Climate Cooled by Particles and Clouds?
(Press Release)
"There's little doubt about the primary effects of increasing greenhouse gases", said Dr. James Hudson of the Desert Research Institute in Reno, NV, "But there's also little doubt that the windows of our house are getting dirtier." At the meeting Dr. William Wilson of the U.S. Environmental Protection Agency reported that . during the past 40 years. spring and summer visibilities in the southeastern U.S. have been cut in half.
"The key factor," said Professor Othmar Preining from the University of Vienna, "Is that exactly the same human activities increasing atmospheric carbon dioxide. namely burning fossil fuels and forests, are also increasing atmospheric particles‹and these may cool the climate."
That particles can cool the climate is well known at least for particles from volcanic sulfur in the upper atmosphere. Professor Alan Robock of the University of Maryland predicts that the worldwide average surface-air temperature next year could be several tenths of a degree Celsius cooler because of this year's eruption of Mt Pinatubo in the Philippines, if initial estimates of its sulfur emissions are correct. As reported by Dr. Larry Stowe of the U.S. National Oceanic and Atmospheric Administration (NOAA) this sulfur has now spread in the stratosphere over approximately 40% of the globe.
Particles in the Lower atmosphere are also known to cool the climate, at least in limited areas, by shading the sun. Ellsworth Dutton of NOAA reported that on one day when the smoke plume from the 1988 Yellowstone National Park fire was over Boulder, CO, about 800 km from the fire, the smoke particles reduced solar radiation by 28%. Similar cooling is now occurring near Kuwait because of the burning oil wells. But at present, there are no data for worldwide cooling by sulfur particles in the lower atmosphere. only predictions from mathematical models; but also, there are no data for warming by greenhouse gases, only predictions from mathematical models.
From one model. developed jointly at Stockholm University and the University of Washington, Prof. Henning Rodhe concluded that burning fossil fuel now releases so much sulfur into the atmosphere that the resulting particles alone are predicted to cool the Northern Hemisphere on average almost as much as the associated carbon dioxide is predicted to warm by the greenhouse effect. Similar results were found in studies at the Institute for Atmospheric Physics in Budapest, Hungary, and at Nanjing University in China.
"Direct cooling by particles is only half the story‹or less than half', said Prof. Glenn Shaw of the University of Alaska at Fairbanks. "By adding more particles to the atmosphere, humans are changing clouds: more particles can cause more cloud droplets: with more droplets clouds can reflect more sunlight. amplifying the cooling."
Pursuing Shaw's ideas. Dr. Yoram Kaufman of NASA-Goddard. MD. and. independently, Dr. Catherine Chuang of Lawrence Livermore National Laboratory, CA, estimate that this increase in cloud reflectivity alone, caused by particles, from current consumption of fossil fuel. may almost cancel greenhouse warming by the past century's accumulation of atmospheric carbon dioxide.
Professor Hans Pruppacher of the University of Mainz was concerned that talk of climate cooling by particles might be used to permit more pollution. "We need to cut back all pollution. both gases and particles. Let's not forget that this same sulfur leads to acid rain." In the long run (time horizon 50 or more years it is likely that the cooling effect of the aerosol will not increase as much as the warming by greenhouse gases, so a net warming ought to result.
Professor Preining summarized, "Until we have better data on atmospheric particles and their interactions, these model results should be treated just as scientific speculations. not facts. Yet many scientists from many nations brought to the conference similar speculations about climate cooling by particles. Nevertheless", Preining noted. Nature isn't very democratic. It wouldn't be the first time she's vetoed even the unanimous opinion of scientists. We'll learn her decision only by collecting more and better data‹but that's much harder to do for particles than for greenhouse gases''.
Prof. Dr. Glenn E. Shaw. Geophysical Institute, University of Alaska, Fairbanks, AK. U.S.A.
Dr. W. George N. Slinn, Pacific Northwest Laboratory, Richland, WA, U.S.A.
For further information contact: Prof. Dr. Othmar Preining. Institut fur Experimentaliphysik der Universität
Wien, Strudlhofgasse 4. A 1090 Wien, Austria. Tel: 43 ] 34 26 30, ext. 214 and 435; Fax: 43 310 2338.