The article speaks of assumptions obtained from tree rings, ice cores, and permafrost.

It's very straightforward to analyze the trees in the forests that stretch across the North Pole to determine such things. And, as most trees in the Arctic live in excess of 1,500 years, we have a very good record. Likewise, it's trivial to examine the permafrost that covers the North Pole and calculate the temperatures from 1,500 years ago.

But, the simplest source of data are ice cores from the extensive and permanent sheets of ice that perpetually cover the North Pole.

No earlier records (or very spotty).

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.