Tropical forests have long thrived in warm and wet conditions, but climate signatures indicate increasing intensity and frequency of drought events coinciding with progressively hotter temperatures. An extensive multi-decadal investigation spanning over 30 years used detailed forest demographic data collected from a selective logging experiment, revealing that tree mortality rates surged conspicuously during periods of intense drought. This mortality was not randomly distributed among species; rather, the fastest-growing pioneers, endowed with lower wood density, succumbed disproportionately. Such selectivity in drought vulnerability casts doubts on forest resilience and future composition under sustained climatic stresses.

Delving deeper into plant physiology, field measurements during recent El Niño–induced drought events of 2015 and 2023 highlighted a critical soil moisture threshold that acts as a tipping point for tree functionality. Above this threshold, trees maintain regular transpiration, facilitating carbon assimilation and cooling. However, upon crossing this boundary, the rate of transpiration plummeted abruptly. This decline implicates a loss of hydraulic function, increasing the likelihood of hydraulic failure—a fatal impairment of water transport within the tree—and a dangerous deficit in carbon resources that can lead to starvation and death.

The implications of this physiological sensitivity extend beyond individual trees. When prolonged dry conditions push soil moisture below critical levels, the entire forest canopy experiences stress, making systemic dieback increasingly probable. The coupling of elevated temperatures with these moisture shortages compounds thermal stress and exacerbates evaporative demand, effectively accelerating the onset of hydraulic dysfunction. This coupling challenges previous assumptions that tropical forests possess robust buffering capacities against climatic extremes, signaling that their physiological thresholds may be narrower than once thought.

Parallel to these field observations, climate model projections from the Coupled Model Intercomparison Project Phase 6 offer a sobering forecast. Under high-emission scenarios—consistent with continued fossil fuel reliance and limited mitigation—large expanses of tropical forest are likely to shift toward what the researchers term a “hypertropical” climate by the year 2100. This novel climate regime will exhibit unprecedented combinations of temperature and moisture extremes, conditions currently without historical analogues. Such a transformation portends a wholesale reorganization of tropical forest ecosystems both structurally and functionally.

Elevated temperatures characteristic of hypertropical climates exacerbate soil desiccation during dry seasons, pushing forests beyond their established physiological limits with greater frequency and intensity. Analysis within this emergent regime predicts that temperature and moisture conditions regularly breach the identified drought-induced mortality thresholds, markedly raising the risk of widespread tree mortality events. Such dieback episodes have cascading consequences, including the release of vast carbon stocks stored in biomass, which could feedback into global climate systems and accelerate warming.

Intriguingly, the ongoing hot droughts already observed represent more than mere anomalies; they serve as vital analogues for the predicted climate state. These episodes furnish researchers with a natural experiment to study responses of tropical forests confronted by extreme heat coupled with water deficits—a scenario expected to become the new normal. Understanding these physiological and ecological responses under current stress conditions is critical to refining models that project tropical forest futures, informing conservation strategies, and guiding climate policy.

The selective vulnerability of tree functional types to drought underscores complexities in ecosystem turnover and regeneration patterns. Pioneer species, typically rapid colonizers following disturbance, showed increased mortality during droughts, which could slow forest recovery and alter successional trajectories. Conversely, species with denser wood, often slower growing, seem better equipped to endure such extremes, hinting at potential shifts in forest species composition driven by climatic filtering.

Moreover, the study emphasizes the integral role of soil moisture in modulating drought impacts. The identified threshold marks a physiologically significant boundary where water availability ceases to sustain normal tree transpiration, triggering a cascade of biochemical and biomechanical failures. These findings spotlight soil hydrology as a critical factor in predicting forest resilience and stress, demanding greater integration of hydrological data into ecosystem response models.

The integrative approach—combining long-term demographic datasets, fine-scale physiological measurements, and sophisticated climate simulations—exemplifies the multifaceted efforts required to disentangle the complex feedbacks at play. This research pushes beyond correlative observations, linking empirical evidence with mechanistic insights and future projections to construct a comprehensive narrative of tropical forest vulnerability.

Crucially, these insights bear significant implications for climate mitigation and adaptation policies. With forests serving as key carbon sinks, their degradation threatens to amplify atmospheric CO2 concentrations, undermining global efforts to curb warming. Protecting and managing tropical forests amid accelerating climate stressors is paramount, necessitating strategies that enhance resilience, such as reducing deforestation, promoting species diversity, and sustaining hydrological regimes.

In conclusion, the research paints a chilling picture of tropical forests confronted by hot droughts that foreshadow a hypertropical future—a regime defined by heightened thermal and hydrological extremes beyond historical experience. These findings not only unravel the physiological thresholds precipitating tree mortality but also project the broader ecosystem consequences under sustained warming trends. As these hot droughts escalate, they demand urgent attention, offering both a warning and a real-world laboratory to understand and mitigate the impacts of climate change on some of Earth’s most vital terrestrial ecosystems.

Subject of Research:
Physiological and ecological responses of tropical forests to hot droughts in the Amazon and projections of future hypertropical climate conditions.

Article Title:
Hot droughts in the Amazon provide a window to a future hypertropical climate.

Article References:
Chambers, J.Q., Nogueira Lima, A.J., Pastorello, G. et al. Hot droughts in the Amazon provide a window to a future hypertropical climate. Nature (2025). https://doi.org/10.1038/s41586-025-09728-y

Image Credits:
AI Generated

DOI:
https://doi.org/10.1038/s41586-025-09728-y

Keywords:
Amazon, hot drought, tropical forest mortality, hydraulic failure, carbon starvation, soil moisture threshold, El Niño, climate change, hypertropical climate, forest dieback, Coupled Model Intercomparison Project, high-emission scenarios

Deforestation within the Brazilian Amazon accounts for an estimated 74.5% of the documented rainfall reduction during the dry season, subtracting roughly 15.8 millimeters of annual precipitation, while global climate change underpins the remaining decrease. The forest loss also explains approximately 16.5% of the observed 2.0 °C temperature increase over the same period, with the greater share attributed to global warming originating largely from industrial activity in Northern Hemisphere nations. These findings represent a crucial “attribution partition,” quantifying the relative influence of local anthropogenic land modifications and worldwide greenhouse gas emissions on Amazonian climate fluctuations.

Professor Luiz Augusto Toledo Machado, who helmed the study from USP’s Physics Institute, emphasizes this work’s significance in disentangling the previously conflated drivers of the Amazon’s changing climate. “While many studies have documented escalating temperatures and diminishing rainfall, this is the first clear breakdown of how much is due to deforestation tied to Brazil itself, versus external global emissions,” he explains. By constructing parametric surface equations that integrate annual variability and deforestation data, the team was able to decompose the cumulative climatic shifts into their component sources, setting a new standard for ecosystem-specific climate attribution.

The researchers underline that the largest climatic disruptions occur early in the deforestation trajectory. Pronounced variations in temperature and precipitation emerge sharply once forest cover is reduced by as little as 10% to 40%. According to co-author Professor Marco Aurélio Franco, “The initial stages of deforestation impose disproportionate impacts, so preserving the standing forest is paramount. Transitioning these lands to pasture or other uses risks triggering amplified local warming and severe rainfall declines.” Their statistical analysis pinpoints this initial deforestation threshold as a climatic tipping point, beyond which recovery of ecosystem equilibrium becomes drastically more difficult.

Remote sensing datasets, including the extensive land-use classifications by the MapBiomas collaborative network, afforded the study a robust spatial and temporal scope over 35 years. These data, combined with long-term reanalyses of atmospheric greenhouse gas concentrations, revealed that atmospheric CO₂ and methane increases in the Amazon are overwhelmingly driven (>99%) by global emissions rather than local deforestation. Though deforestation reduces the forest’s capacity to sink carbon locally, this does not translate into a significant localized elevation of atmospheric CO₂ concentration, given the global scale of greenhouse gas accumulation.

The Amazon’s role in regional and global hydrological cycles is profound, often described through the concept of “flying rivers”—large atmospheric flows of moisture sustained by the forest’s transpiration processes. Trees extract groundwater and release it as vapor, driving cloud formation and precipitation not only locally but throughout South America, including the Cerrado biome. The study confirms that deforestation disrupts this vapor recycling mechanism, intensifying the dry season and exacerbating forest fire frequency, which in turn further degrade the forest’s vegetation and resilience.

Recent international research, including prior work by USP experts, has elucidated how aerosol nanoparticles generated within the Amazon’s atmosphere interplay with electrical discharges and daytime-nighttime chemical reactions to form rain-inducing clouds. This complex “aerosol machine” is tightly linked to forest health. As deforestation escalates, these processes weaken, diminishing cloud formation potential and leading to cascading rainfall deficits. Such physical-chemical insights underscore how land-cover changes ripple through atmospheric chemistry, altering weather and climate patterns in ways that threaten the rainforest’s survival.

The cumulative land degradation between 1985 and 2023 has already resulted in the loss of 14% of the Amazon’s original vegetation, an area roughly equivalent to France. While recent deforestation rates have declined slightly to 4,495 km² annually, the persistence of forest degradation—particularly from recurrent fires—continues to challenge conservationists. The dry season, stretching from June to November, remains the focal window when these impacts are most visible, as precipitation reductions and temperature rises converge to heighten vulnerability.

Looking forward, the researchers warn that the continuation of deforestation at current or higher rates threatens to push the Amazon past critical climate thresholds. Their models project accelerating precipitation declines and temperature increases during dry seasons, intensifying seasonal extremes and undermining the biome’s ecological resilience. These hydrometeorological shifts are already affecting the South American monsoon, leading to drier conditions that imperil the rainforest’s long-term stability and its essential climate regulation functions.

The implications extend beyond local ecosystems. Alterations in the Amazon reverberate across continental weather systems, influencing agriculture, water security, and biodiversity throughout Brazil and neighboring countries. Extreme drought events in 2023 and 2024 serve as ominous indicators of a rapidly shifting baseline, highlighting the urgent need for integrated strategies that address both land-use practices and global greenhouse gas emissions. This new research provides policymakers and environmental stakeholders with a precise “climate ledger” that clarifies responsibilities and informs sustainable development pathways.

This study, supported by the São Paulo Research Foundation (FAPESP) and conducted in collaboration with the Chinese Academy of Sciences, marks a breakthrough in our understanding of the Amazon’s vulnerability amid converging environmental crises. The scientific community now possesses clearer evidence tying local deforestation directly to tangible climatic consequences, alongside the overarching global warming trend. Ultimately, the findings reinforce the critical imperative to protect and sustainably manage the Amazon rainforest to secure its indispensable climate services for Brazil and the world.

—

Subject of Research: The interactive effects of deforestation and global climate change on the Amazon rainforest’s climate, with emphasis on rainfall and temperature changes during the dry season.

Article Title: How climate change and deforestation interact in the transformation of the Amazon rainforest

News Publication Date: 2-Sep-2025

Web References: https://agencia.fapesp.br/54089; https://www.fapesp.br/en

References: DOI 10.1038/s41467-025-63156-0 (Nature Communications)

Image Credits: Luiz Augusto Toledo Machado (IF-USP)