If you have a habit of perusing satellite imagery of the world's oceans--and who doesn't, really?--you might get lucky and spot long, thin clouds, like white slashes across the sea. That's a peculiar phenomenon known as a ship track. As cargo ships chug along, flinging sulfur into the atmosphere, they actually trace their routes for satellites to see. That's because those pollutants rise into low-level clouds and plump them up by acting as nuclei that attract water vapor, which also brightens the clouds. Counterintuitively, these pollution-derived tracks actually have a cooling effect on the climate, since brighter clouds bounce more of the sun's energy back into space.