From California dreaming to California data: Challenging historic models for landfill CH4 emissions

Improved quantification of diverse CH4 sources at the urban scale is needed to guide local GHG mitigation strategies in the Anthropocene. Herein, we focus on landfill CH4 emissions in California, challenging the current IPCC methodology which focuses on a climate dependency for landfill CH4 generation (methanogenesis), but does not explicitly consider climate or soil dependencies for emissions. Relying on a comprehensive California landfill database, a field-validated process-based model for landfill CH4 emissions (CALMIM), and select field measurements at 10 California sites with a variety of methods, we support the contrary position: Limited climate dependency for methanogenesis, but strong climate dependency for landfill CH4 emissions. Contrary to the historic IPCC empirical model for methanogenesis with kinetic constants related to climate, we demonstrate a simpler and more robust linear empirical relationship (r(2) = 0.85; n=128) between waste mass and landfill biogas recovery [126 x 10(-6) Nm(3) CH4 hr(-1) Mg-waste(-1)]. More interestingly, there are no statistically significant relationships with climate, site age, or status (open/closed) for landfill biogas recovery. The current IPCC methodology does not consider soil or climate drivers for gaseous transport or seasonal methanotrophy in different cover soils. On the other hand, we illustrate strong climate and soil dependencies for landfill emissions-e.g., average intermediate cover emissions below 20 g CH4 m(-2) d(-1) when the site's mean annual precipitation is >500 mm y(-1). Thereby, for the California landfill CH4 inventory, the highest-emitting sites shift from landfills containing the largest mass of waste to sites dominated by intermediate cover types having a reduced rate of soil CH4 oxidation during the annual cycle. These differences have profound implications for developing more realistic, science-based urban and regional scale GHG inventories for landfill CH4 while reducing uncertainties for this important anthropogenic source.