Several years ago, Charlson et al. (1987) suggested the plausibility of a multi-step negative feedback phenomenon, whereby tiny marine phytoplankton may moderate the planet's temperature.  The cycle begins with a stimulus that provides an impetus for global warming, such as an increase in the air's CO2 content, which in turn stimulates phytoplanktonic productivity, leading to the production of more of an osmotic-regulatory substance called dimethylsulfonio propionate, which disperses throughout the surface waters of the world's oceans when the plankton that produce it either die or are eaten by zooplankton, whereupon a portion of it slowly decomposes to produce dimethyl sulfide (DMS), part of which diffuses into the atmosphere, where it is rapidly oxidized by OH and NO3 radicals and converted into sulfuric and methanesulfonic acid particles, which are carried aloft to ultimately function as cloud condensation nuclei, some of which produce new clouds and some of which dramatically increase the albedos of preexistent clouds.  The end result of this chain of events is a sizeable cooling effect, which Lovelock (1988) describes as "comparable in magnitude with that of the carbon dioxide greenhouse, but in opposition to it."

Since the publication of Charlson et al.'s hypothesis, much empirical evidence has been gathered in support of its several tenets.  A recent review, for example, states 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" (Ayers and Gillett, 2000).  In addition, 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 large 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 phytoplanktonic succession scenarios and food-web structure.

With respect to the power of the phenomenon, Sciare et al. (2000) examined ten years of DMS data from the vicinity of Amsterdam Island in the southern Indian Ocean, finding that a sea surface temperature increase of only 1°C was sufficient to increase the atmospheric DMS concentration there by as much as 50%, which could provide what they describe as a "very important" negative feedback that could potentially offset the original impetus for warming.  Much the same thing has also been found on the other side of the globe.  At the northernmost continuously-inhabited site in the world - Alert, Northwest Territories, Canada - weekly samples of airborne particulates from the period 1980 to 1991 were analyzed by Hopke et al. (1999).  One of their more interesting discoveries was the identification of biogenic sulfur, including sulfate and methane sulfonate, the concentrations of which were low in winter but high in summer.  They also found that the year-to-year variability in the strength of the biogenic sulfur signal was strongly correlated with the average temperature of the Northern Hemisphere.  In describing the significance of their observations, Hopke et al. said that their data suggest "that as the temperature rises, there is increased biogenic production of the reduced sulfur precursor compounds that are oxidized in the atmosphere to sulfate and methane sulfonate and could be evidence of a negative feedback mechanism in the global climate system."

In addition to unicellular algae, many other plants produce biogenic gases that influence earth's radiation balance in various ways.  Kuhn and Kesselmeier (2000), for example, studied the Lichen Ramalina menziesii, which they collected from an open oak woodland in central California.  When optimally hydrated the lichen absorbed carbonyl sulfide (COS) from the atmosphere at a rate that gradually doubled as air temperature rose from approximately 3 to 25°C, whereupon the absorption rate dropped precipitously, reaching a value of zero at 35°C.  As the absorption of COS thus rapidly declines as temperature rises above 25°C, more COS remains in the atmosphere, which increases the potential for it to make its way into the stratosphere, where it can be converted into sulfate aerosol particles that can reflect more incoming solar radiation back to space and thereby cool the earth.  What is more, since COS is the most stable and abundant reduced sulfur gas in the atmosphere, and since the consumption of COS by lichens is under the physiological control of carbonic anhydrase - which is the key enzyme for COS uptake in all higher plants, algae and soil organisms - we could expect this warming-induced negative feedback phenomenon to be generally operative over much of the earth.  Hence, it is possible that its strength may well rival that of the more commonly discussed negative feedback phenomenon involving DMS.

Trees also participate in various climate stabilization activities, as demonstrated by Kavouras et al. (1998), who measured a number of atmospheric gases and particles in a Eucalyptus forest in Portugal to see if there was any evidence of biologically-produced gases being converted into particles that could function as cloud condensation nuclei.  They found that certain hydrocarbons emitted by the trees do indeed experience gas-to-particle transformations.  In fact, aerosols produced from two specific organic acids comprised as much as 40% of the fine particle mass of the boundary-layer atmosphere during daytime hours.

These observations add a new twist to the climate stabilization picture; for they suggest the existence of a feedback response that is not triggered by warming but is stimulated by what is widely believed to be a primary cause of warming, i.e., rising atmospheric CO2 concentrations.  Independent of any increase in temperature, as the air's CO2 content rises so too does plant productivity rise, which leads to an enhanced evolution of biogenic gases that leads to the production of more cloud condensation nuclei, which leads to the creation of more and more-highly-reflective clouds that tend to cool the earth.  Thus, it is theoretically possible that an increase in the air's CO2 content could actually lead to a decrease in mean global air temperature.

Other studies suggest still further variations on this theme.  With more biogenic aerosols in the atmosphere as a consequence of the aerial fertilization effect of atmospheric CO2 enrichment, the amount of diffuse solar radiation reaching the surface of the earth is significantly enhanced, which reduces the volume of shade in vegetative canopies (Roderick et al., 2001); and with less internal shading, whole-canopy photosynthesis is enhanced (Healey et al., 1998), which results in more CO2 being removed from the air, as noted in our Editorial of 10 October 2001.

Another possible way by which atmospheric CO2 enrichment may reduce greenhouse gas-induced warming is to force a reduction in ecosystem methane emissions.  This concept derives from the work of Schrope et al. (1999), who studied plots of rice grown under conditions of ambient (350 ppm) and enriched (700 ppm) atmospheric CO2 concentrations.  In addition to enhancing aboveground biomass by as much as 35% and belowground biomass by up to 83%, the extra CO2 resulted in methane emissions from the rice plots being 10 to 45 times less than methane emissions from the ambient-treatment plots.  This finding was totally unexpected by the scientists who did the work; but in checking for potential problems with their study, they could identify none.  Hence, they stated for the record that their results "unequivocally support the conclusion that during this study, methane emissions from Oryza sativa plants grown under conditions of elevated CO2 were dramatically reduced relative to plants grown in comparable conditions under ambient levels of CO2."

Broecker and Sanyal (1998) describe yet another way in which atmospheric CO2 enrichment is self-limiting.  They note, first of all, that the full expression of the enhanced growth potential provided by the aerial fertilization effect of elevated levels of atmospheric CO2 requires the enhanced recovery from soils of various essential mineral nutrients, such as phosphorus, potassium and sulfur.  Second, they note that when grown in air enriched with CO2, plants are able to divert extra energy to the nourishment of fungi living on the tips of their rootlets that are "the dominant agents for the chemical weathering of soils."  Then, and completing the negative feedback loop, they note that the resulting enhancement of the chemical weathering reactions results in more CO2 being removed from the atmosphere above and beyond the extra amount that is removed by the CO2-induced increase in vegetative productivity.

In summary, in light of the many and diverse biologically-mediated ways in which increases in air temperature and/or atmospheric CO2 concentration limit the degree of warming the globe can experience, it is difficult to envision how the ongoing rise in the air's CO2 content could be anything but good for the planet's vegetation and its host of plant-supported animal life.  It would seem, therefore, that with respect to human-induced global warming, the only thing we have to fear is fear itself.

References
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.

Broecker, W.S. and Sanyal, A.  1998.  Does atmospheric CO2 police the rate of chemical weathering?  Global Biogeochemical Cycles 12: 403-408.

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.

Healey, K.D., Rickert, K.G., Hammer, G.L. and Bange, M.P.  1998.  Radiation use efficiency increases when the diffuse component of incident radiation is enhanced under shade.  Australian Journal of Agricultural Research 49: 665-672.

Hopke, P.K., Xie, Y. and Paatero, P.  1999.  Mixed multiway analysis of airborne particle composition data.  Journal of Chemometrics 13: 343-352.

Kavouras, I.G., Mihalopoulos, N. and Stephanou, E.G.  1998.  Formation of atmospheric particles from organic acids produced by forests.  Nature 395: 683-686.

Kuhn, U. and Kesselmeier, J.  2000.  Environmental variables controlling the uptake of carbonyl sulfide by lichens.  Journal of Geophysical Research 105: 26,783-26,792.

Lovelock, J.E.  1988.  The Ages of Gaia: A Biography of Our Living Earth.  W.W. Norton & Co., New York, NY.

Roderick, M.L., Farquhar, G.D., Berry, S.L. and Noble, I.R.  2001.  On the direct effect of clouds and atmospheric particles on the productivity and structure of vegetation.  Oecologia 129: 21-30.

Schrope, M.K., Chanton, J.P., Allen, L.H. and Baker, J.T.  1999.  Effect of CO2 enrichment and elevated temperature on methane emissions from rice, Oryza sativa.  Global Change Biology 5: 587-599.

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.

Simo, R. and Pedros-Alio, C.  1999.  Role of vertical mixing in controlling the oceanic production of dimethyl sulphide.  Nature 402: 396-399.