Satellite-based estimate of global aerosol-cloud radiative forcing by marine warm clouds

Changes in aerosol concentrations affect cloud albedo and Earth's radiative balance(1). Aerosol radiative forcing from pre-industrial time to the present due to the effect of atmospheric aerosol levels on the micro- and macrophysics of clouds bears the largest uncertainty among external influences on climate change(1). Of all cloud forms, low-level marine clouds exert the largest impact on the planet's albedo(2). For example, a 6% increase in the albedo of global marine stratiform clouds could offset the warming that would result from a doubling of atmospheric CO2 concentrations(3). Marine warm cloud properties are thought to depend on aerosol levels and large-scale dynamic or thermodynamic states(4-6). Here we present a comprehensive analysis of multiple measurements from the A-Train constellation of Earth-observing satellites, to quantify the radiative forcing exerted by aerosols interacting with marine clouds. Specifically, we analyse observations of co-located aerosols and clouds over the world's oceans for the period August 2006-April 2011, comprising over 7.3 million CloudSat single-layer marine warm cloud pixels. We find that thermodynamic conditions-that is, tropospheric stability and humidity in the free troposphere-and the state of precipitation act together to govern the cloud liquid water responses to the presence of aerosols and the strength of aerosol-cloud radiative forcing.