Modeling Global Wetlands and Their Methane Emissions

Climate model comparisons show wetlands remain a big challenge.

Image courtesy of the U.S. Environmental Protection Agency

The Science

Global wetlands are the largest natural emitters of methane (CH₄), a potent greenhouse gas. Although these wetlands are believed to be climate sensitive, their size, CH₄ emissions, and response to climate change are poorly constrained. Earth system models are now developing the capability to simulate these wetlands and their CH₄ production.

The Impact

Even though modeling of wetland extent and CH₄ emissions has progressed significantly over recent decades, large uncertainties still exist when estimating CH₄ emissions. For example, there is little consensus on model structure or complexity because of knowledge gaps, different model aims, and the range of temporal and spatial resolutions of the models.

Summary

To study the importance of wetlands in the global water and carbon cycles, a variety of hydrological and biogeochemical models have been developed over the last three decades. The Wetland and Wetland CH₄ Intercomparison of Models Project is assessing the ability of these models to simulate large-scale wetland characteristics and their CH₄ emissions, essential for evaluating key uncertainties in the mechanisms and parameters leading to such emissions. Ten modeling groups, including Department of Energy (DOE) researchers at Lawrence Berkeley National Laboratory, ran eight global and two regional models with a common experimental protocol using the same climate and atmospheric carbon dioxide (CO₂) forcing datasets. The models demonstrate extensive disagreement in their simulations of wetland areal extent and CH₄ emissions, in both space and time. All models show a strong positive response to increased atmospheric CO₂ concentrations, in both CH₄ emissions and wetland area. In response to increasing global temperatures, the models, on average, show decreased wetland area and CH₄ fluxes, primarily in the tropics, but the magnitude and sign of the response varied greatly. Models were least sensitive to increased global precipitation, demonstrating a consistent small positive response in CH₄ fluxes and wetland area. Existing wetland CH₄ observation datasets are insufficient for evaluating model fluxes, severely restricting the ability to model global wetland CH₄emissions with confidence. The large range in predicted CH₄ emission rates suggests that there is both substantial parameter and structural uncertainty in large-scale CH₄ emission models, even after uncertainties in wetland areas are accounted for. Clearly, significant research is needed both in modeling and measuring wetland sources of CH₄ and their responses to climate.

Contact

J. R. Melton
ARVE Group, École Polytechnique Fédérale de Lausanne, Switzerland
joe.melton.sci@gmail.com

Funding

This research was supported by the (1) Swiss Ministry for Research and Education (grant C09.0054); (2) Swiss National Science Foundation (grants PP0022 119049 and PP00P2 139193); (3) Climate and Environmental Sciences Division of the Office of Biological and Environmental Research within the U.S. Department of Energy Office of Science under contract no. DE-AC02-05CH11231 to Lawrence Berkeley National Laboratory (IMPACTS and C-Climate Uncertainties projects); (4) NASA’s ROSES program, grant NNX08AH97G; (5) National Science Foundation grant (ARC-1107981); (6) a grant from The Max Planck Society (Germany); (7) President of Russia grant 5467.2012.5 by the Russian Foundation for Basic Research; (8) programs of the Russian Academy of Sciences; (9) Swiss National Science Foundation; (10) European Research Council advanced grant MATRICs (ERC grant agreement no. 226172) under the European Community’s Seventh Framework Programme; (11) a NERC UK grant (NE/I010912/1) and previously through a NERC UK/INSU France QUEST-DESIRE project; and (12) ETH Competence Center Environment and Sustainability’s MAIOLICA grant.

Publications

Wania, R., et al. “Present state of global wetland extent and wetland methane modelling: Methodology of a model intercomparison project (WETCHIMP),” Geosci. Model Dev. 6, 617–641 (2013). [DOI: 10.5194/gmd-6-617-2013], www.geosci-model-dev.net/6/617/2013/.

Melton, J. R., et al. “Present state of global wetland extent and wetland methane modelling: Conclusions from a model inter-comparison project (WETCHIMP),” Biogeosci. 10, 753–788 (2013). [DOI: 10.5194/bg-10-753-2013].