Cloud Sensitivity to Black Carbon and Aerosol Concentrations

While greenhouse gases get most of the attention in studies of global climate change, aerosols—suspensions of small particles in the air, such as dust, sea salt, and soot—can also affect climate in two ways: (1) they scatter and absorb radiation (the direct effect), and (2) they change the microphysical structure of clouds (the indirect effect). Aerosols can act as cloud condensation nuclei, increasing the number and decreasing the size of cloud droplets, a process called the first indirect forcing or Twomey effect; they can also affect the initial size distribution of precipitation drops, thereby influencing the cloud lifetime and liquid water content (the second indirect effect). Human activity, especially fossil fuel and biomass combustion, has greatly increased the amount of aerosols in the atmosphere. Whether these aerosols contribute to climate warming or cooling depends on many factors, and including those factors in global climate models can increase their accuracy.

	Figure 5   Annual average of the simulated first indirect forcing (W m–2) by anthropogenic carbonaceous aerosols (a) without absorption by black carbon in clouds and (b) with absorption. (c) Increases in the magnitude of forcing if the effect of black carbon is not taken into account. Values shown in brackets are global averages.

Chuang et al. have simulated and quantified the first indirect effect of aerosols, its sensitivity to natural and anthropogenic emissions, and the role of absorption by black carbon in clouds. The first indirect effect is generally thought to contribute to cooling by increasing cloud albedo (reflectivity). The sensitivity of cloud albedo to changes in drop number concentration is referred to as cloud susceptibility. But black carbon from combustion counteracts this effect by absorbing radiation and decreasing albedo.

The simulated contribution of anthropogenic aerosols to cloud susceptibility is consistent with satellite data and confirms previous findings that marine stratus clouds are more sensitive to changes in drop concentration than continental clouds. However, if the effect of black carbon absorption in clouds is not included, the first indirect forcing by anthropogenic aerosols may be overestimated by 15–25% in regions where black carbon emissions are pronounced (Figure 5). The first indirect forcing is also highly sensitive to the abundance of natural aerosols; without accounting for natural dust and sea salt particles, it could be overestimated by at least a factor of 2. If half of dust emissions are due to human activities, these might be responsible for additional forcing. The researchers concluded that to quantify the first indirect effect on the radiation budget with a higher level of certainty, we need further understanding of the interactions between anthropogenic components and natural particles, together with a more thorough investigation of aerosol-cloud interactions.

INVESTIGATORS
J. E. Penner, University of Michigan; C. C. Chuang and K. E. Grant, Lawrence Livermore National Laboratory; J. M. Prospero, University of Miami; G. H. Rau, University of California, Santa Cruz; K. Kawamoto, Virginia Polytechnic Institute and NASA Langley Research Center.

PUBLICATION
C. C. Chuang, J. E. Penner, J. M. Prospero, K. E. Grant, G. H. Rau, and K. Kawamoto, “Cloud susceptibility and the first aerosol indirect forcing: Sensitivity to black carbon and aerosol concentrations,” J. Geophys. Res. (in press).

URL
http://aoss.engin.umich.edu/Penner/