Peter Manshausen at the University of Oxford and his colleagues have found that clouds that had appeared unaffected are in fact affected by the aerosols. The researchers mapped the paths of two million ships in the Atlantic Ocean over a six-year period and used a global database for the winds on the trade routes the ships travelled along to see where their emitted aerosols would go. They then used another database of cloud properties to see how these clouds were affected. The researchers found that at the locations they predicted the aerosols would travel to, there were tracks of clouds with fewer droplets but more liquid water. This slightly changes the overall reflectivity of the clouds and, more importantly, a larger water-to-droplet ratio implies a stronger cooling effect, so the clouds would reflect more radiation back towards the sun. This leads to a cooling effect from human-derived aerosols that, measured as the amount of water between two points in the atmosphere, was found to be -0.76 watts per square metre, very different from the heating effect, at 0.2 watts per square metre, in the most recent Intergovernmental Panel on Climate Change report.
