BBC: The melting of Earth’s polar ice sheets has contributed 11 mm, or about one-fifth, of the overall global sea-level rise over the past two decades, according to the most definitive study to date, published online today in Science. The latest finding is the result of the combined efforts of 20 polar research teams, who used improved modeling and data from NASA and European Space Agency satellites. Because of the remoteness and sheer size of the ice sheets, accurate measurements have been a challenge. The satellites have to distinguish snow from ice, and scientists have to account for the “post-glacial rebound,” or the amount the land rises when the ice melts. By pooling all the researchers’ data, the most recent estimate is “two to three times more reliable than the last one,” according to Andrew Shepherd, a professor at the University of Leeds in the UK and lead author of the study. The next big challenge, say the researchers, is to predict what will happen over the next century.