Ice sheets and smaller ice caps appear to behave in dynamically similar ways; both contain slow-moving ice that is probably frozen to the bed, interspersed with fast-flowing ice streams and outlet glaciers that terminate into the ocean. Academy of Sciences Ice Cap (Akademii Nauk ice cap; 5570 km2), Severnaya Zemlya, Russian High Arctic, provides a clear example of this varied flow regime. We have combined satellite measurements of elevation change and surface velocity to show that variable ice-stream dynamics dominate the mass balance of the ice cap. Since 1988, the ice cap has lost 58±16 Gt of ice, corresponding to ~3% of its mass or 0.16mm of sea-level rise. The climatic mass balance is estimated to be close to zero, and terminus positions have remained stable to within a few kilometers, implying that almost all mass loss has occurred through iceberg calving. The ice-cap calving rate increased from ~0.6 Gt a–1 in 1995 to ~3.0 Gt a–1 in 2000–02, but has recently decreased to ~1.4 Gt a–1 due to a likely slowdown of the largest ice stream. Such highly variable calving rates have not been reported before from High Arctic ice caps, suggesting that these ice masses may be less stable than previously thought.