Evolution Earth

[Shane] but this ice world specialist is having to deal with an invader, whose presence signals a problem for our planet.

The new arrival is a wily cousin... the red fox.

[Arctic fox yelps] He, too, is no slouch when it comes to catching lemmings... [sniffing] and he does it in style.

He's bigger and more aggressive, and he bullies the smaller Arctic fox out of his territory... but this competition is not as simple as it seems.

Unlike the Arctic fox, the red fox is not so well adapted to survive the blistering icy winds of the Arctic plains... but he does not come alone.

He's backed up by a force much more powerful than himself.

Moving north with him is an army... a wall of trees.

Warming temperatures are allowing the forest to move north, invading the Arctic, lending enough shelter for the red fox to take over this territory.

Now typically, we think of trees as having a positive impact on the climate, but out here they don't have the effect you might expect.

Snow reflects up to 80% of the sun's rays... but the trees darken the tundra, canceling this reflective quality, so the land absorbs more of the sun's heat.

Trees have been found to raise the temperature in the Arctic by nearly two degrees... and this heating effect is a feedback loop.

The more the land warms, the more the trees advance, accelerating climate change as they go.