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Tropospheric Methane Concentrations and Planetary Health
Volume 10, Number 1: 3 January 2007

The Intergovernmental Panel on Climate Change (IPCC) long predicted that earth's tropospheric methane (CH4) concentration would rise dramatically throughout the 21st century; see, for example, Ehhalt and Prather (2001). So entrenched in both the popular and scientific psyches did this dogma thus become, that when real-world data suddenly suggested otherwise, Simpson et al. (2002) continued to vehemently contend it was "premature to believe that the CH4 burden is ceasing to increase," even though their own data bore witness against them in demonstrating that such was in fact occurring, as we indicated in our Editorial of 8 Jan 2003).

Once glimpsed, however, the truth cannot long be held at bay. Consequently, it was only a few months later that Dlugokencky et al. (2003) reported that the globally-averaged tropospheric methane concentration had been "constant at ~1751 ppb from 1999 through 2002." This observation led them to proclaim that "during this 4-year period the global methane budget has been at steady state," and now, three years later (and with an additional five years of data under their belt), Simpson et al. (2006) have been obliged to admit that this is indeed the case.

Whereas Simpson et al. (2002) had cautioned "against viewing each year of high CH4 growth as an anomaly against a trend of declining CH4 growth," Simpson et al. (2006) did a complete turnabout and acknowledged just the opposite, i.e., that the anomalous CH4 growth spikes to which Simpson et al. (2002) had referred were indeed "superimposed on an overall slowdown of the CH4 growth rate." In fact, the slowdown documented by Simpson et al. (2006) was so significant that they report that "the global growth rate of atmospheric CH4 has been near-zero for the past seven years, averaging 0.7 2.6 ppbv year-1." What is more, they report that "opportunities exist for still further reductions," noting that "CH4 levels may decrease if various CH4 emission mitigation strategies are implemented." In this regard, they further note that "the reduction of fossil fuel leakage has promise," and that "because the leaking fossil fuels have high value in the market, these mitigation steps can in some cases even be economically favorable."

This change in outlook portends much the same outcome as what we proposed in our Editorial of 8 Jan 2003, wherein we opined that "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 [so that] 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."

The ramifications of the playing out of this scenario are huge. Because "methane has been second only to carbon dioxide in enhanced climatic forcing from 1750 to the present," in the words of Simpson et al. (2006), a continuously-negative atmospheric CH4 growth rate could allow the air's CO2 content to continue to rise with little to no net increase in the radiative forcing of climate, while at the same time allowing the aerial fertilization and anti-transpirant effects of atmospheric CO2 enrichment to grow in magnitude and thereby enhance the water and nutrient use efficiencies of both natural and managed ecosystems. In addition, because methane is "an important source of tropospheric ozone," as related by Simpson et al. (2006), the declining CH4 concentration would alleviate much of the damage to earth's vegetation that is routinely caused by this most debilitating of air pollutants.

In concluding, we note that all of these impressive changes for the better could occur without the imposition of any mandated restrictions on anthropogenic CO2 emissions. So let's let nature and the market take their unfettered courses, as they appear to be poised to accomplish, almost automatically, what the Kyoto Protocol and its ilk have no hope of ever even pretending to do.

Sherwood, Keith and Craig Idso

References
Dlugokencky, E.J., Houweling, S., Bruhwiler, L., Masarie, K.A., Lang, P.M., Miller, J.B. and Tans, P.P. 2003. Geophysical Research Letters 30: 10.1029/2003GL018126.

Ehhalt, D.H. and Prather, M. 2001. Atmospheric chemistry and greenhouse gases. In: Climate Change 2001: The Scientific Basis, Cambridge University Press, New York, NY, USA, pp. 245-287.

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.

Simpson, I.J., Rowland, F.S., Meinardi, S. and Blake, D.R. 2006. Influence of biomass burning during recent fluctuations in the slow growth of global tropospheric methane. Geophysical Research Letters 33: 10.1029/2006GL027330.