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Emissions exchange rates: how many tonnes of CO2 for a tonne of methane?

By Don McClatchy - posted Friday, 13 November 2009

Statements like “Agriculture accounts for 14 per cent of global GHG (greenhouse gas) emissions” are not nearly as authoritative or definitive as they may appear.

Let us assume that “agriculture” refers to just the activities of farmers, and does not include “agricultural product” processing, warehousing, transporting and retailing. Farming activities result mainly in emissions of three GHGs: CO2 (carbon dioxide) from the burning of fossil fuels (for example, by tractors); methane from rice paddies, effluent ponds and the “burping” (enteric fermentation) of ruminant farm animals (cattle, sheep and goats mainly); and nitrous oxide from the soil, deriving from urine patches and nitrogenous fertiliser application.

Let us also assume here that all these emissions can be measured with a reasonable degree of accuracy, in tonnes. The three main gases must be added together to determine what agriculture’s “total” GHG emissions are. But not by weight, - this would be meaningless in terms of their contributions to global warming. Such aggregation requires “exchange rates” of “CO2 equivalence” (in terms of effect on global warming), for each GHG other than CO2, which is taken as the reference point.


How are these “exchange rates” determined? Are they really cast in stone? Regardless of the price of carbon, how many tonnes of CO2 (or “carbon credits/debits”) is a tonne of methane, for example, worth? If the atmospheric warming effects of different gases can not be aggregated easily, and in a way that is scientifically robust, is it really necessary that we attempt it? This comment focuses particularly on the methane/CO2 exchange rate, which is of particular concern in countries where ruminant livestock farming and paddy rice are relatively important.


International negotiators, in their efforts to rid the world of “bad” things like trade barriers and greenhouse gas emissions, love numbers - more specifically, single numerical indicators of aggregate performance. They love them because such numbers make it easier for Ministers to explain, and gain public support for, negotiation outcomes. Summary numerical measures facilitate comparisons of the levels of different countries’ transgressions. At times they may even be used to indicate that countries are “paying” for them fairly, equally or proportionately (through the use of negotiated formula reductions).

In the last (Uruguay) Round of multilateral trade negotiations the AMS (aggregate measure of support) was chosen, from among several, as a single numerical indicator of each country’s “domestic support” for agriculture. It had already been agreed that “domestic support” programs, which come in a wide variety of shapes and sizes, significantly distort trade and must be reduced. Having chosen the indicator, the focus of that part of the overall negotiations then moved on to the percentage by which it needed to be reduced by each country, and the time frame for this. Individual countries would have the choice of which programs they cut in order to achieve the required reductions in their overall AMS.

Perhaps with one eye on the Uruguay Round precedent, negotiators of the Kyoto Protocol on climate change in the 1990s also sought a single aggregate measure or “metric”. Luckily (it seemed at the time) such a beast, called GWP (global warming potential), had already been around for a few years, and was proposed by the IPCC (Intergovernmental Panel on Climate Change). Recognised “greenhouse gases”, with very different chemical properties, and atmospheric lives, had all been lumped together in this single indicator. This was possible because some physical scientists, experts in “radiative forcing”, had assigned a value of “CO2e” (carbon dioxide equivalence) to each gas. For example, 1 tonne of methane was determined to be the equivalent of about 25 tonnes of CO2 in its global warming impact. But methane only exists in the atmosphere for an average of about 10 years, while some other important gases, like CO2 and nitrous oxide, “live” for more than 100 years. Apples and oranges indeed!

Concerns about the single GWP metric

No surprise then that over the last decade or so considerable anxiety has been expressed in the academic journals about the shortcomings of the GWP. Several alternatives have been put forward - a 2005 example being Dr Keith Shine’s (University of Reading) GTP (global temperature change potential).

Under the GTP, and still assuming the rather arbitrary 100 year horizon used by GWP (any harmful effects after 100 years not counted), 1 tonne of methane is calculated to be the equivalent of only 4 tonnes of CO2. In other words, if GWP were to be replaced with GTP as the chosen summary indicator, the principal emission from many countries’ agriculture sectors would become only one sixth as serious as previously thought!


In Brazil, in 1994, methane emissions were counted (using GWP) as more important than CO2 in the country’s total emission profile, but would fall to being only 17 per cent of CO2 emissions if “GTP” were used! These numbers illustrate rather dramatically the degree of scientific doubt associated with this single methane/CO2 exchange rate.

International disquiet about the appropriateness of GWP as the negotiating metric led to an Expert Meeting (PDF 146KB) being held, under the auspices of the IPCC, in Norway earlier this year. The key conclusions of the meeting recognised that there are several shortcomings with GWP, that it “was not designed with any particular policy goal in mind”, and that “depending on the specific policy goal or goals, alternative metrics may be preferable”.

Logically, the choice of goal should precede the choice of indicator. Several climate change policy goals are still under discussion, including limits on global temperature rise, or on the rate of growth of that rise, on the concentration of one or more greenhouse gases in the atmosphere, and others.

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About the Author

Dr Don McClatchy is a retired agricultural and trade policy consultant.

Creative Commons LicenseThis work is licensed under a Creative Commons License.

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