From the Arctic: why was it so much warmer in the past?

From this post:

Today we traveled through a forest, or at least the remains of a forest. In a time before the human race evolved there were great pine trees right here on Axle Heiberg thanks to a warmer world and the aid of a micro-climate system that sat above this location. Yes that's right, at one point in time it was warm enough for trees to grow in the high arctic, a place now covered with sparse tundra. Even more amazing is the fact that the trees are still here, in relatively good condition. After the forest died instead of decomposing it went through a process of extreme dryness and cold which mummified the wood, preventing it from rotting. Later it was buried beneath the ground and now as the land is eroding pockets of old wood are showing up.

Its an amazing feeling to be walking though the tundra and to come across tree stumps; the remains of what would have been towering giants, and to picture yourself in the past amongst the trunks of a mighty forest. Who knows, the future may not look that different either.