Elicitation of Expert Judgments of Aerosol Forcing |
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Authors: | M Granger Morgan Peter J Adams David W Keith |
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Institution: | (1) Department of Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, PA 15213, USA;(2) Department of Engineering and Public Policy and Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA;(3) Department of Chemical and Petroleum Engineering and Department of Economics, University of Calgary, Calgary, T2N 1N4, Alberta, Canada |
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Abstract: | A group of twenty-four leading atmospheric and climate scientists provided subjective probability distributions that represent
their current judgment about the value of planetary average direct and indirect radiative forcing from anthropogenic aerosols
at the top of the atmosphere. Separate estimates were obtained for the direct aerosol effect, the semi-direct aerosol effect,
cloud brightness (first aerosol indirect effect), and cloud lifetime/distribution (second aerosol indirect effect). Estimates
were also obtained for total planetary average forcing at the top of the atmosphere and for surface forcing. Consensus was
strongest among the experts in their assessments of the direct aerosol effect and the cloud brightness indirect effect. Forcing
from the semi-direct effect was thought to be small (absolute values of all but one of the experts' best estimates were ≤0.5
W/m2). There was not agreement about the sign of the best estimate of the semi-direct effect, and the uncertainty ranges some
experts gave for this effect did not overlap those given by others. All best estimates of total aerosol forcing were negative,
with values ranging between −0.25 W/m2 and −2.1 W/m2. The range of uncertainty that a number of experts associated with their estimates, especially those for total aerosol forcing
and for surface forcing, was often much larger than that suggested in 2001 by the IPCC Working Group 1 summary figure (IPCC,
2001). |
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