Mass loss from the ice sheets of Antarctica and Greenland is contributing slightly to sea-level rise, but could contribute much more.  Physics and history show that warming melts ice and that too much warming triggers faster iceberg calving raising sea level more rapidly.  Visitors to Glacier Bay in Alaska now sail more than 100 km into a fjord that lay beneath up to 1500 m of ice when George Vancouver visited in 1794, and many other fjords have similarly “unzipped” into their mountains or ice sheet.  If such a retreat is triggered in any of the major Antarctic basins, more than 3 m feet of additional sea-level rise could occur in the following century or less.  Although the basic physical processes are well-understood, accurately projecting future behavior remains challenging.  Penn State is a leader in field work and modeling addressing this challenge.