Present state of global wetland extent and wetland methane modelling: conclusions from a model inter-comparison project (WETCHIMP)

摘要

pstrongAbstract./strong Global wetlands are believed to be climate sensitive, and are the largest natural emitters of methane (CHsub4/sub). Increased wetland CHsub4/sub emissions could act as a positive feedback to future warming. The Wetland and Wetland CHsub4/sub Inter-comparison of Models Project (WETCHIMP) investigated our present ability to simulate large-scale wetland characteristics and corresponding CHsub4/sub emissions. To ensure inter-comparability, we used a common experimental protocol driving all models with the same climate and carbon dioxide (COsub2/sub) 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 COsub2/sub 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. brbr Four major conclusions emerged from the project. First, the suite of models demonstrate extensive disagreement in their simulations of wetland areal extent and CHsub4/sub 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 CHsub4/sub emissions, but large variation between the models remains. For annual global CHsub4/sub emissions, the models vary by ?±40% of the all-model mean (190 Tg CHsub4/sub yrsupa??1/sup). Second, all models show a strong positive response to increased atmospheric COsub2/sub concentrations (857 ppm) in both CHsub4/sub emissions and wetland area. In response to increasing global temperatures (+3.4 ?°C globally spatially uniform), on average, the models decreased wetland area and CHsub4/sub 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 CHsub4/sub 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 CHsub4/sub 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 CHsub4/sub emission rates leads to the conclusion that there is both substantial parameter and structural uncertainty in large-scale CHsub4/sub emission models, even after uncertainties in wetland areas are accounted for./p.