The main source of sulphate particles and CCN over the oceans is the oxidation of DMS. As such the DMS may determine the concentrations and size spectra of cloud droplets and therefore the cloud albedo over large regions of the oceans. Marine stratiform clouds are of particular importance in this regard as they cover about one-quarter of the world's oceans and therefore play a major role in the Earth's radiative balance. This scenario is the basis for what is called the DMS-cloud-climate hypothesis.