There is an important new paper in Science magazine that sheds new insight into the complex role that aerosols play within the climate system. Since aerosols are input into the atmosphere through a variety of human activities (e.g. industrial and vehicular emissions, biomass burning, blowing dust from landscape degradation), this means that the human aerosol effect on climate is not only very significant but also quite complicated in how it affects weather.
The paper is Flood or Drought: How Do Aerosols Affect Precipitation?by Daniel Rosenfeld, Ulrike Lohmann, Graciela B. Raga, Colin D. O’Dowd, Markku Kulmala, Sandro Fuzzi, Anni Reissell, Meinrat O. Andreae, Science 5 September 2008: Vol. 321. no. 5894, pp. 1309 - 1313 DOI: 10.1126/science.1160606.
The abstract reads
“Aerosols serve as cloud condensation nuclei (CCN) and thus have a substantial effect on cloud properties and the initiation of precipitation. Large concentrations of human-made aerosols have been reported to both decrease and increase rainfall as a result of their radiative and CCN activities. At one extreme, pristine tropical clouds with low CCN concentrations rain out too quickly to mature into long-lived clouds. On the other hand, heavily polluted clouds evaporate much of their water before precipitation can occur, if they can form at all given the reduced surface heating resulting from the aerosol haze layer. We propose a conceptual model
that explains this apparent dichotomy.”
An excerpt from the text of the paper reads
“….before humankind started to change the environment, aerosol concentrations were not much greater (up to double) over land than over the oceans… Anthropogenic aerosols alter Earth’s energy budget by scattering and absorbing the solar radiation that energizes the formation of clouds…Because all cloud
droplets must form on preexisting aerosol particles that act as cloud condensation nuclei (CCN), increased aerosols also change the composition of clouds (i.e., the size distribution of cloud droplets). This, in turn, determines to a large extent the precipitation-forming processes.”
The excellent Rosenfeld et al paper is an important new contribution that describes the complexity of how aerosols affect the climate system. Since the multi-decadal global models used to create the 2007 IPCC report did not properly represent these effects, their projections of how precipitation (and thus all other climate variables) are expected to change in the future are clearly inadequate scientifically.