Plants and animals may have flourished back then. At least they didn't have ANY sea ice to contend with. Unfortunately we do now.
The Arctic ice has REDUCED significantly since about 1980 and is melting at a faster rate each year. At current rates could be GONE by 2030. If the Arctic melts the permafrost will release more CO2 into the atmosphere than is already there.
If the Antarctic melts too we are looking at sea levels increasing by about 70 meters.
