What is different about methane (CH4) emissions? The forgotten CO2 in GHG emissions accounting
I would venture to guess that many of you are unaware that most GHG emission inventories and offset project methodologies underestimate actual emissions from fugitive emissions of methane. This is the case because these methodologies systemically forget to include an emissions category: “indirect CO2 from the atmospheric oxidation of CH4.”
The fact is that when methane is anthropogenically emitted, methane is oxidized in the atmosphere a decade or two later. Once oxidized, the carbon in each methane molecule is converted to CO2, which then stays in the atmosphere as CO2 for another century or more. So really, when methane is emitted, you get a double whammy: first from the methane itself followed by the CO2 that results from atmospheric oxidization.
Many of you may assume that the GWP of methane would account for this oxidization, right? Wrong! This effect is not included in the GWP of methane, and it should not be included. Why? Simply stated, the effect depends on the origin of the methane. We have to treat methane from biogenic sources (such as livestock and rice paddies) different from fossil sources (such as coal mines and natural gas leaks), as only methane from fossil fuels result in a net addition of CO2 to the atmosphere following atmospheric oxidation.
Because of this difference, we cannot simply change the GWP value. If we did we would be in the confusing position of having two different GWP values for the same gas, with this variation in accounting tied to where the methane came from.
Indirect CO2 emissions from the atmospheric oxidation of CH4 was basically forgotten about by the IPCC when the original guidelines for GHG emission inventories were developed. However the IPCC has recently targeted the issue and is slowly moving to address it in future work.
What is the magnitude of this accounting discrepancy, you ask? Well, it is just under a percent of global emissions (on a GWP-weighted basis), which is not large. But, it is larger than a lot of other source categories we spend a lot of time worrying about. And, for countries with a larger share of fossil methane emissions it can be closer to 2%. More significantly, offset methodologies that fail to account for the effect in coal mine and natural gas projects may produce estimates that are off by 13%.
If you are interested in reading more on this subject, I wrote an academic paper a couple of years ago on it. The abstract is below as well as the link for the full article:
Gillenwater, Michael, “Forgotten carbon: Indirect CO2 in greenhouse gas emission inventories” Environmental Science and Policy, volume 11, issue 3, May 2008, Pages 195-203.
National governments that are Parties to the United Nations Framework Convention on Climate Change (UNFCCC) are required to submit greenhouse gas (GHG) inventories accounting for the emissions and removals occurring within their geographic territories. The Intergovernmental Panel on Climate Change (IPCC) provides inventory methodology guidance to the Parties of the UNFCCC. This methodology guidance, and national inventories based on it, omits carbon dioxide (CO2) from the atmospheric oxidation of methane, carbon monoxide, and non-methane volatile organic compounds emissions that result from several source categories. The inclusion of this category of “indirect” CO2 in GHG inventories increases global anthropogenic emissions (excluding land use and forestry) between 0.5 and 0.7 percent. However, the effect of inclusion on aggregate UNFCCC Annex I Party GHG emissions would be to reduce the growth of total emissions, from 1990 to 2004, by 0.2 percentage points. The effect on the GHG emissions and emission trends of individual countries varies. The paper includes a methodology for calculating these emissions and discusses uncertainties. Indirect CO2 is equally relevant for GHG inventories at other scales, such as global, regional, organizational, and facility. Similarly, project-based methodologies, such as those used under the Clean Development Mechanism, may need revising to account for indirect CO2.
If you don’t have a subscription to the journal, you can download the pre-publication “discussion paper” version below:
Gillenwater, M., 2007. “Forgotten carbon: Indirect CO2 in greenhouse gas emission inventories“, [Discussion paper] Science Technology and Environmental Policy Program. Princeton University, Princeton, NJ.