The Greenland ice sheet is the largest ice sheet in the Northern Hemisphere. Snow accumulation is high due to the proximity of the North Atlantic, and annual layers have been resolved in ice core records through the last glacial period. Ice flows from the interior toward the margin, where it is lost by surface melting or drains through fast-flowing outlet glaciers into fiords. The Greenland ice sheet is currently losing mass by retreat along the margins and acceleration of outlet glaciers.