The New York Times article, Beavers Emerge as Agents of Arctic Destruction, explains:
… as climate change warms the Arctic and thaws the permafrost, the growing season extends. What was once tundra gives way to brush.
This may allow beavers to move north.
But in the tundra, the vast treeless region in the Far North, beaver behavior creates new water channels that can thaw the permanently frozen ground, or permafrost.
What remains is a pitted landscape, with boggy depressions, that directs warmer water onto the permafrost, leading to further thawing. As permafrost thaws it releases carbon dioxide and methane, which in turn contributes to global warming and helps increase the speed that the Arctic, which is already warming faster than the rest of the planet, defrosts.
This is an interesting article with satellite photos showing how beavers have changed the landscape.