Dangerous human-made interference with climate: a GISS modelE study J. Hansen1,2, M. Sato2, R. Ruedy3, P. Kharecha2, A. Lacis1,4, R. Miller1,5, L. Nazarenko2, K. Lo3, G. A. Schmidt1,4, G. Russell1, I. Aleinov2, S. Bauer2, E. Baum6, B. Cairns5, V. Canuto1, M. Chandler2, Y. Cheng3, A. Cohen6, A. Del Genio1,4, G. Faluvegi2, E. Fleming7, A. Friend8, T. Hall1,5, C. Jackman7, J. Jonas2, M. Kelley8, N. Y. Kiang1, D. Koch2,9, G. Labow7, J. Lerner2, S. Menon10, T. Novakov10, V. Oinas3, Ja. Perlwitz5, Ju. Perlwitz2, D. Rind1,4, A. Romanou1,4, R. Schmunk3, D. Shindell1,4, P. Stone11, S. Sun1,11, D. Streets12, N. Tausnev3, D. Thresher4, N. Unger2, M. Yao3, and S. Zhang2 1NASA Goddard Institute for Space Studies, New York, NY, USA 2Columbia University Earth Institute, New York, NY, USA 3Sigma Space Partners LLC, New York, NY, USA 4Department of Earth and Environmental Sciences, Columbia University, New York, NY, USA 5Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, USA 6Clean Air Task Force, Boston, MA, USA 7NASA Goddard Space Flight Center, Greenbelt, MD, USA 8Laboratoire des Sciences du Climat et de l'Environnement, Orme des Merisiers, Gif-sur-Yvette Cedex, France 9Department of Geology, Yale University, New Haven, CT, USA 10Lawrence Berkeley National Laboratory, Berkeley, CA, USA 11Massachusetts Institute of Technology, Cambridge, MA, USA 12Argonne National Laboratory, Argonne, IL, USA
Abstract. 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 conclude that a
CO2 level exceeding about 450 ppm is "dangerous", but reduction of
non-CO2 forcings can provide modest relief on the CO2 constraint.
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.
Simulated recent ocean warming in the region of Atlantic hurricane formation
is comparable to observations, suggesting that greenhouse gases (GHGs) may
have contributed to a trend toward greater hurricane intensities. Increasing
GHGs cause significant warming in our model in submarine regions of ice
shelves and shallow methane hydrates, raising concern about the potential
for accelerating sea level rise and future positive feedback from methane
release. Growth of non-CO2 forcings has slowed in recent years, but
CO2 emissions are now surging well above the alternative scenario.
Prompt actions to slow CO2 emissions and decrease non-CO2 forcings
are required to achieve the low forcing of the alternative scenario.
Citation: Hansen, J., Sato, M., Ruedy, R., Kharecha, P., Lacis, A., Miller, R., Nazarenko, L., Lo, K., Schmidt, G. A., Russell, G., Aleinov, I., Bauer, S., Baum, E., Cairns, B., Canuto, V., Chandler, M., Cheng, Y., Cohen, A., Del Genio, A., Faluvegi, G., Fleming, E., Friend, A., Hall, T., Jackman, C., Jonas, J., Kelley, M., Kiang, N. Y., Koch, D., Labow, G., Lerner, J., Menon, S., Novakov, T., Oinas, V., Perlwitz, Ja., Perlwitz, Ju., Rind, D., Romanou, A., Schmunk, R., Shindell, D., Stone, P., Sun, S., Streets, D., Tausnev, N., Thresher, D., Unger, N., Yao, M., and Zhang, S.: Dangerous human-made interference with climate: a GISS modelE study, Atmos. Chem. Phys., 7, 2287-2312, doi:10.5194/acp-7-2287-2007, 2007.