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Methane Madness
Volume 6, Number 2: 8 January 2003

In a recent paper published in the American Geophysical Union's Geophysical Research Letters, Simpson et al. (2002) present annual global tropospheric methane (CH4) growth rates for the period 1983-2000, based on measurements taken by their group from the Department of Chemistry at the University of California in Irvine. So what do their measurements show?

What the measurements show is open to debate - a debate the authors clearly want to squelch. Several recent studies, for example, have suggested that the dramatic rise of the atmosphere's methane concentration over the past two centuries or so, which was sparked by the Industrial Revolution, is about to come to an end (Dlugokencky et al., 1998; Francey et al., 1999; Lassey et al., 2000). In their new publication, however, Simpson et al. appear to be hell-bent on driving home a very different message.

Quoting their dogmatic statements in the order in which they appear in their paper, they say: (1) "the CH4 growth rate fluctuates in an unpredictable fashion," (2) "global CH4 concentrations cannot be extrapolated into the future based on past trends," (3) "the slowing of the CH4 growth rate during much of the 1980s and 1990s cannot be used to imply that CH4 will no longer be of concern in greenhouse gas studies during this century," (4) "the global concentration of CH4 has varied in an unpredictable fashion," (5) "we suggest that it is premature to believe that CH4 increases will no longer be of concern in greenhouse gas studies during this century," (6) "future CH4 mixing ratios cannot be extrapolated from past trends," (7) "it is premature to believe that the CH4 burden is ceasing to increase," (8) "the decoupling of the sources of past increases from the sources of present and future increases makes attempting to predict future CH4 mixing ratios based on past changes questionable," (9) "trends in CH4 sources and sinks are changing and unpredictable," (10) "upcoming variations in the global CH4 concentration cannot be estimated in advance," (11) "it is important to continue to allow for changes in the global CH4 concentration, and not assume that CH4 concentrations will cease to grow much above current levels," (12) "future CH4 concentrations cannot be predicted based on past growth rate trends," and (13) "the slowing of the CH4 growth rate during much of the 1980s and 1990s cannot be used to indicate that CH4 will no longer be of concern in greenhouse gas studies."

Why are Simpson et al. so intent on browbeating us into believing we cannot extrapolate the historic trend of the atmosphere's CH4 concentration into the future? Clearly, as we shall demonstrate shortly, it is because any rational projection suggests there will be very little - if any - future increase in this important atmospheric property; and Simpson et al. cannot seem to tolerate that conclusion. In contrast, they suggest the radically different approach of specifying a variety of alternative scenarios, which have little foundation in fact, approvingly noting that "the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios features a wide range of global CH4 emission scenarios, some of which have large CH4 growth rates throughout the 21st century." [Our italics.]

Is this a smoking gun or what? Just as has been the case with periodic projections of future atmospheric CO2 concentrations throughout the IPCC's checkered history, the next major report of that ideologically-driven body (due out in 2007) will - if the likes of Simpson et al. have their way - contain a humongous atmospheric CH4 growth rate scenario (among a variety of others, of course) that politicians can readily use to create concern about a totally unrealistic upper-bound increase in mean global air temperature, which they can then refer to adnauseam in an attempt to justify - in an insidious perversion of the precautionary principle - ever more stringent and coercive energy policies that could well stifle global economic progress for many decades to come and prevent much of the world's poorer nations from escaping the poverty that currently besets them.

Of course, if the data truly justify what Simpson et al. say - over and over, again and again, in their baker's dozen pronouncements - then they indeed are right and we are wrong. So what do the data suggest?

We feel that most people are totally capable of drawing their own conclusions about this matter; and, hence, we have redrawn the key graph of Simpson et al.'s paper, which we reproduce in the figure below, thereby affording everyone the opportunity to do just that, after which we provide our perspective on the subject for comparison.


Figure 1. Global tropospheric methane (CH4) growth rate vs. time. Adapted from Simpson et al. (2002).

With respect to the data of Figure 1, and particularly the data from the 1990s, Simpson et al. say "we caution against viewing each year of high CH4 growth as an anomaly against a trend of declining CH4 growth." Yet that is precisely what the data suggest, to us at least, i.e., a declining baseline upon which are superimposed periodic anomalous CH4 concentration spikes.

In this interpretation, we are not alone. The first of the 1990s' large CH4 spikes is widely recognized as having been caused by the sudden eruption of Mt. Pinatubo in June of 1991 (Bekki et al., 1994; Dlugokencky et al., 1996; Lowe et al., 1997); while the last and most dramatic of the spikes has been associated with the remarkably strong El Nio of 1997-98 (Dlugokencky et al., 2001). Furthermore, as noted earlier, Dlugokencky et al. (1998), Francey et al. (1999) and Lassey et al. (2000) have all felt confident in interpreting the data to suggest that the annual rate-of-rise of the atmosphere's CH4 concentration is indeed declining and leading to a cessation of growth in the atmospheric burden of CH4.

Projecting ahead, therefore, if anomalous events such as those recorded in the 1990s continue to occur at similar intervals, the global atmospheric CH4 concentration should continue to rise - but only very slowly - for just a few more years, after which the declining background CH4 growth rate, which has already turned negative, will have dropped low enough to have the capacity to totally overwhelm any short-term positive impacts of periodic anomalous CH4 spikes. Then we should be able to see an actual decline in the atmosphere's global CH4 concentration, which should gradually accelerate in the negative direction, as subsequent anomalous CH4 spikes fail to penetrate into positive territory.

This projection, in our opinion, is not only plausible, but of all possible projections, it is the one most likely to ultimately be found to be correct, based on what we feel is the most rational interpretation of the data of Figure 1. Let us hope we are correct; for if we are, the declining atmospheric CH4 concentration that will occur over the greater part of the 21st century will be able to offset a large portion of the greenhouse effect produced by rising concentrations of carbon dioxide.

This result, in a way, is like having one's cake and eating it too; for it enables the planet to reap the great biological benefits that come from atmospheric CO2 enrichment without creating a net increase in the atmosphere's greenhouse effect.

Now that's a real win-win situation!

Sherwood, Keith and Craig Idso

References
Bekki, S., Law, K.S. and Pyle, J.A. 1994. Effect of ozone depletion on atmospheric CH4 and CO concentrations. Nature 371: 595-597.

Dlugokencky, E.J., Dutton, E.G., Novelli, P.C., Tans, P.P., Masarie, K.A., Lantz, K.O. and Madronich, S. 1996. Changes in CH4 and CO growth rates after the eruption of Mt. Pinatubo and their link with changes in tropical tropospheric UV flux. Geophysical Research Letters 23: 2761-2764.

Dlugokencky, E.J., Masarie, K.A., Lang, P.M. and Tans, P.P. 1998. Continuing decline in the growth rate of the atmospheric methane burden. Nature 393: 447-450.

Dlugokencky, E.J., Walter, B.P., Masarie, K.A., Lang, P.M. and Kasischke, E.S. 2001. Measurements of an anomalous global methane increase during 1998. Geophysical Research Letters 28: 499-502.

Francey, R.J., Manning, M.R., Allison, C.E., Coram, S.A., Etheridge, D.M., Langenfelds, R.L., Lowe, D.C. and Steele, L.P. 1999. A history of 13C in atmospheric CH4 from the Cape Grim Air Archive and Antarctic firn air. Journal of Geophysical Research 104: 23,631-23,643.

Lassey, K.R., Lowe, D.C. and Manning, M.R. 2000. The trend in atmospheric methane 13C and implications for constraints on the global methane budget. Global Biogeochemical Cycles 14: 41-49.

Lowe, D.C., Manning, M.R., Brailsford, G.W. and Bromley, A.M. 1997. The 1991-1992 atmospheric methane anomaly: Southern hemisphere 13C decrease and growth rate fluctuations. Geophysical Research Letters 24: 857-860.

Simpson, I.J., Blake, D.R. and Rowland, F.S. 2002. Implications of the recent fluctuations in the growth rate of tropospheric methane. Geophysical Research Letters 29: 10.1029/2001GL014521