For decades, Antarctica seemed to defy global warming. Since satellites began monitoring the poles in the late 1970s, the seasonal growth and retreat of Antarctic sea ice – frozen seawater that expands around the continent each winter – appeared remarkably resilient. It was often described as the "heartbeat of the planet". Unlike the Arctic, where sea ice declined rapidly as the planet warmed, Antarctic sea ice showed little overall loss. It even expanded between 2007 and 2015. But that