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Is the Amazon forest approaching a tipping point?

The Amazon has existed as a forest system for millions of years, but now, global warming, regional climatic changes and deforestation may be interacting to accelerate forest loss, pushing the system towards partial or total collapse, which could happen within the 21st century.

Research published today in Nature, has identified potential thresholds of these stressors, showing how their combined effects could produce a ‘tipping point’ – in which the forest is so fragile that just a small disturbance could cause an abrupt and irreversible shift in the ecosystem.

The study was led by the Federal University of Santa Catarina in Brazil, and includes experts from Brazil, Europe and USA. Its authors hope that by understanding the most important stressors on the system, they can develop a pathway for keeping the Amazon forest resilient. 

The thought of an Amazon forest collapse is disturbing for various reasons, but one in particular, has raised worldwide attention – the risk of destabilizing the global climate system. 

Because the Amazon stores massive amounts of carbon, forest loss and carbon emissions could accelerate global warming in 15 – 20 years. Recent observations of carbon flux above the forest revealed that the south-eastern Amazon may already be acting as a carbon source to the atmosphere, likely because of land-use disturbances.

Forest loss across the Amazon reduces atmospheric moisture circulation in the region, and potentially also in other parts of the world, such as Asia or Antarctica. Every day, trees in the forest pump enormous amounts of water (up to 500 liters by a single tree) from the soil into the atmosphere, increasing atmospheric moisture concentration. Together with water, trees release volatile organic compounds that act as cloud condensation nuclei, i.e., they contribute to form clouds and rainfall. Thus, trees produce rainfall, and rainfall maintains forests resilient.

In the Amazon region, winds flow predominantly from east to west, carrying clouds and moisture, and increasing rainfall along the way. This positive feedback mechanism (forests increase rainfall, and rainfall increases forest resilience) is likely the main reason why the Amazon forest persisted, despite large climatic fluctuations in the past. 

In recent decades, however, the region began to face unprecedented stress from climatic and land-use changes. Increasing temperatures, extreme drought events, deforestation and fires are now affecting internal parts of the system. Feedback mechanisms that enhanced forest resilience are losing strength and being replaced by other feedbacks that increase the risk of a critical transition, i.e., a self-reinforced forest loss. 

According to Flores, “compounding disturbances are increasingly common within the core of the Amazon. If these disturbances act in synergy, we may observe unexpected ecosystem transitions in areas previously considered as resilient, such as the moist forests of the western and central Amazon”. Additional forest loss would likely exacerbate regional climate change, thus increasing the risk of a systemic Amazonian transition. 

The authors also analyzed examples of disturbed forests in various parts of the Amazon to understand what could happen to the ecosystem. They identified three main types of ecosystem trajectories, related to environmental conditions and feedback mechanisms. “In some cases, the forest may recover, but remains trapped in a degraded state, dominated by opportunistic plants, such as lianas or bamboos. In other cases, the forest does not recover anymore, and persists trapped in an open-canopy, flammable state”, said Flores. The expansion of open, flammable ecosystems throughout the core of the Amazon forest is particularly concerning because they can spread fires to adjacent forests.

The study also discusses the roles of biodiversity and Indigenous peoples and local communities in shaping Amazonian forest resilience. These elements of the system have contributed to increasing ecosystem adaptability, by providing them with different strategies to cope with climatic fluctuations. “Today, land-use changes in the region are destroying both biodiversity and the ancient ecological knowledge of Amazonian peoples that sustained healthy and resource-rich forests for millennia”, said Carolina Levis, coauthor of the study.

 “The Amazon is a complex system, which makes it extremely challenging to predict how different forest types will respond to global changes. If we want to avoid a systemic transition, we need to take a precautionary approach that will keep forests resilient in the coming decades”, said Marina Hirota, co-author of the paper. 

Based on their findings, the authors highlight several ecological aspects of the Amazon forest system that can be incorporated by Earth System Models to more accurately simulate forest responses to global changes. Co-author Boris Sakschewski underlines. “Most current models oversimplify the Amazon’s complexity. Our findings call for integrating more detailed representations of tree growth, biodiversity, and community adaptation to enhance our predictive capabilities,” Sakschewski notes.

The authors propose a precautionary approach for keeping the Amazon forest resilient in the Anthropocene that will depend on a combination of local and global efforts. Locally, Amazonian countries need to cooperate to end deforestation and degradation and to expand restoration, which will strengthen the forest-rainfall feedback. The study shows how these actions can benefit from strong governance inside Indigenous territories and protected areas. Globally, all countries need to cooperate to stop greenhouse-gas emissions, thus mitigating the impacts of climate change. Both fronts are crucial to maintain the Amazon forest system alive for future generations.

Article: Flores et al. (2024) “Critical transitions in the Amazon forest system”, Nature, DOI:10.1038/S41586-023-06970-0.

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