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Non-Ozone Air Pollution (Effects on Climate) -- Summary
Many air pollutants participate in phenomena that exert a climatological brake upon global warming tendencies by enhancing cloud and planetary albedo.  Some examples are surface-active organic solutes that escape from industrial and agricultural enterprises (Facchini et al., 1999), exhaust from jet aircraft (Meerkotter et al., 1999), and emissions from ships that sail the seas (Ferek et al., 1998; Capaldo et al., 1999).

How does the process work?  As noted in our Editorial of 1 August 2001 - Humanity's Impacts on Clouds: Could They Lead to Global Cooling? - Charlson et al. (2001) briefly summarize the chain of linked events this way: "an increase in atmospheric aerosols from anthropogenic emissions would lead to smaller cloud droplets because the same amount of cloud liquid water is distributed among more condensation nuclei," so that "for the same liquid water content, a cloud with more numerous, but smaller, drops has a higher albedo than one with fewer, larger drops," which leads to a greater reflection of incoming solar radiation and a consequent cooling tendency.

How significant is the process?  Charlson et al. say it is generally believed 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 light of their ongoing intensive research, however, they state that "recent studies indicate that both the [aerosol] forcing and its magnitude may be even larger than anticipated," which could well shift the direction of future anthropogenic-induced climate change to one of cooling.

The planet's plant life also seems to be getting into the act.  In a study of 24 different airborne particulates obtained on a weekly basis from 1980 to 1991 at the northernmost manned site in the world (Alert, Northwest Territories, Canada), Hopke et al. (1999) observed that concentrations of biogenic sulfur-based aerosols, including sulfate and methane sulfonate, were strongly and positively correlated with the average temperature of the Northern Hemisphere.  "This result," they concluded, "suggests that as the temperature rises, there is increased biogenic production of the reduced sulfur precursor compounds that are oxidized in the atmosphere to sulfate and methane sulfonate and could be evidence of a negative feedback mechanism in the global climate system."  Of course, these naturally-produced aerosols would probably not be classified as air pollutants; but they demonstrate that man and nature may both be exerting powerful forces that have a common tendency to keep potential global warming in check.

Getting back to man, Satheesh and Ramanathan (2000) - as noted in our Editorial How Do Human Activities Affect Earth's Climate? - studied the effects of the December-to-April northeastern low-level monsoonal flow of air that transports anthropogenic aerosols - including sulphates, nitrates, organics, soot and fly ash - from the Indian sub-continent and the south Asian region literally thousands of kilometers over the entire north Indian Ocean and as far south as 10 S latitude.  What they found, in their words, was 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, which Schwartz and Buseck (2000) note is "three to seven times as great as global average longwave (infrared) radiative forcing by increases in greenhouse gases over the industrial period ... but opposite in sign."

Although this forcing represents a phenomenal impetus for cooling, Ackerman et al. (2000) have determined that much of it is negated by a concurrent burn-off of low-level cumulus cloud, leaving the net effect unknown.  "Unfortunately for those who would like a quick and accurate assessment of anthropogenic climate forcing over the industrial period," Schwartz and Buseck note, these studies demonstrate "there is much to be learned before such an assessment can confidently be given."  Yet the political cheerleaders of the IPCC process have no trouble at all in declaring a guilty verdict in the case of anthropogenic-induced global warming!

Although the IPCC has nowhere near the evidence needed to declare "a discernable human influence" on the world's climate, there are indications that air pollutants produced by man may be having a discernable influence on certain aspects of weather in the coastal region of the northwest Atlantic Ocean.  Cerveny and Balling (1998), for example, have documented statistically significant weekly cycles in ozone and carbon monoxide concentrations there, as well as similar weekly cycles in precipitation and tropical cyclone wind speeds.  The authors suggest that increased particulate pollution from vehicular traffic and industrial activities, which are greater on weekdays than on weekends, is responsible for their observations; and we have further milked their data to discover a secular decline in maximum tropical cyclone wind speed that may be related to the historical increase in the region's level of pollution.  Hence, in this one instance of possible human-induced weather modification, the change has been for the better, i.e., a trend towards reduced maximum wind speed in tropical cyclones.

References
Ackerman, A.S., Toon, O.B., Stevens, D.E., Heymsfield, A.J., Ramanathan, V. and Welton, E.J.  2000.  Reduction of tropical cloudiness by soot.  Science 288: 1042-1047.

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.

Cerveny, R.S. and Balling Jr., R.C.  1998.  Weekly cycles of air pollutants, precipitation and tropical cyclones in the coastal NW Atlantic region.  Nature 394: 561-563.

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.

Hopke, P.K., Xie, Y. and Paatero, P.  1999.  Mixed multiway analysis of airborne particle composition data.  Journal of Chemometrics 13: 343-352.

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

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.

Schwartz, S.E. and Buseck, P.R.  2000.  Absorbing phenomena.  Science 288: 989-990.