Observed and Projected Ocean Wind Speed Trends and Marine Boundary Layer Clouds

Increases in aerosol concentrations can brighten clouds, alter precipitation processes, and change cloud lifetime. The aerosol therefore has the potential to significantly impact the radiative forcing of the climate system. While evidence for these effects does exist, quantification has proven difficult, in spite of enormous effort. Following Baker and Charlson's pioneering work in 1990, we will show that the boundary layer cloud system presents two metastable states: a non-precipitating, and a precipitating state. It will be argued that these states represent cases where robust relationships between the aerosol, precipitation, and radiative forcing are hard to detect. We will describe a number of factors, including a runaway reduction in the aerosol concentration, that can initiate the transition between these states and result in a dramatic change in cloud microphysical and radiative properties. We will show that the principles of self-organizing systems apply to these metastable states, and the transition between them. Implications for our ability to quantify aerosol effects on the radiative properties of clouds will be discussed.