 Despite their proximity and importance to human populations , the mountain and valley glaciers of tropical and mid @-@ latitude glaciers amount to only a small fraction of glacial ice on the Earth . About 99 percent of all freshwater ice is in the great ice sheets of polar and subpolar Antarctica and Greenland . These continuous continental @-@ scale ice sheets , 3 km ( 1 @.@ 9 mi ) or more in thickness , cap much of the polar and subpolar land masses . Like rivers flowing from an enormous lake , numerous outlet glaciers transport ice from the margins of the ice sheet to the ocean . 
