Over 75 per cent of the world’s tropical forests experienced drought and heatwaves that were more extreme than usual during the El Niño years of 2023 and 2024.
Researchers have warned that under probable emission scenarios, widespread heat experienced during the 2023-2024 El Niño could become the ‘new normal’ for tropical forest regions in less than 20 years.
The findings, published in Earth's Future, used multiple data sets of measured climate between 2000 and 2024 spanning a total area of 15.9 million square km.
This included pantropical forests across the Americas spread over an area of 8.4 million sq km, Africa (3.6 million sq km), and Asia (3.9 million sq km). The researchers used the data to project conditions from 2025 to 2100 to assess how extreme the 2023 and 2024 events were, compared with past and future climates.
The study noted that over three quarters of tropical forests endured an average of 6 months of anomalous drought and heat during the 2023-2024 El Niño.
“When assessing the occurrence of compound hot‐dry periods during the two El Niño years, we found that the American and African tropical forests were most affected by compound hot‐dry events with on average 88 per cent and 89 per cent of the continental forest area, followed by 43 per cent of Asian forests,” the study noted.
It added that Asian tropical forests suffered less than two months of hot-dry periods. This duration stretched between five and nine months in African and American forests respectively.
“In both the American and African tropical forests, compound hot‐dry events impacted larger areas and lasted up to four times longer in 2023-2024, compared to 2015-2016. We found a maximum difference in affected area of almost 45 per cent between the different products, with 77 per cent of pantropical forests experiencing hotter and drier conditions,” it further said.
The authors said all the three assessed continents experienced hot and dry conditions longer, with 19, 17, and 18 hot months out of 2023 and 2024 for the American, African, and Asian tropics, respectively.
The area affected by heat was similar compared to the 2015-16 El Niño years with American forests seeing 100 per cent in 2023-24, compared to 99 per cent in 2015-16.
For Africa, the area was larger by 10 per cent compared to previous El Niño years and 17 per cent wider in Asia.
However, the heat events lasted on average 10-13 months longer in 2023-2024 than 2015-2016 across all three continents, the study observed.
On average, these heatwaves lasted between 17 and 19 months out of the 24-month period across all three tropical continents
Drier-than-usual conditions affected 80 per cent of pantropical forests, impacting 91 per cent of American forests, 92 per cent of African forests, and 48 per cent of Asian forests.
The impacted area also surpassed the previous events.
The study noted that African forests displayed the greatest variability of up to 95 per cent among climate products for all three event types.
Though the El Niño period of 2015-2016 was an exception across all three continents for having widespread dry and especially hot conditions, the researchers found that the spatial extent for Africa surpassed by 20 per cent during the 2023-2024 El Niño, with the largest increase in drought‐affected area in African forests.
For the Americas, the areas was larger by 18 per cent.
“The only exception to this increase occurred in Asian tropical forests, where drought‐affected areas were 22 per cent smaller in 2023-2024 than in 2015-2016. Patterns of compound hot‐dry events largely mirrored those of dry events in the Americas and Africa, indicating that most droughts in these regions coincided with unusually high temperatures,” they noted.
The study further stated that approximately 80 per cent of all anomalously dry and hot months occurred during the wet season. “Wet-season droughts disrupt groundwater recharge needed to sustain trees through subsequent dry months, posing a severe threat to tree growth and survival,” it said.
The researchers said there was an average increase in burned areas and in forest degradation reports during the 2023-24 El Niño period by 9 per cent 19 per cent respectively. This indicates that the drought-fire degradation cycle is accelerating and hampering the ecosystem’s ability to recover.
“The drought resilience has decreased in Southeast Asia and West Africa, implying that even lower levels of drought can now trigger conversion of carbon sinks. Although the probability of 2023-2024‐level drought conditions is projected to decline for these regions, milder droughts might still produce increasingly severe impacts on vegetation. This vulnerability may be further amplified because ecosystem productivity has an optimum temperature, making more frequent heatwaves likely to reduce gross primary productivity (GPP) across tropical forests,” it said.
The scientists warned that annual temperatures matching or exceeding the record highs of 2023-24 are projected to become six times more likely by 2100 under low emission scenario while chances of it becoming an annual phenomenon are 100 per cent under high emission scenario by as early as 2043.
Across the Amazon basin, Central America, and Southern Africa, drought frequency is projected to rise significantly by up to 9 times more likely under high emissions by 2100.
The study underlined that the tropical forests have traditionally served as a major carbon sink, absorbing about 1.2 Pg C year− 1.
However, there is growing evidence that increased tree mortality is reducing this sink or even transitioning forest areas into a carbon source. Under increasingly hot and dry conditions, tropical forests may also cross a critical threshold toward a degraded ecosystem state, due to a self‐ perpetuating decrease in the water recycling feedback loops that sustain these forests, it said.
The authors said that such widespread tree mortality would have catastrophic consequences on the global hydrological and carbon cycles and on the livelihoods of millions of people, making it important to quantify El Niño impacts and their predicted future occurrences.