Around 50 million years ago there were extensive, lush deciduous forests in the polar regions of the Arctic, where today there is sparse vegetation. The forests existed due to the conditions in the Eocene—a combination of a greenhouse climate and almost twice the level of carbon dioxide in the atmosphere as there is today. However, the same extreme light conditions prevailed in these regions of high latitude—permanent darkness for months on end in winter and a sun that never set in summer, even if it was low in the sky.
