Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 6 2006 10.5194/acpd-6-12549-2006 http://www.atmos-chem-phys-discuss.net/6/12549/2006/ http://www.atmos-chem-phys-discuss.net/6/12549/2006/acpd-6-12549-2006.html http://www.atmos-chem-phys-discuss.net/6/12549/2006/acpd-6-12549-2006.pdf 12549 12610 2006-12-05 Dangerous human-made interference with climate: a GISS modelE study J. Hansen jhansen@giss.nasa.gov M. Sato R. Ruedy P. Kharecha A. Lacis R. Miller L. Nazarenko K. Lo G. A. Schmidt G. Russell I. Aleinov S. Bauer E. Baum B. Cairns V. Canuto M. Chandler Y. Cheng A. Cohen A. Del Genio G. Faluvegi E. Fleming A. Friend T. Hall C. Jackman J. Jonas M. Kelley N. Y. Kiang D. Koch G. Labow J. Lerner S. Menon T. Novakov V. Oinas Ja. Perlwitz Ju. Perlwitz D. Rind A. Romanou R. Schmunk D. Shindell P. Stone S. Sun D. Streets N. Tausnev D. Thresher N. Unger M. Yao S. Zhang NASA Goddard Institute for Space Studies, New York, New York, USA Columbia University Earth Institute, New York, New York, USA Sigma Space Partners LLC, New York, New York, USA Department of Earth and Environmental Sciences, Columbia University, New York, New York, USA Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York, USA Clean Air Task Force, Boston, Massachusetts, USA NASA Goddard Space Flight Center, Greenbelt, Maryland, USA Laboratoire des Sciences du Climat et de l'Environnement, Orme des Merisiers, Gif-sur-Yvette Cedex, France We investigate the issue of "dangerous human-made interference with climate" using simulations with GISS modelE driven by measured or estimated forcings for 1880–2003 and extended to 2100 for IPCC greenhouse gas scenarios as well as the "alternative" scenario of Hansen and Sato (2004). Identification of "dangerous" effects is partly subjective, but we find evidence that added global warming of more than 1°C above the level in 2000 has effects that may be highly disruptive. The alternative scenario, with peak added forcing ~1.5 W/m² in 2100, keeps further global warming under 1°C if climate sensitivity is ~3°C or less for doubled CO₂. The alternative scenario keeps mean regional seasonal warming within 2σ (standard deviations) of 20th century variability, but other scenarios yield regional changes of 5–10σ, i.e., mean conditions outside the range of local experience. We discuss three specific sub-global topics: Arctic climate change, tropical storm intensification, and ice sheet stability. We suggest that Arctic climate change has been driven as much by pollutants (O₃, its precursor CH₄, and soot) as by CO₂, offering hope that dual efforts to reduce pollutants and slow CO₂ growth could minimize Arctic change. 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