February 28, 2007
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FROM VOLUME 8, NUMBER 11, NOVEMBER 1995
CLOUDS, AEROSOLS AND CLIMATE
"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.
"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.
"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
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.
"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
"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.
"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
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