Subject: Cooling vs. Warming Newsgroups: sci.environment From: rparson@rintintin.Colorado.EDU (Robert Parson) Message-ID: Organization: University of Colorado, Boulder Date: Mon, 18 Oct 1993 01:21:39 GMT One way to get a sense of what scientists thought was important 20 years ago is to look up conference proceedings. In our library I found the proceedings of a December 1968 AAAS Symposium, _Global Effects of Environmental Pollution_, ed. by S. Fred Singer (who was at the Interior Dept. at that time), Springer, N.Y. 1970. I also found what appears to be a followup volume, also edited by Singer and published by Springer, _The Changing Global Environment_, pub. 1975. Actually the later book seems to be a sort of expanded edition of the former; revised versions of the papers from the first book are reproduced and newer ones added. Incidentally, Singer's editorial remarks in these books are well worth reading, he was a lot less irritating back then ;-). Part III of the second book is entitled "Effects of Atmospheric Pollution on Climate". It contains papers by: Reid Bryson and Wayne Wedland J. Murray Mitchell Syukuro Manabe Vincent Schaefer H. E. Landsberg Hugh Ellsaesser The first 4 were in the earlier volume as well. Singer's introduction to this section summarizes it nicely: --------------------------------- "Human activities are not only increasing the content of carbon dioxide in the atmosphere (see Part I), but also the particle content... "The effects are not at all well understood; even the immediate effects are difficult to predict and our confidence in predicting long-term effects is not high... "Bryson and Wendland attempt to delineate the current thinking and review available scientific studies. They opt for a downward trend in planetary temperatures, over the long run, because of increased albedo effects of atmospheric particulate material... "Mitchell uses a different, but complementary approach, but with differing results. He concludes that the carbon dioxide increase is more effective in raising planetary temperatures than is the human-derived particulate loading in reducing temperatures. Natural dust loads dominate at present; in the future, however, anthropogenic dust effects could eventually have an important effect on climate. "On the other hand, Ellsaesser concludes...that the human-related particle loading is only 13% of the natural loading, and that the anthropogenic contribution is growing slowly, if at all. "Manabe examines some of the fundamental assumptions in a critical manner. The induced temperature changes depend on the optical properties of the particles. Under certain circumstances. the increased particulate loading could even raise planetary temperatures. "Schaefer describes more local effects on the weather due to air pollution... "Many of the foregoing discussions are tied together in the review by Landsberg which deals with climactic change on global, regional, and local scales. It concentrates on the last five centuries and on future changes produced by CO2 and dust..." ------------ I have looked briefly over these papers. They all seem to agree that there are two major anthropogenic impacts on global climate: CO2 and aerosols/particulates. The one causes warming, the other cooling. They disagree substantially about how these stack up against each other. It is clear that Keeling's Mauna Loa and South Pole results were starting to make people think very hard about greenhouse warming; however much less was known about earlier CO2 levels (the ice core data hadn't been collected yet) and it was not known that the CO2 increase was unprecedented in human history. Everyone notes the warming trend before 1940 and the cooling trend since 1940. Some (e.g. Bryson) argue that the cooling could be due to anthropogenic particulates, others that it is more likely due to increased volcanic activity. Nobody has a very good idea of what the anthropogenic contribution to particulates is, or whether it is increasing. Mitchell mentions the Rasool and Schneider study, but he argues that it represents an upper bound to the possible cooling effects. As Singer says above, he thinks that CO2 warming is more important, but he hedges his statements very carefully. Mitchell seems to be closest to the positions that are generally adopted today. The impression that I get from all this is that everyone agreed that CO2 emissions could produce warming and that particle emissions could produce cooling, but that lack of information precluded any definite conclusions about which was more important. By making different assumptions, different researchers came to radically different conclusions. All contributors readily admitted the enormous uncertainties that they were faced with. Nothing remotely resembling the claim that "they were preaching global cooling" is evident in this volume. Some suggest cooling, some suggest warming, some suggest neither, some are not willing to draw any conclusions, and nobody is "preaching".