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 stret

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.