Humans Are Drying Up the Amazon Rainforest

©. Deforested land in the Amazon rainforest in Brazil, on August 23, 2019. (Photo: CARL DE SOUZA/AFP via Getty Images)

NASA finds that over the last 20 years, the atmosphere above the Amazon rainforest has been drying out – here's why.

The Amazon is the largest rainforest on Earth, and as such, it is so much more than just abstract swaths of land in a faraway place. It is a crucial player in the health of the planet. By absorbing billions of tons of carbon dioxide (CO2) a year through photosynthesis, the Amazon helps to keep temperatures down and regulate climate for the rest of us.

Although it is enormous and made of organisms gigantic and small, it is also a delicate system that's highly sensitive to drying and warming trends. Which is a bummer, given what we are doing to it.

According to a new study from NASA, over the last 20 years the atmosphere hovering above the rainforest has been drying out, increasing the need for water and leaving ecosystems susceptible to harm from fires and drought.

For the study, researchers at NASA's Jet Propulsion Laboratory in Pasadena, California, looked at decades of ground and satellite data over the rainforest to track both how much moisture was in the atmosphere and how much moisture the rainforest system needed to function.

The decline of moisture in the air over the Amazon rainforest, particularly across the south and southeastern Amazon, during the dry season months from 1987 to 2016. The measurements are shown in millibars.

NASA/JPL-Caltech, NASA Earth/Public DomainIllustration: The decline of moisture in the air over the Amazon rainforest, particularly across the south and southeastern Amazon, during the dry season months from 1987 to 2016. The measurements are shown in millibars.

"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," said JPL's Armineh Barkhordarian, lead author of the study. "In comparing this trend to data from models that estimate climate variability over thousands of years, we determined that the change in atmospheric aridity is well beyond what would be expected from natural climate variability."

Barkhordarian said that elevated greenhouse gas levels are the cause of around half of the more arid conditions; the rest comes courtesy of ongoing human activity – predominantly from lighting forests on fire to clear land for agriculture and grazing.

"The combination of these activities is causing the Amazon's climate to warm," notes NASA.

The soot from a burning forest releases particles into the atmosphere, including black carbon, also known as soot.

"While bright-colored or translucent aerosols reflect radiation, darker aerosols absorb it," explains NASA. "When the black carbon absorbs heat from the sun, it causes the atmosphere to warm; it can also interfere with cloud formation and, consequently, rainfall."

When left alone, rainforests are a marvel of sufficiency. Trees and plants drink water from the soil and release water vapor through their leaves into the atmosphere, where it cools the air and then rises to become clouds. The clouds do their thing – rain – and the cycle repeats itself. Rainforests create as much as 80 percent of their own rain; hence, the name.

But when that dance it disrupted, problems arise – especially during dry season.

"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," said JPL's Sassan Saatchi, co-author of the study. "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 scientists found that the worst drying of the atmosphere is in the southeast region, the area where most of the deforestation and agricultural expansion is happening.

If this continues, as with all ecosystems, a tipping point will be reached and the rainforest will no longer be able to function properly. As trees die, they will release CO2 into the atmosphere. As NASA puts it:

"The fewer trees there are, the less CO2 the Amazon region would be able to absorb – meaning we'd essentially lose an important element of climate regulation."

The study, "A Recent Systematic Increase in Vapor Pressure Deficit Over Tropical South America," was published in Scientific Reports.