Sensitivity of wetland methane emissions to model assumptions: application and model testing against site observations 1Department of Geography and Environmental Studies Program, Western Michigan University, Kalamazoo, MI 49008, USA
30 Jul 2012
2Department of Biological and Environmental Engineering, Cornell University, Ithaca, NY 14850, USA
3Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14850, USA
4Department of Natural Resources, Cornell University, Ithaca, NY 14850, USA
5Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley CA 94720, USA
6NCAR-CGD, P.O. Box 3000, Boulder, CO 80307, USA
7Department of Forest Sciences, P.O. Box 27, University of Helsinki, Helsinki 00014, Finland
Received: 14 June 2011 – Published in Biogeosciences Discuss.: 30 June 2011 Abstract. Methane emissions from natural wetlands and rice paddies constitute a large
proportion of atmospheric methane, but the magnitude and year-to-year
variation of these methane sources are still unpredictable. Here we describe
and evaluate the integration of a methane biogeochemical model (CLM4Me;
Riley et al., 2011) into the Community Land Model 4.0 (CLM4CN) in order to
better explain spatial and temporal variations in methane emissions. We test
new functions for soil pH and redox potential that impact microbial methane
production in soils. We also constrain aerenchyma in plants in
always-inundated areas in order to better represent wetland vegetation.
Satellite inundated fraction is explicitly prescribed in the model, because
there are large differences between simulated fractional inundation and
satellite observations, and thus we do not use CLM4-simulated hydrology to predict inundated areas. A rice paddy module is also incorporated into the
model, where the fraction of land used for rice production is explicitly
prescribed. The model is evaluated at the site level with vegetation cover
and water table prescribed from measurements. Explicit site level
evaluations of simulated methane emissions are quite different than
evaluating the grid-cell averaged emissions against available measurements.
Using a baseline set of parameter values, our model-estimated average global
wetland emissions for the period 1993–2004 were 256 Tg CH4 yr−1
(including the soil sink) and rice paddy emissions in the year 2000 were 42 Tg CH4 yr−1.
Tropical wetlands contributed 201 Tg CH4 yr−1, or 78% of the global wetland flux. Northern latitude (>50 N)
systems contributed 12 Tg CH4 yr−1. However, sensitivity studies
show a large range (150–346 Tg CH4 yr−1) in predicted global
methane emissions (excluding emissions from rice paddies). The large range
is sensitive to (1) the amount of methane transported through aerenchyma,
(2) soil pH (±100 Tg CH4 yr−1), and (3) redox inhibition (±45 Tg CH4 yr−1).
Results are sensitive to biases in the CLMCN and to
errors in the satellite inundation fraction. In particular, the high latitude
methane emission estimate may be biased low due to both underestimates in
the high-latitude inundated area captured by satellites and unrealistically
low high-latitude productivity and soil carbon predicted by CLM4.
Revised: 13 June 2012 – Accepted: 29 June 2012 – Published: 30 July 2012
Citation: Meng, L., Hess, P. G. M., Mahowald, N. M., Yavitt, J. B., Riley, W. J., Subin, Z. M., Lawrence, D. M., Swenson, S. C., Jauhiainen, J., and Fuka, D. R.: Sensitivity of wetland methane emissions to model assumptions: application and model testing against site observations, Biogeosciences, 9, 2793-2819, doi:10.5194/bg-9-2793-2012, 2012.