Deforestation-induced drying lowers Amazon climate threshold

Humanity is exerting unprecedented pressures on the Amazon forest through global warming and land-use changes. As the ecosystem may undergo self-reinforcing transitions, these pressures could lead to system-wide changes across major parts of the biome. Research using a dynamical systems model assessed local and far-reaching cascading transition risks under different Shared Socioeconomic Pathways (SSPs). Without accounting for deforestation, the forest faces a critical global warming threshold of 3.7–4.0 °C, beyond which up to one-third of the system risks losing stability.

However, when deforestation is factored in, the threshold for a near system-wide transition (affecting 62–77% of the area) drops significantly to a range of 1.5–1.9 °C of global warming combined with 22–28% deforestation. The majority of simulated transitions are caused by spatial knock-on effects from increasing drought intensities, leading to long-ranging cascades spanning hundreds to thousands of kilometres. This highlights the urgent need to keep warming below 1.5 °C and halt deforestation while restoring degraded forests to mitigate transition risks.

Transitions due to global warming

The study quantified spatially resolved transition risks across the Amazon on a 1° × 1° grid. Under the SSP2-4.5 scenario, which is closest to current emission pathways, the transition risk at the end of the century remains very low at 2.8 °C of warming. However, for SSP3-7.0 and SSP5-8.5 scenarios, transition risks increase sharply once global warming reaches the 3.7–4.0 °C range. The largest risks are observed in the western and southwestern parts of the Amazon, which are the primary receivers of forest-generated precipitation.

The relationship between warming and environmental changes is non-trivial. While some areas are at risk due to decreasing mean annual precipitation (MAP), others, particularly in central parts of the forest, are threatened by increased dry season intensity and duration, measured as maximum cumulative water deficit (MCWD). Peak transition risks can emerge even at lower warming levels, such as approximately 2.1 °C in the SSP2-4.5 scenario, depending on specific environmental conditions.

Transitions including deforestation

Deforestation amplifies global warming-induced water stress downwind. By exploring a severe deforestation scenario characterised by strong clearing between 2002 and 2050, the model identifies widespread transitions for all SSPs by the end of the century. Under these conditions, more than 62% of the basin transitions in SSP1-2.6 and SSP2-4.5, rising to 77% in SSP3-7.0 and SSP5-8.5. Crucially, a systemic transition is crossed between 1.5 and 1.9 °C when basin-wide deforestation reaches 22–28%.

Current SSP scenarios often use optimistic land-use assumptions where deforestation remains near historical levels of 15%. If deforestation is kept strictly at these levels, large-scale transitions are largely absent. However, the study emphasises that the location of deforestation is critical; clearing in the east of the Amazon basin is particularly damaging as it avoids downwind cascades along main wind directions that could otherwise be maintained.

Reasons for transitions

The forest maintains its existence through atmospheric moisture recycling. Trees absorb water from soil layers and release it through transpiration, while the canopy contributes via interception evaporation. Locally, up to 50% of precipitation is generated from within the basin. Emergent canopy trees are vital, contributing approximately 71% of transpired water. Overall, trees recycle around 36% of precipitation, and their transpiration in the late dry season determines the onset of the wet season.

In scenarios without deforestation, cascading network effects dominate as the reason for critical transitions, accounting for 87.5% of cases. When deforestation is included, this figure rises to 99.0%. These cascading effects are primarily located downwind of initial transition events triggered by critical MAP or MCWD levels. This indicates that once specific thresholds are crossed, knock-on effects can induce large-scale degradation across the biome.

Discussion

The research concludes that a widespread transition risk for the Amazon is robust in terms of commitment to irreversible change. While a full transition to a degraded state may take decades or centuries, the threshold for such change could be crossed much sooner. Following a high-emission pathway like SSP3-7.0 is highly unsafe even without further land-use change. Severe deforestation could lead to detrimental transitions across 77% of the forest even at moderate warming levels.

There are opportunities to strengthen forest resilience through restoration. Strategic efforts, such as the Brazilian government’s plan to restore 12 million hectares in the ‘Arc of Restoration’, are of global importance. Limiting deforestation and restoring lost cover could recover moisture recycling mechanisms faster than the recovery of biodiversity. The study reinforces the necessity of global cooperation to halt emissions and achieve zero deforestation to safeguard the Amazonian forest system and the regional water security of downwind areas like southern Brazil and Argentina.