The 2015 drought produced by El Niño — the warm phase of the El Niño Southern Oscillation (ENSO), which registers Earth’s climate fluctuations as measured by sea surface temperatures — has killed 65 percent more trees and large branches in the Amazon Rainforest than are typically lost in an average year.
The conclusion comes from an in-depth analysis by scientists at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the Brazilian Agricultural Research Corporation (EMBRAPA), who published their findings in a study featured in the journal New Phytologist.
According to NASA, the research was based on an innovative technique of measuring the living trees and dead debris on the ground of the rainforest, which yielded a 3D model of the Amazon’s forests.
While such measurements are normally made on foot by scientists trekking a portion of the rainforest, the team came up with the brilliant idea to attach a light detection and ranging (LiDAR) instrument onto an airplane and survey the forest canopy from up above.
The data was gathered on three separate flights, in 2013, 2014 and 2016, and led to a 3D reconstruction of the Amazon rainforest.
“With 300,000 laser pulses a second, the LiDAR data provides an incredibly detailed depiction of the forest over a much greater area than they could cover on foot,” NASA officials explained in a news release.
El Niño Killed A Swath Of The Amazon Rainforest The Size Of Wisconsin
The scientists examined a portion of the rainforest that normally goes through a three-month drought season which coincides with peak sea surface temperatures in the Pacific Ocean caused by El Niño.
Just like the cold phase of the ENSO, dubbed La Niña, El Niño represents a “deviation from normal surface temperatures and can have large-scale impacts not only on ocean processes but also on global weather and climate,” notes the National Ocean Service.
In the case of the Amazon rainforest, El Niño has been linked to a delay in the rainy season and an extended drought period. This puts the trees at risk because their towering canopies, which can grow up to 20 stories high, depend on rainfall to get the water they need in order to survive.
LiDAR measurements from 2013 and 2014, taken before El Niño hit, revealed a loss of 1.8 percent of the canopy in the studied area — two 30-mile swaths of forest in the vicinity of Santarém city in the Brazilian state of Pará. That’s the equivalent of more than 38,000 square miles on the scale of the entire Amazon rainforest — an area the size of Kentucky, NASA points out.
But subsequent measurements during the El Niño drought period, from 2014 to 2016, indicated that tree and branch mortality had gone up by 65 percent, spreading on a 65,000-square-mile area the size of Wisconsin.
“Our findings suggest that El Niño conditions accelerated canopy turnover in central Amazon forests, increasing coarse woody debris production by 62 percent,” the authors wrote in their study.
Why It Matters
While the world’s forests are doing us a favor by absorbing carbon dioxide as they grow and releasing oxygen, dead and decomposing trees return carbon dioxide into the atmosphere.
“Because it’s a big forest, even a subtle shift in an El Niño year has a big impact on the total carbon budget of the forest,” said Doug Morton, study co-author and an Earth system scientist at Goddard.
The only good news for the carbon budget of the Amazon rainforest is that the 2015 El Niño drought didn’t take out predominantly large trees, which hold most of the forest’s carbon.
The 3D view of the rainforest constructed by LiDAR data showed that the drought affected both large and small trees in equal measure, which disproves the previous simulation models carried out on small plots.
Nevertheless, the large trees that went down during the El Niño drought accounted for 80 percent of the carbon losses, stated NASA officials.
Ecologist Paulo Brando, from the Woods Hole Research Center in Massachusetts and the Amazon Environmental Research Institute in Brazil, told the space agency that prolonged droughts, such as the one caused by El Niño in 2015, have a massive impact not only on the forest ecosystem but also on what happens in our atmosphere on a global scale.
“If the number of trees present declines on a large scale, that adds up to a lot of carbon dioxide left in the atmosphere to contribute to greenhouse warming, which can feed the cycle of the Amazon seeing more droughts in the future.”