Even without that warm spell, the polar regions had at least a temperate climate. But one unlike any today, because of the long dark winters, and the nightless summers. I remember an episode of Walking with Dinosaurs, which showed what it was like in Antarctica 100 million years ago. There were frosty periods during the dark winter, but these were compensated by the 24 hour daylight of summer.
Interesting programme "Planet Earth" on Prime last night with some weather related stuff, along with some cuddly and not-so-cuddly animals. Good photography.