From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
More than two decades ago, Charlson et al. (1987) discussed the plausibility of a multi-stage negative feedback process, whereby warming-induced increases in the emission of dimethyl sulfide (DMS) from the world’s oceans tend to counteract any initial impetus for warming. The basic tenet of their hypothesis was that the global radiation balance is significantly influenced by the albedo of marine stratus clouds (the greater the cloud albedo, the less the input of solar radiation to the earth’s surface). The albedo of these clouds, in turn, is known to be a function of cloud droplet concentration (the more and smaller the cloud droplets, the greater the cloud albedo and the reflection of solar radiation), which is dependent upon the availability of cloud condensation nuclei on which the droplets form (the more cloud condensation nuclei, the more and smaller the cloud droplets). And in completing the negative feedback loop, Charlson et al. noted that the cloud condensation nuclei concentration often depends upon the flux of biologically produced DMS from the world’s oceans (the higher the sea surface temperature, the greater the sea-to-air flux of DMS).
Since the publication of Charlson et al.’s initial hypothesis, much empirical evidence has been gathered in support of its several tenets. One review, for example, states that “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 and reactive atmospheric chemistry in the remote marine boundary layer of the Southern Hemisphere” (Ayers and Gillett, 2000).
But just how strong is the negative feedback phenomenon proposed by Charlson et al.? Is it powerful enough to counter the threat of greenhouse gas-induced global warming? According to the findings of Sciare et al. (2000), it may well be able to do just that.
In examining 10 years of DMS data from Amsterdam Island in the southern Indian Ocean, these researchers found that a sea surface temperature increase of only 1°C was sufficient to increase the atmospheric DMS concentration by as much as 50 percent. This finding suggests that the degree of warming typically predicted to accompany a doubling of the air’s CO2 content would increase the atmosphere’s DMS concentration by a factor of three or more, providing what they call a “very important” negative feedback that could potentially offset the original impetus for warming.
Other research has shown that this same chain of events can be set in motion by means of phenomena not discussed in Charlson et al.’s original hypothesis. Simo and Pedros-Alio (1999), for example, discovered that the depth of the surface mixing-layer has a substantial influence on DMS yield in the short term, via a number of photo-induced (and thereby mixing-depth mediated) influences on several complex physiological phenomena, as do longer-term seasonal variations in vertical mixing, via their influence on seasonal planktonic succession scenarios and food-web structure.
More directly supportive of Charlson et al.’s hypothesis was the study of Kouvarakis and Mihalopoulos (2002), who measured seasonal variations of gaseous DMS and its oxidation products—non-sea-salt sulfate (nss-SO42-) and methanesulfonic acid (MSA)—at a remote coastal location in the Eastern Mediterranean Sea from May 1997 through October 1999, as well as the diurnal variation of DMS during two intensive measurement campaigns conducted in September 1997. In the seasonal investigation, DMS concentrations tracked sea surface temperature (SST) almost perfectly, going from a low of 0.87 nmol m-3 in the winter to a high of 3.74 nmol m-3 in the summer. Such was also the case in the diurnal studies: DMS concentrations were lowest when it was coldest (just before sunrise), rose rapidly as it warmed thereafter to about 1100, after which they dipped slightly and then experienced a further rise to the time of maximum temperature at 2000, whereupon a decline in both temperature and DMS concentration set in that continued until just before sunrise. Consequently, because concentrations of DMS and its oxidation products (MSA and nss-SO42-) rise dramatically in response to both diurnal and seasonal increases in SST, there is every reason to believe that the same negative feedback phenomenon would operate in the case of the long-term warming that could arise from increasing greenhouse gas concentrations, and that it could substantially mute the climatic impacts of those gases.
Also of note in this regard, Baboukas et al. (2002) report the results of nine years of measurements of methanesulfonate (MS-), an exclusive oxidation product of DMS, in rainwater at Amsterdam Island. Their data, too, revealed “a well distinguished seasonal variation with higher values in summer, in line with the seasonal variation of its gaseous precursor (DMS),” which, in their words, “further confirms the findings of Sciare et al. (2000).” In addition, the MS- anomalies in the rainwater were found to be closely related to SST anomalies; and Baboukas et al. 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.”
In a newer study of this phenomenon, Toole and Siegel (2004) note that it has been shown to operate as described above in the 15 percent of the world’s oceans “consisting primarily of high latitude, continental shelf, and equatorial upwelling regions,” where DMS may be accurately predicted as a function of the ratio of the amount of surface chlorophyll derived from satellite observations to the depth of the climatological mixed layer, which they refer to as the “bloom-forced regime.” For the other 85 percent 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 the warming-induced DMS negative feedback cycle operate in these waters?
For oligotrophic regimes, 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 percent of the variability is explained by UVR(325) [ultraviolet radiation at 325 nm].” This 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 dimethylsulfoniopro-pionate) “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)—suggest, 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 DMS-induced negative-feedback cycle, which operates over 85 percent of the world’s marine waters and complements the first DMS-induced negative-feedback cycle, which operates over the other 15 percent, is another manifestation of the 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’s surface within bounds conducive to the continued existence of life, in all its variety and richness. In addition, it has been suggested that a DMS-induced negative climate feedback phenomenon also operates over the terrestrial surface of the globe, where the volatilization of reduced sulfur gases from soils may be just as important as marine DMS emissions in enhancing cloud albedo (Idso, 1990).
On the basis of experiments that showed soil DMS emissions to be positively correlated with soil organic matter content, for example, and noting that additions of organic matter to a soil tend to increase the amount of sulfur gases emitted therefrom, Idso (1990) hypothesized that because atmospheric CO2 is an effective aerial fertilizer, augmenting its atmospheric concentration and thereby increasing vegetative inputs of organic matter to earth’s soils should also produce an impetus for cooling, even in the absence of surface warming.
Nevertheless, and in spite of the overwhelming empirical evidence for both land- and ocean-based DMS-driven negative feedbacks to global warming, the effects of these processes have not been fully incorporated into today’s state-of-the-art climate models. Hence, the warming they predict in response to future anthropogenic CO2 emissions must be considerably larger than what could actually occur in the real world. It is very possible these biologically driven phenomena could entirely compensate for the warming influence of all greenhouse gas emissions experienced to date, as well as all those anticipated to occur in the future.
In the 2009 NIPCC report, Idso and Singer (2009) discussed the plausibility of a multistage negative feedback process whereby warming-induced increases in the emission of dimethyl sulfide (DMS) from the world’s oceans tend to counteract any initial impetus for warming. The basic tenet of this hypothesis is that the global radiation balance is significantly influenced by the albedo of marine stratus clouds (the greater the cloud albedo, the less the input of solar radiation to the Earth’s surface). The albedo of these clouds, in turn, is known to be a function of cloud droplet concentration (the more and smaller the cloud droplets, the greater the cloud albedo and the reflection of solar radiation), which is dependent upon the availability of cloud condensation nuclei on which the droplets form (the more cloud condensation nuclei, the more and smaller the cloud droplets). And in completing the negative feedback loop, the cloud condensation nuclei concentration often depends upon the flux of biologically produced DMS from the world’s oceans (the higher the sea surface temperature, the greater the sea-to-air flux of DMS).
Since the publication of the 2009 NIPCC report, additional empirical evidence has been found to support the several tenets of the DMS feedback process. Qu and Gabric (2010), for example, introduce their contribution to the subject by stating, “dimethylsulfide (DMS) is the main volatile sulfur [species] released during the formation and decay of microbial ocean biota” and “aerosols formed from the atmospheric conversion of DMS to sulfate and methanesulfonic acid can exert a climate cooling effect directly by scattering and absorbing solar radiation and indirectly by promoting the formation of cloud condensation nuclei and increasing the albedo of clouds, thus reflecting more solar radiation back into space.”
Working with climate and DMS production data from the region of the Barents Sea (70–80°N, 30–35°E) obtained over the period 1998 to 2002, Qu and Gabric employed a genetic algorithm to calibrate chlorophyll-a measurements (obtained from SeaWiFS satellite data) for use in a regional DMS production model. Then, using GCM temperature outputs for the periods 1960–1970 (pre-industry CO2 level) and 2078–2086 (triple the pre-industry CO2 level), they calculated the warming-induced enhancement of the DMS flux from the Barents Sea region.
The two researchers report, “significantly decreasing ice coverage, increasing sea surface temperature and decreasing mixed-layer depth could lead to annual DMS flux increases of more than 100% by the time of equivalent CO2 tripling (the year 2080).” In commenting on their findings, they state, “such a large change would have a great impact on the Arctic energy budget and may offset the effects of anthropogenic warming that are amplified at polar latitudes.” What is more, they write, “many of these physical changes will also promote similar perturbations for other biogenic species (Leck et al., 2004), some of which are now thought to be equally influential to the aerosol climate of the Arctic Ocean.” Thus it can be appreciated that DMS production in a warming world—especially when augmented by analogous biogenic phenomena—may provide a large moderating influence on the primary impetus for warming that is produced by mankind’s emissions of CO2 and other greenhouse gases.
Kim et al. (2010) write that DMS “represents 95% of the natural marine flux of sulfur gases to the atmosphere (Bates et al., 1992; Liss et al., 1997),” and they say it “may be oxidized to form non sea-salt sulfate aerosols, which are known to act as cloud condensation nuclei and thereby exert a cooling effect by absorbing or scattering solar radiation.” They cite Charlson et al. (1987), who first described the intriguing and important chain of events. They also note “DMS is generated by intracellular or extracellular enzymatic cleavage of DMSP [dimethylsulfoniopropionate] by DMSP-lyase, which is synthesized by algae and bacteria, following DMSP secretion from producer cells or release following autolysis or viral attack,” while noting that “grazing activity can also result in DMSP conversion to DMS if DMSP and DMSP-lyase are physically mixed following grazing,” citing Stefels et al., 2007, and Wolfe and Steinke, 1996.
Working in the coastal waters of Korea from 21 November to 11 December 2008, the 14 Korean scientists utilized 2,400-liter mesocosm enclosures to simulate, in triplicate, three sets of environmental conditions—an ambient control (~400 ppm CO2 and ambient temperature), an acidification treatment (~900 ppm CO2 and ambient temperature), and a greenhouse treatment (~900 ppm CO2 and ~3°C warmer-than-ambient temperature)—and within these mesocosms they initiated phytoplankton blooms by adding equal quantities of nutrients to each mesocosm on day 0. For 20 days thereafter they measured numerous pertinent parameters within each mesocosm. This work revealed, as they describe it, that “total accumulated DMS concentrations (integrated over the experimental period) in the acidification and greenhouse mesocosms were approximately 80% and 60% higher than the values measured in the control mesocosms, respectively,” which they attribute to the fact that, in their experiment, “autotrophic nanoflagellates (which are known to be significant DMSP producers) showed increased growth in response to elevated CO2” and “grazing rates [of microzooplankton] were significantly higher in the treatment mesocosms than in the control mesocosms.” In the concluding paragraph of their paper, they write, “the key implication of our results is that DMS production resulting from CO2-induced grazing activity may increase under future high CO2 conditions,” concluding that “DMS production in the ocean may act to counter the effects of global warming in the future.”
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