Source: Scientists Have Uncovered a Disturbing Climate Change Precedent
Looking at evidence about the environments on earth 50 million years ago, it is likely that the earth then had 1,000 parts per million (ppm) of atmospheric carbon dioxide (CO2), arctic and antarctic environments similar to today’s tropics (e.g. the Sahara desert and Amazon rainforest), and possibly dead zones with temperatures above 50 degrees Celsius (122 degrees Fahrenheit) in the tropics.
You can also see a rendering of the earth [here].