How does rising atmospheric CO2 affect marine organisms?

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The Capacity of Man and Nature to Control Climate Change
Volume 7, Number 32: 11 August 2004

Dimethylsulfide (DMS) is a climatically-important trace gas that is produced by various types of marine phytoplankton and is believed to play a major role in maintaining earth's temperature within bounds conducive to the continued existence of life. This CLAW Hypothesis, named for the four scientists who formulated it - Charlson, Lovelock, Andreae and Warren (Charlson et al., 1987) - begins with an initial impetus for warming, such as an increase in the air's CO2 content, which induces an increase in the productivity of marine phytoplankton that results in a greater production of oceanic DMS and its release to the atmosphere, where greater gas-to-particle conversions increase the air's population of cloud condensation nuclei and, ultimately, the albedos of marine stratus and altostratus clouds (via a narrowing of the cloud droplet spectrum and a decrease in the mean radius of the cloud droplets), both of which phenomena tend to counteract the initial impetus for warming and thus complete the negative feedback loop that maintains the planet's mean temperature within a range conducive to the continued existence of life, which objective, to borrow a phrase from the Star Trek television and movie series, we could perhaps describe as the biosphere's prime directive.

About the same time, Martin and Fitzwater (1988) and Martin et al. (1988) developed what has come to be known as the Iron Hypothesis (Martin, 1990), which posits that iron-rich dust swept up from exposed continental shelves during glacial maxima by the greatly enhanced winds of those periods fertilizes the world's oceans to the point where their phytoplanktonic productivity rises so high that it draws the air's CO2 concentration down from typical interglacial values (280 ppm) to the much lower values characteristic of glacials (180 ppm); and shortly thereafter, the IronEx studies of Martin et al. (1994) and Coale et al. (1996) confirmed the fundamental premise of this hypothesis: after fertilizing small patches of seawater in high-nitrate low-chlorophyll (HNLC) regions of the equatorial Pacific with bio-available iron, they found, in the words of Turner et al. (2004), that this procedure "benefited all the major groups of the algal community, including those which produce significant amounts of intracellular dimethylsulfoniopropionate (DMSPp)," the precursor of DMS that also saw its concentration rise as a result of the experimental iron treatment (Turner et al., 1996).

In the aftermath of these latter demonstrations, large-scale ocean fertilization with bio-available iron became a doubly-viable potential strategy for the mitigation of global warming. Not only could its natural or anthropogenic implementation result in the removal of CO2 from the atmosphere at an augmented rate in response to heightened phytoplanktonic productivity, it could also lead to the reflection of more incoming solar radiation back to space as a result of greater DMSPp production (and all that follows it) in response to the same basic phenomenon, i.e., heightened phytoplanktonic productivity, which then activates the CLAW mechanism. However, the studies supporting the second of these two pathways to augmented planetary cooling had been conducted in the equatorial Pacific; and it was not known if the findings of those studies could be extrapolated to other HNLC ecosystems, such as those of the Southern Ocean. Consequently, Turner et al. (2004) conducted two additional iron-release experiments: the Southern Ocean Iron Release Experiment (SOIREE), which took place south of Australia (61S, 140E) in February of 1999, and EisenEx, which took place south of Africa (48S, 21E) in November of 2000.

In both of these studies, Turner et al. say "the experimental patches (~50 km) were created by pumping dissolved iron sulfate into the mixed layer, as the ships sailed on a spiral track out from, and relative to, a buoy." The initial levels of dissolved iron in these patches rapidly decreased, and additional injections were made at their centers during the course of the experiments. As to what occurred thereafter, the scientists report that "in SOIRRE, the major increase in DMS occurred several days after the maximum in DMSPp and by the end of the study DMS levels at 30 m depth were 6.5-fold higher in treated waters than outside," while "in EisenEx, highest observed DMS concentrations [occurred] on days 5 and 12, about 2-fold higher than initial levels." What is more, they say that "a series of ocean color images from SeaWiFS reveals a feature with chlorophyll levels up to 3 g l-1 close to the SOIREE site (Boyd et al., 2000)," and that "Abraham et al. (2000) argue that this was our patch which had spread to cover 1100 km."

How significant are these findings? They are truly huge; for Turner et al. report that "recent coupled ocean-atmosphere modeling studies show that even a relatively small change in marine DMS emissions may have a significant impact on global temperatures: ~1C for a halving or doubling of DMS emissions, respectively (Spall et al., 2001)." In addition, they note that "evidence from ice cores suggests that changes in DMS emission at least as large as this have occurred in the past (Legrand et al., 1991) and so it is easily conceivable that significant changes in DMS emissions would occur in future climate scenarios."

Operating in tandem, it is clear that the marine-productivity-mediated increase in reflected solar radiation to space (via the CLAW mechanism) and the more direct marine-productivity-mediated increase in removal rate of CO2 from the atmosphere possess the capacity (whether naturally or anthropogenically implemented in response to an increase in temperature or iron flux to the world's oceans) to substantially counter whatever warming of the planet might possibly occur in response to future anthropogenic CO2 emissions. Hence, there is reason to be guardedly optimistic about the health of the biosphere, based largely on the ability of marine phytoplankton to safeguard their own future by moderating the nature of their physical environment (in this case, ocean surface temperature).

Sherwood, Keith and Craig Idso

References
Abraham, E.R., Law, C.S., Boyd, P.W., Lavender, S.J., Maldonado, M.T. and Bowie, A.R. 2000. Importance of stirring in the development of an iron-fertilized phytoplankton bloom. Nature 407: 727-730.

Boyd, P.W., Watson, A.J., Law, C.S., Abraham, E.R., Trull, T., Murdoch, R., Bakker, D.C.E., Bowie, A.R., Buesseler, K.O., Chang, H., Charette, M., Croot, P., Downing, K., Frew, R., Gall, M., Hadfield, M., Hall, J., Harvey, M., Jameson, G., LaRoche, J., Liddicoat, M., Ling, R., Maldonado, M.T., McKay, R.M., Nodder, S., Pickmere, S., Pridmore, R., Rintoul, S., Safi, K., Sutton, P., Strzepek, R., Tanneberger, K., Turner, S., Waite, A. and Zeldis, J. 2000. A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization. Nature 407: 695-702.

Charlson, R.J., Lovelock, J.E., Andrea, M.O. and Warren, S.G. 1987. Oceanic phytoplankton, atmospheric sulfur, cloud albedo and climate. Nature 326: 655-661.

Legrand, M., Feniet-Saigne, C., Sattzman, E.S., Germain, C., Barkov, N.I. and Petrov, V.N. 1991. Ice-core record of oceanic emissions of dimethylsulfide during the last climate cycle. Nature 350: 144-146.

Martin, J.H. 1990. Glacial-interglacial CO2 change: The iron hypothesis. Paleoceanography 5: 1-13.

Martin, J.H. and Fitzwater, S.E. 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature 331: 341-343.

Martin, J.H., Gordon, M. and Fitzwater, S. 1988. Oceanic iron distributions in relation to phytoplanktonic productivity. EOS: Transactions of the American Geophysical Union 69: 1045.

Spall, S.A., Jones, A., Roberts, D.L. et al. 2001. Simulating DMS feedbacks on climate with a 3-D coupled atmosphere/ocean climate model. Paper presented at the Joint Oceanographic Assembly, IAPSO, IABO, Mar del Plata, Argentina.

Turner, S.M., Harvey, M.J., Law, C.S., Nightingale, P.D. and Liss, P.S. 2004. Iron-induced changes in oceanic sulfur biogeochemistry. Geophysical Research Letters 31: 10.1029/2004GL020296.

Turner, S.M., Nightingale, P.D., Spokes, L.J. et al. 1996. Increased dimethyl sulphide concentrations in sea water from in situ iron enrichment. Nature 383: 513-517.