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Methane (CH4) flux is the net rate of methane exchange between an ecosystem and the atmosphere. The Western United States was a methane sink with a flux ranging from −3.1 to −2.9 TgCO2-eq/yr in the baseline period (2001-2005), and little change was projected from 2006 to 2050. Both forests and grasslands/shrublands were methane sinks, and wetlands were methane sources. The methane budget varied regionally; Mediterranean California was a source and the other ecoregions were sinks.

Suggested Citation

Zhu, Zhiliang, ed., Bergamaschi, Brian, Bernknopf, Richard, Clow, David, Dye, Dennis, Faulkner, Stephen, Forney, William, Gleason, Robert, Hawbaker, Todd, Liu, Jinxun, Liu, Shuguang, Prisley, Stephen, Reed, Bradley, Reeves, Matthew, Rollins, Matthew, Sleeter, Benjamin, Sohl, Terry, Stackpoole, Sarah, Stehman, Stephen, Striegl, Robert, Wein, Anne, and Zhu, Zhiliang, 2010, A method for assessing carbon stocks, carbon sequestration, and greenhouse-gas fluxes in ecosystems of the United States under present conditions and future scenarios: U.S. Geological Survey Scientific Investigations Report 2010–5233, 188 p. (Available from USGS).


The net rates of methane exchange in terrestrial ecosystems in the western United States were estimated using the Paint-By-Number (PBN) model embedded in the USGS General Ensemble Biogeochemical Modeling System (GEMS). PBN only accounts for the impacts of land cover change defined by scenarios according to storylines A1B, A2, or B1 from the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios (IPCC–SRES). Specifically, the PBN model estimated methane fluxes for different LULC classes and ecoregions using emission factors that were compiled from an extensive review of the literature. Methane fluxes were expressed in carbon dioxide equivalent (CO2-eq) using a factor of 21.