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Isoprene Emissions from Vegetation in a CO2-Acreting Atmosphere: Observations vs. Models
Volume 10, Number 48: 28 November 2007

Why are we concerned about vegetative emissions of isoprene?

We are concerned because (1) isoprene 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, because (2) it has been calculated by Poisson et al. (2000) that current levels of NMHC emissions -- the vast majority of which are isoprene -- may increase surface ozone concentrations by up to 40% in the marine boundary-layer and 50-60% over land, because (3) the current tropospheric ozone content extends the atmospheric lifetime of methane -- one of the world's most powerful greenhouse gases -- by approximately 14%, because (4) anything that reduces vegetative isoprene emissions tends to ameliorate these several problematic phenomena, because (5) several studies have indicated that atmospheric CO2 enrichment reduces plant isoprene emissions, and because (6) the work of Monson et al. (2007) suggests, in their words, that "the current generation of models intended to predict the response of isoprene emission to future global change probably contain large errors," which clearly need to be corrected.

So what did Monson et al. do in this regard? ... and what did they learn?

First of all, they measured leaf isoprene emission rates (Is) at three long-term global-change experiment sites in the United States: (1) the Warming and Rainfall Manipulation (WaRM) experiment near College Station, Texas, (2) the Free-Air CO2 Enrichment (FACE) experiment at the Oak Ridge National Environmental Research Park in Tennessee, and (3) the Aspen FACE experiment near Rhinelander, Wisconsin. Their Texas measurements indicated that oak leaves exhibited a slight increase in Is when the atmospheric CO2 concentration was decreased instantaneously from ambient values, while their measurements "in both a sweetgum forest in Tennessee and aspen stands in Wisconsin revealed evidence of an active down regulation of Is during growth in an atmosphere of increased [our italics] CO2 concentration," which latter observations, in their words, "are among the first to show a consistent down regulation of Is in response to growth at elevated CO2, and they emphasize that while many past studies show an instantaneous inhibition of elevated CO2 on Is when measured in a leaf cuvette, a response in the similar direction is evident on whole forest stands exposed to elevated CO2 under natural field growth conditions."

In discussing the context of their work, the twelve researchers write that "most global or regional models of present or future isoprene emissions are based on relationships among climate change, increases in atmospheric CO2 concentration and changes in net primary productivity (NPP)," and that "the fundamental logic of such models is that changes in NPP will produce more or less biomass capable of emitting isoprene, and changes in climate will stimulate or inhibit emissions per unit of biomass." Hence, as they continue, "these models tend to ignore the discovery that there are direct effects of changes in the atmospheric CO2 concentration on isoprene emission that tend to work in the opposite direction [our italics] to that of stimulated NPP." For additional evidence of this fact, see Isoprene in our Subject Index. Furthermore, their results showed, in their words, "that growth in an atmosphere of elevated CO2 inhibited the emission of isoprene at levels that completely compensate [our italics] for possible increases in emission due to increases in aboveground NPP."

In lamenting this sorry state of global-change modeling, Monson et al. say that, "to a large extent, the modeling has 'raced ahead' of our mechanistic understanding of how isoprene emissions will respond to the fundamental drivers of global change," and that "without inclusion of these effects in the current array of models being used to predict changes in atmospheric chemistry due to global change, one has to question the relevance of the predictions."

We believe that this criticism applies equally well to many aspects of the work of the Intergovernmental Panel on Climate Change, as several of the processes involved in their massive international modeling effort possess biological components that are either wrongly parameterized or wrongly ignored altogether for lack of sufficient real-world knowledge. Progress, of course, will be continually made; and that progress could well lead to far different conclusions about the future state of earth's climate than those that presently hold sway within high political circles.

Sherwood, Keith and Craig Idso

Chameides, W.L., Lindsay, R.W., Richardson, J. and Kiang, C.S. 1988. The role of biogenic hydrocarbons in urban photochemical smog: Atlanta as a case study. Science 241: 1473-1475.

Harley, P.C., Monson, R.K. and Lerdau, M.T. 1999. Ecological and evolutionary aspects of isoprene emission from plants. Oecologia 118: 109-123.

Monson, R.K., Trahan, N., Rosenstiel, T.N., Veres, P., Moore, D., Wilkinson, M., Norby, R.J., Volder, A., Tjoelker, M.G., Briske, D.D., Karnosky, D.F. and Fall, R. 2007. Isoprene emission from terrestrial ecosystems in response to global change: minding the gap between models and observations. Philosophical Transactions of the Royal Society A 365: 1677-1695.

Poisson, N., Kanakidou, M. and Crutzen, P.J. 2000. Impact of non-methane hydrocarbons on tropospheric chemistry and the oxidizing power of the global troposphere: 3-dimensional modeling results. Journal of Atmospheric Chemistry 36: 157-230.