Some time ago, Idso (1990) suggested that the volatilization of reduced sulfur gases from earth's soils may be just as important as dimethyl sulfide (DMS) emissions from the world's oceans in enhancing cloud albedo and thereby cooling the planet and providing a natural brake on the tendency for anthropogenically-enhanced greenhouse gases to drive global warming (see Dimethyl Sulfide in our Subject Index). On the basis of experiments that showed soil DMS emissions to be positively correlated with soil organic matter content, and noting that additions of organic matter to soils tend to increase the amount of sulfur gases they emit, Idso hypothesized that because atmospheric CO2 enrichment augments plant growth and, as a result, vegetative inputs of organic matter to earth's soils, this phenomenon should produce an impetus for cooling, even in the absence of the surface warming that sets in motion the chain of events that produce the oceanic DMS-induced negative-feedback that tends to cool the planet.
Two years later, Idso (1992) expanded this concept to include another biologically-produced sulfur gas that is emitted from soils (carbonyl sulfide or OCS), noting that it too is likely to be emitted in increasingly greater quantities as earth's vegetation responds to the aerial fertilization effect of the ongoing rise in the air's CO2 content, while pointing out that OCS is relatively inert in the troposphere, but that it eventually makes its way into the stratosphere, where it is transformed into solar-radiation-reflecting sulfate aerosol particles. He consequently concluded that the CO2-induced augmentation of soil OCS emissions constitutes a mechanism that can cool the planet's surface (1) in the absence of an impetus for warming (2) without producing additional clouds or (3) making them any brighter.
So what have we subsequently learned about the latter of these two natural cooling phenomena, i.e., the one that involves biologically-mediated increases in carbonyl sulfide emissions? One important thing is that the OCS-induced cooling mechanism also operates at sea, just as the DMS-induced cooling mechanism does, and that it too possesses a warming-induced component in addition to its CO2-induced component.
In a study contemporary with that of Idso (1992), ocean-surface OCS concentrations were demonstrated by Andreae and Ferek (1992) to be highly correlated with surface-water primary productivity. So strong is this correlation, in fact, that Erickson and Eaton (1993) developed an empirical model for computing ocean-surface OCS concentrations based solely on surface-water chlorophyll concentrations and values of incoming solar radiation. It has also been learned that an even greater portion of naturally-produced OCS is created in the atmosphere, where carbon disulfide and dimethyl sulfide - also largely of oceanic origin (Aydin et al., 2002) - undergo photochemical oxidation (Khalil and Rasmussen, 1984; Barnes et al., 1994). Hence, the majority of the tropospheric burden of OCS is ultimately dependent upon photosynthetic activity occurring near the surface of the world's oceans.
Why is this important? It is important because the tropospheric OCS concentration has risen by approximately 30% since the 1600s, from a mean value of 373 ppt over the period 1616-1694 to something on the order of 485 ppt today. This is a sizeable increase; and Aydin et al. (2002) note that only a fourth of it can be attributed to anthropogenic sources. Consequently, the rest of the observed OCS increase must have had a natural origin, a large portion of which must have ultimately been derived from the products and byproducts of marine photosynthetic activity, which must have increased substantially over the last three centuries. What is more, a solid case can be made (see our Editorial of 23 Oct 2002) for the proposition that both the increase in atmospheric CO2 concentration and the increase in temperature experienced over this period were the driving forces for the concomitant increase in tropospheric OCS concentration and its likely subsequent transport to the stratosphere, where it could exert the cooling influence on the earth that is described in the second paragraph of this review and that may have kept the warming of the globe considerably below what it might otherwise have been in the absence of this chain of events.
Another fascinating aspect of this multifaceted global "biothermostat" was revealed in a laboratory study of samples of the lichen Ramalina menziesii, which were collected from an open oak woodland in central California, USA, by Kuhn and Kesselmeier (2000). They found that when the lichens were optimally hydrated, they absorbed OCS from the air at a rate that gradually doubled as air temperature rose from approximately 3 to 25°C, whereupon their rate of OCS absorption began a precipitous decline that led to zero OCS absorption at 35°C.
The first portion of this response can be explained by the fact that most terrestrial plants prefer much warmer temperatures than a mere 3°C, so that as their surroundings warm and they grow better, they extract more OCS from the atmosphere in an attempt to promote even more warming and grow better still. At the point where warming becomes a detriment to them, however, they reverse this course of action and begin to rapidly reduce their rates of OCS absorption in an attempt to forestall warming-induced death. And since the consumption of OCS by lichens is under the physiological control of carbonic anhydrase - which is the key enzyme for OCS uptake in all higher plants, algae and soil organisms - we could expect this phenomenon to be generally operative over most of the earth. Hence, this thermoregulatory function of the biosphere may well be powerful enough to define an upper limit above which the surface air temperature of the planet may be restricted from rising, even when changes in other forcing factors, such as increases in greenhouse gas concentrations, produce an impetus for it to do so.
Clearly, this multifaceted phenomenon is extremely complex, with different biological entities tending to both increase and decrease atmospheric OCS concentrations at one and the same time, while periodically reversing directions in this regard in response to climate changes that push the temperatures of their respective environments either above or below the various thermal optima at which they function best. This being the case, there is obviously much more we need to learn about the many plant physiological mechanisms that may be involved. Nevertheless, it should be clear to everyone that we already know enough to realize that state-of-the-art climate models totally neglect these and many other vital biological processes (some of which may yet be undiscovered) that combine to determine the mean state of earth's climate; and until we fully understand the ultimate impact of the rest of the biosphere on the climate of the globe, we should not be rushing to impose a "remedy" upon the nations of the world for what we currently believe (likely erroneously) we may be doing to its climate.
References
Andreae, M.O. and Ferek, R.J. 1992. Photochemical production of carbonyl sulfide in seawater and its emission to the atmosphere. Global Biogeochemical Cycles 6: 175-183.
Aydin, M., De Bruyn, W.J. and Saltzman, E.S. 2002. Preindustrial atmospheric carbonyl sulfide (OCS) from an Antarctic ice core. Geophysical Research Letters 29: 10.1029/2002GL014796.
Barnes, I., Becker, K.H. and Petroescu, I. 1994. The tropospheric oxidation of DMS: a new source of OCS. Geophysical Research Letters 21: 2389-2392.
Erickson III, D.J. and Eaton, B.E. 1993. Global biogeochemical cycling estimates with CZCS satellite data and general circulation models. Geophysical Research Letters 20: 683-686.
Idso, S.B. 1990. A role for soil microbes in moderating the carbon dioxide greenhouse effect? Soil Science 149: 179-180.
Idso, S.B. 1992. The DMS-cloud albedo feedback effect: Greatly underestimated? Climatic Change 21: 429-433.
Khalil, M.A.K. and Rasmussen, R.A. 1984. Global sources, lifetimes, and mass balances of carbonyl sulfide (OCS) and carbon disulfide (CS2) in the earth's atmosphere. Atmospheric Environment 18: 1805-1813.
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.