That depends on what you mean when you say "forest".How big does a forest have to be to be a forest? And over what timescale - do the destruction of the great forests of the fossil record count?

I would wager a guess - and this is a guess - that somewhere around a third to a half of the world's forests that existed ten thousand years ago do not exist today. Much of that destruction has been caused by humans, but with the climate change over the last ten thousand years associated with the end of the last ice advance, even what is due to us is hard to say.

That's a difficult question to answer, because forests come and go naturally over the ages as climate and animal populations change. Here are some numbers for you, though.

From the beginning of recorded history through the beginning of the agricultural revolution (roughly 1700), about 34% of Temperate forests, 23% of Subtropical forests, and 5% of Tropical forests were cut down. These trees were used for building materials, ship building,and fuel. The tropics weren't so badly damaged because most people using the trees during those times lived in temperate Europe or subtropical regions of Asia.

From 1700 to today, though, 19% more of our forests were destroyed. Most of these were tropical rain forests. That's because people in tropical regions are now industrialized, and because many areas of temperate forest are now gone, so if those people need wood, they buy it from the tropics. What's frightening is that we've destroyed almost as much forest in the last 300 years (since 1700) as we did in the 3,000 years before 1700!