Volume 7, Number 24: 16 June 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 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, that 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 CLAW's negative climate feedback loop.
In a new study of this phenomenon, Toole and Siegel (2004) note that this particular negative feedback cycle operates in the 15% of the world's oceans "consisting primarily of high latitude, continental shelf, and equatorial upwelling regions," where "DMS is accurately predicted as a function of the ratio of [the amount of] surface chlorophyll derived from satellite to [the depth of the] climatological mixed layer," which they refer to as the "bloom-forced regime." For the other 85% of the world's marine waters, they demonstrate that modeled surface DMS concentrations are independent of chlorophyll and are a function of the mixed layer depth alone, which they call the "stress-forced regime." So how does this latter regime operate?
For oligotrophic waters, Toole and Siegel find that "DMS biological production rates are negatively or insignificantly correlated with phytoplankton and bacterial indices for abundance and productivity while more than 82% of the variability is explained by UVR(325) [ultraviolet radiation at 325 nm]." This latter relationship, in their words, is "consistent with recent laboratory results (e.g., Sunda et al., 2002)," who demonstrated that intracellular DMS concentration and its biological precursors (particulate and dissolved dimethylsulfoniopropionate) "dramatically increase under conditions of acute oxidative stress such as exposure to high levels of UVR," which "are a function of mixed layer depth."
These results, which Toole and Siegel confirmed via an analysis of the Dacey et al. (1998) 1992-1994 organic sulfur time-series that was sampled in concert with the U.S. JGOFS Bermuda Atlantic Time-series Study (Steinberg et al., 2001), suggests, in their words, "the potential of a global change-DMS-climate feedback." Specifically, they say that "UVR doses will increase as a result of observed decreases in stratospheric ozone and the shoaling of ocean mixed layers as a result of global warming (e.g., Boyd and Doney, 2002)," and that "in response, open-ocean phytoplankton communities should increase their DMS production and ventilation to the atmosphere, increasing cloud condensing nuclei, and potentially playing out a coupled global change-DMS-climate feedback."
This second CLAW cycle, which operates over 85% of the world's marine waters and complements the first CLAW cycle, which operates over the other 15%, is but another manifestation of the wonderful capacity of earth's biosphere to regulate its affairs in such a way as to maintain climatic conditions over the vast majority of the planet within bounds conducive to the continued existence of life, in all its variety and richness, in the face of such major threats to its well-being as greenhouse gas-induced global warming. Without the host of biological phenomena that help to maintain a reasonable thermal homeostasis over the face of the earth [see Forcing Factors (Aerosols - Biological: Aquatic and Terrestrial, as well as Methane in our Subject Index], this anthropogenic-induced potential for significant climate change might otherwise spell the doom of many of the planet's lifeforms and confirm the worst fears (or pseudo-fears) of the world's climate alarmists. Thank goodness nature knows better than they do.
| Sherwood, Keith and Craig Idso |
References
Boyd, P.W. and Doney, S.C. 2002. Modeling regional responses by marine pelagic ecosystems to global climate change. Geophysical Research Letters 29: 10.1029/2001GL014130.
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.
Dacey, J.W.H., Howse, F.A., Michaels, A.F. and Wakeham, S.G. 1998. Temporal variability of dimethylsulfide and dimethylsulfoniopropionate in the Sagasso Sea. Deep Sea Research 45: 2085-2104.
Steinberg, D.K., Carlson, C.A., Bates, N.R., Johnson, R.J., Michaels, A.F. and Knap, A.H. 2001. Overview of the US JGOFS Bermuda Atlantic Time-series Study (BATS): a decade-scale look at ocean biology and biogeochemistry. Deep Sea Research Part II: Topical Studies in Oceanography 48: 1405-1447.
Sunda, W., Kieber, D.J., Kiene, R.P. and Huntsman, S. 2002. An antioxidant function for DMSP and DMS in marine algae. Nature 418: 317-320.
Toole, D.A. and Siegel, D.A. 2004. Light-driven cycling of dimethylsulfide (DMS) in the Sargasso Sea: Closing the loop. Geophysical Research Letters 31: 10.1029/2004GL019581.