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

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.

Background

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.

Several speakers noted that there are economic and policy dimensions to the choice of metrics, inferring that it is not a task which should be left to physical scientists alone. Despite recognising these problems with GWP, the meeting did not recommend discarding it, instead just encouraging further research into the merits of alternative metrics as information about future policy goals becomes available. Apparently, no alternative single measure stood out as being undeniably superior to GWP. There may also have been pressure from above for the meeting rapporteurs not to “upset the apple cart”, by writing anything which might risk stalling the ongoing negotiations. Perhaps surprisingly, the Meeting Report did not address the question of whether the use of more than one metric - say one for short-lived greenhouse gases and another for long-lived ones - would be more appropriate in the climate change negotiations.

The IPCC, in its “First Assessment Report” (1990), at first tentatively embraced the GWP concept, according to Dr Shine who was one of its authors. He recently quoted from that report (in “The global warming potential - the need for an interdisciplinary retrial”, Climatic Change (2009)):

It must be stressed that there is no universally accepted methodology for combining all the relevant factors into a single (metric) … A simple approach (i.e. the GWP) has been adopted here to illustrate the difficulties inherent in the concept.

Subsequently, these cautions seem to have been overlooked. IPCC endorsed GWP, and this provided a mechanism for the Kyoto Protocol to be a multi-gas treaty, based on a single, all-embracing metric.

It is with a sense of urgency that the world’s governments are trying to negotiate a successor agreement to the Kyoto Protocol, which will be more comprehensive and effective in reducing global GHG emissions and thus slowing global warming. The way agricultural emissions (particularly of methane) are addressed in such an agreement has the potential to create a serious political snag to its acceptance in many countries. If the use of “GWP” is retained, farmers organisations can be expected to argue that, with all the uncertainty surrounding its logical and scientific basis, it is unacceptable to them. This would risk jeopardising the whole agreement.

What is the way forward?

If Kyoto II follows Kyoto I in establishing just single declining limits on each country’s overall GHG emissions, then individual country governments will have the option of reducing emissions in non-agricultural sectors more, in order to partially or completely let agriculture off the hook. This may be what politically powerful farmers’ organisations in many countries are counting on happening. If some major countries decide to exempt their agricultural sector from emission reductions to some extent, then it will be politically very difficult for other governments not to follow suit, despite the implied added costs to non-agricultural sectors and to their taxpayers. Such an outcome would put predominantly agricultural countries, including many developing countries, in a very difficult position, so they will then favour minimal targets for emission cuts.

A second possibility is that it will be agreed internationally to negotiate reductions in agricultural emissions separately, following a precedent for special sector treatment established more than 50 years ago in the first GATT agreement. There will be arguments for this based on food security concerns, and the essential nature of farm production. This would imply an international acceptance of lower reductions in agricultural emissions. Problems associated with the choice of a multi-gas metric would remain, however, providing grounds for different groups within agriculture, and in the non-agricultural sectors, to dispute whether they were being fairly treated.

In both of these first two cases where the use of a multi-gas metric is to be retained, there would appear to be a strong logical case for first negotiating the particular climate target which countries are collectively trying to achieve, and then choosing a metric designed for that target. Choice of more than one target may well imply the choice of more than one metric. Replacement of GWP with one or more other, more focused metrics would also be implied.

Perhaps a more logical alternative to the second possibility above would be to negotiate commitments on the principal GHGs separately. Then the problems associated with the use of a single metric could be avoided altogether. Justification for different targets for different gases could take into consideration the availability of alternative technologies for emissions reductions or avoidance in each case, while still implicitly taking into account the food security arguments. As a simplification of this, it may prove acceptable to classify GHGs into groups - say into “very-short-lived”, “short-lived” (including methane) and “long-lived” (including CO2 and nitrous oxide) GHGs - and to negotiate reduction commitments applying to each group.

But it is certainly time that these issues deriving from the choice and use (until now) of GWP were brought to the fore in the negotiations. They can surely be overlooked no longer, if problems of selling the climate negotiations to the world’s farmers are to be overcome.