After writing about the lingering terror that is the potential for Arctic methane releases yesterday, I discovered a seven-page paper that discusses some interesting aspects of that gas as a forcing agent. The paper is Global Warming: The Significance of Methane [small PDF], and it “was peer-reviewed and published in the scientific magazine ‘La Recherche’ in March 2008.

The authors provide a background on the practice of measuring non-CO2 greenhouse gases in terms of CO2 equivalent units, based on their GWP (global warming potential), which is reason enough for an armchair climate scientist to read the paper. But they go into some detail on what this use of GWP and CO2 equivalence means, including (pages 2-3, emphasis added):

Whereas the First Conference of the Parties (COP 1 1995) merely stated that “Parties may use global warming potentials to reflect their inventories and projections in carbon-dioxide equivalent terms. In such cases, the 100-year time-horizon values provided by the Intergovernmental Panel on Climate Change in its 1994 Special Report should be used”, the use of GWPs over a 100-year period very quickly became the norm. The pulse emission of 1 t of CH4 in 2000 is counted as 21 t CO2 eq2 on the basis of the cumulative effects respectively of CH4 and CO2 between 2000 and 2100, and the emission of 1 t of CH4 in 2020 for example is counted as 21 t CO2 eq on the basis of the cumulative effects respectively of CH4 and CO2 between 2020 and 2120: the impacts of a CH4 emission compared to those of an emission of the same volume of CO2 are each year put back 100 years.

Adopting such a rule has significant consequences on the relative assessment of the role of the different GHGs. While the use of the concept of CO2 equivalent, as previously shown, does not present any ambiguity to estimate concentrations, using it to estimate emissions necessarily implies that a reference is made to an integration period from when the emission is made.

As the atmospheric lifetime of CH4 is short compared to that of CO2, the GWP of CH4 varies considerably depending on the period of time chosen. With the rule of the equivalence coefficient being 21 (GWP over a 100-year period following the date of emission), it is therefore impossible to estimate the impact at a given time horizon (2020, 2050, 2100) of a CH4 emission. To make this estimate, it is necessary to take into account the difference between the year of emission and the year of the time horizon since the equivalence coefficient (the GWP) rapidly varies depending on the time period chosen to measure the respective impacts of CO2 and CH4 on global warming.

Furthermore, it is vital to bear in mind the fact that the GWP concept applies to climate impacts of a pulse emission at a given point in time. To apply it without caution to measures which continue over time in order to estimate the impact at a given time horizon may thus lead to serious errors of assessment.

On page 4 the authors show graphs for the amount of CO2 and methane in the atmosphere over time after a one-time release and the AGWP (absolute global warming potential) of those gases over time. Page 5 has a data table and graph depicting the GWP of methane for various time horizons, and the numbers are pretty scary: 101 at 5 years, 90 at 10 years, 80 at 15 years, 72 at 20 years, and so on, up to 18 at 150 years.

So, we have two distinct factors at play here:

The planning horizon effect, in which we artificially limit our view to the year 2100, which allows us to ignore 40% of the CO2-induced warming which will take place after that year and is caused by emissions released before that date. (I mentioned David Archer’s discussion of this fact in The CO2 Countdown Clock.) In the case of methane, we will see considerable warming effect well after the 100-year horizon, a point that I suspect many amateurs (like me) studying this material would not consider, based on methane’s average atmospheric lifetime of under 10 years.

The pulse vs. stream effect. This is the issue I alluded to yesterday when I talked about the impact of yearly methane releases from the Arctic hydrates and/or permafrost, and is addressed by the authors of this paper in the quote above and in a detailed example on page 5:

The example given below shows the order of magnitude of the assessment errors that are likely to be made by using “the 100 years equivalence”. We consider two measures to reduce CH4 and CO2 emissions:

a) firstly, in the year 0, putting a permanent end to the source of an annual emission of 1 kg of CH4 (which would continue if this measure were not implemented), ie 21 kg CO2 eq according to current methodology). We call this “CH4 measure”: from year 1, the CH4 emission avoided is thus 1 kg each year.

b) secondly, in the same year 0, putting a permanent end to the source of an annual emission of 1 kg of CO2 (which would be permanent if this measure were not implemented). We call this “CO2 measure”: from year 1, the CO2 emission avoided is thus 1 kg each year.

We calculate the compared impacts on global warming of each measure at different time horizons starting from the horizon year 1.

The respective cumulative effects of each emission avoided during the whole of the period between the year in which the measure was implemented and the horizon year is obtained by adding together the “absolute” GWPs of CH4 and CO2.

The ratio of the cumulative effects allows us to draw a comparison between a permanent CH4 emission reduction measure and a permanent CO2 emission reduction measure.

The resulting graph shows that the effective GWP of this series of methane releases begins around 100 and doesn’t decline to the IPCC value of 21 (for a 100-year horizon) until the year 250. The authors’ comment on these results is that:

At 20 and 50 year time horizons, the underestimated impacts of using the GWP of 21 is thus highly significant (respectively a factor of 3.9 and 2.7). It is still a factor of 1.9 at a horizon of 100 years and does not reach the value of 1 until 250 years have elapsed.

Consider these results in light of how hard it will be to significantly reduce anthropogenic methane emissions, and the potential for a sustained series of large natural methane emissions, and I think one conclusion and one question leap to mind:

We should be paying much more attention to methane than we have to date. CO2 emissions and their effect on the global climate are without doubt an immense and very serious problem and deserving of considerable attention, but we will be making an enormous mistake if we relegate methane to a minor role in our public policy.

Why isn’t this result (meaning the authors’ example illustrating pulse vs. stream effects) much more widely known, nearly 18 months after being published? Is there some flaw in their methodology or the implications they draw from their calculations? Or is this merely another example of how changes in the scientific mind set can take a surprisingly long time to become known and accepted?

I don’t often say this explicitly, but this is one time when I hope that the authors of a study got it very wrong.

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