There are three substantial ice sheets on Earth: Greenland, West Antarctica, and East Antarctica. They contain enough frozen water that, if all of it melted, global sea level would rise about 65 meters. They respond to warming on different timescales, and the partial collapse of any one of them is among the largest single risks in twenty-first-century planetary science. Greenland and West Antarctica are now losing ice at a combined rate of ~500 Gt per year. East Antarctica, the largest, is roughly stable but contains glaciers — Totten, Wilkes — whose marine-grounded sectors may be more vulnerable than thought a decade ago. The timescale for full collapse is centuries to millennia; the timescale for committed collapse may be much shorter.
What makes ice-sheet collapse non-linear is marine-grounded retrograde-bed instability. A glacier whose grounding line — where ice transitions from resting on bedrock to floating on the ocean — sits on a bed that deepens inland is geometrically unstable: as the grounding line retreats, the ice column at the new boundary is thicker, discharge increases, retreat accelerates. Once the threshold is crossed, the dynamics carry the system regardless of further forcing. West Antarctica's Thwaites and Pine Island glaciers are the most-watched indicators; both are retreating, and 2014 papers (Joughin, Rignot) argued collapse may already be unavoidable over centuries. The full West Antarctic Ice Sheet would add ~3 meters to sea level. Greenland has different dynamics — its bedrock mostly rises inland — but the surface is increasingly in contact with above-freezing summer air, and surface mass balance has gone from broadly balanced in the 1990s to substantially negative. The threshold for committed Greenland decay is estimated at roughly 1.5°C of warming; full collapse adds ~7 meters over millennia.
Global sea level is rising at ~3.4 mm per year (1993–2023, satellite altimetry) — doubled since the early 1990s. Contributors by importance: thermal expansion of the warming ocean, Greenland mass loss, mountain glaciers (Patagonia, Alaska, the Himalayas, the Alps), and Antarctica. High-emissions pathways project 0.6 to 1 m of rise by 2100, plausibly more. The committed sea level — the rise the planet is now locked into regardless of emissions — is the more troubling figure: ice sheets respond slowly. Coastal infrastructure, low-lying deltas (the Nile, Ganges-Brahmaputra, Mekong, Mississippi), and small island states face the consequences first; several hundred million people now live within reach of plausible 21st-century rise.
ITGC — the International Thwaites Glacier Collaboration, 2018+ — is the largest single Antarctic field campaign in decades, measuring grounding-line dynamics, ice-shelf melt rates, and ocean conditions under the floating ice tongue. NASA's GRACE-FO tracks ice-sheet mass balance from space at monthly resolution. The Topex-Jason-Sentinel altimetry chain has measured global sea level continuously since 1992. National adaptation programs explicitly account for committed sea-level rise: tide gates in Boston, raised road grades in Miami, planned population relocation in Indonesia. Whether the world's coasts can adapt at the pace the dynamics impose is increasingly a question about engineering and political will rather than climate science.