Volume 12, Number 35: 2 September 2009
Isoprene (C5H8 or 2-methyl-1,3-butadiene) is a highly reactive non-methane hydrocarbon (NMHC) that is emitted in copious quantities by vegetation and is responsible for the production of vast amounts of tropospheric ozone (Chameides et al., 1988; Harley et al., 1999), which is a debilitating scourge of plant and animal life alike. In addition, it has been calculated by Poisson et al. (2000) that current levels of NMHC emissions -- the vast majority of which are isoprene, accounting for more than twice as much as all other NMHCs combined -- may increase surface ozone concentrations by up to 40% in the marine boundary-layer and 50-60% over land, and that the current tropospheric ozone content extends the atmospheric lifetime of methane -- one of the world's most powerful greenhouse gases -- by approximately 14%. Thus, one could conclude that anything that reduces isoprene emissions from vegetation will also reduce the negative consequences of these other undesirable trace components of the atmosphere. But that conclusion may or may not be correct, according to the recent analysis of Young et al. (2009).
The five researchers -- hailing from Finland, New Zealand, Sweden, the UK, and the USA -- used a chemistry/climate model that employed isoprene emissions calculated from the vegetation-isoprene emission model of Arneth et al. (2007) to investigate the impact of the isoprene/CO2 effect (described in the figure below) on "tropospheric composition predictions for the late 21st century."
Figure 1. Field and laboratory observations of leaf isoprene emissions from plants grown in a variety of atmospheric CO2 concentrations (Ca), normalized to a value of unity at Ca = 370 ppm. Adapted from Young et al. (2009).
The results of this exercise, in Young et al.'s words, "resulted in opposing responses in polluted (O3 decreases of up to 10 ppb) vs. less polluted (O3 increases of up to 10 ppb) source regions," with the globally-averaged response to less isoprene emissions in the future being "an increase in ozone, rather than the decrease noted by some other studies (Sanderson et al., 2003; Hauglustaine et al., 2005)."
So why the unexpected and large differences? Although Young et al. say they "tried to rationalize the differences by appealing to differences in the model isoprene schemes," they concluded that current models are simply too "limited in their ability to represent isoprene chemistry accurately," and that "new mechanisms need to interface with new laboratory measurements for evaluation," further suggesting that "the net effect of interactions between BVOC [biogenic volatile organic compound] emissions, tropospheric ozone and plant productivity are as yet unresolved." And on top of all of these problems, they state that their work highlights only "one of the many uncertainties in the overall response of future BVOC emissions, atmospheric chemistry and climate."
Clearly, there remains a tremendous amount of work to be done in studying a number of different facets of just this one important area of inquiry that needs to be more thoroughly researched before we can reliably calculate the ultimate climatic response of the earth to anthropogenic CO2 emissions; and there are many other areas of research where equally significant uncertainties -- and even unknowns -- persist. These considerations should temper the enthusiasm of people who may be inclined to believe that we need to act now, and in a most draconian way, to "save the planet" from the catastrophic thermal outcome suggested by such ill-defined computational schemes as those parameterized -- or not even included -- in today's state-of-the-art climate models.
Sherwood, Keith and Craig Idso
References
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