What is the meaning of 1,500 years? Are there no earlier records or was the Arctic declining faster 1,500 years ago? If the later, do we know the effects?
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.
If there's one thing that sea captains were good at it was keeping excruciatingly detailed logs.