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

Abstract

Global wetlands are believed to be climate sensitive, and are the largest natural emitters of methane (CH<sub>4</sub>). Increased wetland CH<sub>4</sub> emissions could act as a positive feedback to future warming. The Wetland and Wetland CH<sub>4</sub> Inter-comparison of Models Project (WETCHIMP) investigated our present ability to simulate large-scale wetland characteristics and corresponding CH<sub>4</sub> emissions. To ensure inter-comparability, we used a common experimental protocol driving all models with the same climate and carbon dioxide (CO<sub>2</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 CO<sub>2</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. <br><br> Four major conclusions emerged from the project. First, the suite of models demonstrate extensive disagreement in their simulations of wetland areal extent and CH<sub>4</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 CH<sub>4</sub> emissions, but large variation between the models remains. For annual global CH<sub>4</sub> emissions, the models vary by ±40% of the all-model mean (190 Tg CH<sub>4</sub> yr<sup>−1</sup>). Second, all models show a strong positive response to increased atmospheric CO<sub>2</sub> concentrations (857 ppm) in both CH<sub>4</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 CH<sub>4</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 CH<sub>4</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 c...

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

“…As briefly described in Melton et al (2013), WETCHIMP consisted of six experiments, including both a transient simulation and several equilibrium state simulations with stepchanges to climate forcing. The first experiment ("Experiment 1-equil") was an equilibrium simulation under repeating 1901-1931 climate and a carbon dioxide concentration ([CO 2 ]) of 303 ppmv.…”

“…In all cases, the step increases were applied to all months and grid cells uniformly. While actual changes in climate are projected to vary in both space and time, these uniform changes are suitable for the purpose of sensitivity tests (Melton et al, 2013). An overview of which groups conducted which simulations is shown in Table 2.…”

“…This approach allows us to study the sources of uncertainties and spatial and temporal differences in model behaviour. The results of this multi-model comparison are presented in Melton et al (2013). In this paper, we provide the technical background for WETCHIMP, presenting details of the modelling protocol (Sect.…”

“…Following the definitions set out in Melton et al (2013), we define wetlands for the purpose of large-scale modelling as grid cells, or fractions thereof, where the land surface has inundated, or saturated, conditions. Peatlands are a form of wetlands characterized by fixed extent, at least on timescales of decades, and contrasting hydrologic and nutrient regimes between dry nutrient-poor bogs and wet nutrient-rich fens (Melton et al, 2013). Inundated areas are assumed to be wetlands (unless masked out with a rice agriculture or lake dataset) with the water table above, or at the soil surface, but do not include areas that are unsaturated at the soil surface but saturated at depth.…”

“…Ten modelling groups joined WETCHIMP to run eight global and two regional models with a common experimental protocol using the same climate and atmospheric carbon dioxide (CO 2 ) forcing datasets. We reported the main conclusions from the intercomparison effort in a companion paper (Melton et al, 2013). Here we provide technical details for the six experiments, which included an equilibrium, a transient, and an optimized run plus three sensitivity experiments (temperature, precipitation, and atmospheric CO 2 concentration).…”