The climatic significance of iodinated compounds or iodocompounds was first described in the pages of Nature by O'Dowd et al. (2002). As related by Kolb (2002) in an accompanying perspective on their work, the ten-member research team discovered "a previously unrecognized source of aerosol particles" by unraveling "a photochemical phenomenon that occurs in sea air and produces aerosol particles composed largely of iodine oxides." Specifically, the team used a smog chamber operated under coastal atmospheric conditions to demonstrate, as they report, that "new particles can form from condensable iodine-containing vapors, which are the photolysis products of biogenic iodocarbons emitted from marine algae." With the help of aerosol formation models, they also demonstrated that concentrations of condensable iodine-containing vapors over the open ocean "are sufficient to influence marine particle formation."
The significance of this work, of course, is that the aerosol particles O'Dowd et al. discovered can function as cloud condensation nuclei (CCN), helping to create new clouds that reflect more incoming solar radiation back to space and thereby cool the planet. With respect to the negative feedback nature of this phenomenon, O'Dowd et al. cite the work of Laturnus et al. (2000), which demonstrates that emissions of iodocarbons from marine biota "can increase by up to 5 times as a result of changes in environmental conditions associated with global change." Therefore, as O'Dowd et al. continue, "increasing the source rate of condensable iodine vapors will result in an increase in marine aerosol and CCN concentrations of the order of 20 - 60%." Furthermore, they note that "changes in cloud albedo resulting from changes in CCN concentrations of this magnitude can lead to an increase in global raidative forcing similar in magnitude, but opposite in sign [our italics], to the forcing induced by greenhouse gases."
Four years later, Smythe-Wright et al. (2006) measured trace gas and pigment concentrations in seawater, while identifying and enumerating picophytoprokaryotes during two ship cruises in the Atlantic Ocean and one in the Indian Ocean, where they focused "on methyl iodide production and the importance of a biologically related source." In doing so, they encountered methyl iodide concentrations as great as 45 pmol per liter in the top 150 meters of the oceanic water column that correlated well with the abundance of Prochlorococcus, which they report "can account for >80% of the variability in the methyl iodide concentrations," they add that they "have confirmed the release of methyl iodide by this species in laboratory culture experiments."
Extrapolating their findings to the globe as a whole, the six researchers "estimate the global ocean flux of iodine [I] to the marine boundary layer from this single source to be 5.3 x 1011 g I year-1," which they say "is a large fraction of the total estimated global flux of iodine (1011-1012 g I year-1)." This observation is extremely important, because volatile iodinated compounds, in Smythe-Wright et al.'s words, "play a part in the formation of new particles and cloud condensation nuclei (CCN)," and because "an increase in the production of iodocompounds and the subsequent production of CCN would potentially result in a net cooling of the earth system and hence in a negative climate feedback mechanism, mitigating global warming." More specifically, they suggest that "as ocean waters become warmer and more stratified, nutrient concentrations will fall and there will likely be a regime shift away from microalgae toward Prochlorococcus," such that "colonization within the <50° latitude band will result in a ~15% increase in the release of iodine to the atmosphere," with consequent "important implications for global climate change," which, as previously noted, tend to counteract global warming.
Most recently, as part of the Third Pelagic Ecosystem CO2 Enrichment Study, Wingenter et al. (2007) investigated the effects of atmospheric CO2 enrichment on marine microorganisms in nine marine mesocosms maintained within two-meter-diameter polyethylene bags submerged to a depth of ten meters in a fjord at the Large-Scale Facilities of the Biological Station of the University of Bergen in Espegrend, Norway. Three of these mesocosms were maintained at ambient levels of CO2 (~375 ppm or base CO2), three were maintained at levels expected to prevail at the end of the current century (760 ppm or 2xCO2), and three were maintained at levels predicted for the middle of the next century (1150 ppm or 3xCO2). During the 25 days of this experiment, the twelve researchers followed the development and subsequent decline of an induced bloom of the coccolithophorid Emiliania huxleyi, carefully measuring several physical, chemical and biological parameters along the way. This work revealed that the iodocarbon chloroiodomethane (CH2CII) experienced its peak concentration about six to ten days after the coccolithophorid's chlorophyll-a maximum, and that its estimated abundance was 46% higher in the 2xCO2 mesocosms and 131% higher in the 3xCO2 mesocosms.
The international team of scientists concluded that the differences in the CH2CII concentrations "may be viewed as a result of changes to the ecosystems as a whole brought on by the CO2 perturbations." And because emissions of various iodocarbons have been found to lead to an enhancement of cloud condensation nuclei in the marine atmosphere, as demonstrated by O'Dowd et al. (2002) and Jimenez et al. (2003), it can be appreciated that the CO2-induced stimulation of the marine emissions of these substances provides a natural brake on the tendency for global warming to occur as a consequence of any forcing, as iodocarbons lead to the creation of more-highly-reflective clouds over greater areas of the world's oceans.
In conclusion, as Wingenter et al. sum things up, the processes described above "may help contribute to the homeostasis of the planet." And the finding of O'Dowd et al. that changes in cloud albedo "associated with global change" can lead to an increase in global radiative forcing that is "similar in magnitude, but opposite in sign, to the forcing induced by greenhouse gases," suggests that CO2-induced increases in marine iodocarbon emissions likely contribute to maintaining that homeostasis in a really big way.
References
Jimenez, J.L., Bahreini, R., Cocker III, D.R., Zhuang, H., Varutbangkul, V., Flagan, R.C., Seinfeld, J.H., O'Dowd, C.D. and Hoffmann, T. 2003. New particle formation from photooxidation of diiodomethane (CH2I2). Journal of Geophysical Research 108: 10.1029/2002JD002452.
Kolb, C.E. 2002. Iodine's air of importance. Nature 417: 597-598.
Laturnus F., Giese, B., Wiencke, C. and Adams, F.C. 2000. Low-molecular-weight organoiodine and organobromine compounds released by polar macroalgae -- The influence of abiotic factors. Fresenius' Journal of Analytical Chemistry 368: 297-302.
O'Dowd, C.D., Jimenez, J.L., Bahreini, R., Flagan, R.C., Seinfeld, J.H., Hameri, K., Pirjola, L., Kulmala, M., Jennings, S.G. and Hoffmann, T. 2002. Marine aerosol formation from biogenic iodine emissions. Nature 417: 632-636.
Smythe-Wright, D., Boswell, S.M., Breithaupt, P., Davidson, R.D., Dimmer, C.H. and Eiras Diaz, L.B. 2006. Methyl iodide production in the ocean: Implications for climate change. Global Biogeochemical Cycles 20: 10.1029/2005GB002642.
Wingenter, O.W., Haase, K.B., Zeigler, M., Blake, D.R., Rowland, F.S., Sive, B.C., Paulino, A., Thyrhaug, R., Larsen A., Schulz, K., Meyerhofer, M. and Riebesell, U. 2007. Unexpected consequences of increasing CO2 and ocean acidity on marine production of DMS and CH2CII: Potential climate impacts. Geophysical Research Letters 34: 10.1029/2006GL028139.
Last updated 26 December 2007