Hansen 2003 : Soot Twice As Effective As CO2

Soot climate forcing via snow and ice albedos  James Hansen*  †‡  and Larissa Nazarenko*  †

Plausible estimates for the effect of soot on snow and ice albedos (1.5% in the Arctic and 3% in Northern Hemisphere land areas) yield a climate forcing of
0.3 W
m2 in the Northern Hemisphere. The ‘‘efficacy’’ of this forcing is
2, i.e., for a given forcing it is twice as effective as CO2 in altering global surface air temperature.

Global Warming. Soot snow
ice albedo climate forcing is not included in Intergovernmental Panel on Climate Change evaluations.  This forcing is unusually effective, causing twice as much global  warming as a CO2 forcing of the same magnitude. This high efficacy  is a straight-forward consequence of positive albedo feedbacks and  atmospheric stability at high latitudes.  Our estimate for the mean soot effect on spectrally integrated  albedos in the Arctic (1.5%) and Northern Hemisphere land areas  (3%) yields a Northern Hemisphere forcing of 0.3 W
m2  or an  effective hemispheric forcing of 0.6 W
m2  . The calculated global  warming in an 1880–2000 simulation is about one quarter of  observed global warming

pubs.giss.nasa.gov/docs/2004/2004_Hansen_Nazarenko.pdf