Researchers have investigated the disappearance of trees in the southern Amazon region. According to their findings, the southern Amazon rainforest now emits significantly more carbon dioxide (CO2) than it absorbs due to forest damage.
This is the result of an analysis of detailed aerial photographs taken in the Brazilian states of Rondônia, Mato Grosso and Pará between 2016 and 2018. As the study, led by Ovidiu Csillik from the California Institute of Technology in Pasadena, shows, the forest damage has very different causes, with humans often playing a role.
The Amazon rainforest in South America is home to more than ten percent of all terrestrial species living in the world, according to a study recently published in the journal Nature.
According to the study, the area contributes to the stabilization of the Earth's climate through the net cooling effect of water evaporation. In addition, the rainforest stores an amount of carbon equivalent to 15 to 20 years of global CO2 emissions.
"Green lung" under threat
However, deforestation and other forest degradation are threatening this role that the Amazon rainforest plays as the Earth's "green lung" for the global climate. However, measures taken by the current Brazilian government have recently significantly reduced the destruction of the forest.
"Despite their greater range, satellite-based approaches suffer from a coarse resolution that makes it difficult to quantify the extent and intensity of forest degradation," write the authors of the current study, which is reported in the journal "Proceedings of the National Academy of Sciences" ("PNAS").
Determining the height of tree crowns from an airplane
They therefore used images from aircraft that flew over the study area at an altitude of 600 meters in 99 measurement strips. This involved the use of so-called lidar technology: a method related to radar that is roughly equivalent to three-dimensional laser scanning. Among other things, the research group used this method to determine the height of the tree crowns. The study area was flown over twice at intervals of one to one and a half years.
The study region covers an area of 544,300 square kilometers, which corresponds to 8.2 percent of the entire Amazon region (around 6,600,000 square kilometers). Csillik and his team discovered forest damage on 21.6 percent of the area studied. Of this, 0.7 percent was caused by logging, 0.7 percent by reclamation for agriculture and 2.8 percent by fire. According to the research group, almost all of the fires in the Amazon region are ignited by humans. This results in 4.2 percent of the area being damaged by human activity.
Surprisingly much wind breakage
Csillik and his team attribute the damage to the remaining 14.7 percent of the damaged area to minor natural and man-made disturbances that could not be identified with a high degree of certainty. The scientists were surprised by the large proportion of damage caused by wind breakage - i.e. strong winds - at 2.7 percent. On 62.1 percent of the area, there was no change between the two surveys. The researchers also found clearly recognizable forest growth on 16.3 percent of the area.
However, according to the study, this is not enough to offset the carbon emissions from the damaged areas. During the study period, emissions totalled 134.6 million tons of carbon, while forest growth absorbed 44.1 million tons from the air. This results in annual emissions of 90.5 million tons of carbon between 2106 and 2018 on the area studied. However, the carbon balance of the soil was not taken into account. The authors conclude: "This study highlights the role of forest degradation in the carbon balance of this critical region in the Earth system."