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Cloud Condensation Nuclei (Climatic Effects of Biologically-Produced Aerosols and Gases) Summary
Cloud condensation nuclei are small particles in the atmosphere about which water vapor may condense to form clouds.  These particles may be of either natural or anthropogenic origin; and they can influence the amount and transfer of heat within the earth-ocean-atmosphere system by altering cloud formation, type, albedo and duration, thereby exerting a significant force upon the planet's climate and impacting both aquatic and terrestrial ecosystems.  In this Summary, we thus distill the results of scientific papers we have reviewed that deal with the climatic effects of plant-produced aerosols and gases.

Kavouras et al. (1998) 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 to particles that could function as cloud condensation nuclei.  Their work revealed that certain hydrocarbons emitted by vegetation (isoprene and terpenes, in particular) do indeed experience gas-to-particle transformations.  In fact, aerosols produced from two of these organic acids (cis- and trans-pinonic acid) comprised as much as 40% of the fine-particle atmospheric mass above the forest during daytime hours.

Similar results were obtained by O'Dowd et al. (2002), who measured aerosol electrical-mobility size-distributions before and during an atmospheric nucleation event over a boreal forest in Finland, while simultaneously measuring organic vapor growth rates of particles that nucleated into organic cloud-droplets in the flow-tube cloud chamber of a modified condensation-particle counter.  They also demonstrated that newly-formed aerosol particles over forested areas "are composed primarily of organic species, such as cis-pinonic acid and pinic acid, produced by oxidation of terpenes in organic vapours released from the canopy," further noting that "aerosol particles produced over forested areas may affect climate by acting as nuclei for cloud condensation."

These findings clearly demonstrate that the terrestrial plant life of the planet can influence earth's climate.  Specifically, they reveal a direct connection between the metabolic activity of trees and the propensity for the atmosphere to produce clouds.  What is more, the relationship is one that is self-protecting of the biosphere: as the air's CO2 content rises, plant productivity rises, which leads to an enhanced evolution of biogenic gases, which leads to the production of more cloud condensation nuclei, which leads to the creation of more and longer-lived brighter clouds that reflect more solar radiation back to space, which tends to counteract any increase in the strength of the atmosphere's greenhouse effect that may have been produced by the increase in the air's CO2 content.  In addition, Idso (1990) has indicated that similar phenomena operate all the way down to the level of soil microbes.

Much the same thing occurs at sea.  As a specific example, literally hundreds of scientific studies have shown how an initial impetus for warming tends to (1) stimulate primary production in marine phytoplankton, which (2) results in the production of more copious quantities of dimethylsulphoniopropionate, which (3) leads to the evolution of greater amounts of dimethylsulphide or DMS in the surface waters of the world's oceans, which (4) diffuses into the atmosphere, where the DMS (5) is oxidized, which (6) leads to the creation of acidic aerosols, which (7) function as cloud condensation nuclei, which (8) create more longer-lived and brighter clouds, which (9) reflect more incoming solar radiation back to space, which (10) cools the planet and thereby counters the impetus for warming. What is more, the study of Simo and Pedros-Allo (1999) additionally demonstrates how warming-induced changes in mixing-layer depth promote this same chain of events via a number of phenomena not previously elucidated.  Hence, there is greater reason than ever to believe that intricately-related biological-physical processes tend to maintain earth's surface temperature regime within limits conducive to the continued existence of life; and the chief implication of this world-view of the matter is that rising levels of atmospheric CO2 should not lead to deleterious global warming.

In a review of the multitude of papers that have focused on this long chain of events and that trace their origins back to the original seminal study of Charlson et al. (1987), Ayers and Gillett (2000) marshal empirical evidence in support of Charlson et al.'s hypothesis from data collected at Cape Grim, Tasmania since 1988, as well as from what has been reported in prior peer-reviewed papers on the subject.  They find, in their words, that "major links in the feedback chain proposed by Charlson et al. 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."

Also studying DMS were Sciare et al. (2000), who made continuous measurements of atmospheric DMS concentrations over a 10-year period (1990-1999) at Amsterdam Island in the southern Indian Ocean and compared their results with a number of other environmental parameters measured over the same period.  They found that atmospheric DMS concentration showed "a clear seasonal variation with a factor of 20 in amplitude between its maximum in January (austral summer) and minimum in July-August (austral winter)."  In addition, DMS anomalies were found to be "closely related to sea surface temperature anomalies, clearly indicating a link between DMS and climate changes."  Indeed, they determined that a sea surface temperature (SST) increase of only 1C was sufficient to increase the atmospheric DMS concentration by as much as 50% on a monthly basis.

"To our knowledge," say Sciare et al., "this is the first time that a direct link between SSTs and atmospheric DMS is established for a large oceanic area."  And the effect they documented is huge.  If the relationship between DMS and SST is linear, for example - and their data suggest that it likely is - the degree of warming 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.  And in the estimation of Sciare et al., such an increase in DMS would have a "very important" negative feedback on the original impetus for warming.  In fact, it could easily overwhelm it.

In conclusion, nearly all of earth's plants, both terrestrial and aquatic and from the most highly developed all the way down to the unicellular level, appear to operate in such a manner that in response to an impetus for warming, they intensify various biological activities that ultimately result in the production of greater quantities of cloud condensation nuclei, which phenomenon leads to the creation of (1) more clouds, (2) longer-lived clouds and (3) brighter clouds, all of which changes result in the reflection of greater amounts of solar radiation back to space and the countering or negation of the impetus for warming.  Consequently, and in view of the fact that anthropogenically-produced cloud condensation nuclei exert a similar influence on earth's climate (see Cloud Condensation Nuclei (Climatic Effects of Anthropogenic Aerosols and Gases) in our Subject Index), there is little reason to believe there will ever be a significant CO2-induced warming of the globe.

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.

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.

Idso, S.B.  1990.  A role for soil microbes in moderating the carbon dioxide greenhouse effect?  Soil Science 149: 179-180.

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

O'Dowd, C.D., Aalto, P., Hameri, K., Kulmala, M. and Hoffmann , T.  2002.  Atmospheric particles from organic vapours.  Nature 416: 497-498.

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.

Last updated 10 August 2005