This is interesting not only to penguins and polar bears.
Fresh water is less dense than seawater.
Differences in density to a large extent drive the global circulation of the oceans. Unfortunately, the oceans, with their corrosive salt water, are a hostile environment for all observational apparatus, so it is difficult to measure precipitation over them except from passing ships. To get round this problem, the authors of the Nature report, Nathaniel L. Bindoff of the Antarctic Co-operative Research Centre, Hobart, Tasmania, Australia, and colleagues, applied a neat trick. They used the fact that in the cold, high latitudes, water masses form with distinct temperature and salinity signatures. Both processes add to the density of seawater.