What is different about methane (CH4) emissions? The forgotten CO2 in GHG emissions accounting

July 13, 2010, by Michael Gillenwater

In this third blog post on greenhouse gas (GHG) emissions and Global Warning Potential (GWP) values, I want to focus on a unique characteristic of methane (CH4).

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 inventoriesEnvironmental 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.

8 responses to “What is different about methane (CH4) emissions? The forgotten CO2 in GHG emissions accounting”

  1. Andrew Green says:

    Thanks Michael – I was indeed unaware!

  2. Prakash J. Lakhapate says:

    Good article and an eye opener.
    I have the following query.
    CH4 is considered to have 21 times CO2 equivalent.
    What is the basis for this?

  3. Prakash,

    You can refer to a previous blog post here for information on GWPs as well as links to the originating IPCC reports that describe in detail how GWPs are defined and estimated.


  4. Jane says:

    But the C from CH4 from biomass burning of long-lived vegetation, such as peat or trees, should be counted, shouldn’t it? Just not the biogenic CH4 where the cycle is very short, I would think. Am I wrong? What about CH4 released from flooded lands, such as when reservoirs or dams are built?

  5. Jane,

    The question of whether biogenic (versus fossil) carbon, when added to the atmosphere, should be “counted” depends on whether we know (or assume) it is “offset” by CO2 uptake by new biomass growth. We know the CO2 concentration in the atmosphere goes up in the fall (of the northern hemisphere) each year by a significant amount. But we don’t count that because then it goes back down by a roughly equivalent amount in the summer as new growth pulls CO2 from the atmosphere. Similarly, if burn corn husks then regrow the corn next year in a repeated cycle, that would not be adding any CO2 to the atmosphere on a net basis (although it might be adding other pollutants). In the case of trees, we are just talking about a slightly longer time frame because trees take longer to regrow, but the same concept applies.

    Now peat is an interesting case. It is an example of something sort of inbetween what we think of as biogenic and fossil. Peat takes a long time to replenish (from my understanding) so it borders on not something that would not fit my definition of biogenic in this context. We have to think in terms of the time scale of the questions we are dealing with. In terms of climate change we are talking about decades. Trees can regrow in that amount of time (assuming you allow them to). But peat bogs, from my understanding, can take longer to recover their carbon sequestration function.

    But we are just talking direct CO2 fluxes so far. For the indirect additions of CO2 to the atmosphere from methane, you can just think of the fact that the carbon starts out as methane delaying how quickly the carbon can be cycled back into biomass because if first has to live one lifetime in a methane molecule. I am not saying that just because the carbon originates from biomass that we can definitely ignore it. Obviously if the carbon comes from biomass, the biomass is burned and methane is emitted, then that methane will be oxidized to CO2 in the atmosphere, and then if new biomass is not regrown to replace the original biomass, then even this indirect CO2 from biogenic sources would be a net addition to the atmosphere (even though it is not from a fossil source).

    None of this is really an accounting problem, however. Because we typically assume that all of the carbon (except that remaining solid as ash, etc.) is converted to CO2 when things burn. In reality, some of this carbon is really converted first to methane, CO, and NMVOCs. But as I said, these are later oxidized to CO2 in the atmosphere. So for combustion (biomass and fossil fuels) we take care of this issue in GHG emission inventories. It is only for methane sources other than from combustion it is a problem.

    CH4 from flooded lands is an example of biogenic carbon that is not replaced. But again, we can just assume all that carbon is oxidized (except for that which remains in the lake).

  6. Dusan Vacha says:

    Hello Michael, I do not understand why indirect emissions of CO2 and N2O are differently treated in IPCC 2006 Gl. For indirect N2O emissions, methodology is provided and emissions estimates are included in the N2O trend tables. CO2 indirect emissions are only mentioned (and mixed with precursors inventory) with the remark that “The 2006 Guidelines estimate carbon emissions in terms of the species which are emitted.” What’s the difference and reason for that? Thanks, Dusan

  7. Archie Quinn says:

    Very good article and well researched; there may be some well constructed thought having gone into the lack of submission as this is a measurable quanta, my first thought is that we would have to remove methane (CH4 )as a list component to negate any double whammy effect on carbon emissions calculations. That being said, I do see your point regarding the amount created being far in excess of the value placed on the methane; however, being a basic logic thinker in puzzle solutions it would also appear that according to the same sources that define a destruction of natural habits as being the same sources on which we rely as accurate concerning the effects of and dissemination of CO2, we must therefore use those same calculations backward to the start of the industrial revolution and say that our planet has x cubic metres of plant life as carbon sink, less the amounts noted by our green brothers and sisters as being destroyed per cubic metre per day. This massive total is why we have carbon counting and the prelude to carbon trading; however, my puzzle master of a brain says you cannot have one without the other. The loss of this massive carbon sink also equates to the identical equal cubic metres of biomass that is “not now decaying” annually and producing the methane to which you refer. My point being that in fact before man came, the “methane level” (not CO2) was far greater than it is today, because the biomass level decaying on a daily basis from the uncultivated and unstripped planet. I am not saying your point and math is not correct in respect to methane, my point is that you are not considering that this form of Carbon has existed previously to mankind in far greater levels. This is shown in the ice core samples that show the massive increase in CO2 prior to every ice age when the planet was at the most lush and green hothouse conditions. True, we can no longer measure what is from us and what the natural level should be, but I would stand in common sense and logic that no amount of crops, animals or biomass produced by us now would equate to the extraordinary volume of missing biomass by natural decay that we have stripped from our planet. Your math is correct in methane to carbon, however you fail to consider the minimal gross volume of the methane compared to what it should be or was, the additional, and we need to repeat that word, “additional” carbon is what has caused our problem. I do not feel that any production of a natural material or organic matter in plant or animal should be considered as having contributed to global warming, for the legal definition of contributing is “adding to” or additional whereas the math for which you are so good at shows that natural bio organic methane levels, must by sheer loss of biomass be lower than the would otherwise be without other pollutants. Remove all mankind polluters other than methane producing industry from the point of the industrial revolution and our CO2 levels would by logic be lower with the loss of such vast forest areas, in short or simple form, you cannot cut down 10,000 acres of self decaying biomass, and say an acre of crops for ethanol creates more, and according to the used data regarding forest destruction that is about the ratio. Your point is proven, and your research into the eventual result of the methane is correct, however we need to get rid of the additional non methane polluters to make a difference, not punish farmers and green energy companies by greater carbon tax costs. I think you are a genius to have worked it out and published it, I just wish you would direct your efforts to the bad guys not the little guys.

  8. Jessica says:

    Hello Michael, I am also interested in this. (Take a look at my most recent blog post for details: http://www.scrapcarz.co.uk/free-collection/hampshire/southampton/winchester.) This makes for very engaging reading, you have most certainly provided me with some food for thought!

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