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Demise of "The CLAW" Greatly Exaggerated
Volume 6, Number 1: 1 January 2003

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 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 climate feedback cycle.

In the decade following its initial description, the CLAW hypothesis was the subject of over 700 scientific papers describing the biogeochemistry of DMS, its precursors, and their connection to earth's climate; but because of the great complexity of the concept's many linked processes, Andreae and Crutzen (1997) admitted fully ten years after its formulation that "even the overall sign of the feedback cannot be deduced with certainty, because it is not yet known if a warming climate would result in an increase or decrease of DMS emissions."

This admission was a great stimulus to further work on the hypothesis, one of the most recent examples of which is the study of van Rijssel and Gieskes (2002), who conclude that DMS does not "play the role in climate regulation formulated in the CLAW hypothesis that proposes a feedback mechanism in which elevated temperatures lead to an increase in albedo via DMS-derived cloud condensation nuclei." Hence, we thought it important to review what the two marine biologists did, as well as what some other scientists have learned about the subject.

Van Rijssel and Gieskes measured various growth characteristics and the production of dimethylsulfoniopropionate (DMSP), the immediate precursor of DMS, in a strain of Emiliania huxleyi (a coccolithophorid originally isolated from the Oslo Fjord) in flasks filled with 250 ml of artificial seawater placed within controlled environment cabinets maintained at temperatures of 4.0, 9.0, 12.0, 14.8, 18.2, 20.5 and 23.2C. Their findings? Initial growth rate increased by a factor of seven in going from 4.0 to 23.2C, although final end values of biomass were equivalent in all but the 4C treatment, where they were lower than the rest. The scientists also noted that individual cells were smaller at the higher temperatures. Most important of all, however - according to the authors - "there was a negative effect of temperature on the DMSP content expressed per [cellular] volume," such that over the temperature range in which the coccolithophorid is found in natural situations (5-15C), there would be a "two-fold drop in DMSP per volume" with a 10C warming.

If van Rijssel and Gieskes had concluded their discussion at that point, we would have had no problem with their paper. However, they apparently felt a need to extrapolate their findings worldwide, saying "on the assumption that the response of E. huxleyi is characteristic of DMSP-producing microalgae, a decrease rather than an increase of the DMSP concentration in the cells must be expected upon the temperature rise of the ocean now so often discussed under the heading 'Global Warming'," stating further that "this result seems to contradict the feedback mechanism proposed by Charlson et al. (1987)," and finally concluding that global warming "does certainly not necessarily result in increased sea-to-air emission of DMS."

What is wrong with this reasoning? First of all, it rests on the assumption that the response of E. huxlei to warming is characteristic of DMSP-producing microalgae in general, which is highly debatable. In their major review of the CLAW hypothesis, for example, Andreae and Crutzen (1997) note that the intracellular concentration of DMSP varies among different phytoplankton species over a range of five orders of magnitude, which suggests to us that no single species can be described as being "characteristic" of the rest, in perhaps any regard. Although it is clear that some taxonomic groups typically contain higher amounts of DMSP, as Andreae and Crutzen note, they also report that "these relations are by no means clear cut." In addition, it does not seem reasonable to expect that a coccolithophorid isolated from waters that typically experience temperatures of 5-15C would necessarily exhibit a DMSP response to warming of the same nature as that of marine microalgae found in much warmer waters, where keeping temperatures from rising higher would appear to be a much more crucial and highly-valued enterprise, as evidenced by the fact that the algal symbionts of the corals of these regions appear to be living very close to the upper limit of their thermal tolerance range.

It is also unrealistic to presume that results obtained from so unnatural and limited a setting as that employed by van Rijssel and Gieskes (a single strain of coccolithophorid growing in 250 ml of artificial seawater in flasks containing no other life forms) could be extrapolated to infer an end result - the validity or invalidity of the CLAW hypothesis - characteristic of the real-world world of nature, where according to Andreae and Crutzen "the release of DMSP into the water column is controlled by senescence or by grazing by viruses, bacteria, and zooplankton, which in turn is influenced by the dynamics of the phytoplankton population" - none of which phenomena were studied by van Rijssel and Gieskes - and where the subsequent breakdown of DMSP to DMS "is microbially mediated" and the liberated DMS is subject to a number of removal mechanisms in addition to emission to the atmosphere, "including bacterial and photochemical decomposition ... and downward mixing."

Yes, the world of nature is so complex that a multi-step negative feedback phenomenon, such as that envisioned in the CLAW hypothesis, cannot be adequately evaluated in the laboratory; it must be done in situ. So what have people found when this approach has been employed?

In another review of this rapidly-evolving field of research that was published just three years after Andreae and Crutzen's review, enough had been learned that Ayers and Gillett (2000) were able to report that the "major links in the feedback chain proposed by Charlson et al. (1987) have a sound physical basis," and that there is "compelling observational evidence to suggest that DMS and its atmospheric products participate significantly in processes of climate regulation."

In that same year, Sciare et al. (2000) reported the results of a ten-year-long set of measurements of atmospheric DMS concentration at Amsterdam Island in the southern Indian Ocean, where they observed "a clear seasonal variation with a factor of 20 in amplitude between its maximum in January (austral summer) and minimum in July-August (austral winter)." In addition, DMS anomalies were found to be "closely related to sea surface temperature anomalies, clearly indicating a link between DMS and climate changes." In fact, they found that a sea surface temperature increase of only 1C was sufficient to increase the atmospheric DMS concentration by as much as 50% on a monthly basis.

Most recently, Baboukas et al. (2002) reported the results of nine years of measurements of methanesulfonate (MS-), an exclusive oxidation product of DMS, in rainwater at Amsterdam Island. Their data, too, reveal "a well distinguished seasonal variation with higher values in summer, in line with the seasonal variation of its gaseous precursor (DMS)," which, as they say, "further confirms the findings of Sciare et al. (2000)." In addition, the MS- anomalies in the rainwater were found to be closely related to sea surface temperature anomalies; and the authors say this observation provides even more support for "the existence of a positive ocean-atmosphere feedback on the biogenic sulfur cycle above the Austral Ocean, one of the most important DMS sources of the world."

Clearly, the studies of Sciare et al. and Baboukas et al. leave no doubt but what the CLAW hypothesis is very much alive and well. Compared to their decade of daily measurements performed on the real-world air above the Austral Ocean, the real-world rain that falls upon it, and the real-world waters that comprise its body, van Rijssel and Gieskes' month-long laboratory study of one specific strain of coccolithophorid growing in 250 ml portions of artificial seawater in a set of flasks in controlled environment cabinets fades into insignificance. The work of the latter authors only touches on one small aspect of the vast and complicated set of phenomena that combine to produce "the CLAW that keeps earth's climate cool." It can in no way be construed to weaken that hypothesis.

Sherwood, Keith and Craig Idso

Andreae, M.O. and Crutzen, P.J. 1997. Atmospheric aerosols: biogeochemical sources and role in atmospheric chemistry. Science 276: 1052-1058.

Ayers, G.P. and Gillett, R.W. 2000. DMS and its oxidation products in the remote marine atmosphere: implications for climate and atmospheric chemistry. Journal of Sea Research 43: 275-286.

Baboukas, E., Sciare, J. and Mihalopoulos, N. 2002. Interannual variability of methanesulfonate in rainwater at Amsterdam Island (Southern Indian Ocean). Atmospheric Environment 36: 5131-5139.

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.

Sciare, J., Mihalopoulos, N. and Dentener, F.J. 2000. Interannual variability of atmospheric dimethylsulfide in the southern Indian Ocean. Journal of Geophysical Research 105: 26,369-26,377.

van Rijssel, M. and Gieskes, W.W.C. 2002. Temperature, light, and the dimethylsulfoniopropionate (DMSP) content of Emiliania huxleyi (Prymnesiophyceae). Journal of Sea Research 48: 17-27.