These sorts of records depended on explorers. I'm supposing someone had to read through endless ship logs to find data on ice patterns.

If there's one thing that sea captains were good at it was keeping excruciatingly detailed logs.

Indeed. The logs of sea captains sailing across the Arctic ice during the past 1,500 are endless. The entire 1492 hoopla was kind of overrated in light of the fact that endless amounts of ships were zipping across the Arctic Ocean to North America (taking excellent ice-sheet measurement as they went) for a thousand years before Columbus.