Over the last two decades, Brazil's government has pushed to curb deforestation. By official accounts, their progress as been commendable, but new research suggests a significant portion of deforestation has gone undetected by Brazil's satellite monitoring system, called PRODES.

Researchers published their findings this week in the journal Conservation Letters.

"PRODES has been an incredible monitoring tool and has facilitated the successful enforcement of policies," Leah VanWey, co-author of the new study and director at the Institute at Brown for Environment and Society, said in a press release. "But we show evidence that landowners are working around it in ways that are destroying important forests."

PRODES has been witness to a significant reduction in the amount of rain forest lost to agriculture each year. But it's missing a portion of what's being lost because its focus is on primary rain forest -- ignoring other important ecosystems, dry forests and secondary forests.

"PRODES essentially masks out these regions and treats them as non-forest," VanWey said. "We wanted to compare the PRODES maps with satellite sources that just look at canopy cover, without those exclusions. We showed that while deforestation in large plots of primary rainforests has declined, it has expanded in these areas not tracked by PRODES."

PRODES doesn't just measure deforestation. It also tallies Brazil's greenhouse gas emissions caused by the burning of biomass. Its arithmetic may be way off.

VanWey and her colleagues say deforestation-related CO2 emissions are really twice as high as PRODES estimates.

The findings are a reminder that it's time to re-calibrate PRODES.

"There's been a transformation in the way people do agriculture," VanWey said. "At the same time, the landscape has changed with emergence in recent decades of these secondary forests. We suggest that enforcement and monitoring regimes need to be updated for that new reality of land management."