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Routine observations of Arctic sea ice thickness only began about a decade ago, experts use the age of the ice pack—which has been observed since the 1980s—as a proxy for ice thickness. In mid-March 1985 (left), the winter maximum ice pack was dominated by ice at least 4 years old (white). In 2021 (right), only a small strip of very old ice remained tucked up against the islands of the Canadian Arctic. More than half of the winter ice pack was less than a year old (dark blue). NOAA Climate.gov image, based on data from the National Snow and Ice Data Center.