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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 11, NUMBER 7, JULY 1998

PROFESSIONAL PUBLICATIONS...
CLOUDS, AEROSOLS AND CLIMATE

Item #d98jul46

"Uncertainties of Aerosol Effects in Global Climate Models," J.S. Gaffney (Argonne Natl. Lab., 9700 S. Cass Ave., Argonne IL 60439), N.A. Marley,Atmos. Environ., 32(16), 2873-2874, Aug. 1998.

This commentary emphasizes the urgency of including the various effects of aerosols in climate models, which will require better measurements of aerosol chemical compositions and radiative properties as a function of size, in addition to temporal and spatial aerosol distributions.

Item #d98jul47

"Intercomparison of Models Representing Direct Shortwave Radiative Forcing by Sulfate Aerosols," O. Boucher (Lab. d'Optique Atmos., Univ. de Lille-I, 59655 Villeneuve d'Ascq Cedex, France; e-mail: boucher@loa.univ-lille1.fr), S.E. Schwartz et al.,J. Geophys. Res., 103(D14), 16,979-16,998, July 27, 1998.

The comparison involved 12 groups using 15 models to examine radiative forcing by sulfate aerosol for a wide range of particle radius, aerosol optical depth, surface albedo, and solar zenith angle. All models simulated forcings of comparable amplitude and exhibit a similar dependence on input parameters.

Item #d98jul48

"Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS)," O.B. Toon (LASP, Univ. Colorado, Boulder CO 80309), R.C. Miake-Lye,Geophys. Res. Lett., 25(8), 1109-1112, Apr. 15, 1998.

Overviews results of a multi-aircraft field campaign held in 1996, presented in detail in a special section of this issue (and to be continued in future issues).

Item #d98jul49

"Global Average Climate Forcing and Temperature Response Since 1750," P.R. Rowntree (25 Bramblegate, Crowthorne, Berkshire RG45 6JA, UK),Intl. J. Climatology, 18(4), 355-377, Mar. 30, 1998.

Estimates major radiative forcing factors in climate change for the period since 1750. Fossil fuel burning had a minor role until the mid-twentieth century, with the relatively small effects of fossil fuel CO₂ being cancelled by sulfate aerosols from the same source. Shows that a simple box-diffusion model simulates many features of the climate record, without including either variations in solar intensity or internal variability, but the simulation does include volcanic aerosols, which most studies have neglected.

Item #d98jul50

"Clouds, Contrails and Climate," J.H. Seinfeld (Chem. Engin. (104-44), Calif. Inst. Technol., Pasadena CA 91125; e-mail: seinfeld@cco.caltech.edu),Nature, 391(6670), 837-838, Feb. 26, 1998.

The formation of cirrus clouds, which warm the Earth, is incompletely understood, but recent studies hint that they can evolve from jet contrails. This is an unsuspected but possibly important effect of human activity on climate, offering a new way to study the processes involved.

Item #d98jul51

"Effect of Black Carbon and Sulfate Aerosols on the Global Radiation Budget," I. Schult, J. Feichter (M. Planck Inst. Meteor., Bundestr. 55, D-20146 Hamburg, Ger.; e-mail: feichter@dkrz.de), W.F. Cooke,J. Geophys. Res., 102(D25), 30,107-30,117, Dec. 27, 1997.

Calculations based on global distributions show that the presence of black carbon, the main absorbing component of anthropogenic aerosol, may reduce the cooling effect of aerosol, thus leading to an increase in greenhouse warming.

Item #d98jul52

"Supercooled Cirrus Cloud Formation Modified by Nitric Acid Pollution of the Upper Troposphere," A. Laaksonen (Dept. Physics, POB 9, 00014 Univ. Helsinki, Finland; e-mail: laaksone@rock.helsinki.fi), J. Hienola, M. Kulmala,Geophys. Res. Lett., 24(23), 3009-3012, Dec. 1, 1997.

Theoretical model investigations indicate that nitric acid may substantially modify supercooled cirrus cloud at temperatures above -40°C, and thereby influence climate.

Item #d98jul53

"Multi-Spectral Calculations of the Direct Radiative Forcing of Tropospheric Sulfate and Soot Aerosols Using a Column Model," J.M. Haywood (NOAA/GFDL, POB 308, Princeton NJ 08542), K.P. Shine,Quart. J. Royal Meteor. Soc., 123(543), 1907-1930, Oct. 1997 Part A.

Results from a newly developed multi-spectral radiative transfer code show that subgrid-scale variations in relative humidity and the spatial correlation between clouds and areas of high humidity should be considered in GCM simulations. Recent estimates of the climate response to direct forcing may be too large. The role of soot is discussed.

Item #d98jul54

"Climate Change and Energy Policy: The Impacts and Implications of Aerosols," J.J. West (Dept. Civil Eng., Carnegie Mellon Univ., Pittsburgh PA 15213; e-mail: jw6z@andrew.cmu.edu), C. Hope, S.N. Lane,Energy Policy, 25(11), 923-939, Sep. 1997.

Isolates the effects of aerosols by treating their emissions as a policy variable separate from greenhouse gas emissions, but linked through energy policy. Using a simple climate model, questions the conclusion that aerosols are beneficial to climate because they counteract greenhouse warming. Instead, there is greater uncertainty in mean temperature, and a greater likelihood of changes in other climate parameters.

Item #d98jul55

"Contrail-Cirrus and Their Potential for Regional Climate Change," K. Sassen (Dept. Meteor., Univ. Utah, Salt Lake City UT 84112; e-mail: ksassen@atmos.met.utah.edu),Bull. Amer. Meteor. Soc., 78(9), 1885-1903, Sep. 1997.

Reviews the indirect evidence for the regional climatic impact of contrail-generated cirrus clouds, and presents new measurements. The evidence indicates that the direct radiative effects of contrails display the potential for regional climate change at many midlatitude locations. The sign of the impact may be uncertain, but new information favors albedo cooling over a greenhouse warming effect.

Item #d98jul56

"General Circulation Model Calculations of the Direct Radiative Forcing by Anthropogenic Sulfate and Fossil-Fuel Soot Aerosol," J.M. Haywood (NOAA/GFDL, POB 308, Princeton NJ 08542; e-mail: jmh@gfdl.gov), D.L. Roberts et al.,J. Clim., 10(7), 1562-1577, July 1997.

Uses a new radiation code to assess the direct solar and thermal radiative forcing of these species. Results are extremely sensitive to the adopted soot/sulfate ratio and the assumed vertical profile, but they indicate that fossil fuel soot aerosol may exert a nonneglible radiative forcing, and emphasize the need to consider each anthropogenic aerosol species.

Item #d98jul57

"Volcanic Sulfur Emissions: Estimates of Source Strength and Its Contribution to the Global Sulfate Distribution," H.-F. Graf (M. Planck Inst. Meteor., Bundestr. 55, D-20146 Hamburg, Ger.; e-mail: graf@dkrz.de), J. Feichter, B. Langmann,J. Geophys. Res., 102(D9), 10,727-10,738, May 20, 1997.

Uses an atmospheric GCM with a full sulfur cycle and prescribed source distributions. Results show that natural S sources are at least as important as the anthropogenic ones, even though their source strength is smaller, because of different lifetimes resulting from different production and emission processes. Volatile volcanic sources should be studied more.

Item #d98jul58

"Aerosols and Climate: Anthropogenic Emissions and Trends for 50 Years," M.E. Wolf (Radian Intl., 10389 Old Placerville Rd., Sacramento CA 95827; e-mail: marty_wolf@radian.com), G.M. Hidy,J. Geophys. Res., 102(D10), 11,113-11,121, May 27, 1997.

Describes a global inventory of anthropogenic particulate emissions for the period 1990-2040, including both primary particulate emissions and secondary contributions from atmospheric chemical reactions, particularly those involving SO₂. Emissions worldwide, which are dominated by fossil fuel combustion and biomass burning, are projected to grow by a factor of 1.5 to 2.5 by 2040, largely from fossil fuel combustion in the developing countries. Present anthropogenic emissions appear to be a small fraction of emissions from natural sources, but could rival them by 2040. The resulting increased haziness will alter the radiation budget of the Earth, in a spatially non-uniform way.

Item #d98jul59

"On Modification of Global Warming by Sulfate Aerosols," J.F.B. Mitchell (Hadley Ctr., Meteor. Off., London Rd., Bracknell, Berkshire RG12 2SY, UK; e-mail: jfbmitchell@meto.gov.uk), T.C. Johns,J. Clim., 10(2), 245-267, Feb. 1997

Compares the patterns of response in the surface climatology of a coupled ocean-atmosphere GCM forced by increases in CO₂ alone, to those associated with forcing by both CO₂ and aerosols. In winter, the cooling due to aerosols merely tends to reduce the response to CO₂, whereas in summer, it weakens the monsoon circulations and reverses some of the changes in the hydrological cycle. Results of a non-intervention scenario for CO₂ and aerosols suggests that future changes in aerosol concentrations would have a major effect on regional climate, especially over Europe and Southeast Asia.

Item #d98jul60

Special Issue: "Atmospheric Chemistry of Sulphur in Relation to Aerosols, Clouds, and Climate," Phil. Trans. Royal Soc. London, Series B, 352, Feb. 28, 1997. Contains 12 papers from a discussion held July 1996, including the following:

"Climate Model Studies of Sulphate Aerosols and Clouds," A. Jones (Hadley Ctr., Meteor. Off., London Rd., Bracknell, Berkshire RG12 2SY, UK), A. Slingo, 221-229. Summarizes recent work at the Hadley Centre on the indirect effect of sulfate aerosols, which illustrates the considerable uncertainty in estimating the indirect effect.

"The Missing Climate Forcing," J. Hansen (NASA Goddard Inst. Space Studies, 2880 Broadway, New York NY 10025), M. Sato et al., 231-240. Since the industrial revolution began, global warming has been only half that expected due to the principal forcing, increasing greenhouse gases. This paper argues on the basis of several lines of indirect evidence that aerosol effects on clouds have caused a large negative forcing that has substantially offset global warming. Observing and studying this effect are extremely difficult, but are essential for the prognosis of future climate change.

"Atmospheric Sulfur and Climate—What Have We Learned?" R.A. Cox (Ctr. Atmos. Sci., Univ. Cambridge, Lensfield Rd., Cambridge CB2 1EW, UK), 251-254. Summarizes the rest of the papers from the meeting. It appears that the global distribution of atmospheric sulfur is such that significant influences on atmospheric radiative transfer are possible, and that the basic idea of a coupling between biological systems and climate via atmospheric chemistry and clouds seems to be robust. But the processes involved are even more complex than was first suggested by Charlson et al. in their 1987 paper.

Item #d98jul61

"Uncertainty in Climate Change Caused by Aerosols," S.E. Schwartz (Brookhaven Natl. Lab, Upton NY 11973; e-mail: ses@bnl.gov), M.O. Andreae,Science, 272(5265), 1121-1122, May 24, 1996.

A critique of the National Research Council report A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change (April, 1996). The report, while basically sound, should have stressed the urgency of reducing uncertainty in the role of aerosols in climate, and seriously underestimates the research effort required to reduce this uncertainty.