Present state of global wetland extent and wetland methane modelling: conclusions from a model intercomparison 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 % 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 spatia...

Response of methane emissions from wetlands to the Last Glacial Maximum and an idealized Dansgaard–Oeschger climate event: insights from two models of different complexity

“…Instead of this, the model has been independently tuned to reproduce the wetland extent against remote sensing data and the CH 4 flux densities against sites measurements. This underlines the uncertainty linked to the contribution of the wetlands to the global CH 4 budget (Melton et al, 2012a andS. Kirschke, personal communication, 2012).…”

“…the methanogenesis sensitivity to the temperature). Currently, an intercomparison between many global wetland CH 4 emission models focusing on the current time period is in progress (WETCHIMP, Melton et al, 2012a).…”

The decadal state of the terrestrial carbon cycle: Global retrievals of terrestrial carbon allocation, pools, and residence times

“…For example, global changes in photosynthetic uptake could lead to a rapid response from short-lived C pools (such as foliage, fine roots, and litter) or a prolonged response from the long-lived C pools (such as woody biomass and soil C), with very different outcomes on ecosystem source-sink behavior. Quantitative knowledge of terrestrial C pathways is, therefore, central to understanding the temporal responses of the major terrestrial C fluxes-including heterotrophic respiration (13), fires (14,15), and wetland CH 4 emissions (16,17)-to interannual variations in C uptake.…”

Modeling CH4 Emissions from Natural Wetlands on the Tibetan Plateau over the Past 60 Years: Influence of Climate Change and Wetland Loss

“…For example, Kleinen [45] and Melton [87] showed reasonable validation by comparing the monthly global distribution of the water table with remote sensing data [76,84] as well as the GLWD map [86]. Similar validations conducted on the regional and global scale have also been reported in previous studies [88,89]. However, the limited resolution would inevitably induce bias into the water table variations-especially on TP, where the natural wetland has a large degree of microscale topographic variation [17].…”

“…The latter method uses a model to simulate the dynamical wetland extent. However, both methods may produce uncertainties [88]. Improving the ability to obtain accurate data on the distribution and extent of wetlands should be a research priority in the future [63].…”