Anomalously high activities of short-lived radium isotope ²²⁴Ra, measured in the summer surface water of the Arctic Ocean, have long been enigmatic. Through experiments, we demonstrate that sea ice effectively absorbs dissolved ²²⁸Ra during formation, enabling in-situ production of ²²⁴Ra via a complete chain of radioactive decay within the ice matrix. The accumulated ²²⁴Ra in sea ice gets released to surface water upon melting. We develop a coupled sea ice-radium dynamics model to simulate this seasonal production-and-release mechanism. The model reproduces the observed ²²⁴Ra signals, as amplified by the seasonal sea ice freezing-melting cycle. Using satellite-derived sea ice records along with model predictions, we further show that shifts in sea ice extent and seasonality caused by climate change drive significant changes of ²²⁴Ra dynamics, in addition to the effect due to rising continental input of ²²⁸Ra from thawing permafrost. The ²²⁴Ra activity encodes the cumulative effects of seasonal ice freezing–melting cycle shift and permafrost thaw, and serves as an integrative tracer of climate change impacts on the Arctic cryosphere and hydrosphere.