Soot climate forcing via snow and ice albedos

Abstract

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/m(2) in the Northern Hemisphere. The "efficacy" of this forcing is approximately 2, i.e., for a given forcing it is twice as effective as CO(2) in altering global surface air temperature. This indirect soot forcing may have contributed to global warming of the past century, including the trend toward early springs in the Northern Hemisphere, thinning Arctic sea ice, and melting land ice and permafrost. If, as we suggest, melting ice and sea level rise define the level of dangerous anthropogenic interference with the climate system, then reducing soot emissions, thus restoring snow albedos to pristine high values, would have the double benefit of reducing global warming and raising the global temperature level at which dangerous anthropogenic interference occurs. However, soot contributions to climate change do not alter the conclusion that anthropogenic greenhouse gases have been the main cause of recent global warming and will be the predominant climate forcing in the future.

Fig. 1.

Climate forcing in W/m² (Upper) and equilibrium annual-mean T_s response in °C (Lower) for changes of snow and ice albedos specified in Table 2. Numbers on the upper right are global means.

Fig. 2.

Equilibrium T_s (°C) response to the snow/ice albedo forcings of Fig. 1 as a function of month and latitude.

Fig. 3.

Observed 1880–2002 T_s change based on adjusted meteorological station data over land (28) and sea T_s data for the ocean (29, 30).

Fig. 4.

Simulated 1880–2002 T_s change for the transient BC snow/ice albedo forcing that peaks in the 1990s with 83% of the case 1 forcing of Fig. 1.