Caldeira Lab

Dependence of Climate Forcing and Response on the Altitude of Black Carbon Aerosols

George Ban-Weiss, Long Cao, Govindasamy Bala & Ken Caldeira

Black carbon aerosols absorb solar radiation, which contributes to climate warming. Here we model the relationship between the altitude of black carbon and global mean surface temperature. We find that black carbon near the surface causes strong surface heating and increases in precipitation. However, black carbon higher in the atmosphere can cause surface cooling and decreases in precipitation.

Ban-Weiss, G. A., L. Cao, G. Bala, and K. Caldeira, 2012: Dependence of climate forcing and response on the altitude of black carbon aerosols. Climate Dynamics, 38, 897–911, doi:10.1007/s00382-011-1052-y.



Black carbon aerosols absorb solar radiation and decrease planetary albedo, and thus can contribute to climate warming. In this paper, the dependence of equilibrium climate response on the altitude of black carbon is explored using an atmospheric general circulation model coupled to a mixed layer ocean model. The simulations model aerosol direct and semi-direct effects, but not indirect effects. Aerosol concentrations are prescribed and not interactive. It is shown that climate response of black carbon is highly dependent on the altitude of the aerosol. As the altitude of black carbon increases, surface temperatures decrease; black carbon near the surface causes surface warming, whereas black carbon near the tropopause and in the stratosphere causes surface cooling. This cooling occurs despite increasing planetary absorption of sunlight (i.e. decreasing planetary albedo). We find that the trend in surface air temperature response versus the altitude of black carbon is consistent with our calculations of radiative forcing after the troposphere, stratosphere, and land surface have undergone rapid adjustment, calculated as “regressed” radiative forcing. The variation in climate response from black carbon at different altitudes occurs largely from different fast climate responses; temperature dependent feedbacks are not statistically distinguishable. Impacts of black carbon at various altitudes on the hydrological cycle are also discussed; black carbon in the lowest atmospheric layer increases precipitation despite reductions in solar radiation reaching the surface, whereas black carbon at higher altitudes decreases precipitation.


Figure: Changes (relative to the control) for each simulation in global and annual-mean surface air temperature and precipitation. In each simulation the concentration of black carbon is increased by 1 Mt at a different horizontal layer in the atmosphere. Approximate altitudes of layers with additional black carbon are indicated in the legend. Uncertainty is given by the standard error computed from 70 annual means using the Student t test with 95% confidence interval. The standard error is corrected for autocorrelation (Zwiers and von Storch 1995)