The Amazon Is Drying Out Due to Human Activities Making It More Vulnerable to Fires, NASA Scientists Say

The atmosphere above the Amazon rainforest has become increasingly dry over the past two decades—and this process is primarily due to human activities, a study involving NASA scientists has found.

The drying of the atmosphere could harm the ability of the world's largest rainforest to sustain itself by increasing demand for water and making it more vulnerable to droughts and fires

According to the study published in the journal Scientific Reports, researchers observed an increasing trend in a measure known as vapor pressure deficit (VPD) over tropical South America in dry season months. They say that the values they observed were well beyond the scope of natural variability, indicating strong human influence.

VPD is just one of many variables that scientists can use to assess the functioning of ecosystems—it can reveal the dryness of the atmosphere and help scientists to predict how much water plants are losing. VPD is essentially the difference between the water vapor pressure in the air when it is completely saturated, and the actual water vapor pressure for a given temperature.

Higher VPD indicates a decline in atmospheric moisture.

By looking at this measure, the scientists—led by by Armineh Barkhordarian from the University of California, Los Angeles, and NASA's Jet Propulsion Laboratory (JPL)—were able able to examine how moisture in the atmosphere changed over time and, furthermore, how much is needed to sustain the forest.

"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," Barkhordarian said in a statement. "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."

The researchers found that the trend of increasing VPD is widespread across the southeastern Amazon, but has been driven by several severe droughts which affected the northwest of the rainforest in 2005, 2010 and 2015.

They say around half of the increase in the dryness of the atmosphere above the rainforest is due to elevated levels of greenhouse gases—a result of man-made fossil fuel emissions. Other human activities are responsible for the remainder of the increase—the burning of forests to clear land for farming and other uses in particular.

Higher levels of greenhouse gases—which trap heat—in combination with these human activities in the region are also causing the Amazon climate to become warmer. For example, the fires that are widely used to clear land pump large quantities of tiny black carbon particles—known as aerosols—into the atmosphere, which absorb heat from the sun. Furthermore, these particles can have an affect on the amount of rainfall that the forest produces—a key factor in its survival.

Amazon rainforest, French Guiana
Aerial view taken on October 3, 2008 over the Amazon in French Guiana. JODY AMIET/AFP via Getty Images

The Amazon rainforest is important to the global climate because of the impact it has on greenhouse gases in the atmosphere, specifically, the ability of its plants to store carbon dioxide via the process of photosynthesis—thus mitigating the impact of global warming.

"The key role of tropical forests is that they absorb and store large amounts of carbon dioxide," Sandra Steingraber, a professor from Ithaca College, who was not involved in the study, told Newsweek. "Fully half of the the carbon dioxide that is pulled out of atmosphere by the biosphere on land is pulled out by tropical forests."

Furthermore, the Amazon also plays an important role in a regional sense by regulating rainfall.

"The Amazon has a capacity to cycle water between the forest at the atmosphere via rainfall and transpiration of leaves, basically leading to a great freshwater ocean in South America—the rivers and ground water—that is responsible for maintaining rainfall in South America's more southerly agricultural regions," Scott Stark from Michigan State University, who was also not involved in the study, told Newsweek.

However, the rainforest is extremely vulnerable to increases in atmospheric dryness and warming.

Plants suck up water—which they need for photosynthesis and to keep themselves cool—through their roots, eventually releasing it into the atmosphere as vapor. This process leads to the formation of clouds that produce rain. This falls back to the ground and gets absorbed by the plant roots again, completing the cycle.

In fact, rainforests are thought to produce up to 80 percent of their own rainfall.

But an increase in atmospheric dryness and temperature, for example, can disrupt this process, leaving the plants lacking the water they need—particularly during the dry season—increasing the risk of mortality.

"It's a matter of supply and demand," co-author of the study Sassan Saatchi, from JPL, said in a statement. "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.

"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."

These effects are especially worrying given that the plants are already vulnerable to droughts—and the associated risk of wildfires—which are only expected to increase in frequency and severity in the region as climate change takes hold.

This has significant implications because if we lost the forest, we would also lose all the crucial ecosystem services it provides.

Amazon Rainforest, air moisture
The image shows the decline of moisture in the air over the Amazon rainforest, particularly across the south and southeastern Amazon, during the dry season months—August through October—from 1987 to 2016. The measurements are shown in millibars. NASA/JPL-Caltech, NASA Earth Observatory

"The Amazon is a big carbon sink but its ability to scrub carbon dioxide from the atmosphere is declining," Steingraber said. "This will contribute to climate chaos, turning tropical forests from a global carbon sink to a global carbon source."

Meanwhile, Philip Fearnside, a professor at Brazil's National Institute of Amazonian Research, said the Amazon's ability to supply rainfall will also be hampered.

"[The Amazon] has a vital climate role for Brazil and for neighboring countries like Argentina in supplying water vapor that is essential for rainfall in heavily populated areas like São Paulo," he told Newsweek.

"During the peak of the rainy season in São Paulo, when reservoirs fill within a few weeks, up to 70 percent of the rain is derived from Amazonian water vapor. If current trends continue and the forest is destroyed, rain water in Amazonia will not be recycled through the trees, and will be drained away to the Atlantic Ocean via the Amazon River rather than being carried to São Paulo by the 'flying rivers.'"