- 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