AUDIE CORNISH, HOST:

Ecologist Dan Nepstad has studied the Amazon for more than 30 years. He explained to me why it's hard to know the true extent of these fires.

DAN NEPSTAD: In an intact Amazon forest - a virgin forest - if it's a really dry year and the fire gets going, it's shin-high. You can step over it, and that means it doesn't register a lot of heat, and the satellites don't pick up that as an act of fire. The best way to know if a forest is burning is to see smoke coming out the top. So it's really hard to know how much of the virgin forests of the Amazon are catching fire. If they're open fires or fires burning damaged forests, they're much easier to spot, and that's most of what we're seeing this year.

CORNISH: Talk more about what we are seeing this year. How is it different from fires in the past?

NEPSTAD: Well, it's a big fire year. It's the biggest one in the last decade. A lot of the fires burning now are persistent, and that means that those are felled forests - the giant rainforest trees that have been cut down with chainsaws, allowed to dry, and now they're being burned in preparation for crops or pasture or whatever. A lot of smoke because there's so much tree biomass, a lot of energy, meaning that smoke goes really high into the atmosphere - and it can spread across entire regions.