Up to half of the Amazon rainforest could hit a tipping point by 2050 as a result of water stress, land clearance and climate disruption, warns a new study (Critical transitions in the Amazon forest system, Nature volume 626, Wednesday, 14 Feb. 2024).
The possibility that the Amazon forest system could soon reach a tipping point, inducing large-scale collapse, has raised global concern.
The study report, which is the most comprehensive to date in its analysis of the compounding impacts of local human activity and the global climate crisis, warned that the forest had already passed a safe boundary and urged remedial action to restore degraded areas and improve the resilience of the ecosystem.
The study report by Bernardo M. Flores, Encarni Montoya, Boris Sakschewski, Nathália Nascimento, Arie Staal, Richard A. Betts, Carolina Levis, David M. Lapola, Adriane Esquível-Muelbert, Catarina Jakovac, Carlos A. Nobre, Rafael S. Oliveira, Laura S. Borma, Da Nian, Niklas Boers, Susanna B. Hecht, Hans ter Steege, Julia Arieira, Isabella L. Lucas, Erika Berenguer, José A. Marengo, Luciana V. Gatti, Caio R. C. Mattos & Marina Hirota said:
‘The Amazon forest is a complex system of interconnected species, ecosystems and human cultures that contributes to the well-being of people globally. The Amazon forest holds more than 10% of Earth’s terrestrial biodiversity, stores an amount of carbon equivalent to 15–20 years of global CO 2 emissions (150–200 Pg C), and has a net cooling effect (from evapotranspiration) that helps to stabilize the Earth’s climate. The forest contributes up to 50% of rainfall in the region and is crucial for moisture supply across South America, allowing other biomes and economic activities to thrive in regions that would otherwise be more arid, such as the Pantanal wetlands and the La Plata river basin. Large parts of the Amazon forest, however, are projected to experience mass mortality events due to climatic and land use-related disturbances in the coming decades, potentially accelerating climate change through carbon emissions and feedbacks with the climate system. These impacts would also involve irreversible loss of biodiversity, socioeconomic and cultural values.’
A report by The Guardian said:
‘Bernardo Flores of the Federal University of Santa Catarina, Brazil, the lead author of the study, said he was surprised by the results, which projected a potential shift from slow to rapid forest decline earlier than he had expected.
‘The forest was already becoming weaker and more homogenous, he said. “By 2050, it will accelerate rapidly. We need to respond now. Once we pass the tipping point, we will lose control of how the system will behave.”
‘This requires international action because even a local halt to deforestation would not prevent collapse without a global reduction in the CO 2 emissions that are disrupting the climate.
‘For 65 million years, Amazonian forests have withstood climatic variability, but the region is now exposed to unprecedented stress from drought, heat, fire and land clearance, which are penetrating even the deep central areas of the biome. This is altering the functioning of the forest, which in many areas is producing less rain than before, and turning a carbon sink into a carbon emitter.
‘Concerns about an Amazon tipping point have been discussed for the past two decades, with previous models suggesting this could come when 20% to 25% of the forest is cleared. The new study, published in Nature, went further in its complexity, analysing evidence for five drivers of water stress and identifying critical thresholds that, if crossed, could trigger local, regional or even biome-wide forest collapse.
‘It estimated that by 2050, 10% to 47% of Amazonian forests would be exposed to compounding disturbances that might trigger unexpected ecosystem-wide transitions and have an adverse knock-on effect for regional climate change.
‘To prevent this, the study found that a safe boundary, which included a buffer zone, would be needed to keep deforestation to 10% of the Amazon region, and to keep global heating within 1.5C above pre-industrial levels.
‘But overshoot has already happened. The study found 15% of the Amazon had already been cleared and another 17% had been degraded by human activity, such as logging, fires and under-canopy extraction. A further 38% of the Amazon may be weakened as a result of the prolonged droughts over the past decade.
‘Using recent data collected on the ground, proxy indicators of ancient trends, and computer modelling that incorporates regional and global climate trends, the study traced three plausible ecosystem trajectories: a white-sand savanna, a degraded open canopy and a degraded forest – all of which would bring more fire and drought.
‘Dry season temperatures are already 2C higher than they were 40 years ago in central and southern parts of the Amazon. By 2050, the models projected between 10 and 30 more dry days than now, and an increase in annual maximum temperatures of between 2C and 4C. The paper said this would expose “the forest and local peoples to potentially unbearable heat” and potentially reduce forest productivity and carbon storage capacity.
‘Rainfall patterns are shifting. Since the early 1980s, swathes of the central and peripheral Amazon forest have become drier. Annual rainfall in the southern Bolivian Amazon has declined by up to 20mm. By contrast, western and eastern Amazon regions are becoming wetter. If these trends continued, the paper said, ecosystem resilience would be reshaped. Some regions would become savanna, whereas most of the rest of the Amazon was likely to persist in a degraded state.
‘This will have a profound impact on local and regional populations. The Amazon is home to more than 10% of the Earth’s terrestrial biodiversity, stores 15-20 years’ worth of global CO 2 emissions, contributes up to 50% of rainfall in the region and is crucial for moisture supply across South America. Its evapotranspiration helps to cool and stabilize the world’s climate. But its importance and complexity are not fully understood.’
It said:
‘The paper noted that existing computer climate models did not adequately reflect how different types of disturbances such as fire, drought and land clearance compound one another, nor did they take into account the different effects experienced by different types of forest; or the plans for new roads, such as the proposed BR319 which would open up a huge area to illegal mining and land grabbing; or how forest degradation contributes to rain recycling; or whether the extra CO 2 in the atmosphere is strengthening or weakening forest resilience.
‘The lack of complexity in existing models can create unpleasant surprises, such as last year’s devastating drought. “The recent El Niño shows how everything is happening now faster than we think,” Flores said. “We have to expect things happening earlier than we thought. We need to address this with a very precautionary approach. We must reach net zero emissions and net zero deforestation as quickly as possible. It needs to be done now. If we lose the Amazon, it would be problematic for humanity.”’
The study report, said:
‘A major question is whether a large-scale collapse of the Amazon forest system could actually happen within the twenty-first century, and if this would be associated with a particular tipping point.’
The scientists synthesized evidence from paleorecords, observational data and modelling studies of critical drivers of stress on the system, and assessed potential thresholds of those drivers and the main feedbacks that could push the Amazon forest towards a tipping point.
From examples of disturbed forests across the Amazon, the scientists analyzed the most plausible ecosystem trajectories that may lead to alternative stable states. They also identified climatic and land use boundaries that reveal a safe operating space for the Amazon forest system in the Anthropocene epoch.
The study report said:
A ‘tipping point’ is the critical threshold value of an environmental stressing condition at which a small disturbance may cause an abrupt shift in the ecosystem state, accelerated by positive feedbacks. This type of behaviour in which the system gets into a phase of self-reinforcing (runaway) change is often referred to as ‘critical transition’. As ecosystems approach a tipping point, they often lose resilience while still remaining close to equilibrium. Thus, monitoring changes in ecosystem resilience and in key environmental conditions may enable societies to manage and avoid critical transitions. The scientists adopt the concept of ‘ecological resilience’ (hereafter ‘resilience’), which refers to the ability of an ecosystem to persist with similar structure, functioning and interactions, despite disturbances that push it to an alternative stable state. The possibility that alternative stable states (or bistability) may exist in a system has important implications, because the crossing of tipping points may be irreversible for the time scales that matter to societies. Tropical terrestrial ecosystems are a well-known case in which critical transitions between alternative stable states may occur.
The report said:
‘The Amazon system has been mostly covered by forest throughout the Cenozoic era (for 65 million years). Seven million years ago, the Amazon River began to drain the massive wetlands that covered most of the western Amazon, allowing forests to expand over grasslands in that region. More recently, during the drier and cooler conditions of the Last Glacial Maximum (LGM) (around 21,000 years ago) and of the mid-Holocene epoch (around 6,000 years ago), forests persisted even when humans were already present in the landscape. Nonetheless, savannas expanded in peripheral parts of the southern Amazon basin during the LGM and mid-Holocene, as well as in the northeastern Amazon during the early Holocene (around 11,000 years ago), probably influenced by drier climatic conditions and fires ignited by humans. Throughout the core of the Amazon forest biome, patches of white-sand savanna also expanded in the past 20,000–7,000 years, driven by sediment deposition along ancient rivers, and more recently (around 800 years ago) owing to Indigenous fires. However, during the past 3,000 years, forests have been mostly expanding over savanna in the southern Amazon driven by increasingly wet conditions.
‘Although palaeorecords suggest that a large-scale Amazon forest collapse did not occur within the past 65 million years, they indicate that savannas expanded locally, particularly in the more seasonal peripheral regions when fires ignited by humans were frequent. Patches of white-sand savanna also expanded within the Amazon forest owing to geomorphological dynamics and fires. Past drought periods were usually associated with much lower atmospheric CO 2 concentrations, which may have reduced water-use efficiency of trees (that is, trees assimilated less carbon during transpiration). However, these periods also coincided with cooler temperatures, which probably reduced water demand by trees. Past drier climatic conditions were therefore very different from the current climatic conditions, in which observed warming trends may exacerbate drought impacts on the forest by exposing trees to unprecedented levels of water stress.’
The study report said:
‘Satellite observations from across the Amazon suggest that forest resilience has been decreasing since the early 2000s, possibly as a result of global changes. In this section, we synthesize three global change impacts that vary spatially and temporally across the Amazon system, affecting forest resilience and the risk of critical transitions.’
It said:
‘Within the twenty-first century, global warming may cause long-term changes in Amazonian climatic conditions. Human greenhouse gas emissions continue to intensify global warming, but the warming rate also depends on feedbacks in the climate system that remains uncertain. Recent climate models of the 6th phase of the Coupled Model Intercomparison Project (CMIP6) agree that in the coming decades, rainfall conditions will become more seasonal in the eastern and southern Amazonian regions, and temperatures will become higher across the entire Amazon. By 2050, models project that a significant increase in the number of consecutive dry days by 10−30 days and in annual maximum temperatures by 2–4 °C, depending on the greenhouse gas emission scenario. These climatic conditions could expose the forest to unprecedented levels of vapour pressure deficit and consequently water stress.’
The report said:
‘Since the early 1980s, the Amazonian region has been warming significantly at an average rate of 0.27 °C per decade during the dry season, with the highest rates of up to 0.6 °C per decade in the centre and southeast of the biome. Only a few small areas in the west of the biome are significantly cooling by around 0.1 °C per decade. Dry season mean temperature is now more than 2 °C higher than it was 40 years ago in large parts of the central and southeastern Amazon. If trends continue, these areas could potentially warm by over 4 °C by 2050. Maximum temperatures during the dry season follow a similar trend, rising across most of the biome, exposing the forest and local peoples to potentially unbearable heat. Rising temperatures will increase thermal stress, potentially reducing forest productivity and carbon storage capacity and causing widespread leaf damage.
Fig. 1: Exploring ecosystem transition potential across the Amazon forest biome as a result of compounding disturbances.
a, Changes in the dry season (July–October) mean temperature reveal widespread warming, estimated using simple regressions between time and temperature observed between 1981 and 2020 (with P < 0.1). b, Potential ecosystem stability classes estimated for year 2050, adapted from current stability classes (Extended Data Fig. 1b) by considering only areas with significant regression slopes between time and annual rainfall observed from 1981 through 2020 (with P < 0.1) (see Extended Data Fig. 3 for areas with significant changes). c, Repeated extreme drought events between 2001–2018 (adapted from ref. 39). d, Road network from where illegal deforestation and degradation may spread. e, Protected areas and Indigenous territories reduce deforestation and fire disturbances. f, Ecosystem transition potential (the possibility of forest shifting into an alternative structural or compositional state) across the Amazon biome by year 2050 inferred from compounding disturbances (a – d) and high-governance areas (e). We excluded accumulated deforestation until 2020 and savannas. Transition potential rises with compounding disturbances and varies as follows: less than 0 (in blue) as low; between 1 and 2 as moderate (in yellow); more than 2 as high (orange–red). Transition potential represents the sum of: (1) slopes of dry season mean temperature (as in a, multiplied by 10); (2) ecosystem stability classes estimated for year 2050 (as in b), with 0 for stable forest, 1 for bistable and 2 for stable savanna; (3) accumulated impacts from extreme drought events, with 0.2 for each event; (4) road proximity as proxy for degrading activities, with 1 for pixels within 10 km from a road; (5) areas with higher governance within protected areas and Indigenous territories, with −1 for pixels inside these areas.
‘Since the early 1980s, rainfall conditions have also changed. Peripheral and central parts of the Amazon forest are drying significantly, such as in the southern Bolivian Amazon, where annual rainfall reduced by up to 20 mm yr −1. By contrast, parts of the western and eastern Amazon forest are becoming wetter, with annual rainfall increasing by up to 20 mm yr −1. If these trends continue, ecosystem stability (Fig. 1) will probably change in parts of the Amazon by 2050, reshaping forest resilience to disturbances (Fig. 1b and Extended Data Fig. 3b). For example, 6% of the biome may change from stable forest to a bistable regime in parts of the southern and central Amazon. Another 3% of the biome may pass the critical threshold in annual rainfall into stable savanna in the southern Bolivian Amazon. Bistable areas covering 8% of the biome may turn into stable forest in the western Amazon (Peru and Bolivia), thus becoming more resilient to disturbances.’
The report said:
‘Within the remaining Amazon forest area, 17% has been degraded by human disturbances, such as logging, edge effects and understory fires, but if we consider also the impacts from repeated extreme drought events in the past decades, 38% of the Amazon could be degraded. Increasing rainfall variability is causing extreme drought events to become more widespread and frequent across the Amazon (Fig. 1c), together with extreme wet events and convective storms that result in more windthrow disturbances. Drought regimes are intensifying across the region, possibly due to deforestation that continues to expand within the system. As a result, new fire regimes are burning larger forest areas, emitting more carbon to the atmosphere and forcing IPLCs to readapt. Road networks (Fig. 1d) facilitate illegal activities, promoting more deforestation, logging and fire spread throughout the core of the Amazon forest. The impacts of these pervasive disturbances on biodiversity and on IPLCs will probably affect ecosystem adaptability, and consequently forest resilience to global changes.’
It said:
‘Currently, 86% of the Amazon biome may be in a stable forest state, but some of these stable forests are showing signs of fragility. For instance, field evidence from long-term monitoring sites across the Amazon shows that tree mortality rates are increasing in most sites, reducing carbon storage, while favouring the replacement by drought-affiliated species.’
The scientists ‘found that 10% of the Amazon forest biome has a relatively high transition potential (more than 2 disturbance types; Fig. 1f), including bistable forests that could transition into a low tree cover state near savannas of Guyana, Venezuela, Colombia and Peru, as well as stable forests that could transition into alternative compositional states within the central Amazon, such as along the BR319 and Trans-Amazonian highways.’
The scientists define ‘ecosystem adaptability’ as the capacity of an ecosystem to reorganize and persist in the face of environmental changes. In the past, many internal mechanisms have probably contributed to ecosystem adaptability, allowing Amazonian forests to persist during times of climate change. In this section we synthesize two of these internal mechanisms, which are now being undermined by global change.
The report said:
‘Amazonian forests are home to more than 15,000 tree species, of which 1% are dominant and the other 99% are mostly rare. A single forest hectare in the central and northwestern Amazon can contain more than 300 tree species. Such tremendous tree species diversity can increase forest resilience by different mechanisms. Tree species complementarity increases carbon storage, accelerating forest recovery after disturbances. Tree functional diversity increases forest adaptability to climate chance by offering various possibilities of functioning. Rare species provide ‘ecological redundancy’, increasing opportunities for replacement of lost functions when dominant species disappear. Diverse forests are also more likely to resist severe disturbances owing to ‘response diversity’—that is, some species may die, while others persist. For instance, in the rainy western Amazon, drought-resistant species are rare but present within tree communities, implying that they could replace the dominant drought-sensitive species in a drier future. Diversity of other organisms, such as frugivores and pollinators, also increases forest resilience by stabilizing ecological networks. Considering that half of Amazonian tree species are estimated to become threatened (IUCN Red list) by 2050 owing to climate change, deforestation and degradation, biodiversity losses could contribute to further reducing forest resilience.’
The study report said:
‘Globally, Indigenous peoples and local communities (IPLCs) have a key role in maintaining ecosystems resilient to global change. Humans have been present in the Amazon for at least 12,000 years and extensively managing landscapes for 6,000 years. Through diverse ecosystem management practices, humans built thousands of earthworks and ‘Amazon Dark Earth’ sites, and domesticated plants and landscapes across the Amazon forest. By creating new cultural niches, humans partly modified the Amazonian flora, increasing their food security even during times of past climate change without the need for large-scale deforestation. Today, IPLCs have diverse ecological knowledge about Amazonian plants, animals and landscapes, which allows them to quickly identify and respond to environmental changes with mitigation and adaptation practices. IPLCs defend their territories against illegal deforestation and land use disturbances, and they also promote forest restoration by expanding diverse agroforestry systems. Amazonian regions with the highest linguistic diversity (a proxy for ecological knowledge diversity) are found in peripheral parts of the system, particularly in the north-west). However, consistent loss of Amazonian languages is causing an irreversible disruption of ecological knowledge systems, mostly driven by road construction. Continued loss of ecological knowledge will undermine the capacity of IPLCs to manage and protect Amazonian forests, further reducing their resilience to global changes.’
It said:
‘Rising atmospheric CO 2 concentrations are expected to increase the photosynthetic rates of trees, accelerating forest growth and biomass accumulation on a global scale. In addition, CO 2 may reduce water stress by increasing tree water-use efficiency. As result, a ‘CO 2 fertilization effect’ could increase forest resilience to climatic variability. However, observations from across the Amazon suggest that CO 2 -driven accelerations of tree growth may have contributed to increasing tree mortality rates (trees grow faster but also die earlier), which could eventually neutralize the forest carbon sink in the coming decades. Moreover, increases in tree water-use efficiency may reduce forest transpiration and consequently atmospheric moisture flow across the Amazon, potentially reducing forest resilience in the southwest of the biome.’
It said:
‘It is possible that in the fertile soils of the western Amazon and Várzea floodplains, forests may gain resilience from increasing atmospheric CO 2 (depending on how it affects tree mortality rates), whereas on the weathered (nutrient-poor) soils across most of the Amazon basin, forests might not respond to atmospheric CO 2 increase, particularly on eroded soils within deforestation frontiers. In sum, owing to multiple interacting factors, potential responses of Amazonian forests to CO 2 fertilization are still poorly understood. Forest responses depend on scale, with resilience possibly increasing at the local scale on relatively more fertile soils, but decreasing at the regional scale due to reduced atmospheric moisture flow.’
The report said:
‘Environmental heterogeneity can reduce the risk of systemic transition (large-scale forest collapse) because when stressing conditions intensify (for example, rainfall declines), heterogeneous forests may transition gradually (first the less resilient forest patches, followed by the more resilient ones), compared to homogeneous forests that may transition more abruptly (all forests transition in synchrony). Amazonian forests are heterogeneous in their resilience to disturbances, which may have contributed to buffering large-scale transitions in the past. At the regional scale, a fundamental heterogeneity factor is rainfall and how it translates into water stress. Northwestern forests rarely experience water stress, which makes them relatively more resilient than southeastern forests that may experience water stress in the dry season, and therefore are more likely to shift into a low tree cover state. As a result of low exposure to water deficit, most northwestern forests have trees with low drought resistance and could suffer massive mortality if suddenly exposed to severe water stress. However, this scenario seems unlikely to occur in the near future (Fig. 1). By contrast, most seasonal forest trees have various strategies to cope with water deficit owing to evolutionary and adaptive responses to historical drought events. These strategies may allow seasonal forests to resist current levels of rainfall fluctuations, but seasonal forests are also closer to the critical rainfall thresholds (Extended Data Fig. 1) and may experience unprecedented water stress in the coming decades (Fig. 1).’
The study report said:
‘Future changes in rainfall regimes will probably affect hydrological regimes, exposing plateau (hilltop) forests to unprecedented water stress, and floodplain forests to extended floods, droughts and wildfires. Soil fertility is another heterogeneity factor that may affect forest resilience, and which may be undermined by disturbances that cause topsoil erosion. Moreover, as human disturbances intensify throughout the Amazon (Fig. 1), the spread of invasive grasses and fires can make the system increasingly homogeneous. Effects of heterogeneity on Amazon forest resilience have been poorly investigated so far and many questions remain open, such as how much heterogeneity exists in the system and whether it can mitigate a systemic transition.’
The report said:
‘Our findings suggest that interactions and synergies among different disturbances (for example, frequent extreme hot droughts and forest fires) could trigger unexpected ecosystem transitions even in remote and central parts of the system.’
It said:
‘Recent policy and approaches to Amazon development, however, accelerated deforestation that reached 13,000 km 2 in the Brazilian Amazon in 2021 (http://terrabrasilis.dpi.inpe.br). The southeastern region has already turned into a source of greenhouse gases to the atmosphere. The consequences of losing the Amazon forest, or even parts of it, imply that we must follow a precautionary approach — that is, we must take actions that contribute to maintain the Amazon forest within safe boundaries. Keeping the Amazon forest resilient depends firstly on humanity’s ability to stop greenhouse gas emissions, mitigating the impacts of global warming on regional climatic conditions. At the local scale, two practical and effective actions need to be addressed to reinforce forest–rainfall feedbacks that are crucial for the resilience of the Amazon forest: (1) ending deforestation and forest degradation; and (2) promoting forest restoration in degraded areas. Expanding protected areas and Indigenous territories can largely contribute to these actions. Our findings suggest a list of thresholds, disturbances and feedbacks that, if well managed, can help maintain the Amazon forest within a safe operating space for future generations.’