Fires below the forest canopy, or "understory fires," have until now been hidden from NASA satellites that detect actively burning fires.
Understory fires appear "unremarkable when you see them burning," said Doug Morton of NASA's Goddard Space Flight Center. Flames only reach a few feet high, but they may burn for weeks at a time, spreading only a few feet per minute.
But the Amazon's trees are not adapted to fire, and the long, slow burn can claim 10 to 50 percent of the burn area's trees.
Researchers analyzed observations collected by the Moderate Resolution Imaging Spectroradiometer, or MODIS, instrument on NASA's Terra satellite and found that between 1999 and 2010, understory forest fires burned more than 33,000 square miles, or 2.8 percent of the forest.
Surprisingly, their results show no connection between understory fires and deforestation activity. Morton said that despite the fact that "deforestation fires are massive, towering infernos" that spew sparks and cinders, the adjacent forest wasn't at any higher risk.
Instead, understory fires coincide with low nighttime humidity, measured by the Atmospheric Infrared Sounder (AIRS) instrument aboard NASA's Aqua satellite.
The first forecast in 2012 and new forecast for 2013, led by Jim Randerson at UC Irvine, are based on a model that primarily considers historical fire data from MODIS instruments along with sea surface temperature data from NOAA.
Researchers predict that the severity of the 2013 fire season will be considerably higher than in 2012 for many Amazon forests.
Sea surface temperatures in the Central Pacific and North Atlantic were cooler than normal in 2012, which lead to increased rainfall across the southern Amazon in the months preceding the fire season.
"With this forecasting system we're hoping to build some advanced warning about whether the Amazon region is facing a fire year or a flood year," Morton said. "This year, plan for fires."