Over recent years, the Arctic has warmed much like it did during the natural Medieval Warming Period - during prior warm phases, mother nature melted the Northwest Passage sea ice completely, but not yet in the modern era
Read here. Canadian Arctic experts have conclusively determined that the Arctic's Northwest Passage has frequently been devoid of sea ice during summer months.
This lack of sea ice was the result of natural warming that produced excessive temperatures. They conclude that temperatures during the Medieval Period, and multiple earlier periods, were significantly warmer than modern temperatures - from 1 to 3 degrees warmer.
"Numerous sites have been surveyed along the length of the Northwest Passage. The eastern and western approaches have become reliably ice-free in summer under historical climatic conditions, whereas in the central part summer sea ice has been persistent. The radiocarbon-dated bowhead whale remains indicate that the whales were able to range along the length of the Passage during two intervals (centered on 9000 years ago and 1000 years ago) and that they were able to access the central part from the east about 4000 years ago. During the first of these intervals (9000 before present) ice cores indicate that summer temperatures were about 3°C warmer than mid 20th Century. Therefore, a warming of 3°C exceeds the opening threshold. Medieval Warm Period temperatures were probably about 1°C warmer than mid-20th Century, which is likely close to threshold conditions for an opening of the passage."
When compared to the historical and ancient past, the modern Arctic warming and subsequent summer sea ice melt is not unusual, and is likely the result of the same natural climatic patterns and oscillations that produced such conditions in the past.
As revealed in the adjacent series of images, the modern sea melt (image3 at the bottom) still has not freed the Northwest Passage of summer ice as the experts believe the earlier warming periods easily did (image1 and image2).
Conclusions: Modern Arctic warming is not unusual versus the Medieval Warming. Current Arctic and Northwest Passage sea ice melt is less than what occurred in the past, per the empirical evidence. Since both modern and historical Arctic warming, and sea ice melting, are similar, one can surmise that natural warming is the principal cause of today's Arctic conditions. Plus, anthropogenic black soot and greenhouse gases may have exacerbated the current sea ice melting.