Fires could lock vast parts of the Amazon into ‘treeless state’

Human-ignited fires play a decisive role in shaping the Amazon’s future, potentially locking more than three-quarters of the rainforest into a “treeless state”, a new study finds. 

The new paper, published in Communications Earth & Environment, examines how the interplay of fire, climate and land-use change impacts the Amazon’s ability to recover and regenerate.

It finds that, depending on atmospheric CO2 levels, fire can prevent the regrowth of as much as 82% of the natural forest area.

The researchers also investigate the effects of “bi-stabilities” – the existence of two stable states, forest and grassland, under the same climatic conditions. 

They find that bi-stabilities are highly important in determining the future recovery of the forest, especially in the so-called “arc of deforestation”, a key region of the Amazon home to the region’s highest rates of deforestation.

One researcher who was not involved in the study tells Carbon Brief that “although there is a lot of uncertainty” around the Amazon’s tipping point, “it is very certain that the socioeconomic consequences of this process for the more immediate future are dramatic” – especially for Indigenous peoples and local communities.

Tipping point

Experts have warned that the combination of human-caused climate change and deforestation could push the Amazon rainforest past a “tipping point“. This would result in a large-scale “dieback” of the Amazon and the conversion of areas previously covered by tropical forest into grasslands. 

Existing research often emphasises the Amazon forest’s regrowth potential, or the ecosystem’s ability to recover after disturbances. But the new study investigates the impact of recurring fires, not just one-off events. 

The researchers use a fire-enabled Earth system model that couples climate, vegetation and fire dynamics to link fire activity and climate change to the Amazon’s resilience and recovery potential. 

The research starts with two hypothetical “extreme” scenarios: one that assumes the Amazon basin to be completely deforested into grassland, and another one that assumes the Amazon to be a wholly intact forest.

Each scenario is simulated both with and without fire, with each set of experiments performed under four different levels of atmospheric CO2 concentration – 284, 450, 750 and 1200 parts per million (ppm) – in order to assess the influence of climate and CO2 fertilisation on forest recovery.

The map below shows the regrowth of the rainforest in scenarios without fire (top) and with fire (bottom), for the four different atmospheric CO2 concentrations. Darker green colours represent a higher amount of above-ground biomass, indicating a stronger recovery.

In each scenario, the researchers find that in the absence of fire, the forest made a near-complete and uniform recovery from the grassland state. But in the fire-enabled experiments, the forest did not return to its original state. Instead, the forest  crossed “an irreversible fire controlled tipping point”, thus locking it into a grassland state that leaves the region vulnerable to more frequent and intense fires.

The researchers call this a “lock-in effect”, noting that it disrupts essential moisture-recycling processes and prevents tropical forests from regenerating. They estimate that in 56-82% of the Amazon forest – covering 353-515m hectares – the damage may be “irreversible”. The extent of this damage depends on the trajectory of atmospheric CO2 levels.

Dr Kirsten Thonicke, one of the paper’s authors and the deputy head of the Earth-systems analysis group at the Potsdam Institute for Climate Impact Research (PIK), says that human activities have a “clear” influence on the recovery prospects of the Amazon. She tells Carbon Brief that the possibility of the tropical rainforest’s recovery largely “depends on humans setting fire [to it] or not”.

Thonicke emphasises that fire-management policies are “crucial” measures in forest restoration plans. She adds: 

“New initiatives, like turning the arc of deforestation into an arc of restoration, are very useful.”

The “arcs of restoration” is an initiative aimed at halting deforestation and degradation, while restoring already degraded forests in the Amazon. The Science Panel for the Amazon (SPA) unveiled a policy brief about the strategy at COP27 in Sharm el-Sheikh last November. 

Bi-stability

Thonicke and her team also use the model to identify and quantify “bi-stabilities” within the Amazon ecosystem – the ability for two stable states, open grassland and closed forest, to exist side by side under the same climate.

The existence of these two potential states means that a shock to the system – such as a fire – could tip an area from a forested to a grassland state. And since forested and grassland areas can coexist in close proximity, fires can more easily spread from one to the other, further impacting their ability to recover.

Frequent fires disrupt tree growth, stabilise grasslands and change vegetation-climate interactions, thus impacting forest recovery. This is particularly true in transition zones between the forest and grasslands, which tend to be fragmented and degraded, and therefore more likely to burn.

The researchers found that the extent of these bi-stabilities also depends on the atmospheric CO2 levels. As the concentration increases, the area over which both states can coexist becomes larger, thus increasing the forested area at risk. Thonicke tells Carbon Brief: 

“We found that the eastern and south-eastern Amazon region shows always or mostly bi-stable states. This is the region that is mostly threatened by deforestation and fire already today, because it fragments and degrades the forests considerably.”

Forest recovery and regeneration

Tropical forests play a significant role in climate change adaptation and mitigation efforts. According to the sixth assessment report of the Intergovernmental Panel on Climate Change (IPCC), climate change is already impacting the Amazon forest by causing more extreme weather events, such as heatwaves and droughts. These, in turn, are leading to an increased – and irreversible – loss of biodiversity.

Dr Nathalia Nascimento, a postdoctoral researcher at the University of São Paulo, tells Carbon Brief that many nations have committed to “ambitious targets” around forest restoration as part of their climate change mitigation and adaptation plans.

But although much is said about forest resilience and recovery, she says, “forest restoration is not a simple process”. She adds:

“Areas with a long history of land use or significant alterations in vegetation structure are particularly difficult to regenerate naturally, demanding human intervention through tree planting and monitoring to enable vegetation regeneration. This process requires study, planning, time and financial investment, which can make forest regeneration initiatives potentially laborious and costly.”

One limitation of the study, the authors note, is that the model is a simplification of the diverse nature of the Amazon, and does not necessarily capture the differences between individual tree species. This is partly due to the limited availability of observed data around burned areas and fire activity.

Dr Patricia Pinho, the deputy science director at the Amazon Environmental Research Institute (IPAM), tells Carbon Brief:

“The ‘point of no return’ for the Amazon is not something unique and homogeneous. It will happen in different parts of the region, at different times and time scales.”

Regenerating the Amazon will be challenging, Pinho says, because 38% of the remaining forest “exists in a state of degradation” due to climate change and deforestation. She adds: 

“[Today] we have a much drier and more flammable Amazon…The Amazon no longer supports any process associated with deforestation. There has to be investment in recovery, in large-scale reforestation, which is something very challenging for the Amazon precisely because of the altered climatic conditions that are already underway.”

Mitigation measures

The study finds that the recovery of the Amazon rainforest hinges on a crucial factor – “human-caused ignitions” 

Previous research has found that fires and burned area both increase in the Brazilian Amazon during high-deforestation years and exceptionally high drought years – as predicted for 2023 with the onset of El Niño. According to Thonicke, that study highlights the clear indication of human influence on the occurrence of fires in the Amazon, as the fires mainly occur “along major road networks and in deforestation frontier regions”.

The map below shows the location (red) of the nearly 1,000 major fires that burned across the Amazon during the 2022 fire season. The inset chart shows the area of the Brazilian Amazon burned during each fire season over the last decade.

Although ignition forces are the major cause for fire occurrence, Thonicke says that fire-mitigation efforts are just as important for protecting the rainforest. To avoid fires spreading into the neighbouring forests, restoration plans should go beyond putting out fires. These, she adds, should be considered alongside “alternative management methods and tools in renewing pastures and on agricultural land”. She adds: 

“This way, this unique socio-biodiverse habitat can be strengthened to be more resilient.”

Alternative techniques of forest management combining scientific and Indigenous knowledge have been the focus of previous work. A 2021 study notes that traditional communities within the Amazon are known for possessing “a deep knowledge of ecological processes, biodiversity and the use and management of fire”.

Pinho emphasises the risks of separating forest maintenance from the communities living in the Amazon rainforest. She tells Carbon Brief:

“Although there is a lot of uncertainty [around] when, where and how the dieback or the point of no return will happen, it is very certain that the socioeconomic consequences of this process for the more immediate future are dramatic, especially for the most vulnerable, for Indigenous Peoples and traditional populations that occupy and inhabit the Amazon and that are less responsible for all the destabilisation of both crises, be it biodiversity or climate.”