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, USA now sail more than 100km into a fjord that held ice more than 1,500m 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 3m of additional sea-level rise could occur in the following century or less. 'Tipping point' behaviour, involving ice-shelf loss and retreat from a stabilizing ridge, is likely to be especially important, with West Antarctica's Thwaites Glacier most vulnerable. The International Thwaites Glacier Collaboration is making rapid progress, but the glacier is also changing rapidly, posing problems for communications about risk.
Richard Alley is a Roy Chapman Andrews Society 'Distinguished Explorer'. Richard has spent years working in the field in Antarctica and Greenland, collecting data to improve understanding of the 'flow and stability of the world's vast ice sheets', and the data he collected on these explorations was incorporated into several IPCC reports (one of which won the Nobel Peace Prize).