The research, published April 22 in the journal Philosophical Transactions of the Royal Society B, used satellite data to show that in some years, understory fires burn a far larger extent of forest than the area deforested for agriculture and cattle pasture. Yet the study found no correlation between understory fires to deforestation.

"You would think that deforestation activity would significantly increase the risk of fires in the adjacent forested area because deforestation fires are massive, towering infernos," said lead author Doug Morton of NASA's Goddard Space Flight Center in a statement. "You make a bonfire that is a square kilometer in size, throwing ash and live cinders and preheating the adjacent forest. Why didn't we have more understory fires in 2003 and 2004, when deforestation rates were so high?"

 

The answer lay in humidity data from the Atmospheric Infrared Sounder (AIRS) instrument aboard NASA's Aqua satellite. Understory fire frequency coincides with low nighttime humidity, which allows the low-intensity surface fires to continue burning. In other words, climate conditions seem to be the most important factor in the area affected by understory fires.