Efficacy of climate forcings

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Read Full Article (file size: 21552991 bytes) Cited by JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, D18104, doi:10.1029/2005JD005776, 2005 Efficacy of climate forcings J. Hansen NASA Goddard Institute for Space Studies, New York, New York, USA Columbia University Earth Institute, New York, New York, USA M. Sato Columbia University Earth Institute, New York, New York, USA R. Ruedy SGT Incorporated, New York, New York, USA L. Nazarenko Columbia University Earth Institute, New York, New York, USA A. Lacis NASA Goddard Institute for Space Studies, New York, New York, USA Department of Earth and Environmental Sciences, Columbia University, New York, New York, USA G. A. Schmidt NASA Goddard Institute for Space Studies, New York, New York, USA Department of Earth and Environmental Sciences, Columbia University, New York, New York, USA G. Russell NASA Goddard Institute for Space Studies, New York, New York, USA I. Aleinov Columbia University Earth Institute, New York, New York, USA M. Bauer Columbia University Earth Institute, New York, New York, USA S. Bauer Columbia University Earth Institute, New York, New York, USA N. Bell Columbia University Earth Institute, New York, New York, USA B. Cairns Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York, USA V. Canuto NASA Goddard Institute for Space Studies, New York, New York, USA M. Chandler Columbia University Earth Institute, New York, New York, USA Y. Cheng SGT Incorporated, New York, New York, USA A. Del Genio NASA Goddard Institute for Space Studies, New York, New York, USA Department of Earth and Environmental Sciences, Columbia University, New York, New York, USA G. Faluvegi Columbia University Earth Institute, New York, New York, USA E. Fleming NASA Goddard Space Flight Center, Greenbelt, Maryland, USA A. Friend Laboratoire des Sciences du Climat et de l'Environnement, Orme des Merisiers, Gif-sur-Yvette, France T. Hall NASA Goddard Institute for Space Studies, New York, New York, USA Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York, USA C. Jackman NASA Goddard Space Flight Center, Greenbelt, Maryland, USA M. Kelley Laboratoire des Sciences du Climat et de l'Environnement, Orme des Merisiers, Gif-sur-Yvette, France N. Kiang NASA Goddard Institute for Space Studies, New York, New York, USA D. Koch Columbia University Earth Institute, New York, New York, USA Department of Geology, Yale University, New Haven, Connecticut, USA J. Lean Naval Research Laboratory, Washington, D. C., USA J. Lerner Columbia University Earth Institute, New York, New York, USA K. Lo SGT Incorporated, New York, New York, USA S. Menon Lawrence Berkeley National Laboratory, Berkeley, California, USA R. Miller NASA Goddard Institute for Space Studies, New York, New York, USA Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York, USA P. Minnis NASA Langley Research Center, Hampton, Virginia, USA T. Novakov Lawrence Berkeley National Laboratory, Berkeley, California, USA V. Oinas SGT Incorporated, New York, New York, USA Ja. Perlwitz Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York, USA Ju. Perlwitz Columbia University Earth Institute, New York, New York, USA D. Rind NASA Goddard Institute for Space Studies, New York, New York, USA Department of Earth and Environmental Sciences, Columbia University, New York, New York, USA A. Romanou NASA Goddard Institute for Space Studies, New York, New York, USA Department of Earth and Environmental Sciences, Columbia University, New York, New York, USA D. Shindell NASA Goddard Institute for Space Studies, New York, New York, USA Department of Earth and Environmental Sciences, Columbia University, New York, New York, USA P. Stone Center for Meteorology, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA S. Sun NASA Goddard Institute for Space Studies, New York, New York, USA Center for Meteorology, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA N. Tausnev SGT Incorporated, New York, New York, USA D. Thresher Department of Earth and Environmental Sciences, Columbia University, New York, New York, USA B. Wielicki NASA Langley Research Center, Hampton, Virginia, USA T. Wong NASA Langley Research Center, Hampton, Virginia, USA M. Yao SGT Incorporated, New York, New York, USA S. Zhang Columbia University Earth Institute, New York, New York, USA Abstract We use a global climate model to compare the effectiveness of many climate forcing agents for producing climate change. We find a substantial range in the “efficacy” of different forcings, where the efficacy is the global temperature response per unit forcing relative to the response to CO₂ forcing. Anthropogenic CH₄ has efficacy ∼110%, which increases to ∼145% when its indirect effects on stratospheric H₂O and tropospheric O₃ are included, yielding an effective climate forcing of ∼0.8 W/m² for the period 1750–2000 and making CH₄ the largest anthropogenic climate forcing other than CO₂. Black carbon (BC) aerosols from biomass burning have a calculated efficacy ∼58%, while fossil fuel BC has an efficacy ∼78%. Accounting for forcing efficacies and for indirect effects via snow albedo and cloud changes, we find that fossil fuel soot, defined as BC + OC (organic carbon), has a net positive forcing while biomass burning BC + OC has a negative forcing. We show that replacement of the traditional instantaneous and adjusted forcings, Fi and Fa, with an easily computed alternative, Fs, yields a better predictor of climate change, i.e., its efficacies are closer to unity. Fs is inferred from flux and temperature changes in a fixed-ocean model run. There is remarkable congruence in the spatial distribution of climate change, normalized to the same forcing Fs, for most climate forcing agents, suggesting that the global forcing has more relevance to regional climate change than may have been anticipated. Increasing greenhouse gases intensify the Hadley circulation in our model, increasing rainfall in the Intertropical Convergence Zone (ITCZ), Eastern United States, and East Asia, while intensifying dry conditions in the subtropics including the Southwest United States, the Mediterranean region, the Middle East, and an expanding Sahel. These features survive in model simulations that use all estimated forcings for the period 1880–2000. Responses to localized forcings, such as land use change and heavy regional concentrations of BC aerosols, include more specific regional characteristics. We suggest that anthropogenic tropospheric O₃ and the BC snow albedo effect contribute substantially to rapid warming and sea ice loss in the Arctic. As a complement to a priori forcings, such as Fi, Fa, and Fs, we tabulate the a posteriori effective forcing, Fe, which is the product of the forcing and its efficacy. Fe requires calculation of the climate response and introduces greater model dependence, but once it is calculated for a given amount of a forcing agent it provides a good prediction of the response to other forcing amounts. Received 7 January 2005; accepted 27 June 2005; published 28 September 2005. Keywords: climate forcings; climate models; greenhouse gases. Index Terms: 1622 Global Change: Earth system modeling (1225); 1620 Global Change: Climate dynamics (0429, 3309); 1616 Global Change: Climate variability (1635, 3305, 3309, 4215, 4513); 1637 Global Change: Regional climate change. Read Full Article (file size: 21552991 bytes) Cited by Citation: Hansen, J., et al. (2005), Efficacy of climate forcings, J. Geophys. Res., 110, D18104, doi:10.1029/2005JD005776. Copyright 2005 by the American Geophysical Union.

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