Before the exodus, for many years, all the world blew poisons into the sky. Forests died. The world grew warmer. Since the exodus the forests had returned and the world begun to cool, but the old poisons were still there, dormant.
This line reminds me of a current problem occurring in this modern day: climate change. Global warming is the warming of the Earth and this has caused many drastic changes in different environments, creating many problems such as the melting of icebergs which rises sea levels.