Farek et al. (1998), for example, observed an increase in the reflectance of solar radiation from clouds exposed to the airborne effluents of ships; and Capaldo et al. (1999) determined that this phenomenon creates a significant cooling influence over water surfaces in both the Northern and Southern Hemispheres.  In addition, Facchini et al. (1999) report that organic solutes evolving from agricultural/industrial regions tend to enhance cloud reflectance over land.

In discussing this subject in more detail (see our Editorials of 25 July 2001 and 1 August 2001), Charlson et al. (2001) say that droplet clouds "are the most important factor controlling the albedo (reflectivity) and hence the temperature of our planet," and that man-made aerosols "have a strong influence on cloud albedo, with a global mean forcing estimated to be of the same order (but opposite in sign) as that of greenhouse gases."  It can readily be appreciated, therefore, that even a small change in cloud properties could well determine whether the combined influence of all anthropogenic activities results in a net warming or cooling of the planet.

What is particularly interesting in this regard is that the results of several recent empirical studies lead Charlson et al. to conclude that the anthropogenic impetus for cooling "may be even larger than anticipated."  Anticipated by who?  By those who prepared the most recent IPCC reports, of course.  Specifically, Charlson and his colleagues say the IPCC assessments of the situation "do not include the combined influences of some recently identified chemical factors, each of which [our italics] leads to additional negative forcing (cooling) on top of that currently estimated."

What are some examples of the things to which Charlson et al. allude?  "It has recently become clear," they say, "that soluble gases, slightly soluble solutes [aerosols], and surface tension depression by organic substances also influence the formation of cloud droplets."  And the ways in which mankind's activities influence these processes all tend to produce extra cloud cooling power that is nowhere to be found in IPCC analyses of cloud effects on climate.  Yet even all of these phenomena are but the tip of the climate-cooling iceberg of negative feedbacks to global warming that are basically ignored by the IPCC.

Consider, for example, the hypothesis developed by Charlson et al. (1987) - which has inspired literally hundreds of subsequent confirmatory studies - wherein biology plays an integral role in mitigating global warming.  This scenario begins with an initial impetus for warming that (1) stimulates primary production in marine phytoplankton, which (2) results in the production of more copious quantities of dimethylsulphoniopropionate (DMSP), which (3) leads to the evolution of greater amounts of dimethyl sulphide (DMS) in the surface waters of the world's oceans, which (4) diffuse into the atmosphere, where (5) the DMS is oxidized, which (6) leads to the creation of acidic aerosols, which (7) function as cloud condensation nuclei, which (8) create more and brighter clouds of higher albedo, which (9) reflect more incoming solar radiation back to space, which (10) cools the planet and thereby counters the initial impetus for warming.

Simo and Pedros-Alio (1999) have recently added even more complexity to this scenario by describing a number of short-term photo-induced (and, thereby, mixing-depth mediated) influences on several complex physiological phenomena manifest in marine phytoplankton, as well as longer-term variations in vertical mixing that influence planktonic succession and food-web structure.  In addition, Ayers and Gillett (2000) have recently summarized empirical evidence in support of Charlson et al.'s hypothesis obtained from data collected at Cape Grim, Tasmania since 1988, as well as from what has been reported in prior studies of the subject.  They too conclude 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."

In a somewhat analogous study, Sciare et al. (2000) made continuous measurements of atmospheric DMS concentration from 1990 to 1999 at Amsterdam Island in the southern Indian Ocean, along with concomitant measurements of a number of environmental parameters, finding a clear seasonal variation with a factor of 20 difference in amplitude between the maximum atmospheric DMS concentration in austral summer and the minimum in austral winter.  Most importantly, DMS anomalies in their study 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 1°C was sufficient to increase the atmospheric DMS concentration by as much as 50% on a monthly basis, providing what they called a "very important" albedo-moderated negative feedback on the original impetus for warming.

In addition to DMS there is COS, or carbonyl sulfide, which operates in a somewhat similar earth albedo-enhancing fashion over land.  This substance is the most stable and abundant reduced sulfur gas in the atmosphere and a major player in determining earth's radiation balance.  After making its way into the stratosphere, for example, it can be photodissociated, as well as oxidized, to form SO2, which is typically converted to sulfate aerosol particles that are highly reflective of incoming solar radiation and, therefore, have the capacity to significantly cool the earth.  And, like DMS, COS is heavily influenced by planetary biology.  Here's how it works.

In a study of COS uptake by a lichen species found in an open oak woodland in central California, Kuhn and Kesselmeier (2000) observed the rate of absorption of COS from the atmosphere by this species to decline dramatically once air temperature rose above 25°C.  Hence, when temperatures begin to become uncomfortable for this and many other species of plants (and animals), more COS remains in the air, which increases the potential for more of 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.  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 phenomenon to be generally operative over much of the planet.  Hence, this biological "thermostat" may well be powerful enough to define an upper limit above which the surface air temperature of the earth may be restricted from rising, even when changes in other forcing factors, such as greenhouse gases, produce a substantial impetus for warming.

That several of the above-described phenomena - as well as others yet to be elucidated - may well be occurring at the present time is suggested by the study of Herman et al. (2001), who used satellite data to determine changes in radiation reflected back to space over the period 1979 to 1992.  Their data indicate "there have been increases in reflectivity (cloudiness) poleward of 40°N and 30°S, with some smaller but significant changes occurring in the equatorial and lower middle latitudes."  The overall long-term effect, they report, is for an increase in radiation reflected back to space of 2.8 Wm^-2 per decade, from which they conclude "there is a likely cooling effect" that is provided "by changes in the amount of snow/ice, cloudiness, and aerosols."

In light of these multitudinous observations, we conclude there is a plethora of natural and anthropogenic-induced negative feedbacks to purported global warming that are more than capable of maintaining the climate of the globe within a temperature range conducive to the continued well-being of all forms of life currently found upon the face of the earth ... and in the sea, the soil, and the air.

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.

Capaldo, K., Corbett, J.J., Kasibhatla, P., Fischbeck, P. and Pandis, S.N.  1999.  Effects of ship emissions on sulphur cycling and radiative climate forcing over the ocean.  Nature 400: 743-746.

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.

Charlson, R.J., Seinfeld, J.H., Nenes, A., Kulmala, M., Laaksonen, A. and Facchini, M.C.  2001.  Reshaping the theory of cloud formation.  Science 292: 2025-2026.

Facchini, M.C., Mircea, M., Fuzzi, S. and Charlson, R.J.  1999.  Cloud albedo enhancement by surface-active organic solutes in growing droplets.  Nature 401: 257-259.

Ferek, R.J., Hegg, D.A., Hobbs, P.V., Durkee, P. and Nielsen, K.  1998.  Measurements of ship-induced tracks in clouds off the Washington coast.  Journal of Geophysical Research 103: 23,199-23,206.

Herman, J.R., Larko, D., Celarier, E. and Ziemke, J.  2001.  Changes in the Earth's UV reflectivity from the surface, clouds, and aerosols.  Journal of Geophysical Research 106: 5353-5368.

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.

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.