Seasonal and interannual variability in wetland methane emissions simulated by CLM4Me' and CAM-chem and comparisons to observations of concentrations

[摘要] Understanding the temporal and spatial variation of wetland methane emissionsis essential to the estimation of the global methane budget. Our goal forthis study is three-fold: (i) to evaluate the wetland methane fluxessimulated in two versions of the Community Land Model, the Carbon-Nitrogen(CN; i.e., CLM4.0) and the Biogeochemistry (BGC; i.e., CLM4.5) versions usingthe methane emission model CLM4Me' so as to determine thesensitivity of the emissions to the underlying carbon model; (ii) to comparethe simulated atmospheric methane concentrations to observations, includinglatitudinal gradients and interannual variability so as to determine theextent to which the atmospheric observations constrain the emissions; (iii)to understand the drivers of seasonal and interannual variability inatmospheric methane concentrations. Simulations of the transport and removalof methane use the Community Atmosphere Model with chemistry (CAM-chem) model in conjunctionwith CLM4Me' methane emissions from both CN and BGC simulations andother methane emission sources from literature. In each case we compare model-simulated atmospheric methane concentration with observations. In addition,we simulate the atmospheric concentrations based on the TransCom wetland andrice paddy emissions derived from a different terrestrial ecosystem model,Vegetation Integrative Simulator for Trace gases (VISIT). Our analysis indicates CN wetland methane emissions are higher in thetropics and lower at high latitudes than emissions from BGC. In CN, methaneemissions decrease from 1993 to 2004 while this trend does not appear in theBGC version. In the CN version, methane emission variations followsatellite-derived inundation wetlands closely. However, they are dissimilarin BGC due to its different carbon cycle. CAM-chem simulations withCLM4Me' methane emissions suggest that both prescribed anthropogenicand predicted wetlands methane emissions contribute substantially to seasonaland interannual variability in atmospheric methane concentration. Simulatedatmospheric CH₄ concentrations in CAM-chem are highly correlated withobservations at most of the 14 measurement stations evaluated with an averagecorrelation between 0.71 and 0.80 depending on the simulation (for the periodof 1993–2004 for most stations based on data availability). Our resultssuggest that different spatial patterns of wetland emissions can havesignificant impacts on Northern and Southern hemisphere (N–S) atmospheric CH₄ concentration gradients andgrowth rates. This study suggests that both anthropogenic and wetlandemissions have significant contributions to seasonal and interannualvariations in atmospheric CH₄ concentrations. However, our analysis alsoindicates the existence of large uncertainties in terms of spatial patternsand magnitude of global wetland methane budgets, and that substantialuncertainty comes from the carbon model underlying the methane flux modules.

[学科分类] 地球化学与岩石