance/isotope composition may provide
a substantial insight on C exchanges in the Earth System (ES). When simulated
in the climate models, CH4 helps to identify climate parameters transitions via
triggering of its different (natural) sources, with a proviso that its sinks are adequately
represented in the model. The latter are still a matter of large uncertainty in the
studies focussing on the interpretation of CH4 evolution throughout Last Glacial
Maximum (LGM), judging the conferred span of tropospheric CH4 lifetime (λ) of 3-16
yr [1-4]. In this study, we attempt to: (i) deliver the most adequate estimate of
the LGM atmospheric sink of CH4 in the EMAC AC-GCM [5] equipped with the
comprehensive representation of atmospheric chemistry [6], (ii) reveal the ES and
CH4 emission parameters that are most influential for λ and (iii) based on these
findings, suggest a parameterisation for λ that may be consistently used in climate
models.
In pursuing (i) we have tuned the EMAC model for simulating LGM atmospheric
chemistry state, including careful revisiting of the trace gases emissions from the biosphere,
biomass burning/lightning source, etc. The latter affect the key simulated component bound
with λ, viz. the abundance and distribution of the hydroxyl radicals (OH) which, upon
reacting with CH4, constitute its main tropospheric sink. Our preliminary findings
suggest that OH is buffered in the atmosphere in a similar fashion to preindustrial
climate, which in line with the recent studies employing comprehensive chemistry
mechanisms (e.g., [3]). The analysis in (ii) suggests that tropospheric λ values may be
qualitatively described as a convolution of values typical for zonal domain with high
and low photolytic recycling rates (i.e. tropics and extra-tropics), as in the latter a
dependence of the zonal average λ value on the CH4 emission strength exists. We
further use the extensive diagnostic in EMAC to infer the sensitivity of zonal OH to
changes in various component of the ES, e.g. in stratospheric O3 input and dynamics.
Finally, we discuss the potential set of parameters required for efficient λ and/or
OH parameterisation implementation in models dealing with (transient) climate
simulations.
References
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termination, Nature, 452, 864-867, doi: 10.1038/nature06825, 2008.
2. Kaplan, J. O., Folberth, G.,and Hauglustaine, D. A.: Role of methane and biogenic volatile
organic compound sources in late glacial and Holocene fluctuations of atmospheric methane
concentrations, Global Biogeochemical Cycles, 20, n/a-n/a, doi: 10.1029/2005GB002590,
2006.
3. Murray, L. T., et al.: Factors controlling variability in the oxidative capacity of the troposphere
since the Last Glacial Maximum, Atmos. Chem. Phys., 14, 3589-3622, doi: 10.5194/acp-14-3589-2014,
2014.
4. Valdes, P. J., Beerling, D. J.,and Johnson, C. E.: The ice age methane budget, Geophysical
Research Letters, 32, n/a-n/a, doi: 10.1029/2004GL021004, 2005.
5. Jöckel, P., et al.: Development cycle 2 of the Modular Earth Submodel System (MESSy2),
Geosci. Model Dev., 3, 717-752, doi: 10.5194/gmd-3-717-2010, 2010.
6. Lelieveld, J., et al.: Global tropospheric hydroxyl distribution, budget and reactivity, Atmos.
Chem. Phys., 16, 12477-12493, doi: 10.5194/acp-16-12477-2016, 2016. |