Which of satellite- or model-based estimates is closer to reality for aerosol indirect forcing?

Quaas, J., Boucher, O., Bellouin, N. ORCID: https://orcid.org/0000-0003-2109-9559 and Kinne, S. (2011) Which of satellite- or model-based estimates is closer to reality for aerosol indirect forcing? Proceedings of the National Academy of Sciences of the United States of America, 108 (46). E1099-E1099. ISSN 0027-8424 doi: 10.1073/pnas.1114634108

Abstract/Summary

In their contribution to PNAS, Penner et al. (1) used a climate model to estimate the radiative forcing by the aerosol first indirect effect (cloud albedo effect) in two different ways: first, by deriving a statistical relationship between the logarithm of cloud droplet number concentration, ln Nc, and the logarithm of aerosol optical depth, ln AOD (or the logarithm of the aerosol index, ln AI) for present-day and preindustrial aerosol fields, a method that was applied earlier to satellite data (2), and, second, by computing the radiative flux perturbation between two simulations with and without anthropogenic aerosol sources. They find a radiative forcing that is a factor of 3 lower in the former approach than in the latter [as Penner et al. (1) correctly noted, only their “inline” results are useful for the comparison]. This study is a very interesting contribution, but we believe it deserves several clarifications.