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Aerosols (Non-Biological - Anthropogenic) -- Summary
There are numerous ways in which the activities of man lead to the creation of aerosols that have the potential to alter earth's radiation balance and affect its climate.  Contrails created in the wake of emissions from jet aircraft are one example.  They have been implicated in both the cooling of the planet's surface during daylight hours and its warming at night (Meerkotter et al., 1999).  Ship tracks, or bright streaks that form in layers of marine stratus clouds, are another example.  They are created by emissions from ocean-going vessels; and these persistent and highly-reflective linear patches of low-level clouds generally always tend to cool the planet (Ferek et al., 1998).  Averaged over the surface of the earth both day and night and over the year, for example, Capaldo et al. (1999) have calculated that this phenomenon creates a mean negative radiative forcing of -0.16 Wm-2 in the Northern Hemisphere and -0.06 Wm-2 in the Southern Hemisphere, which values are to be compared to the much larger positive radiative forcing of approximately 4 Wm-2 due to a 300 ppm increase in the atmosphere's CO2 concentration.

In some cases, the atmosphere over the sea also carries a considerable burden of anthropogenically-produced aerosols that come from terrestrial sites.  In recent years, attention to this topic has centered on highly-polluted air from south and southeast Asia that makes its way over the northern Indian Ocean during the dry monsoon season.  There has been much discussion about the impact of this phenomenon on regional climate (see our Editorial of 1 June 2000); and within this context, Norris (2001) has looked at cloud cover as the ultimate arbiter of the various competing hypotheses, finding that daytime low-level oceanic cloud cover increased substantially over the last half of the past century in both the Northern and Southern Hemispheres at essentially all hours of the day.  This finding is indicative of a pervasive net cooling effect.

Over land, aerosol-generating human activities also have a significant impact on local, as well as more wide-ranging, climatic phenomena.  Most interesting in the local context is the study of Sahai (1998), who found that although suburban areas of Nagpur, India had warmed over recent decades, the central part of the city had cooled, especially during the day, because of "increasing concentrations of suspended particulate matter."  Likewise, outside of, but adjacent to, industrial complexes in the Po Valley of Italy, Facchini et al. (1999) found that water vapor was more likely to form on aerosols that had been altered by human-produced organic solutes, and that this phenomenon led to the creation of more numerous and more-highly-reflective cloud droplets that had a tendency to cool the surface below them.

In a similar vein, as described in our Editorial of 1 May 2000, Rosenfield (2000) studied terrestrial analogues of ship tracks downwind of urban/industrial complexes in Turkey, Canada and Australia, to which he gave the name pollution tracks.  His findings indicated that the clouds comprising these pollution tracks were composed of droplets of reduced size that suppressed precipitation by inhibiting further coalescence and ice precipitation formation.  In commenting on this research, Toon (2000) pointed out that when clouds are composed of smaller droplets, they will not "rain out" as quickly and will therefore last longer and cover more of the earth, both of which effects tend to cool the globe.

In reviewing these and other advances in the field of anthropogenic aerosol impacts on clouds, Charlson et al. (2001) - as described in our Editorials of 25 July 2001 and
1 August 2001 - note that droplet clouds "are the most important factor controlling the albedo (reflectivity) and hence the temperature of our planet."  Furthermore, he and his coauthors state 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."  In fact, in reviewing the very newest advances in this field of research, which have yet to be incorporated into either the analyses or recommendations of the Intergovernmental Panel on Climate Change, Charlson et al. conclude that "both the forcing [of this man-induced impetus for cooling] and its magnitude may be even larger than anticipated."  Hence, they rightly warn us that lack of inclusion of the consequences of these important phenomena in climate change deliberations "poses additional uncertainty beyond that already recognized by the Intergovernmental Panel on Climate Change, making the largest uncertainty in estimating climate forcing even larger."

Another assessment of the issue was provided by Ghan et al. (2001), who studied both the positive radiative forcings of greenhouse gases and the negative radiative forcings of anthropogenic aerosols and reported that current best estimates of "the total global mean present-day anthropogenic forcing range from 3 Wm-2 to -1 Wm-2," which represents everything from a modest warming to a slight cooling.  After performing their own analysis of the problem, they reduced the magnitude of this range somewhat; but the end result still stretched from a small cooling influence to a modest impetus for warming.  "Clearly," they thus concluded, "the great uncertainty in the radiative forcing must be reduced if the observed climate record is to be reconciled with model predictions and if estimates of future climate change are to be useful in formulating emission policies."

Another pertinent observation comes from Stanhill and Cohen (2001), who reviewed numerous solar radiation measurement programs around the world to see if there had been any trend in the mean amount of solar radiation falling on the surface of the earth over the past half-century.  In a finding so stunning that it stretches one's credulity, they determined there was a significant 50-year downward trend in this parameter that "has globally averaged 0.51 0.05 Wm-2 per year, equivalent to a reduction of 2.7% per decade, [which] now totals 20 Wm-2."  They also concluded that the most probable explanation for this observation "is that increases in man made aerosols and other air pollutants have changed the optical properties of the atmosphere, in particular those of clouds."

Although this surface-cooling influence is huge, it falls right in the mid-range of a similar solar radiative perturbation documented by Satheesh and Ramanathan (2000) in their study of the effects of human-induced pollution over the tropical northern Indian Ocean (see our Editorial of 1 June 2000), where they determined that "mean clear-sky solar radiative heating for the winters of 1998 and 1999 decreased at the ocean surface by 12 to 30 Wm-2."  Hence, the decline in solar radiation reception discovered by Stanhill and Cohen could well be real.  And if it is, it represents a tremendous counter-influence to the enhanced greenhouse effect produced by the contemporaneous increase in atmospheric CO2 concentration.  In fact, it overwhelms it.

Last, but not least, we come to the subject of the possible health effects of anthropogenic aerosols, which we discuss in our Editorials of 5 September 2001 and
7 November 2001.  In these essays, we describe the ground-breaking analyses of Smith et al. (1999) and the intriguing research work of Keatinge and Donaldson (2001) that challenge the claim that high levels of atmospheric particulate matter (PM) of diameter less than 10 µm (PM10) create a host of ill effects (some of which are deadly) in humans.  As is made very clear in these studies, this subject is extremely complex; and we end up agreeing with Smith et al. that there are "too many uncertain issues to allow us to make definitive statements about a causal relationship between PM10 and mortality."

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.

Ghan, S.J., Easter, R.C., Chapman, E.G., Abdul-Razzak, H., Zhang, Y., Leung, L.R., Laulainen, N.S., Saylor, R.D. and Zaveri, R.A.  2001.  A physically based estimate of radiative forcing by anthropogenic sulfate aerosol.  Journal of Geophysical Research 106: 5279-5293.

Keatinge, W.R. and Donaldson, G.C.  2001.  Mortality related to cold and air pollution in London after allowance for effects of associated weather patterns.  Environmental Research 86A: 209-216.

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

Norris, J.R.  2001.  Has northern Indian Ocean cloud cover changed due to increasing anthropogenic aerosol?  Geophysical Research Letters 28: 3271-3274.

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

Sahai, A.K.  1998.  Climate change: a case study over India.  Theoretical and Applied Climatology 61: 9-18.

Satheesh, S.K. and Ramanathan, V.  2000.  Large differences in tropical aerosol forcing at the top of the atmosphere and Earth's surface.  Nature 405: 60-63.

Smith, R.L., Davis, J.M. and Speckman, P.  1999.  Assessing the human health risk of atmospheric particles.  Environmental Statistics: Analyzing Data for Environmental Policy Novartis Foundation Symposium 220: 59-79.

Stanhill, G. and Cohen, S.  2001.  Global dimming: a review of the evidence for a widespread and significant reduction in global radiation with discussion of its probable causes and possible agricultural consequences.  Agricultural and Forest Meteorology 107: 255-278.

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