OCEAN SCIENCE: Ocean Freshening, Sea Level Rising

Global sea levels have been rising over the past century. It remains hotly debated, however, how fast this rise has been and which of the two causes—expansion of ocean waters due to warming, or freshwater input from the continents-has dominated. In his Perspective, Munk argues that a recent survey of global ocean salinity may point to a dominant contribution from freshwater input. He cautions, however, that uncertainties in sea ice melting rates and possible biases in tide gage data do not yet allow a definitive answer. Even with global satellite coverage, several decades of data will be required for good estimates of the role of global warming in sea level rise.