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.”

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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.

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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.

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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