Is this a forest? That depends on what you mean.

Historically, the definition of forests has been a lot like the definition of pornography: We know it when we see it. “Yes,” one might say to oneself as they stand in the middle of a patch of green, noting the presence of trees and wildlife, and the absence of buildings and sidewalks. “I see that I am in a forest.”


But if you’re looking not around you but below you, like in a satellite picture designed to measure the amount of forest cover in the world, that process rapidly falls to pieces. Is that mostly green patch of trees a forest? What about that much more green patch a couple 100 miles away from it? Do both count? Should one count more than the other?



Two of the more common definitions that ecologists have settled on are: 1) A forest is a place with more than 10 percent tree cover and 2) A forest is a place with more than 30 percent tree cover. As you might imagine these two definitions obviously create a big discrepancy in how much forest we have today, to the tune of about 19,000,000 square kilometers, according to a study published in Nature Climate Change today.


NASA’s Earth Observatory also put together these two maps showing how much forest we would have under each of the two warring definitions. Under the 10+ percent definition, things are looking pretty good for the Earth:

With the 30+ percent definition, though, it becomes pretty obvious that forests are actually pretty rare:


Of course, one solution is simply to settle on a definition and use that one, but that kind of worldwide standardization is hardly simple.

Also, although these two definitions have created quite a bit of confusion, they’ve also made one thing exceptionally clear: The problem is not just decreasing forest area, it’s decreasing forest density—and that loss may turn out to be as big of a problem as any the world’s forests face.


Top image: Humboldt Forest / Scrubhiker (USCdyer) Maps: Jesse Allen / NASA Earth Observatory