The answer is that the world is home to over three trillion trees—with almost half of them living in tropical or subtropical forests. There are roughly 400 trees for every human. 

12,000 years ago, before the advent of agriculture, Earth had twice as many trees as it does now. Currently, our planet is losing 10 billion* trees a year. 

Researchers represented the number of trees across the globe using bars that are taller for denser forests.

The report was published in Nature on September 2, 2015.

*Editor’s Note (11/27/18): This sentence was edited after posting to correct the statistic.