The Amazon Rainforest’s Sinking Carbon Sink

Aerial photograph of the Amazon rainforest taken near Manaus, Brazil. By Neil Palmer/CIAT, CC BY-SA 2.0, via Wikimedia Commons.

The Amazon rainforest is one of the world’s greatest natural wonders. It is the largest tropical rainforest on Earth, spanning 2.6 million square miles (roughly 70% the size of the United States). The Amazon contains at least 10% of the world’s known species, and, for many years, it has functioned as an important carbon sink. If you’ve been following the news on climate change, you probably know that trees are one of the good guys in the story, able to store some of the carbon that we emit and keep it from polluting our atmosphere.

However, according to a study published this summer by Luciana V. Gatti et. al., the Amazon has switched from being a carbon sink to a carbon source, releasing more carbon into the atmosphere than it takes in.

In the study, planes were used to sample the air above the Amazon, collecting data spanning nine years and four test sites. They measured carbon flux, which indicates how much carbon is entering or leaving an ecosystem, with positive values corresponding to net emission (a carbon source) and negative to net absorption (a carbon sink). Carbon flux caused by fires, often intentionally set to free up land, was isolated by tracking levels of carbon monoxide, which is emitted by fires. This was set apart from the natural, biological carbon flux of the ecosystem, which involves factors like trees using up carbon dioxide in photosynthesis or bacterial and fungal decomposers releasing carbon dioxide as they break down dead trees and animals. They found that the total carbon flux was positive for all four regions they tested. Even more astonishingly, in the southeastern part of the Amazon, the ecosystem itself, even after removing the effects of fires, has become a carbon source.

The answer is a combination of climate change and direct human interference.

The figure above shows the four regions sampled in the study, with bars representing average carbon fluxes from 2010 to 2018. Carbon flux from fires is in red, carbon flux from the ecosystem is in green, and total carbon flux (adding the green and red bars) is in blue. Figure by Johanna L. Miller, “The Amazon is reaching its carbon tipping point” in Physics Today.

So how did the Amazon become a carbon source? The answer is a combination of climate change and direct human interference. As the climate has warmed, the Amazon has become hotter and drier, particularly during the dry-season months of August, September, and October. Together with deforestation, this has created a degrading forest, where trees are either performing photosynthesis (the very process by which trees remove carbon from the atmosphere) at a slower rate, or just dying off.

This figure maps deforestation, dry-season temperatures, and dry-season precipitation together with the average carbon fluxes from the previous figure. ASO stands for August, September and October. Figure by Gatti et al., “Amazonia as a carbon source linked to deforestation and climate change,” Nature.

These forces of climate change and deforestation are interrelated, working together to create a less healthy, less resilient ecosystem. For example, the forest needs evapotranspiration — water evaporation from plants and soil — to supply rainfall and cool the trees. The southeast portion, which has fewer lakes and rivers, is particularly reliant on this mechanism. Fewer trees means less evapotranspiration, increasing heat and drought in the area.

With the transformation of the Amazon into a carbon source, we have lost yet another asset in the battle against climate change, not to mention the direct danger to one of the greatest forests on Earth. As these natural ecosystems become less and less able to withstand the changing climate, the urgency of curbing our carbon emissions is greater than ever.

This article was edited by Emi Krishnamurthy and Anagha Aneesh.