The research, published Wednesday in the scientific journal Nature, says the Amazon's vital role as a carbon sink -- absorbing massive amounts of heat-trapping carbon dioxide from the atmosphere, helping to cool the Earth -- is under threat.

"This carbon sink seems to be in decline," the study said. "Over the past 40 years, eastern Amazonia has been subjected to more deforestation, warming and moisture stress than the western part, especially during the dry season."

Over nine years, researchers led by Brazil's National Institute for Space Research conducted close to 600 flights over four main sites in the Brazilian Amazon, collecting data on the amount of carbon dioxide and carbon monoxide in the atmosphere.

They found that collectively, these four sites emit 410 million metric tons of carbon dioxide per year, caused mainly by large fires -- often set by humans. These emissions are partially offset by the same regions absorbing about 120 million metric tons of carbon per year; but that's still 290 million metric tons in net emissions -- about the same as the entire country of Thailand produces in a year.