Just 5.53 million square miles of ice had formed as of March 22, the smallest maximum extent since satellite recordkeeping began in the 1970s.

Sea ice plays many important roles for the global climate: Its white surface can reflect energy back into space, helping the planet cool. It also acts like a blanket for the ocean, insulating it and preventing ocean heat from reaching the atmosphere. Less sea ice means more heat goes in Earth’s systems, warming the atmosphere and the oceans.

Arctic sea ice had its weakest winter buildup since record-keeping began 47 years ago, a symptom of climate change that will have repercussions globally, scientists said Thursday. The Arctic reaches its maximum sea ice in March each year and then starts a six-month melt season.