Mass loss from the ice sheets of Antarctica and Greenland is contributing to sea-level rise. History and physics show that warming melts ice and that too much warming triggers rapid iceberg calving. Visitors to Glacier Bay in Alaska now sail more than 60 miles into a fjord that lay beneath up to a mile of ice when George Vancouver visited in 1794, and many other fjords have rapidly “unzipped” into their mountains or ice sheet. If a similar retreat is triggered in any of the major Antarctic basins holding far more ice, more than 10 feet of additional sea-level rise could occur in the next century or less. Exciting scientific advances will be needed to reduce the remaining large uncertainties.