(CNN) A newly-released NASA study has found the atmosphere over the Amazon rainforest in South America has been drying out over the past 20 years and that human activity is the primary cause.

Scientists at NASA's Jet Propulsion Laboratory looked at decades' worth of ground data and satellite data to track the amount of moisture in the air over the Amazon and the amount of moisture needed to maintain the rainforest system.

"We observed that in the last two decades, there has been a significant increase in dryness in the atmosphere as well as in the atmospheric demand for water above the rainforest," JPL's Armineh Barkhordarian said in a statement. She is the lead author of the study, which was published last month in the journal Scientific Reports

Her team compared the data to climate models that estimated climate variation over thousands of years.

"We determined that the change in atmospheric aridity is well beyond what would be expected from natural climate variability," she said.