Tree cover has nearly halved since the start of human civilisation and the pace of deforestation has not abated, with 15 billion trees cut down every year.
A 15-nation team - led by Yale University experts - have used a combination of old-fashioned headcounts and state-of-the-art satellite and supercomputer technology to produce the most comprehensive tree census ever.
It revealed there were about three trillion trees on Earth, or roughly 422 for every person.
But the researchers said humans were currently felling some 15 billion trees every year, due to deforestation, land-use change, and forest management.
The highest densities of trees were found in the boreal forests in the sub-arctic regions of Russia, Scandinavia, and North America, but the largest forest areas, by far, are in the tropics, which are home to about 43 percent of the world's trees.
The study was inspired by a request by Plant for the Planet, a global youth initiative that leads the United Nations Environment Programme's "Billion Tree Campaign." The group approached reserachers two years ago, asking for estimates of tree numbers at regional and global scales, so they could better evaluate the contribution of their efforts and set targets for future tree-planting initiatives.
At the time, the only global estimate was just over 400 billion trees worldwide, or about 61 trees for every person on Earth. That prediction was generated using satellite imagery and estimates of forest area, but did not incorporate any information from the ground.