In August, the skies over São Paulo turned black with the smoke of tens of thousands of fires burning through the Brazilian Amazon thousands of kilometers away. Experts quickly linked the fires to deforestation practices in which rain forest is razed and set ablaze to clear land for crops or livestock. Indeed, a September study showed significant overlap between the 125,000 hectares of forest that were cleared in early 2019 and where fire “hot spots” appeared in the summer.
Much of the international outcry focused on the impact the fires would have on the Amazon’s role as a carbon sink. As the largest rain forest in the world, the Amazon accounts for a quarter of all greenhouse gases absorbed by the world’s forests every year.
But the Amazon also has a critical role in Earth’s water cycle, releasing water vapor into the atmosphere that can travel hundreds or thousands of kilometers before falling to the ground.
A new study finds that converted land is much less efficient at supplying this atmospheric river than intact rain forest. This reduced efficiency is most evident during droughts, which are expected to become longer and more frequent as climate change progresses.
The study, published in Ecohydrology, considered what impact land use changes might have on the local energy balance—the processes by which trees and other vegetation either reflect energy from the Sun or turn it into heat or water vapor. Land use changes might include the conversion of rain forest to agricultural land or pasture.
“It’s a really nice contribution that lets us understand how ecology matters for these energy balance processes and the sensitivity of the forest to climate change,” said Scott Stark, an assistant professor at Michigan State University who was not involved in the study.
A Closer Look
The researchers used satellite observations of both intact and disturbed rain forest areas in Rondônia, a state in northeastern Brazil. Rondônia falls within the “arc of deforestation,” where human settlements and activities are putting increased pressure on natural ecosystems.
Though researchers have used satellite imagery to study the Amazon in the past, most of the images produced have a very coarse resolution—on the scale of 500 meters up to 1 kilometer. What sets this study apart is the high spatial resolution, according to Stark.
Here the researchers used data collected by a satellite-based sensor, the Advanced Spaceborne Thermal Emission and Reflection Radiometer, which allowed the team to zoom in on the landscape down to 15 meters.
Even with the increased resolution, getting clear images of the forest in the region is a challenge. “It’s very hard to get data from this sensor in the Amazon rain forest because of cloud cover,” said Gabriel de Oliveira, a postdoctoral researcher at the University of Kansas and lead author on the new study. “We were lucky to find images.”
The team compared the satellite data to measurements from a flux tower on the ground. A flux tower is a structure built up through the forest canopy that allows researchers to track meteorological conditions and the exchange of both water and carbon dioxide between the forest and the atmosphere.
The team found that cropland and pastures tended to have higher soil and air temperatures, which could exacerbate drought conditions, according to de Oliveira. The results also showed that forested areas had roughly 3 times higher rates of evapotranspiration—the process by which water evaporates from soil and leaves.
These findings could have serious implications for precipitation around the world. “All the water that the forest is pumping back into the atmosphere will go to the equator first, the tropics, and then will be released all over the world,” de Oliveira said. “So if you have this decrease in evaporation, or in the water that comes back to the atmosphere, you have a problem in the flux of water in the tropics, the equator, and maybe reaching the polar region.”
How Deforestation Exacerbates Drought
The team also looked at how drought might affect these energy balance processes in both intact forest and converted land, comparing a relatively wet year to “one of the worst droughts that’s ever been observed in the Amazon,” according to Stark. The results showed that evapotranspiration rates in primary forests were higher during the drought than the wet year, suggesting that old-growth forests in this region may be quite resistant to drought.
So even during dry spells, the rain forest continues pumping water into the atmosphere. But the same cannot be said of croplands and pastures. Deforested land converted more of the Sun’s energy to heat, effectively aggravating the drought conditions.
That’s in line with previous research linking deforestation and drought in the region. “There have been these megadroughts in the agricultural regions in southern Brazil and northern Argentina, and people have already linked that with some reasonable certainty to deforestation,” Stark said, “because you just lose this ability to have that conveyor belt of water that can take rainfall and redistribute it.”
Now researchers have a better sense of the connection between deforestation and drought on an ecological scale. That’s a critical insight for researchers wondering what’s in store for the Amazon as global warming progresses and droughts in the region likely become longer, more intense, and more frequent.
The main takeaway for de Oliveira is that the study could help improve regional and global models of water fluxes in the Amazon. “Some models consider all of the Amazon as forest, but it’s not,” he said. “The biome nowadays is very degraded. You have a lot of pasture areas, agriculture areas, and this has to be taken into consideration in these models; otherwise, our representations are wrong.”
Ultimately, the effects of water flux in the Amazon will ripple beyond the rain forest and around the world. It’s a “major concern,” Stark said, “in terms of the ability for the basin to continue its key function as a hydrological pump that cools the global atmosphere.”
—Kate Wheeling (@katewheeling), Freelance Writer