Forest occupies about 30% of the land surface of the earth.  It is assumed that even in historic times it declined from about 50%. I met someone who heard a presentation from a speaker at a recent environmental conference in Abu Dhabi who claimed that our use of fossil fuels had been fine; that had we not used them then the pressure on the forest would have been much worse, that much more would have been lost. Given the proportion of total biodiversity locked up in the forest, perhaps we would have reached a critical environmental tipping even earlier than we are now at risk of doing.