Do Shiptracks Really Make a Difference Globally?

Ship-tracks are an iconic demonstration of aerosol-cloud interactions. They have been observed in satellite images for more than 60 years. Many studies have resulted from analyses of remote sensing, in-situ, and modeling results. Much have been learnt from these efforts. However, some of the basic questions remain to be answered. One of them is whether they matter on a global scale. For that global observations of ship-tracks are needed as a first step. In this study, we present results of a first global survey of ship-tracks using MODIS nighttime data. MODIS radiance data for the full year of 2011 are analyzed to find ship-tracks within major marine low cloud fields. We characterize the global distribution of ship-tracks and its variability on monthly to seasonal time scales. A significant portion (up to 5%) of the low clouds are affected by instantaneous ship-tracks. We also present results regarding the distribution of ship-track properties such as their length, width, and overall size (in terms of number of pixels) for different regions and seasons. Based on millions of ship-tracks we are able to find, it becomes possible to start answering the title question in a quantitative way with observational data.