Sea level is rising because of human-caused warming, impacting coastal communities. Shrinkage of the ice sheets of Antarctica and Greenland is contributing, and could accelerate in the future. 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 held ice up to a mile thick 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 following century or less. Exciting scientific advances will be needed to reduce the remaining large uncertainties.