Together, the Antarctic and Greenland Ice Sheets contain more than 99 percent of freshwater ice on Earth. If they both completely melted, they would raise sea level by an estimated 67.4 meters (223 feet). Long-term satellite data indicate that through most of the twentieth century, the ice sheets made very little contribution to sea level, and were nearly in balance in annual snowfall gain and ice or meltwater loss. However, the stability of the ice sheets has changed considerably in the twenty-first century.