Lakes across the globe that once froze solid all winter are melting faster than ever before and, in some cases, are not freezing at all, altering long-standing ways of life for people and disrupting lake ecosystems, National Geographic reports. Researchers visited 513 lakes in the Northern Hemisphere to assess how their patterns of freezing and thawing had changed since 1970. If the average annual air temperature surrounding a lake that usually froze all winter exceeded 8°C then the lake would not stay frozen all winter, and at 10°C or hotter the lake was unlikely to freeze in the first place, the researchers reported earlier this week in Nature Climate Change. From these observations, the team estimated that roughly 15,000 lakes around the world already freeze less than they used to. The authors suggest lake ice could become scarce within the next generation, permanently canceling winter activities such as ice skating and ice fishing.