Is the world’s largest rainforest about to turn into a net carbon emitter? New research suggests that the Amazon is entering into a “hypertropical” state a climate regime hotter and drier than anything Earth has seen in at least 10 million years. The root of this transformation lies in the rise of intensifying heat and drought already evident in changing forest structure, species composition, and its critical role in regulating the planet’s carbon cycle.
To refer to conditions that extend beyond the confines of the current tropical climates, scientists adopted the term “hypertropical.” In this condition, the occurrence of “hot droughts,” very hot temperatures concurrent with an extreme deficiency of water, becomes common-even during the wet season. Jeff Chambers, from the University of California, Berkeley, says, “When these hot droughts occur, that’s the climate that we associate with a hypertropical forest, because it’s beyond the boundary of what we consider to be tropical forest now.” Thirty years of field data confirm these events no longer are sporadic anomalies but repeated stressors within the basin.
The physiological implications for the trees are severe. Once the soil moisture drops to about one-third of capacity, two lethal mechanisms often come into play. Hydraulic failure occurs when air bubbles-or embolisms-form in the xylem, blocking the transport of water much like clots in human blood vessels. Carbon starvation happens when trees close their stomata to conserve water, shutting off CO₂ intake and closing down photosynthesis. Both mechanisms have been observed at numerous Amazon locations during recent El Niño-driven droughts with remarkable consistency in soil moisture thresholds for collapse-around 0.32 to 0.33-across years and sites.
Fast-growing species with low wood density are most at risk. Typical of secondary forests, such trees make less investment in structural resilience but need plenty of water and carbon to keep their fast growth going. Among them, mortality increases sharply under hypertropical stress. This could imply that, if continued, the climate extremes will favor a shift in forest composition toward slower-growing, denser-wood species-if those can adapt fast enough to the surging heat and drought.
The carbon cycle consequences are severe. The Amazon historically has acted like a giant carbon sink, storing between 150-200 petagrams of carbon and accounting for roughly 12% of the terrestrial carbon sink. But during the 2015-2016 El Niño, the basin emitted almost 1 gigaton of carbon, and total carbon stocks had still not recovered even at the end of 2018. The combination of reduced photosynthesis, increased tree mortality, and accelerated decay in drought regions flips areas of forest from sink to source. This feedback loop-in which warming drives forest stress that leads to carbon release, feeding further warming-is a core concern for climate modelers.
Remote sensing and long-term plot inventories indicate that the drought impacts go way beyond the immediate die-offs. Canopy height, as measured with LiDAR after extreme droughts, is reduced for years thereafter-a legacy effect on forest structure. In the southern Amazon, where the dry season may extend to seven months, species that have low drought tolerance are already suffering mortality beyond their thresholds of adaptability. These structure and composition changes dampen evapotranspiration, hence weakening the forest’s role of rain recycling and cooling of the atmosphere-critical not only for the local climate but also throughout South America for moisture supply.
Climate projections, barring steep emission cuts, suggest that hot drought conditions could strike as many as 150 days per year by 2100, saturating both dry and wet seasons. That would drive large expanses of the Amazon into the hypertropics-defined here as areas that are hotter than the 99th percentile of the historical tropical climates and that are often hit by strong, prolonged droughts. Similar shifts are projected to occur in tropical Africa and Southeast Asia, imperiling other major carbon sinks.
Understanding the mechanisms of tree mortality is key to predicting and mitigating these impacts. Analyses at the basin scale link species mean growth rate, wood density, and drought tolerance more strongly with mortality risk than either individual tree size or growth rate alone. Fast-growing species are at a higher hazard, underscoring a growth-survival trade-off which stands across diverse Amazonian habitats. Integration of these empirical relationships into vegetation and Earth-system models will sharpen forecasts of forest responses under future climate scenarios.
The message is crystal clear: for climate scientists, ecologists, and policymakers alike, the Amazon’s hypertropical trajectory is not a matter of distant possibility but of unfolding reality. Its fate-and by extension, a significant chunk of the planet’s carbon budget-depends on rapid and coordinated actions to reduce greenhouse gas emissions and enhance forest resilience before these extreme conditions become the norm.
