Simpson et al. (2002) present annual global tropospheric methane (CH4) growth-rate data for the period 1983-2000, based on measurements made by the Department of Chemistry of the University of California at Irvine. What the measurements show, however, is open to debate -- a debate the authors clearly want to squelch. Several earlier studies, for example, had suggested that the past two centuries' dramatic increase in the atmosphere's methane content, 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); but in their publication, Simpson et al. seem hell-bent on driving home a very different message.
Quoting their strident 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 project the past two decades' behavior of the atmosphere's methane 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 the concentration of this important greenhouse gas; and Simpson et al. cannot seem to tolerate that conclusion. In contrast, they promote the radically different approach of simply specifying a variety of alternative scenarios, which have little foundation in observational 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 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 refer to adnauseam in an attempt to justify - in an insidious perversion of the precautionary principle - ever more restrictive and coercive energy policies that could stifle global economic progress for decades to come and prevent many of the world's poorer nations from escaping the crushing poverty that currently thwarts their efforts to progress and improve the environment.
Of course, if the data truly justify what Simpson et al. say -- over and over, again and again, in their baker's dozen pronouncements -- they could in fact be right. 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 think their own thoughts about it, after which we provide our perspective 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 especially 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 growth-rate 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 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 linked to the remarkably strong El Niño 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 in such a way as 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 methane.
Projecting ahead, therefore - in spite of Simpson et al.'s warning to refrain from doing so - 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 may have 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 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.
Subsequent to the publication of Simpson et al.'s study, Dlugokencky et al. (2003) revisited the subject with an additional two years' of data. Based on measurements from 43 globally-distributed remote boundary-layer sites that were obtained by means of the methods of Dlugokencky et al. (1994), they defined an evenly-spaced matrix of surface CH4 mole fractions as a function of time and latitude, from which they calculated global CH4 concentration averages for the years 1984-2002. We have extracted their results from their graphical presentation and re-plotted them as shown in the figure below, where it can be seen that they fall into three natural groupings: initial and latter stages, to which we have fit linear regressions, and an intervening middle stage, to which we have fit a second-order polynomial.
Figure 2. Global tropospheric methane (CH4) concentration vs. time. Adapted from Dlugokencky et al. (2003).
With respect to these data, Dlugokencky et al. note that the globally-averaged atmospheric methane concentration "was constant at ~1751 ppb from 1999 through 2002," which suggests, in their words, that "during this 4-year period the global methane budget has been at steady state." They caution, however, that "our understanding is still not sufficient to tell if the prolonged pause in CH4 increase is temporary or permanent." We agree. However, based on the fact that the data describe an initial stage of significant yearly CH4 increase, a subsequent stage of much smaller yearly CH4 increase, and a latter stage of no yearly CH4 increase, we feel confident in suggesting that if the recent pause in CH4 increase is indeed temporary, it will likely be followed by a decrease in CH4 concentration, since that would be the next logical step in the observed progression from significant, to much smaller to no yearly CH4 increase.
Why is it important to have a reasonably accurate representation of the likely future course of atmospheric methane concentration? It is important because, as Dulgokencky et al. report, "atmospheric methane's contribution to anthropogenic climate forcing is about half that from CO2 when direct and indirect components to its forcing are summed (Hansen and Sato, 2001)." In addition, they note that "all CH4 emission scenarios considered by the IPCC Special Report on Emission Scenarios (Nakicenovic et al., 2000) resulted in increasing atmospheric CH4 for at least the next 3 decades, and many of the scenarios projected large increases through the 21st century (Prather et al., 2001)." Hence, if we are correct in our assessment of the future course of atmospheric CH4 concentration, the IPCC is way off base on this matter, and there will be a significant natural amelioration of a large portion of the many deleterious consequences the IPCC continually projects in its never-ending campaign to justify a host of international treaties, laws and regulations designed to reduce the potential for both CO2- and CH4-induced global warming.
So what has been responsible for the recent dramatic slowdown -- and possible ultimate cessation -- of the post-Little Ice Age upward trend in the air's CH4 concentration? We believe that some significant portion of the welcome development can be attributed to the cumulative effect of a number of indirect impacts of the concomitant increase in the air's CO2 concentration, as described in several of our Subject Index Summaries.
In the Isoprene section of our Subject Index, we describe how elevated levels of atmospheric CO2 may lead to a reduction in global vegetative isoprene emissions, which are responsible for creating a sizeable portion of the troposphere's burden of ozone, the presence of which tends to lengthen the lifetime of atmospheric methane. Hence, less isoprene production leads to less ozone production, and less ozone in the air leads to less methane in the air.
In the Tannins section of our Subject Index, we describe how elevated levels of atmospheric CO2 - and possibly temperature - may lead to increased foliar concentrations of condensed tannins, and how the eating of such vegetation by the world's ruminants -- the agricultural component of which is primarily represented by cattle and sheep -- reduces the amount of methane they release to the atmosphere.
In the Methane (Agricultural Emissions) section of our Subject Index, we additionally describe how elevated levels of atmospheric CO2 might possibly be reducing methane emissions from certain types of rice culture, and how simple fish-oils added to the feed of domestic ruminants may reduce their methane emissions in much the same way that increases in forage tannin concentrations do.
Last of all, in the Methane (Extractions from the Atmosphere) section of our Subject Index, we describe how warming-induced shifts in the latitudinal location of different forest types, together with CO2-induced changes in forest litter characteristics, likely increase the rate at which methane is removed from the atmosphere via oxidation by bacteria living in the aerobic zones of forest soils.
Other phenomena are undoubtedly helping to reduce the air's methane concentration as well; and it will be exciting to see, in the days and years ahead, if their combined influence will actually lead to a sustained downward trend in the concentration of this important greenhouse gas. Such a result would be like having one's cake and eating it too; for it would enable the planet to reap the great biological benefits that come from atmospheric CO2 enrichment without creating a significant net increase in the atmosphere's greenhouse effect.
References
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Dlugokencky, E.J., Houweling, S., Bruhwiler, L., Masarie, K.A., Lang, P.M., Miller, J.B. and Tans, P.P. 2003. Atmospheric methane levels off: Temporary pause or a new steady-state? Geophysical Research Letters 30: 10.1029/2003GL018126.
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