Description:
Use substitute Global Warming Potential (GWP) factor for landfill methane gas emissions, based on 20 years rather than 100 years
Issues:
The 100-year measurement period for GHG emissions that is commonly used in GHG inventories is based on the atmospheric residency of carbon dioxide. However, this is not a relevant time period over which to measure landfill methane gas emissions, because unlike carbon dioxide methane remains resident in the atmosphere for only 12 years. Yet the established general frameworks for climate change (e.g., US EPA's Inventory of U.S. Greenhouse Gas Emissions and Sinks) typically use the 100-year time horizon established by the Intergovernmental Panel on Climate Change (IPCC) Assessment Reports, and thus the 100-year time horizon is the default measurement period value for most GHG inventory boiler plate protocols.
Suggestions:
When conducting a GHG inventory, rather than allocating the climate change impact of landfill methane gas emissions over a 100-year period by applying the commonly-used GHG multiplier of 21x (i.e., methane is 21 times more powerful a GHG than carbon dioxide, or has a GWP of 21), instead use a substitute factor of 72x (GWP of 72) which allocates methane's climate impact over a 20-year period. The purpose of GHG inventories that are conducted to guide development of local/regional Climate Action Plans (CAPs) is to inform establishment of action steps over a relevant policy implementation period AND a relevant climate protection action impact period. Since methane remains resident in the atmosphere for only 12 years, its impacts are far greater over the short term. Subsequently, actions to reduce methane have far greater impacts over the short term. 20 years (rather than 100 years) is more in line with the implementation and measurement timeframes of local/regional CAPs. Therefore, when conducting a GHG inventory whose purpose is to guide development CAP action steps, it is equally (if not more) valid to use a GWP of 72 (over 20 years) for landfill methane gas emissions rather than a GWP of 21 (over 100 years).
Additional Comments:
For resources, see Intergovernmental Panel on Climate Change (IPCC), “Table 2.14,” p. 212, Forster, P., et al, 2007: Changes in Atmospheric Constituents and in Radiative Forcing in Climate Change 2007: The Physical Science Basis., showing methane's GWP of 72 over 20 years. See the same table, footnote "c" listing methane's atmospheric lifetime of 12 years.