IPY occurs amidst abundant evidence of changes in snow and ice, including reductions in extent and mass of glaciers and ice sheets, reductions in area, timing, and duration of snow cover, and reductions in extent and thickness of sea ice. Some changes show clear downward or decreasing trends over recent years and decades. Other changes, for example net mass changes of the massive Greenland and Antarctic ice sheets, remain uncertain in magnitude and direction. Changes in snow cover and sea ice have immediate consequences for land and ocean heating, and for terrestrial and marine ecosystems. Investigators will seek to understand origins of, and connections among, Antarctic sub-glacial lakes and recent evidence of sub-glacial flood-like outflows. The observations and modeling of IPY will document and quantify the extent, rate and impact of snow and ice changes and their integrated impact on global sea level.