Sea ice is getting thinner and younger. A new NASA study combined satellite data and declassified submarine sonar records and found Arctic sea ice cover is much thinner overall. Approximately 70 percent of all Arctic sea ice is now seasonal. This ice forms in the winter and melts in the summer, instead of lasting from year to year. That’s an important change. Seasonal ice is thinner and weaker than perennial ice, which builds up and lasts for many years. For 40 years, NASA has tracked the extent of the Arctic sea ice as it reaches a minimum every September. This annual minimum has been trending lower for decades because of climate change. However, the loss of most thick perennial ice might change that trend. The thinner ice is more vulnerable to weather and wind, so the observed changes are now more variable, not just dominated by warming.