Extreme drought and record heat in tropical forests: implications for future climate

75% of the world's tropical forests experienced at least six consecutive months of unprecedented drought and heat during the last El Niño. This dual stressor affected ecosystems that had previously benefited from near-constant humidity, offering a worrying glimpse into the future climate if greenhouse gas emissions are not controlled.

Record drought and heat during El Niño: extent and key figures

The 2023-2024 El Niño event triggered an oceanic surface temperature anomaly that propagated heat waves across the tropical belt. According to a study published in Earth's Future, 75% of tropical forest areas experienced at least six months where precipitation fell below 50% of historical averages, while air temperatures exceeded 30°C during the same period. These conditions are described as 'unprecedented simultaneously' by researchers, as tropical forests are typically protected by regular rain cycles.

Satellite data from Copernicus, combined with predictive models based on neural networks, allowed for global-scale mapping of hydrological stress intensity. Automated learning algorithms identified critical zones, notably in the Amazon, Congo basin, and Southeast Asia, where the combination of heat and precipitation deficit led to a loss of soil hydraulic conductivity, reducing trees' ability to access water. This loss of moisture also amplified wildfire risks, already observed in several regions.

Scientific mechanisms of El Niño and amplification of climate stress

El Niño is characterized by an abnormal warming of surface waters in the central and eastern Pacific. This warming modifies atmospheric pressure gradients, weakening trade winds that normally push warm waters westward and favor rainfall in tropical regions. When trade winds weaken, warm waters extend eastward, increasing evaporation and strengthening descending air currents (subsidence) above forests. Subsidence creates stable conditions where air warms and cloud formation is inhibited, decreasing precipitation.

In the current context of increasing greenhouse gases, the 'background warming' of the climate makes these El Niño episodes more intense and frequent, according to ECMWF models. Thus, even in the absence of El Niño, tropical forests would already be experiencing longer and more intense droughts, but the combination of the two factors creates a synergy that far exceeds the simple addition of their effects. This synergy translates into an increase in prediction uncertainty, making it more difficult to plan ahead.