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Isoprene -- Summary
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 fact, 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%. Consequently, it can be appreciated that anything that reduces isoprene emissions from vegetation is something to be desired; and in this research Summary we briefly review what has been learned in this regard about the ongoing rise in the atmosphere's CO2 concentration.

Although a few experiments conducted on certain plant species have suggested that elevated concentrations of atmospheric CO2 have little to no effect on their emissions of isoprene (Buckley, 2001; Baraldi et al., 2004; Rapparini et al., 2004), a much larger number of other experiments are suggestive of substantial CO2-induced reductions in isoprene emissions, as demonstrated by the work of Monson and Fall (1989), Loreto and Sharkey (1990), Sharkey et al. (1991) and Loreto et al. (2001). Short synopses of several other such studies are therefore briefly described in the following paragraphs, along with some of the implications of their more robust findings.

Rosentiel et al. (2003) studied three 50-tree cottonwood plantations growing in separate mesocosms within the forestry section of the Biosphere 2 facility near Oracle, Arizona, USA, one of which mesocosms was maintained at an atmospheric CO2 concentration of 430 ppm, while the other two were enriched to concentrations of 800 and 1200 ppm for one entire growing season. Integrated over that period, the total above-ground biomass of the trees in the latter two mesocosms was increased by 60% and 82%, respectively, while their production of isoprene was decreased by 21% and 41%, respectively.

Scholefield et al. (2004) measured isoprene emissions from Phragmites australis plants (one of the world's most important natural grasses) growing at different distances from a natural CO2 spring in central Italy. At the specific locations they chose to make their measurements, atmospheric CO2 concentrations of approximately 350, 400, 550 and 800 ppm had likely prevailed for the entire lifetimes of the plants. Across this CO2 gradient, plant isoprene emissions dropped ever lower as the air's CO2 concentration rose ever higher. Over the first 50-ppm CO2 increase, isoprene emissions were reduced to approximately 65% of what they were at ambient CO2, while for CO2 increases of 200 and 450 ppm, they were reduced to only about 30% and 7% of what they were in the 350-ppm-CO2 air. The researchers note that these reductions were likely caused by reductions in leaf isoprene synthase, which was observed to be highly correlated with isoprene emissions, leading them to conclude that "elevated CO2 generally inhibits the expression of isoprenoid synthesis genes and isoprene synthase activity which may, in turn, limit formation of every chloroplast-derived isoprenoid." Hence, they state that the "basal emission rate of isoprene is likely to be reduced under future elevated CO2 levels."

Centritto et al. (2004) grew hybrid poplar saplings for one full growing season in a FACE facility located at Rapolano, Italy, where the air's CO2 concentration was increased by approximately 200 ppm. Their study demonstrated, in their words, that "isoprene emission is reduced in elevated CO2, in terms of both maximum values of isoprene emission rate and isoprene emission per unit of leaf area averaged across the total number of leaves per plant," which in their case amounted to a reduction of approximately 34%. When isoprene emission was summed over the entire plant profile, however, the reduction was not nearly so great (only 6%), because of the greater number of leaves on the CO2-enriched saplings. "However," as they state, "Centritto et al. (1999), in a study with potted cherry seedlings grown in open-top chambers, and Gielen et al. (2001), in a study with poplar saplings exposed to FACE, showed that the stimulation of total leaf area in response to elevated CO2 was a transient effect, because it occurred only during the first year of growth." Hence, they concluded "it may be expected that with similar levels of leaf area, the integrated emission of isoprene would have been much lower in elevated CO2." Indeed, they say that their data, "as well as that reported by Scholefield et al. (2004), in a companion experiment on Phragmites growing in a nearby CO2 spring, mostly confirm that isoprene emission is inversely dependent on CO2 [concentration] when this is above ambient, and suggests that a lower fraction of C will be re-emitted in the atmosphere as isoprene by single leaves in the future."

Possell et al. (2004) grew seedlings of English oak (Quercus robur), one to a mesocosm (16 cm diameter, 60 cm deep), in either fertilized or unfertilized soil in solardomes maintained at atmospheric CO2 concentrations of either ambient or ambient plus 300 ppm for one full year, at the conclusion of which period they measured rates of isoprene emissions from the trees' foliage together with their rates of photosynthesis. In the unfertilized trees, this work revealed that the 300-ppm increase in the air's CO2 concentration reduced isoprene emissions by 63% on a leaf area basis and 64% on a biomass basis, while in the fertilized trees the extra CO2 reduced isoprene emissions by 70% on a leaf area basis and 74% on a biomass basis. In addition, the extra CO2 boosted leaf photosynthesis rates by 17% in the unfertilized trees and 13% in the fertilized trees. At one and the same time, therefore, the CO2-enriched air of this experiment did a number of positive things. It (1) enhanced the photosynthetic rates of the trees' leaves, while it (2) reduced their rates of isoprene emission, which tends to (3) reduce the (a) lifetime and (b) atmospheric concentration of (i) methane and (ii) other radiatively active trace gases of the atmosphere, as well as (4) reduce the atmospheric concentration of radiatively-active and phytotoxic ozone, which accomplishments are rather astounding for a trace gas (CO2) that climate alarmists are trying to characterize as a pollutant.

Possell et al. (2005) performed multiple three-week-long experiments with two known isoprene-emitting herbaceous species (Mucuna pruriens and Arundo donax), which they grew in controlled environment chambers that were maintained at two different sets of day/night temperatures (29/24C and 24/18C) and atmospheric CO2 concentrations characteristic of glacial (180 ppm), pre-industrial (280 ppm) and current (366 ppm) conditions, where canopy isoprene emission rates were measured on the final day of each experiment. In doing so, they obtained what they describe as "the first empirical evidence for the enhancement of isoprene production, on a unit leaf area basis, by plants that grew and developed in [a] CO2-depleted atmosphere," which results, in their words, "support earlier findings from short-term studies with woody species (Monson and Fall, 1989; Loreto and Sharkey, 1990)." Then, combining their emission rate data with those of Rosenstiel et al. (2003) for Populus deltoides, Centritto et al. (2004) for Populus x euroamericana and Scholefield et al. (2004) for Phragmites australis, they developed a single downward-trending isoprene emissions curve that stretches all the way from 180 to 1200 ppm CO2, where it asymptotically approaches a value that is an order of magnitude less than what it is at 180 ppm.

Working at the Biosphere 2 facility near Oracle, Arizona, USA, in enclosed ultraviolet light-depleted mesocosms (to minimize isoprene depletion by atmospheric oxidative reactions such as those involving OH), Pegoraro et al. (2005) studied the effects of atmospheric CO2 enrichment (1200 ppm compared to an ambient concentration of 430 ppm) and drought on the emission of isoprene from cottonwood (Populus deltoides Bartr.) foliage and its absorption by the underlying soil for both well-watered and drought conditions. In doing so, they found that "under well-watered conditions in the agriforest stands, gross isoprene production (i.e., the total production flux minus the soil uptake) was inhibited by elevated CO2 and the highest emission fluxes of isoprene were attained in the lowest CO2 treatment." In more quantitative terms, it was determined that the elevated CO2 treatment resulted in a 46% reduction in gross isoprene production. In addition, it was found that drought suppressed the isoprene sink capacity of the soil beneath the trees, but that "the full sink capacity of dry soil was recovered within a few hours upon rewetting."

Putting a slightly negative slant on their findings, Pegoraro et al. suggested that "in future, potentially hotter, drier environments, higher CO2 may not mitigate isoprene emission as much as previously suggested." However, we note that climate models generally predict an intensification of the hydrologic cycle in response to rising atmospheric CO2 concentrations, and that the anti-transpirant effect of atmospheric CO2 enrichment typically leads to increases in the moisture contents of soils beneath vegetation (see Soil Water Status Field Studies and Growth Chamber Studies in our Subject Index). Also, we note that over the latter decades of the 20th century, when climate alarmists claim the earth warmed at a rate and to a level that were unprecedented over the past two millennia, soil moisture data from all around the world tended to display upward trends. Robock et al. (2000), for example, developed a massive collection of soil moisture data from over 600 stations spread across a variety of climatic regimes, including the former Soviet Union, China, Mongolia, India and the United States, determining that "in contrast to predictions of summer desiccation with increasing temperatures, for the stations with the longest records, summer soil moisture in the top 1 m has increased while temperatures have risen." And in a subsequent study of "45 years of gravimetrically-observed plant available soil moisture for the top 1 m of soil, observed every 10 days for April-October for 141 stations from fields with either winter or spring cereals from the Ukraine for 1958-2002," Robock et al. (2005) discovered that these real-world observations "show a positive soil moisture trend for the entire period of observation," noting that "even though for the entire period there is a small upward trend in temperature and a downward trend in summer precipitation, the soil moisture still has an upward trend for both winter and summer cereals." Consequently, in a CO2-enriched world of the future, we likely will have the best of both aspects of isoprene activity: less production by vegetation and more consumption by soils.

In view of these many encouraging findings, it would appear that the ongoing rise in the atmosphere's CO2 concentration will lead to ever greater reductions in atmospheric isoprene concentrations; and as noted in the introductory paragraph of this Summary, such a consequence would be very welcome news for man and nature alike.

References
Baraldi, R., Rapparini, F., Oechel, W.C., Hastings, S.J., Bryant, P., Cheng, Y. and Miglietta, F. 2004. Monoterpene emission responses to elevated CO2 in a Mediterranean-type ecosystem. New Phytologist 161: 17-21.

Buckley, P.T. 2001. Isoprene emissions from a Florida scrub oak species grown in ambient and elevated carbon dioxide. Atmospheric Environment 35: 631-634.

Centritto, M., Lee, H. and Jarvis, P. 1999. Interactive effects of elevated [CO2] and water stress on cherry (Prunus avium) seedlings. I. Growth, total plant water use efficiency and uptake. New Phytologist 141: 129-140.

Centritto, M., Nascetti, P., Petrilli, L., Raschi, A. and Loreto, F. 2004. Profiles of isoprene emission and photosynthetic parameters in hybrid poplars exposed to free-air CO2 enrichment. Plant, Cell and Environment 27: 403-412.

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.

Gielen, B., Calfapietra, C., Sabatti, M. and Ceulemans, R. 2001. Leaf area dynamics in a poplar plantation under free-air carbon dioxide enrichment. Tree Physiology 21: 1245-1255.

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

Loreto, F., Fischbach, R.J., Schnitzler, J.-P., Ciccioli, P., Brancaleoni, E., Calfapietra, C. and Seufert, G. 2001. Monoterpene emission and monoterpene synthase activities in the Mediterranean evergreen oak Quercus ilex L. grown at elevated CO2 concentrations. Global Change Biology 7: 709-717.

Loreto F. and Sharkey, T.D. 1990. A gas exchange study of photosynthesis and isoprene emission in red oak (Quercus rubra L.). Planta 182: 523-531.

Monson, R.K. and Fall, R. 1989. Isoprene emission from aspen leaves. Plant Physiology 90: 267-274.

Pegoraro, E., Abrell, L., van Haren, J., Barron-Gafford, G., Grieve, K.A., Malhi, Y., Murthy, R. and Lin, G. 2005. The effect of elevated atmospheric CO2 and drought on sources and sinks of isoprene in a temperate and tropical rainforest mesocosm. Global Change Biology 11: 1234-1246.

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.

Possell, M., Heath, J., Hewitt, C.N., Ayres, E. and Kerstiens, G. 2004. Interactive effects of elevated CO2 and soil fertility on isoprene emissions from Quercus robur. Global Change Biology 10: 1835-1843.

Possell, M., Hewitt, C.N. and Beerling, D.J. 2005. The effects of glacial atmospheric CO2 concentrations and climate on isoprene emissions by vascular plants. Global Change Biology 11: 60-69.

Rapparini, F., Baraldi, R., Miglietta, F. and Loreto, F. 2004. Isoprenoid emission in trees of Quercus pubescens and Quercus ilex with lifetime exposure to naturally high CO2 environment. Plant, Cell and Environment 27: 381-391.

Robock, A., Mu, M., Vinnikov, K., Trofimova, I.V. and Adamenko, T.I. 2005. Forty-five years of observed soil moisture in the Ukraine: No summer desiccation (yet). Geophysical Research Letters 32: 10.1029/2004GL021914.

Robock, A., Vinnikov, K.Y., Srinivasan, G., Entin, J.K., Hollinger, S.E., Speranskaya, N.A., Liu, S. and Namkhai, A. 2000. The global soil moisture data bank. Bulletin of the American Meteorological Society 81: 1281-1299.

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).

Scholefield, P.A., Doick, K.J., Herbert, B.M.J., Hewitt, C.N.S., Schnitzler, J.-P., Pinelli, P. and Loreto, F. 2004. Impact of rising CO2 on emissions of volatile organic compounds: isoprene emission from Phragmites australis growing at elevated CO2 in a natural carbon dioxide spring. Plant, Cell and Environment 27: 393-401.

Sharkey, T.D., Loreto, F. and Delwiche, C.F. 1991. High carbon dioxide and sun/shade effect on isoprene emissions from oak and aspen tree leaves. Plant, Cell and Environment 14: 333-338.

Last updated 26 September 2007