  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. Identification of 'dangerous'
effects is partly subjective, but we find evidence that added global warming of
more than 1 degree 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 degree C if climate sensitivity is
\~3 degrees C or less for doubled CO2. We discuss three specific sub-global
topics: Arctic climate change, tropical storm intensification, and ice sheet
stability. 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 needed to achieve the
low forcing of the alternative scenario.
