1499. Tropical Forests Approaching a “Dangerous Tipping Point” as Drought Intensifies

Often referred to as the “lungs of the planet,” tropical forests play a critical role in stabilizing the global climate by absorbing vast amounts of atmospheric carbon dioxide (CO₂). However, new research suggests that intensifying drought linked to climate change is beginning to undermine this vital function.

A study published in Geophysical Research Letters in 2026 analyzed satellite observations and climate records from 1982 to 2019 and found significant increases in vegetation drought across tropical regions worldwide. African tropical forests showed the most severe expansion in both drought intensity and affected area, making them the hardest-hit region globally.

Rather than relying on conventional drought assessments based mainly on rainfall deficits, the research team developed a new framework focused on the physiological stress experienced by plants themselves. The method isolated the impacts of atmospheric dryness, measured as vapor pressure deficit (VPD), and soil moisture (SM) deficits on vegetation growth, using satellite-derived NDVI and LAI data to assess actual vegetation stress.

The results showed that while atmospheric drying has intensified across the tropics, soil moisture has steadily declined, leading to increasing water stress on vegetation. Machine-learning attribution analysis further revealed that more than half of the intensification in vegetation drought could be explained by declining soil moisture, making it the dominant driver across the pantropical region.

In tropical forests, prolonged water stress causes trees to close leaf stomata to conserve moisture, thereby reducing photosynthesis and limiting CO₂ uptake. Continued drought can also slow tree growth, increase leaf shedding, elevate wildfire risk, and ultimately raise tree mortality. Although the study does not suggest that tropical forests have already collapsed, it warns that some regions may be approaching ecological “tipping points,” beyond which ecosystems could rapidly shift into a fundamentally altered state.

The study also evaluated current CMIP6 climate models and found that they poorly reproduce the observed long-term trends in vegetation drought. According to the authors, existing models may insufficiently represent key processes such as plant hydraulic responses and land–atmosphere interactions, highlighting an urgent need for model improvement to enhance future climate projections.

The findings reinforce the importance of maintaining healthy tropical forests for climate change mitigation and emphasize the need to reduce deforestation and greenhouse gas emissions. Intact, undisturbed forests are believed to be more resilient to drought stress, making their conservation increasingly critical.

(Reference)
Shuai Cheng et al., Observed Increase in Tropical Vegetation Droughts Over the Past Three Decades, Geophysical Research Letters (2026). DOI: 10.1029/2025GL121172

Contributor: Miyuki Iiyama, Strategic Coordination Office