The widespread encroachment on the complex forest system appears to be taking its toll: The ability of the world’s most biodiverse jungle to store greenhouse gases has been severely depleted.
Due to human interference, the Amazon rainforest now appears to release more climate-warming gases than it stores.
The Amazon rainforest is now most likely contributing to the warming of the planet. This is the conclusion of an unprecedented analysis by more than 30 scientists.
For years, researchers have expressed concern that rising temperatures, drought and deforestation are reducing the ability of the world’s largest rainforest to absorb carbon dioxide from the atmosphere and offset emissions from burning fossil fuels. Recent studies even suggest that some parts of the tropical landscape are already releasing more carbon than they store.
But inhaling and exhaling CO2 is just one way this humid jungle – the most biodiverse on Earth – affects global climate. Both natural and human-caused activities in the Amazon can significantly shift the rainforest’s contribution by directly warming the air or releasing other greenhouse gases that do so.
The drying of wetlands and soil compaction from deforestation, for example, can increase emissions of nitrous oxide, also known as laughing gas. Slash-and-burn forests release small soot particles that absorb sunlight and increase warming. Deforestation may be altering rainfall patterns, causing the forest to continue to dry out and heat up. Regular flooding and dam construction release methane, as does cattle ranching, which is one of the main reasons for forest destruction. About 3.5 percent of the methane released worldwide is secreted naturally by Amazon trees.
However, no team had yet attempted to assess the cumulative impacts of these processes, especially as the region is rapidly changing. The unprecedented research was supported by the National Geographic Society and published in Frontiers in Forests and Global Change. It estimates that atmospheric warming from all these sources combined now appears to be outpacing the natural cooling effect of the forest.
"Cutting down the forest disrupts its carbon uptake, which is a problem," says lead author Kristofer Covey, a professor of environmental studies at Skidmore College in New York. "But when you look at these other factors besides CO2, it’s hard to see how the net effect can be anything other than that the Amazon as a whole is actually warming the global climate."
Gallery: Amazon rainforest – ecosystem could tip over
The damage can still be reversed, he and his colleagues say. Stopping global emissions of coal, oil and natural gas would help restore balance. But curbing Amazon deforestation is a must, along with reducing dam construction and stronger reforestation efforts. If deforestation were to continue at the current rate, it will certainly worsen warming for the entire world.
"We have this system and we have relied on it to buffer our own mistakes. But "we’ve gone beyond the capacity of the system to provide a reliable service," says co-author Fiona Soper, an assistant professor at McGill University.
The rainforest – a highly complex system
The same richness that makes the Amazon so wonderfully biodiverse, home to tens of thousands of insects per square kilometer, makes it extremely hard to understand. Shimmering green leaves suck CO2 from the air and convert it through photosynthesis into carbohydrates that end up in the trunks and branches of trees. In trees and carbon-rich soils, the Amazon stores the equivalent of four or five years of man-made carbon emissions – up to 200 gigatons of CO2.
But the Amazon is also tremendously wet. Every year, floods several meters high flood the forest floor. Microbes in these sodden soils produce methane, a greenhouse gas 28 to 86 times more potent than CO2. Trees act like chimneys, releasing this methane into the atmosphere.
Meanwhile, moisture from the Atlantic Ocean that falls as rain is absorbed by plants and used for photosynthesis. Subsequently, leaves exhale it through the same pores that absorb CO2. Back in the atmosphere, it falls again as rain.
Humans are complicating these natural cycles not only through climate change, but also through deforestation, reservoir construction, mining and agriculture. Deforestation in Brazil has exploded in recent years, reaching a 12-year high in 2020 – a nearly 10 percent increase from the previous year.
Some of these processes pull greenhouse gases out of the atmosphere, while others cause the gases to rise, and they all affect each other. Until recently, no one had tried to understand this balance. "It’s a system of interacting parts, all measured in different ways, on different time scales, by different people," Soper says.
What is clear is that the forest has changed rapidly and in alarming ways. Rain now falls more frequently than in the past in massive bursts, triggering record floods. Droughts are occurring more frequently and lasting longer in some areas.
Trees that do better in moist places are being displaced by tall, drought-tolerant species. Illegally set fires are on the rise again. In 2019, about two million hectares burned, an area about the size of Saxony-Anhalt, Germany.
So in 2019, the National Geographic Society brought together Covey, Soper and a team of other Amazon experts to study how all these pieces fit together. They didn’t take new measurements – they looked for new ways to analyze existing data in terms of a comprehensive picture.
Greenhouse gas problem: More than CO2
Although the results include some uncertainty, they make it clear that focusing on a single metric – CO2 – simply doesn’t paint an accurate picture. "As important as carbon is in the Amazon, it’s not the only thing that matters there," says Tom Lovejoy, a senior fellow for biodiversity at the United Nations Foundation who has worked in the Brazilian Amazon for decades. "The only surprise, if you can call it that, is how much more there is when you add it all up."
Resource extraction, damming of rivers, and conversion of forests for soybean production and livestock are changing natural systems in many ways. But most of these changes are promoting a warming climate. Methane is a particularly important player. While the largest sources of methane are still natural forest processes, the Amazon’s ability to sequester carbon used to do much more to offset methane emissions. Man has diminished this capacity.
Rob Jackson, an earth systems scientist at Stanford University and a leading expert on global greenhouse gas emissions, thinks the new research is a worthwhile contribution. "The Amazon is endangered, and we tend to get tunnel vision on a single greenhouse gas," he says.
Gallery: The people of the Amazon and their monkeys
Patrick Megonigal, deputy director for research at the Smithsonian Environmental Research Center, agreed. "What the authors are doing is important to extend the conversation beyond CO2, which is what 90 percent of the public conversation is about," he says.
"CO2 is not a lone actor. When you look at the whole gang of other actors, the prognosis for the Amazon is that the impacts of human activity will be worse than we realize."
Many questions remain. The biggest one for Megonigal is one Lovejoy also worries about: how do all these factors affect the local Amazon climate? This is important, because the Amazon produces much of its own moisture. A single molecule of water passes through the forest five times or more as moist air moves west across the continent from the Atlantic Ocean.
Here’s a disturbing analysis by Lovejoy and Carlos Nobre, a climate scientist at the Institute for Advanced Studies at the University of Sao Paulo. It suggests that increased deforestation could alter the flow of that moisture in a way that could turn large parts of the Amazon into a permanently drier forest savanna. The duo believes that this tipping point could be reached as soon as 20 to 25 percent of the rainforest is cut down.