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.
The study found that the vapor pressure deficit, which measures the difference between the amount of air that is in the atmosphere and the maximum amount of moisture it can hold, has gone up, particularly across the south and southeastern Amazon, during the dry season months of August through October.
Elevated greenhouse gas levels are responsible for about half of the increased aridity, according to Barkhordarian. Human actions such as burning forests to clear land for farming and grazing are responsible for the rest of the change.
The fires give off soot and other particles, known as aerosols, which absorb heat from the sun and contribute to the warming of the atmosphere.
The scientists found that the drying of the atmosphere was worse in the southeast region, where the most of grazing and agricultural expansion is happening.
The northwest Amazon doesn’t typically have a dry season, but it has experienced severe droughts over the past two decades, according to the study.
There has been a dramatic increase in the number of Amazon fires this year. Brazil’s National Institute for Space Research (INPE) said the number of fires was up 85% from last year.
In August, Brazilian President Jair Bolsonaro imposed a 60-day ban on using fire to clear land in the country.
An increased vapor pressure deficit in California has helped drive the increase in ferocity of wildfires there, scientists say.
The trees pull water from the ground to cool themselves during photosynthesis and release water vapor into the atmosphere through their leaves, the study said. That water vapor turns into clouds, which return the water to the ground as rain.
The drier conditions could disrupt that cycle.
“It’s a matter of supply and demand. With the increase in temperature and drying of the air above the trees, the trees need to transpire to cool themselves and to add more water vapor into the atmosphere. But the soil doesn’t have extra water for the trees to pull in,” co-author Sassan Saatchi said in a statement. “Our study shows that the demand is increasing, the supply is decreasing and if this continues, the forest may no longer be able to sustain itself.”
The Amazon is the world’s largest rainforest and covers 40% of South America, according to the World Wildlife Fund. It absorbs billions of tons of carbon dioxide, which helps regulate climate change.