The late Cretaceous probably saw the earth entirely ice-free.  Atmospheric carbon levels were around 1000 ppm at the time.  Today they are around 400 and steadily climbing.   The problem is adaptation.  Most species cannot adapt fast enough to keep up with dramatic environmental changes and it's likely that we will lose up to a quarter of earth's species or more by the end of the century.  When the web of life breaks down all species are threatened with extinction, including humans.  It's Easter Island on a planetary scale whether one wants to deny this or not.