U.S. Department of Energy Office of Biological and Environmental Research

PI-Submitted Research Highlights for
Terrestrial Ecosystem Science Program

A multi-scale comparison of modeled and observed seasonal methane emissions in northern wetlands

William J. Riley
Lawrence Berkeley National Laboratory

Highlight

We compared model predictions against seventeen sites in Northern Hemisphere.

From Xu et al. 2016 (this publication—see attached PDF).

Simulated August CH4 emissions show large spatial heterogeneity across Alaska.

From Xu et al. 2016 (this publication—see attached PDF).

Seasonal methane emissions in northern wetlands

The Science                       
Wetlands are the largest global natural methane (CH4) source, yet predictive capability of land models is low. We improved the methane module in CLM and ALM and compared predictions with tower and aircraft observations and atmospheric inversions, and highlight new observations and model requirements to improve global CH4 predictions.

The Impact
Model changes substantially improved CH4 emission predictions compared to observations. Cold season CH4 emissions estimates remain biased low, motivating more observations during this period. Large CH4 emissions uncertainties are also generated by uncertainties in wetland hydrology.

Summary
We compared wetland CH4 emission model predictions with site to regional scale observations. A comparison of the CH4 fluxes with eddy flux data highlighted needed changes to the model’s estimate of aerenchyma area, which we implemented and tested. The model modification substantially reduced biases in CH4 emissions when compared with CarbonTracker CH4 predictions. CLM4.5 CH4 emission predictions agree well with Alaskan growing season (May-September) CarbonTracker CH4 predictions and site-level observations. However, the model underestimated CH4 emissions in the cold season (October-April). The monthly atmospheric CH4 mole fraction enhancements due to wetland emissions were also assessed using the WRF-STILT Lagrangian transport model and compared with measurements from the Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE) campaign. Both the tower and aircraft analyses confirm the underestimate of cold season CH4 emissions. The greatest uncertainties in predicting the seasonal CH4 cycle are from the wetland extent, cold season CH4 production, and CH4 transport processes. Predicted CH4 emissions remain uncertain, but we show here that benchmarking against observations across spatial scales can inform model structural and parameter improvements.

Contacts (BER PM)
Daniel Stover, Jared DeForest, and Renu Joseph
SC-23.1
Daniel.Stover@science.doe.gov (301-903-0289), Jared.DeForest@science.doe.gov (301-903-1678), and renu.joseph@science.doe.gov

(PI Contact)
William J. Riley
Lawrence Berkeley National Laboratory
wjriley@lbl.gov

Funding
Funding for this study was provided by the US Department of Energy, BER, under the RGCM program and NGEE-Arctic project under contract # DE-AC02-05CH11231. |

Publications
Xu, X., W.J. Riley, C.D. Koven, et al. (2016) A multi-scale comparison of modeled and observed seasonal methane emissions in northern wetlands, Biogeosciences, doi:10.5194/bg-13-5043-2016.

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