The Three Major Determinants of Terrestrial Isoprene Emissions

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, which is a debilitating scourge of plant and animal life alike. It has been calculated by Poisson et al. (2000), for example, that current levels of NMHC emissions may increase surface ozone concentrations by up to 40% in the marine boundary-layer and by 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%. Consequently, it can be understood that anything that reduces isoprene emissions from vegetation is something to be desired.

In a new paper on the subject, Lathiere et al. (2010) (1) describe the development and analysis of a new model based on the Model of Emissions of Gases and Aerosols from Nature (MEGAN) -- which was developed by Guenther et al. (2006) for estimating isoprene emissions from terrestrial vegetation -- (2) validate the new model with compilations of published field-based canopy-scale observations, and (3) use the new model to calculate changes in isoprene emissions from the terrestrial biosphere in response to climate change, atmospheric CO₂ increase, and land use change throughout the 20th century.

Results indicated that between 1901 and 2002, climate change at the global scale, in the words of the authors, "was responsible for a 7% increase in isoprene emissions," but that "rising atmospheric CO₂ caused a 21% reduction," and that "by the end of the 20th century (2002), anthropogenic cropland expansion had the largest impact, reducing isoprene emissions by 15%," so that "overall, these factors combined to cause a 24% decrease in global isoprene emissions during the 20th century."

Such findings represent good news for the planet, as they should mitigate against the undesirable consequences of increases in tropospheric ozone and methane concentrations. The three researchers warn, however, that "the possible rapid expansion of biofuel production with high isoprene-emitting plant species (e.g., oil palm, willow and poplar) may reverse the trend by which conversion of land to food crops leads to lower isoprene emissions." This being the case, there exists yet another reason to reject biofuels as replacements for fossil fuels.

Additional References
Guenther, A., Karl, T., Harley, P., Wiedinmyer, C., Palmer, P.I. and Geron, C. 2006. Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature). Atmospheric Chemistry and Physics 6: 3181-3210.

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.