Ferek et al. (1998) studied distinct ship tracks in a uniform layer of marine stratus clouds that were created by two ships that left the mouth of the USA's Columbia River early in the morning of 26 August 1992, utilizing data obtained from Advanced Very High Resolution Radiometers on U.S. Government satellites, as well as in situ measurements obtained during repeated aircraft flights through the ship tracks.  These efforts resulted in what they call "the most complete data set described to date of ship-track formation produced by specific ships."  Chemical analyses of cloud water in and out of the ship tracks showed that cloud condensation nuclei in the effluents from the ships were responsible for creating them.  Furthermore, cloud droplet spectra measured in the ship tracks showed higher number concentrations and lower droplet effective radii than spectra measured in adjacent non-ship-impacted clouds, both of which alterations typically make clouds brighter and more persistent and, therefore, better able to cool the part of the planet over which they float.

Exploring the global climatic implications of this phenomenon, Capaldo et al. (1999) used a "data-plus-model approach" to evaluate the contributions of ship emissions to atmospheric SO2, sulphate and cloud condensation nuclei, as well as the impacts of these factors on the surface radiation balance over the sea.  They concluded that ship sulphur emissions are nearly equal to the natural sulphur flux from the ocean to the atmosphere in many areas, and that averaged over the surface of the earth, the radiative impact of ship tracks near the end of the 20th century was -0.16 Wm^-2 in the Northern Hemisphere and -0.06 Wm^-2 in the Southern Hemisphere.

Higher in the atmosphere, jet aircraft contrails also impact earth's climate.  Based on what was known about their radiative properties at the turn of the century, Meerkotter et al. (1999) conducted a model study of their consequences for temperatures at the earth's surface.  The model they used suggested that the presence of contrails tends to cool the planet's surface during daylight hours but warm it at night.  They also concluded that aircraft emissions may cause additional indirect climate forcing by changing the particle size distribution of natural cirrus clouds; and, as they describe it, "this indirect forcing may be comparable to the direct forcing due to additional contrail cloud cover."

After a further half-decade of progress, quite different results were obtained by Minnis et al. (2004), who analyzed surface-based measurements of cirrus coverage (CC) for different parts of the world for the period 1971-1995, while employing similar measurements obtained from the International Satellite Cloud Climatology Project for 1984-1996 as a consistency check on them.  The linear trends derived from the data were then input to a general circulation model (GCM) of the atmosphere to calculate their climatic impacts over the different parts of the globe to which they pertained.  So what did they find?

Minnis et al. report that "values of CC increased over the United States, the North Atlantic and Pacific, and Japan, but dropped over most of Asia, Europe, Africa, and South America," making particular note of the fact that "the largest concentrated increases occurred over the northern Pacific and Atlantic and roughly correspond to the major air traffic routes."  Their GCM simulations additionally indicated that the cirrus trends over the United States caused a tropospheric warming of 0.2-0.3°C per decade, a range that they say "includes the observed tropospheric temperature trend of 0.27°C per decade between 1975 and 1994."

According to these GCM-derived results, essentially all of the surface warming observed over the United States between 1975 and 1994, which Minnis et al. report to be 0.54°C, may be explained by aircraft-induced increases in cirrus cloud coverage.  If true, this result would imply that essentially none of the observed warming could be attributed to the concomitant increase in the air's CO2 content, which GCMs also suggest is substantial.  These results would thus appear to present a major problem for the world's climate alarmists, at least in the case of the United States, since they typically claim that what they call the unprecedented warming of the late 20th century was caused primarily by rising greenhouse gas concentrations.  Clearly, they can't have it both ways at all times and in all places.

As the above diversity of examples illustrates, understanding the mechanisms responsible for the formation of clouds and what determines their reflectance characteristics has long been a major but difficult-to-achieve goal of climate change science; yet still other studies have revealed still other levels of complexity, as in the case of Facchini et al. (1999), who explored the potential for organic solutes to alter surface tension characteristics of cloud condensation nuclei and thereby influence cloud droplet formation and albedo.  Focusing on the effects of atmospheric solutes collected from cloud water (fog) in the Po Valley of Italy, they determined that the effect of organic matter solutes was indeed significant, and that water vapor was more likely to condense on organic solute-affected aerosols of lower surface tension, creating more numerous and more highly-reflective cloud droplets.

How significant are these findings?  In the words of Facchini et al., "the error produced in ignoring this [cooling] effect is estimated to be comparable to other calculated direct and indirect influences of aerosols on scattering and absorption of solar radiation."  Also, they note that the organic fractions and concentrations in the aerosols they studied "are similar to air in or downwind of other large agricultural-industrial regions."  Hence, as the human population of the planet continues to grow, and as agricultural-industrial enterprises grow with it, we should see the cooling effect discussed in this study grow ever stronger.

Another person to broach this subject was Rosenfeld (2000), who looked for terrestrial analogues of ship tracks, having previously demonstrated the existence of a precipitation-suppressing effect of the smoke particles emitted by burning vegetation.  Satellite visualizations produced from the mission data used in that earlier study clearly revealed the existence of enhanced cloud trails downwind of urban and industrial complexes in Turkey, Canada and Australia, to which Rosenfeld gave the name "pollution tracks."  He also determined that the clouds comprising these pollution tracks were composed of droplets of reduced size that did indeed suppress precipitation by inhibiting further coalescence and ice precipitation.

According to Rosenfeld, the results of his study indicate that "human activity may be altering clouds and natural precipitation on a global scale."  It is clear, however, that the anthropogenic-induced reduction in global precipitation implied by his findings would tend to ameliorate the human-induced increase in the activity of the hydrologic cycle that is predicted by climate models to accompany greenhouse gas-induced global warming.  Greater numbers of smaller and more-highly-reflective cloud particles would also tend to reflect more incoming solar radiation back to space and thereby cool the planet, which would tend to blunt the impact of the human-intensified greenhouse effect.  In addition, Toon (2000) notes that with smaller cloud droplets, clouds will not "rain out" as quickly and will therefore last longer and cover more of the earth, both of which effects would also tend to cool the globe.  Hence, whereas one consequence of the industrial activities of man, i.e., greenhouse gas emissions, may tend to warm and wet the world, other consequences of his industrial activities, as described here, tend to cool and dry it.

Further exploring this subject were Loflund et al. (2002), who analyzed the carbon and inorganic ion contents of cloud water at a continental background site about 70 km southwest of Vienna, Austria during two intensive field campaigns (11-26 April 1999 and 6-29 March 2000), wherein they found that the total carbon concentration in the cloud water they analyzed was similar to the individual concentrations of the main inorganic compounds (nitrate and sulphate).  More specifically, black carbon was found to comprise 22% of the total carbon, while six carboxylic acids (acetic, formic, oxalic, succinic, malonic and pyruvic) comprised an average of 11% of the remaining organic carbon.  It was also determined that black carbon can be used as a tracer for the six carboxylic acids, as the sum of the identified organic carbon rose in linear fashion in response to increases in cloud water black carbon content.

In discussing the implications of many of these findings, Charlson et al. (2001) note that droplet clouds, as opposed to cirrus clouds, "are the most important factor controlling the albedo (reflectivity) and hence the temperature of our planet."  And in the case of droplet clouds, man-made aerosols, according to them, "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."

Knowledge of the existence of the delicate balance between these two opposing forces is all that is needed to refute the conclusions of radical environmentalists who claim that we currently know enough about the intricate workings of earth's climate system to mandate the types of anthropogenic CO2 emissions reductions they are attempting to impose upon the nations of the world.  It is abundantly clear, for example, that a slight perturbation of but a single factor on one side or the other of this balance could readily produce either a slight warming or a slight cooling.  What is more, in view of the findings of a long series of empirical studies, Charlson et al. concluded some five years ago that the magnitude of the man-made impetus for cooling "may be even larger than anticipated."

More recently, in 2004, V. Ramanathan (Director of the Center for Clouds, Chemistry and Climate of Scripps Institution of Oceanography) stated even more forcefully (http://explorations.ucsd.edu/indoex/) that "until about five years ago, we thought that greenhouse warming was the dominant effect in the atmosphere," adding that "now, new research has shown that the aerosol effect could dominate."  Consequently, it is readily evident that man's net climatic influence may actually be one of cooling, which is a far, far cry from the predicted catastrophic warming that many people are attempting to use as a lever to make reducing CO2 emissions the ultimate issue of public concern throughout the entire world.

So what are some of the things that are causing many atmospheric scientists to rethink their views on the subject?  The standard theory of cloud droplet creation, which is utilized in current IPCC climate projections, assumes that water droplets form around aerosol nuclei composed of soluble inorganic salts, and that the drops are activated, or grow spontaneously, after they reach a certain critical size in air that is supersaturated with water vapor.  However, "it has recently become clear," as Charlson et al. note, "that soluble gases, slightly soluble solutes [aerosols], and surface tension depression by organic substances also influence the formation of cloud droplets," all of which processes produce extra cloud cooling power that is nowhere to be found in IPCC analyses of cloud effects on climate.

Consider, for example, the impact of the presence of a water-soluble trace gas, such as HNO₃ or HCl, on the activation or transformation of atmospheric aerosols into cloud droplets. Kulmala et al. (1993) studied this phenomenon with respect to nitric acid vapor via numerical simulation, finding that the presence of the trace gas depresses the effective vapor pressure of water over the growing solution droplets, and that "as a result, a higher fraction of aerosol particles can serve as cloud condensation nuclei than in an acid-free atmosphere," which tends to decrease the mean size of the cloud droplets, as the available water vapor is distributed among a larger number of activated particles.

What are the ultimate climatic consequences of this phenomenon?  For starters, the more numerous and smaller cloud droplets reflect more incoming solar radiation.  Then there are several effects that lengthen the period of time over which this enhanced cooling process operates.  First of all, in the words of Kulmala et al., "it is likely that the smaller droplet size will decrease precipitation so that the clouds will have a longer lifetime."  Second, "the cloud formation can take place at smaller saturation ratios of water vapor," which allows clouds to form at earlier times or in places where they would not otherwise form.  And third, "with increased HNO3 concentrations the disappearance of the cloud droplets due to evaporation is slower."

How significant are these effects?  Although nitric acid concentrations in marine regions are usually several orders of magnitude lower than what is needed to significantly change the numbers of cloud condensation nuclei there, Leaitch et al. (1992) have concluded that over North America the increased radiative cooling power due to just the increase in cloud albedo that results from pollution-induced increases in cloud droplet concentrations is about 2 Wm^-2, which is to be compared to a global warming power of 4 Wm^-2 due to a doubling of the air's CO2 concentration.  Hence, since Kulmala et al. report that global emissions of nitrogen oxides increased by 3.4% per year from 1860 to 1980, and by nearly a third from 1970 to 1986, while CO2 emissions are rising by less than 1% per year, the "balance of power" between warming and cooling due to the burning of fossil fuels is clearly shifting away from warming towards cooling, due to this single cooling effect of but one specific by-product (HNO₃) of the combustion process.

There are several other ways (Kulmala et al. list three) by which increases in atmospheric nitric acid may lead to increases in the cooling power of earth's cloud cover.  There are also several other water-soluble trace gases that behave similarly.  In addition, there are complementary effects that derive from increases in various types of partly-water-soluble aerosols, as well as many different water-soluble organic compounds.  Hence, there are all kinds of cooling influences that can be added to this solitary example.

In conclusion, as scientists learn ever more about the numerous highly diverse and complex climatic effects of anthropogenically produced or altered aerosols, it is becoming ever more clear that the argument for reducing anthropogenic CO2 emissions in an effort to avert catastrophic global warming is becoming ever less defensible.  In fact, it is looking rather naïve and even foolish.

References
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., 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.

Kulmala, M., Laaksonen, A., Korhonen, P., Vesala, T. and Ahonen, T.  1993.  The effect of atmospheric nitric acid vapor on cloud condensation nucleus activation.  Journal of Geophysical Research 98: 22,949-22,958.

Leaitch, W.R., Isaac, G.A., Strapp, J.W., Banic, C.M. and Wiebe, H.A.  1992.  The relationship between cloud droplet number concentrations and anthropogenic pollution: Observations and climatic implications.  Journal of Geophysical Research 97: 2463-2474.

Loflund, M., Kasper-Giebl, A., Schuster, B., Giebl, H., Hitzenberger, R. and Puxbaum, H.  2002.  Formic, acetic, oxalic, malonic and succinic acid concentrations and their contribution to organic carbon in cloud water.  Atmospheric Environment 36: 1553-1558.

Meerkotter, R., Schumann, U., Doelling, D.R., Minnis, P., Nakajima, T. and Tsushima, Y.  1999.  Radiative forcing by contrails.  Annales Geophysicae 17: 1080-1094.

Minnis, P., Ayers, J.K., Palikonda, R. and Phan, D.  2004.  Contrails, cirrus trends, and climate.  Journal of Climate 17: 1671-1685.

Rosenfeld, D.  2000.  Suppression of rain and snow by urban and industrial air pollution.  Science 287: 1793-1796.

Toon, O. W.  2000.  How pollution suppresses rain.  Science 287: 1763-1765.