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Sea ice is getting thinner and younger.

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A new NASA study combined satellite data and declassified submarine sonar records and found Arctic sea ice cover is much thinner overall.

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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.

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That's an important change. Seasonal ice is thinner and weaker than perennial ice, which builds up and lasts for many years.

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For 40 years, NASA has tracked the extent of the Arctic sea ice as it reaches a minimum every September.