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Humanity's Impacts on Clouds:
Could They Lead to Global Cooling?

Volume 4, Number 31: 1 August 2001

In last week's Editorial, we bristled at the hubris of Wigley and Raper (2001) in saying there is a 90% probability there will be a mean global warming somewhere in the range of 1.7 to 4.9C over the period 1990 to 2100 if the nations of the earth do not reduce their reliance on fossil fuels. In response to this unwarranted assertion, we described how our present knowledge of a single class of phenomena, i.e., human influences on cloud characteristics, totally invalidates this claim, citing as evidence the analysis of Charlson et al. (2001), wherein we are reminded 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."

Taken at face value, Charlson et al.'s assessment of our current knowledge of the subject indicates there is a 50-50 chance the activities of man may actually lead to a slight cooling of the globe. However, they note that "uncertainties associated with the aerosol forcing are large," thereby providing proponents of CO2-induced global warming an excuse to discount the possibility of anthropogenic-induced cooling on the assumption that further research will resolve the uncertainty in favor of warming. Unfortunately for the global pessimists -- but much to the benefit of the biosphere -- Charlson et al. additionally report that "recent studies indicate that both the forcing and its magnitude may be even larger than anticipated," which shifts the delicate balance of future climate change back to cooling.

This conclusion is radically different from the story that is repeatedly promulgated by the major print and broadcast media, as well as globalist politicians worldwide; and, hence, its underlying foundation deserves to be described in some detail. Consequently, when we introduced this subject in last week's Editorial, we promised we would periodically report the results of real-world physical measurements that help to elucidate the nature of the neglected phenomenon of anthropogenic-induced cooling via human impacts on cloud characteristics. This week, we begin to make good on our word.

In its most simple incarnation, the theory of man-made cooling via alterations in earth's cloud cover suggests, in the words of Charlson et al., "that 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." And, as they further note, "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 of the planet.

This phenomenon, originally suggested by Twomey (1977) and known as the first indirect climatic effect of aerosols, is included in current IPCC estimates of future climate change; but as Charlson et al. point out, these estimates "do not include the combined influences of some recently identified chemical factors, each of which leads to additional negative forcing (cooling) on top of that currently estimated." And it is the extra push provided by these additional factors that tilts the delicate balance between anthropogenic-induced warming and anthropogenic-induced cooling in favor of cooling.

So how does it all work? 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.

Let us begin by considering the consequences 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; and we will be discussing these other factors in future Editorials.

So keep your cool about global warming, and hang in there with us in not capitulating to the gnashing of teeth that has arisen in the wake of the United States' decision to abandon the Kyoto Protocol. The earth has many ways of turning down the planetary thermostat and can well take care of itself in this regard.

Dr. Craig D. Idso
Dr. Keith E. Idso
Vice President

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.

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.

Twomey, S. 1977. The influence of pollution on the short-wave albedo of clouds. Journal of the Atmospheric Sciences 34: 1149-1152.

Wigley, T.M.L. and Raper, S.C.B. 2001. Interpretation of high projections for global-mean warming. Science 293: 451-454.