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Foliar Isoprene Emissions: Now and in the Future
Volume 12, Number 28: 15 July 2009

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. In fact, Poisson et al. (2000) have calculated 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 by 50-60% over land. In addition, they note that the current tropospheric ozone content extends the atmospheric lifetime of methane (one of the world's most powerful greenhouse gases) by approximately 15%. Consequently, it can be appreciated that anything that helps to reduce vegetative isoprene emissions is something to be highly prized.

In light of these several observations, Wilkinson et al. (2009) decided to examine an important effect of a phenomenon that influences foliar isoprene emissions and is destined to impact the world for decades, and possibly centuries, to come: the historical and still-ongoing rise in the atmosphere's CO2 concentration. This they did by growing three-year-old cottonwood trees and two-year-old aspen trees in controlled-environment chambers maintained for several weeks at atmospheric CO2 concentrations of 400 and 800 ppm, with the aspen trees also exposed to CO2 concentrations of 600 and 1200 ppm. In addition, they grew two-year-old sweetgum and eucalyptus trees in another controlled-environment facility maintained at CO2 concentrations of 240 and 380 ppm. And in all of these different situations, they measured the isoprene emission rate (IS) of the trees' leaves.

In the case of the latter experiment, the nine researchers found that "IS was approximately 30% and 18% lower, respectively, for eucalyptus and sweetgum trees grown at 520 ppm CO2, compared with trees grown at 240 ppm CO2." They also found, in the other study, that the cottonwood and aspen trees "exhibited a 30-40% reduction in isoprene emission rate when grown at 800 ppm CO2, compared with 400 ppm CO2," and that the aspen trees "exhibited a 33% reduction when grown at 1200 ppm CO2, compared with 600 ppm CO2."

Based on these findings, Wilkinson et al. "used current models of leaf isoprene emission to demonstrate that significant errors occur if the CO2 inhibition of isoprene is not taken into account," developing in the process "a quantitative algorithm that can be used to scale IS at the leaf level to changes in atmospheric and intercellular CO2" that can be "incorporated into larger-scale models that aim to predict regional or global patterns in IS," which can further be used to address "important questions concerning atmospheric chemistry in the face of future global change projections."

In a related paper published in the same issue of Global Change Biology that takes this important additional step, Heald et al. (2009) incorporate an empirical model of observed isoprene emissions response to changes in atmospheric CO2 concentration in the long-term growth environment and short-term changes in intercellular CO2 concentrations into a biogenic emission model embedded within the Community Land Model of the global NCAR Community Climate System Model. And in doing so, they find that "the large increases in future isoprene emissions typically predicted in models, which are due to a projected warmer climate, are entirely offset [our italics] by including the CO2 effects."

So what other global warming phenomena might be "entirely offset" by including still other biological "CO2 effects" that may perturb them? We'll never really know until they are all identified and properly incorporated into the ever-evolving suite of climate models that drive U.S. and global energy policy, which is something that is not likely to happen anytime soon, as it could well upset some very important apple carts that are carrying hundreds of pages of hoped-for U.S. federal legislation along the arduous-but-recently-paved trail that leads to its becoming the law of the land.

Sherwood, Keith and Craig Idso

References
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.

Heald, C.L., Wilkinson, M.J., Monson, R.K., Alo, C.A., Wang, G. and Guenther, A. 2009. Response of isoprene emission to ambient CO2 changes and implications for global budgets. Global Change Biology 15: 1127-1140.

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.

Wilkinson, M.J., Monson, R.K, Trahan, N., Lee, S., Brown E., Jackson, R.B., Polley, H.W., Fay, P.A. and Fall, R. 2009. Leaf isoprene emission rate as a function of atmospheric CO2 concentration. Global Change Biology 15: 1189-1200.