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Anthropogenic CO2 Emissions May Reduce Tropospheric Ozone and Methane Concentrations, Providing Multiple Benefits to the Biosphere
Volume 6, Number 3: 15 January 2003

A few years ago, Rosenstiel et al. (2003) established three 50-tree cottonwood (Populus deltoides Bartr.) plantations in separate mesocosms located within the forestry section of the Biosphere 2 facility.  One of these mesocosms was maintained at an atmospheric CO2 concentration of 430 ppm, while one was enriched to a concentration of 800 ppm and one to 1200 ppm for an entire growing season.  Integrated over that growing season, the net ecosystem production of isoprene in the 800- and 1200-ppm mesocosms was reduced by 21% and 41%, respectively.  Why is this finding important?

Isoprene (C5H8 or 2-methyl-1,3-butadiene) is a highly reactive non-methane hydrocarbon (NMHC) that is emitted in copious quantities by vegetation -- especially by trees -- which is responsible for the production of vast amounts of ozone (Chameides et al., 1988; Harley et al., 1999).  It has been calculated by Poisson et al. (2000), for example, that real-world levels of NMHC emissions -- the vast majority of which are isoprene (more than twice as much as all other NMHCs combined) -- have the capacity to increase surface ozone concentrations by 50-60% over land and by as much as 40% in the marine boundary-layer.  Hence, CO2-induced reductions in forest isoprene emissions should substantially reduce the aerial concentration of this destructive air pollutant, which harms both plants and animals, including humans.

Biogenic NMHCs (again, comprised predominantly of isoprene) also play a major role the global tropospheric chemistry of methane, boosting its atmospheric lifetime by approximately 14%, from 6.5 to 7.4 years (Poisson et al., 2000).  Hence, CO2-induced reductions in vegetative isoprene emissions have the capacity to significantly reduce atmospheric methane concentrations and thereby significantly reduce the total radiative forcing provided by the atmosphere's suite of greenhouse gases, as methane is many times more powerful in this regard than is carbon dioxide.

Is there any evidence these beneficial consequences of atmospheric CO2 enrichment may already be occurring in the real world of nature in response to the ongoing rise in the air's CO2 content?

With respect to methane, the answer appears to be at least a partial yes.  In our Editorial of 8 Jan 2003, for example, we report on the slowing of the tropospheric methane growth rate experienced during the 1980s and 1990s, which rate actually turned negative in 2000 for the first time since modern measurements began.  In fact, the trend that we see in the baseline methane growth rate data of Simpson et al. (2002) suggests that whatever is causing the decline -- which may include processes in addition to the one caused by CO2-induced reductions in isoprene emissions from plants -- is so powerful that it may well lead to regular yearly decreases in the atmosphere's methane concentration in the not too distant future.

With respect to ozone, the effects of CO2-induced decreases in vegetative isoprene emissions are not yet unambiguously evident, since tropospheric ozone concentrations continue to rise as a consequence of expanding anthropogenic activities.  However, there is strong evidence, particularly from Europe, that the multitude of biological benefits provided by the ongoing increase in the air's CO2 content are overpowering the negative effects of the concomitant increase in ozone.  In our Editorial of 26 Dec 2001, for example, we note that ozone exposures that are more than sevenfold greater than the critical level that is known to decrease tree productivity by 10% are occurring all across the Austrian Forest Inventory Grid.  However, there is little sign of any significant forest damage as a result of this increase in ozone exposure.  In fact, Herman et al. (2001) report that not only are there no dramatic reductions in tree health and productivity, such as normally would be expected in response to extended exposure to high concentrations of ozone, there appear to be no deleterious consequences at all, and in many areas actual improvements in forest health are observed.

Clearly, the findings of Rosenstiel et al. (2003) have tremendous positive implications for the biology and climate of the entire planet; but they are particularly important for the world's commercial tree plantations.  Short-rotation agriforests, in their words, "provide an increasing array of human services - including lumber, pulp and paper production, biofuels for renewable energy and, potentially, carbon sequestration."  Because of their great economic and social significance, these enterprises are proliferating around the globe at a rapid and accelerating rate; and until the publication of Rosenstiel et al.'s paper, this phenomenon would have been expected to significantly increase tropospheric ozone concentrations, especially since nearly all commercial forest species emit large amounts of isoprene in ambient air.  Now, however, it can be appreciated that such an outcome is highly unlikely, as the earth appears to be implementing its own Clean Air Act by reducing its production of ozone-generating isoprene in response to man's enriching of the air with CO2.

And there's one last thing we almost forgot.  The Biosphere 2 scientists additionally determined that the extra CO2 in the air of the 800- and 1200-ppm mesocosms increased total above-ground forest biomass production by 60% and 82%, respectively.

Yes, the ongoing rise in the air's CO2 content is truly a blessing.  As the new research results from the Biosphere 2 Center clearly demonstrate, elevated levels of atmospheric CO2 (1) dramatically boost plant productivity, (2) reduce the production an important precursor of one of the planet's worst air pollutants, and (3) provide a means for naturally lowering the concentration of one of the atmosphere's most powerful greenhouse gases, thereby reducing the impetus for global warming.  What more could one ask of a single compound?

CO2 - What a gas!

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.

Herman, F., Smidt, S., Huber, S., Englisch, M. and Knoflacher, M.  2001.  Evaluation of pollution-related stress factors for forest ecosystems in central Europe.  Environmental Science & Pollution Research 8: 231-242.

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.

Rosentiel, T.N., Potosnak, M.J., Griffin, K.L., Fall, R. and Monson, R.K.  2003.  Increased CO2 uncouples growth from isoprene emission in an agriforest ecosystem.  Nature advance online publication, 5 January 2003 (doi:10.1038/nature 01312).

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.