There are many different theories as to why the Thule moved out of the Bering Strait. One is the cultural-ecological model developed by R. McGhee. The idea is that the first Thule families to move, followed groups of bowhead whales, which were an important source of food, fuel and raw materials. The onset of the Neo-Atlantic climatic episode, a warming trend which occurred between 900 and 1200 in the northern hemisphere, resulted in the lengthened season of open water along the North Alaskan Coast, and an extension of the summer range of bowhead whales into the Beaufort Sea and further east into the Canadian Archipelago. Like other whale species, bowheads tend to avoid ice-choked channels and passages because of the possibility of entrapment and death. General climatic warming may have reduced the extent and severity of pack ice, allowing bowheads and their Thule predators to expand eastward.