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Global Climate Change Digest
A Guide to Information on Greenhouse Gases and Ozone Depletion
Published July 1988 through June 1999

FROM VOLUME 8, NUMBER 11, NOVEMBER 1995

PROFESSIONAL PUBLICATIONS...
CLOUDS, AEROSOLS AND CLIMATE

Item #d95nov42

"Warm Days, Hot Nights," L.D.D. Harvey (Dept. Geog., Univ. Toronto, Toronto ON M5S 1A1, Can.), Nature, 377(6544), 15-16, Sep. 7, 1995.

Although computer simulations of increasing atmospheric CO₂ show similar warming for day and night, temperature records for the last four decades show that the average (nighttime) minimum temperature has risen three times faster than the (daytime) maximum. This research perspective comments on progress made in resolving the conflict, focusing mainly on recent computer simulations by Hansen et al. (Atmos. Res., 37, 175-209, 1995), which examine the combined effects of increased CO₂, cloudiness, and sulfate aerosols. Hansen et al. conclude that the observed mean warming and lower diurnal temperature range can only be explained by a combination of regional increases in aerosol optical depth and low-level cloudiness, and a large-scale warming factor such as increases in greenhouse gases.

Item #d95nov43

"The Impact of Atmospheric Aerosols on Global Climate: A Review," R.D. Thompson (Dept. Geog., Univ. Reading, Whiteknights, POB 227, Reading RG6 2AB, UK), Prog. Phys. Geog., 19(3), 336-350, Sep. 1995.

Reviews the cooling or dust-veil effect of volcanic aerosols on surface temperature. Discusses the causes and evidence of the volcanic signal and emphasizes its moderation through El Niño events. Concludes with a brief analysis of the contributions made by particulate matter released into the lower troposphere from human activities.

Item #d95nov44

"The Sulfate-CCN-Cloud Albedo Effect: A Sensitivity Study with Two General Circulation Models," O. Boucher (Lab. Météor. Dynamique, CNRS, Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris cedex 05, France), U. Lohmann, Tellus, 47B(3), 281-300, July 1995.

Anthropogenic sulfate particles have an indirect cooling effect because they induce the formation cloud droplets, which reflect sunlight. This study estimates the magnitude of the effect by empirically relating the cloud droplet number concentration to the sulfate aerosol mass concentration derived from a chemical transport model. Experiments using this information in two GCMs indicate a globally-averaged indirect forcing of about -1 W m-2. The value in different experiments ranged from -0.5 to -1.5 W m-2, but the range of uncertainty is certainly greater. The highest forcings occur in and off the coasts of the polluted regions of the Northern Hemisphere.

Item #d95nov45

"GCM Estimate of the Indirect Aerosol Forcing Using Satellite-Retrieved Cloud Droplet Effect Radii," O. Boucher (address ibid.), J. Clim., 8(5), Pt. 2, 1403-1409, May 1995.

Recently, Han et al. analyzed satellite data radiances and reported that the mean cloud droplet radius is smaller in the Northern Hemisphere than in the Southern Hemisphere, suggesting the presence of an effect on cloud droplet size from anthropogenic aerosols. This study constrains a climate model with those satellite observations and estimates an indirect radiative forcing by anthropogenic aerosols of about -.6 W m-2 in the 0° -50° N latitude band. The uncertainty of the estimate is difficult to assess but is at least 50%.

Item #d95nov46

"Biogenic Sulfur Emissions and Aerosols over the Tropical Atlantic. 3. Atmospheric Dimethylsulfide, Aerosols and Cloud Condensation Nuclei," M.O. Andreae (Biogeochem. Dept., M. Planck Inst. Chem., POB 3060, D-55020 Mainz, Ger.), J. Geophys. Res., 100(D6), 11,335-11,356, June 20, 1995.

Reports field measurements bearing on the hypothesis originally proposed by Charlson et al. (1987), that production of dimethylsulfide (DMS) by marine phytoplankton influences global climate through a multistep feedback process involving sulfate aerosol and cloud albedo. Results provide strong support for several parts of the hypothesis.

Item #d95nov47

"The Effect of Anthropogenic Sulfate and Soot Aerosol on the Clear Sky Planetary Radiation Budget," J.M. Haywood (Dept. Meteor., Univ. Reading, 2 Earley Gate, Whiteknights, Reading RG6 2AU, UK), K.P. Shine, Geophys. Res. Lett., 22(5), 603-606, Mar 1, 1995.

Uses radiation calculations and model-derived sulfate aerosol data to estimate that fossil fuel derived soot causes a positive global-mean radiative forcing that could be comparable in magnitude to the negative forcing of sulfate aerosol. The nature and amount of soot in the atmosphere must be well established if the climatic role of tropospheric aerosols is to be fully understood.