a persistent body of dense ice formed from compressed snow over centuries
covers ~10% of Earth's land surface, stores ~69% of freshwater
two types: continental ice sheets (Greenland, Antarctica) and alpine/valley glaciers
flow under their own weight through internal deformation and basal sliding
carve landscapes: U-shaped valleys, cirques, moraines, fjords, drumlins
glacial advance and retreat tracks climate cycles over millennia
ice cores provide a climate archive: trapped atmosphere bubbles record past CO2 and temperature
meltwater feeds rivers that sustain billions of people (Ganges, Indus, Yangtze)
current retreat is accelerating due to rising global temperature
sea level rise from ice sheet loss threatens coastal populations worldwide
connected to the water cycle as frozen reservoirs and to the carbon cycle through albedo feedback