Arctic ice lower limit
We may not have satellite measurements earlier than 1979, but we do know that the Northwest Passage has not been sufficiently ice-free to allow easy ship travel anytime in the last couple of hundred years. Maybe we don't have every single summer covered but overall it seems like a reasonably clear record. Not that a few hundred years is all that long, but it's certainly more than 30.
The sort of "landscape melting" the article describes might also leave signs that would be apparent for quite some time, but without knowing more I wouldn't want to guess how long.