evidence: The only ice sheets on Earth currently cover Antarctica and Greenland. Ice sheets are enormous continental masses of glacial ice and snow expanding over 50,000 square kilometers (19,300 square miles). The ice sheet on Antarctica is over 4.7 kilometers (3 miles) thick in some areas, covering nearly all the land features except the Transantarctic Mountains, which protrude above the ice. Another example is the Greenland Ice Sheet. In the past ice ages, huge ice sheets also covered most of Canada (the Laurentide Ice Sheet) and Scandinavia (the Scandinavian Ice Sheet), but these have now disappeared, leaving only a few ice caps and mountain glaciers behind. [...] Scientists have adopted three general approaches to ice sheet mass balance measurement: comparing outflow and melt to snowfall accumulation (the mass budget method), observing changes in glacier elevation (volume change or geodetic method), and detecting changes in Earth’s gravity field over the ice sheet (gravimetric method). [...] Satellite radar altimetry mapping and change detection, developed in the early to mid-1990s, allowed the research community to finally extract reliable quantitative information regarding the overall growth or reduction of the volume of the ice sheets. Like the 2002 publication, in which researchers were able to report that both large ice sheets were losing mass.