This is the summary for the video "Military submarines have been measuring the Arctic ice cap since the beginning of the Cold War. What they've discovered is startling. According to their measurements, the thickness of the ice cap has diminished by half since 1980". It doesn't look like it's receding from above but the effect is at the bottom. The US military frequently surveys the area as it's the closet path from Russia.
The holy doctrine of the church of climate change requires that all measurements, data, and trends start at 1979.