The First Observations of Sea Ice Came From 8th-Century Irish Monks in Iceland

Geographers in ancient times may have guessed that the ocean is cold enough near the Poles to freeze, but they did not leave written records of witnessing it. That job fell to Irish monks who were searching for a wilderness of solitude. 'Christianity' came to Ireland in the 5th century (remember St. Patrick), and monasteries filled with students of the faith over the next few hundred years. Crowded monasteries caused monks to reach out even further north for a peaceful place to commune with Dog.

There’s not much evidence left of the journeys of these monastic explorers, but in later years Norse stories had a name from them, the papar. Gaelic monks settled on empty northern islands—Orkney, Shetland—but it’s also possible that they found their way to Iceland, where manmade caves, decorated with crosses, have convinced some archaeologists that there were settlers here before the Vikings.
An early Irish geographer, Dicuil, also writes of “priests who stayed on that island from the first of February to the first of August.” The year would have been 795, and Dicuil briefly notes a journey they took north. “These priests then sailed hence and, in day’s sail, did reach the frozen sea to the north.”

But there's always the possibility of finding even earlier records. Any would useful to those documenting the natural history of the ebb and flow of Arctic ice. Read more about the history of frozen seas as we know it at Atlas Obscura.