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

Kurt Spokas, Jean Bogner, Meg Corcoran, Scott Walker
Elementa: Science of the Anthropocene 2015 v.3 no.000051 pp. -
anthropogenic activities, atmospheric precipitation, biogas, climate, cold, databases, greenhouse gas emissions, greenhouse gases, inventories, landfills, methane, methane production, models, oxidation, soil, temperature, wastes, California
Improved quantification of diverse CH4 sources at the urban scale is needed to guide local greenhouse gas (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 but does not explicitly consider site-specific climate or soil dependencies for emissions. Relying on a new comprehensive California landfill database, a field-validated process-based model for landfill CH4 emissions (CALMIM), and field measurements at 10 California sites, we validate the contrary position: no climate dependency for CH4 generation with a strong climate dependency for CH4 emissions. Contrary to the historic IPCC first order model for methanogenesis with kinetic constants related to climate, we demonstrate a robust linear empirical relationship (r2 = 0.85; n=128) between waste mass and biogas recovery [@242 Nm3 biogas hr-1 at 50% CH4 per million Mgwaste-1], with no statistically significant relationships with climate [mean annual temperature (MAT) and mean annual precipitation (MAP)], site age, or status (open/closed). For emissions, the current IPCC methodology does not consider soil or climate drivers for gaseous transport or seasonal methanotrophy in daily, intermediate, and final cover soils, allowing only 10% annual oxidation. On the other hand, we confirm 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 MAP >500 mm y-1. Cover-specific fractional oxidation can realistically range from negligible to 100%. Thereby, the predicted highest-emitting sites shift from landfills containing the largest mass of waste in the current inventory to sites with large areas of thinner intermediate cover and sites with periodically reduced rates of soil CH4 oxidation during the annual cycle (too hot/dry/cold). 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.