The size of the Amazon rainforest has been shrinking – and not just because of traditional deforestation.

Fires that creep slowly and low in the forest understory burned nearly 3 percent of the world’s largest tropical rainforest in a little more than a decade, scientists at NASA say.

Because they are hard to measure from space, “we've never known the regional extent or frequency of these understory fires," said Doug Morton of NASA's Goddard Space Flight Center in Greenbelt, Md., the lead author of a study publicized by the space agency Friday.

The study estimated that between 1999 and 2010, understory forest fires burned more than 33,000 square miles (85,500 square kilometers), or 2.8 percent of the forest.

The fires are typically caused by human activities such as cooking, cigarettes, or agricultural waste burning.

But they’re not directly related to deforestation activity, which can include fires that Mr. Morton describes as “massive, towering infernos.” Rather, an important indicator of risk for understory fires is dryness.

Frequent understory fire activity coincides with low nighttime humidity, the NASA research found.

By contrast, in some of the peak years for forest-clearing activity (2003 and 2004), adjacent forests had low rates of understory fires.

In understory fires, flames generally reach only a few feet high. They often burn for weeks at a time, spreading a few feet per minute.

To gauge the scale of understory fire activity, Morton and colleagues used observations from early in the dry season, from June to August, collected by MODIS – the Moderate Resolution Imaging Spectroradiometer instrument on NASA's Terra satellite.

The researchers tracked the timing of damage and recovery in various disturbed areas. Areas of deforestation lack signs of recovery for at least two years. But degradation from understory fires is visible in the year after the burn, then dissipates quickly as the forest regrows.

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Separately, NASA and academic researchers on Friday released predictions that the 2013 fire season will be “considerably higher” than in 2011 and 2012 in many parts of the Amazon.

The scope of the understory fires doesn’t make deforestation less important, but it suggests that such fires “are important source of [carbon] emissions that we need to consider," Morton said.