A listening post for the forest
Deep within the Caxiuanã National Forest, a protected area of 300,000 hectares in Brazil's Pará state, a slender tower rises above the canopy. This is not an observation deck for tourists, but a sophisticated scientific instrument. It is an eddy covariance flux tower, one of several in the Amazon basin that are part of the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA). The LBA is a large, international research initiative designed to study how the Amazon functions and how it is affected by climate and land-use changes.
The tower is equipped with high-frequency sensors that continuously monitor the exchange of gases between the forest and the atmosphere. A sonic anemometer measures vertical wind speed in three dimensions, tracking the movement of invisible air parcels called eddies. At the same time, an infrared gas analyzer measures the concentration of carbon dioxide and water vapor within those eddies. By correlating the vertical wind speed with the gas concentrations, scientists can calculate the net flux—the overall movement of CO2 into or out of the forest ecosystem below. This technique provides a real-time measurement of the forest's "breathing."
From carbon sink to carbon source
For decades, the Amazon rainforest was considered a reliable carbon sink, absorbing more CO2 from the atmosphere than it released. Data from the Caxiuanã tower and others like it have shown that this is changing. Research led by scientists like Luciana Gatti of Brazil's National Institute for Space Research (INPE) combined tower data with aircraft measurements to analyze the region's carbon budget.
The findings show that parts of the Amazon, especially in the heavily deforested eastern and southeastern regions, have become a net source of carbon. Between 2010 and 2018, analysis revealed that the Brazilian Amazon emitted more carbon than it absorbed. This shift is driven by a combination of factors. Direct emissions from deforestation and fires release vast amounts of stored carbon. Increased temperatures and reduced rainfall, linked to both global climate change and regional deforestation, stress the remaining forest. This stress makes the forest less efficient at photosynthesis and increases tree mortality, further changing the balance from a carbon sink to a source. The southeastern Amazon, which has experienced around 30% deforestation, now emits ten times more carbon than the western part of the forest, which has seen 11% deforestation.