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Biogeosciences, 10, 753-788, 2013
© Author(s) 2013. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
04 Feb 2013
Present state of global wetland extent and wetland methane modelling: conclusions from a model inter-comparison project (WETCHIMP)
J. R. Melton1,*, R. Wania2,**, E. L. Hodson3,***, B. Poulter4, B. Ringeval4,5,6, R. Spahni7, T. Bohn8, C. A. Avis9, D. J. Beerling10, G. Chen11, A. V. Eliseev12,13, S. N. Denisov12, P. O. Hopcroft5, D. P. Lettenmaier8, W. J. Riley14, J. S. Singarayer5, Z. M. Subin14, H. Tian11, S. Zürcher7, V. Brovkin15, P. M. van Bodegom16, T. Kleinen15, Z. C. Yu17, and J. O. Kaplan1
1ARVE Group, École Polytechnique Fédérale de Lausanne, Switzerland
2Institut des Sciences de l'Evolution (UMR 5554, CNRS), Université Montpellier 2, Place Eugène Bataillon, 34090 Montpellier, France
3Swiss Federal Research Institute WSL, Switzerland
4Laboratoire des Sciences du Climat et de l'Environment, CNRS-CEA, UVSQ, Gif-sur Yvette, France
5BRIDGE, School of Geographical Sciences, Univerity of Bristol, UK
6VU University Amsterdam, Department of Earth Sciences, Amsterdam, The Netherlands
7Climate and Environmental Physics, Physics Institute & Oeschger Centre for Climate Change Research, University of Bern, Switzerland
8Dept. of Civil and Environmental Engineering, University of Washington, USA
9School of Earth and Ocean Sciences, University of Victoria, Canada
10Dept. of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK
11International Center for Climate and Global Change Research and School of Forestry and Wildlife Sciences, Auburn University, Auburn, AL 36849, USA
12A.M. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Russia
13Kazan (Volga Region) Federal University. Kazan, Russia
14Earth Sciences Division (ESD) Lawrence Berkeley National Lab, USA
15Max Planck Institute for Meteorology, Hamburg, Germany
16Department of Ecological Sciences, VU University, Amsterdam, The Netherlands
17Department of Earth and Environmental Sciences, Lehigh University, USA
*now at: Canadian Centre for Climate Modelling and Analysis, Environment Canada, Victoria, BC, V8W 2Y2, Canada
**Lanser Strasse 30, 6080 Igls, Austria
***now at: AAAS Science and Technology Policy Fellow, Office of Climate Change Policy and Technology, US Department of Energy, USA

Abstract. Global wetlands are believed to be climate sensitive, and are the largest natural emitters of methane (CH4). Increased wetland CH4 emissions could act as a positive feedback to future warming. The Wetland and Wetland CH4 Inter-comparison of Models Project (WETCHIMP) investigated our present ability to simulate large-scale wetland characteristics and corresponding CH4 emissions. To ensure inter-comparability, we used a common experimental protocol driving all models with the same climate and carbon dioxide (CO2) forcing datasets. The WETCHIMP experiments were conducted for model equilibrium states as well as transient simulations covering the last century. Sensitivity experiments investigated model response to changes in selected forcing inputs (precipitation, temperature, and atmospheric CO2 concentration). Ten models participated, covering the spectrum from simple to relatively complex, including models tailored either for regional or global simulations. The models also varied in methods to calculate wetland size and location, with some models simulating wetland area prognostically, while other models relied on remotely sensed inundation datasets, or an approach intermediate between the two.

Four major conclusions emerged from the project. First, the suite of models demonstrate extensive disagreement in their simulations of wetland areal extent and CH4 emissions, in both space and time. Simple metrics of wetland area, such as the latitudinal gradient, show large variability, principally between models that use inundation dataset information and those that independently determine wetland area. Agreement between the models improves for zonally summed CH4 emissions, but large variation between the models remains. For annual global CH4 emissions, the models vary by ±40% of the all-model mean (190 Tg CH4 yr−1). Second, all models show a strong positive response to increased atmospheric CO2 concentrations (857 ppm) in both CH4 emissions and wetland area. In response to increasing global temperatures (+3.4 °C globally spatially uniform), on average, the models 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 (+3.9 % globally spatially uniform) with a consistent small positive response in CH4 fluxes and wetland area. Results from the 20th century transient simulation show that interactions between climate forcings could have strong non-linear effects. Third, we presently do not have sufficient wetland methane observation datasets adequate to evaluate model fluxes at a spatial scale comparable to model grid cells (commonly 0.5°). This limitation severely restricts our ability to model global wetland CH4 emissions with confidence. Our simulated wetland extents are also difficult to evaluate due to extensive disagreements between wetland mapping and remotely sensed inundation datasets. Fourth, the large range in predicted CH4 emission rates leads to the conclusion that there is both substantial parameter and structural uncertainty in large-scale CH4 emission models, even after uncertainties in wetland areas are accounted for.

Citation: Melton, J. R., Wania, R., Hodson, E. L., Poulter, B., Ringeval, B., Spahni, R., Bohn, T., Avis, C. A., Beerling, D. J., Chen, G., Eliseev, A. V., Denisov, S. N., Hopcroft, P. O., Lettenmaier, D. P., Riley, W. J., Singarayer, J. S., Subin, Z. M., Tian, H., Zürcher, S., Brovkin, V., van Bodegom, P. M., Kleinen, T., Yu, Z. C., and Kaplan, J. O.: Present state of global wetland extent and wetland methane modelling: conclusions from a model inter-comparison project (WETCHIMP), Biogeosciences, 10, 753-788, doi:10.5194/bg-10-753-2013, 2013.
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