“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.” So if it’s not natural, what’s causing it? When a forest burns, it releases particles called aerosols into the atmosphere — among them, black carbon, commonly referred to as soot. While bright-colored or translucent aerosols reflect radiation, darker aerosols absorb it. 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.

Why It Matters

But it’s a delicate system that’s highly sensitive to drying and warming trends. Trees and plants need water for photosynthesis and to cool themselves down when they get too warm. They pull in water from the soil through their roots and release water vapor through pores on their leaves into the atmosphere, where it cools the air and eventually rises to form clouds. The clouds produce rain that replenishes the water in the soil, allowing the cycle to continue. Rainforests generate as much as 80% of their own rain, especially during the 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 study, “A Recent Systematic Increase in Vapor Pressure Deficit Over Tropical South America,” was published in October in Scientific Reports. The science team used data from NASA’s Atmospheric Infrared Sounder (AIRS) instrument aboard the Terra satellite. Reference: “A Recent Systematic Increase in Vapor Pressure Deficit over Tropical South America” by Armineh Barkhordarian, Sassan S. Saatchi, Ali Behrangi, Paul C. Loikith and Carlos R. Mechoso, 25 October 2019, Scientific Reports.DOI: 10.1038/s41598-019-51857-8