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How Do Human Activities Affect Earth's Climate?
Bob Dylan Said It Best: The Answer is Blowin' in the Wind
Volume 3, Number 11: 1 June 2000

"The effect of radiative forcing by anthropogenic aerosols is one of the largest sources of uncertainty in climate prediction." So wrote Satheesh and Ramanathan (2000) in the 4 May issue of Nature in summarizing the results of their study of the effects of human-induced pollution of the atmosphere over the tropical northern Indian Ocean. Taking a cue from our father's long-standing quest to utilize "natural experiments" to quantify the effects of various climate-perturbing phenomena (Idso, 1998), the two researchers determined the clear-sky radiative consequences 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.

So what did Satheesh and Ramanathan learn about the radiative consequences of these aerosols? In their own words, "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 they suggested would likely lead to a sizeable reduction of evaporation from the ocean's surface and a significant decrease in the hydrologic cycle over a large portion of the planet. However, they also noted that the increased absorption of solar radiation by the anthropogenic haze might be capable of "burning off" low-level trade-wind cumulus clouds, which would allow more solar radiation to reach and warm the ocean surface, counteracting the consequences of the first perturbation.

Eight days later, in the 12 May issue of Science, Ackerman et al. (2000) concluded much the same thing regarding low-level cumulus cloud burn-off. From data obtained over the same oceanic region, they calculated that the anthropogenic haze of 1998 likely reduced fractional cloud coverage there by 25%, and that the even heavier haze of 1999 may have reduced it by 40%.

These are large effects. In fact, Schwartz and Buseck (2000) note that the anthropogenic aerosol-induced clear-sky radiative forcing found by Satheesh and Ramanathan 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." Yet enhanced by the cloud burn-off effects of the aerosols, the total warming effect may approach the magnitude of the cooling effect, depending on a number of assumptions about the chemical and physical characteristics of the aerosols.

The upshot of all these findings, in the words of Schwartz and Buseck, is that "unfortunately for those who would like a quick and accurate assessment of anthropogenic climate forcing over the industrial period, the studies ? demonstrate that there is much to be learned before such an assessment can confidently be given."

Clearly, as all involved in these studies have unhesitatingly affirmed, we are far from knowing even the sign of the net anthropogenic impact on climate since we began to mine and burn coal, gas and oil in prodigious quantities. Yet whoever is responsible for the ultimate synthesis of the massive IPCC reports continues to claim that the balance of evidence suggests a human impact on earth's climate over this period, which is further claimed to be an unprecedented warming of the globe.

Where is the attention of these folks to the findings of studies such as these, which clearly demonstrate that we are not yet anywhere close to being able to make such a stunning determination? In contemplating this question, all we can do is shake our heads and ask, Where's their's?

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

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.

Idso, S.B. 1998. CO2-induced global warming: a skeptic's view of potential climate change. Climate Research 10: 69-82.

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.