I can see 1 degree being enough for tree growth. It's not ALWAYS snowy in the arctic, and as the article says, the tree problem isn't just that its too hot, its that its "summery" arctic temperature for longer. Obviously we don't have trees sprouting at the North Pole in December, but tree-friendly territory is spreading, the glaciers are melting and the grizzlies are taking over polar bear territory.
so if its not caused by global warming, then what?