Luciana Constantino | Agência FAPESP – During the 2023–2024 El Niño, about 75% of the world’s tropical forest areas experienced at least six months of unprecedented simultaneous drought and heat. According to a study published in the October 7 issue of the journal Earth’s Future, this scenario offers more than just a snapshot of an extreme event; it offers a preview of the “new normal” for the next two decades if greenhouse gas emissions are not reduced.

The Amazon is the region that is most vulnerable to this new climate pattern, raising the greatest concern. If global greenhouse gas emissions continue to rise, the likelihood of the biome experiencing droughts as severe as those of the past two years will increase ninefold by the end of the century, even in years not influenced by El Niño.

Under thermal stress of this magnitude, tropical forests undergo functional collapse. According to the researchers, instead of absorbing the gases that warm the planet (acting as sinks), these areas temporarily begin to emit carbon into the atmosphere. Faced with this vicious cycle, the study reinforces the urgency of rapidly and sustainably reducing global emissions.

The need for mitigation is even more urgent in light of the impending 2026–2027 El Niño event. Its effects are already being felt in South America, and forecasts indicate that it will reach the “very strong” category (the highest classification) by the start of summer. Fueled by the climate crisis, this El Niño is on track to be the most intense ever recorded.

Overall, the study found that high average temperatures are expected to become at least six times more likely across all tropical regions, even under a scenario of lower global emissions. This implies that by around 2075, the world will emit an amount of carbon equivalent to what it removes from the atmosphere – a condition known as net-zero CO₂ emissions.

“During the 2023–2024 El Niño, the area and duration of droughts and heat waves were the largest and most extensive in the past 25 years. When we apply the same models to the future, we see that temperatures are rising. We’re concerned because, unfortunately, it could be even worse now,” says Johanna Van Passel, the corresponding author of the article. Van Passel is a scientist at Ghent University in Belgium and a visiting researcher at the Institute of Biology at the State University of Campinas (IB-UNICAMP) in Brazil.

El Niño is a natural phenomenon characterized by the abnormal warming of the tropical Pacific waters, particularly off the coast of South America and in the central region of the ocean. The heat accumulated in the ocean alters atmospheric circulation, contributes to rising global temperatures, and changes rainfall patterns in different parts of the planet. In Brazil, El Niño leads to above-average rainfall in the South, reduced precipitation in parts of the North and Northeast, and higher temperatures in several states across the country.

Cross-referencing data

To arrive at these results, the scientists combined satellite and observational data with projections from 24 climate models for four emissions scenarios in past (2000–2024) and future (through 2100) contexts. They also used an anomaly-based approach, which takes into account each region’s baseline climate, to define extreme events. This allowed them to examine how tropical forests on different continents adapt to distinct climatic conditions and classify an event as extreme compared to what typically occurs in each location.

Biologist Peter Groenendijk, a professor at IB-UNICAMP and co-author of the study, points out that one of the innovations of the study was comparing drought and heat during a single El Niño event with future climate scenarios. This allowed them to identify a “new normal.” In his view, using multiple models lent robustness to the results.

Groenendijk, an expert in dendroecology (the study of ecological processes using tree growth rings), emphasizes the importance of the findings for understanding the ecosystem.

“El Niño is one of the major modulators of the global climate. By seeking to understand its impacts, we can uncover various implications for forests, such as the mechanisms that lead to tree mortality,” says Groenendijk, who received funding from FAPESP through a Young Investigator Grant. 

Van Passel emphasizes that these findings will contribute to his research, which focuses on the effects of heat and extreme drought on tree growth and mortality, particularly in the Amazon.

Precipitation

The study found that, according to all climate models, approximately 80% of months with extreme heat occurred during the rainy season. This finding is significant because it suggests that photosynthesis may be impacted differently, affecting the carbon balance of trees.

A warmer, drier, and less cloudy period during the rainy season increases photosynthesis because there is more light and water available for transpiration and cooling the leaves. However, if this occurs during the dry season, the effect is negative. Reduced soil moisture prevents cooling, and water transport through the xylem may fail. Consequently, part of the canopy dies. When drought and heat occur together for an extended period, the damage intensifies and weakens the trees. Nevertheless, the mechanisms behind this entire process remain poorly understood.

Unlike temperature projections, precipitation projections showed greater regional variation. According to the findings, the Amazon, Central America, southern Africa, and parts of eastern Asia are expected to become drier, while the rest of the tropics will likely become wetter.

Conversely, the results indicate that, by 2100, the low rainfall levels recorded in some regions in 2023–2024 could become between two times (in the Brazilian Northeast) and nine times (in the Amazon) more likely under the two highest greenhouse gas emission scenarios.

This trend aligns with two other studies by scientists from Brazil’s National Institute for Space Research (INPE) that specifically analyzed the Brazilian Amazon. These studies indicate that the biome is already experiencing conditions previously projected for the coming decades, including longer dry seasons (increasing from four to six months) and changes in rainfall patterns (water deficits exceeding -150 millimeters). Additionally, there was a 9% average increase in burned areas and a 19% increase in forest degradation alerts during the 2023–2024 El Niño period. This demonstrates that the drought-fire-degradation cycle is intensifying and reducing the ecosystem’s ability to recover (read more at https://agencia.fapesp.br/58183). 

Consequences

The loss of resilience in tropical forests has global impacts. It alters key ecosystem services, including climate regulation, the carbon cycle, and biodiversity maintenance. This affects everything from the livelihoods of local populations to the health of the planet and the global economy.

Several areas in Brazil were affected by the 2023–2024 El Niño weather conditions. For example, river levels in the state of Amazonas dropped to record lows, which isolated communities. As lake waters warmed, more than 300 river dolphins were reported dead, an unprecedented number. Additionally, more than one-third (38%) of human settlements in the Amazon region – 2,172 out of 5,673 localities – were highly exposed to drought from September through November 2024. This information comes from a MapBiomas survey. MapBiomas is a collaborative network of non-governmental organizations, universities, and technology startups that maps land cover and land use in Brazil.

The main factors determining the intensity and effects of El Niño are the extent, depth, and magnitude of Pacific Ocean warming, as well as the response of winds and the atmosphere. Over the past century, events classified as “very strong” – commonly referred to as “super El Niño” – were recorded in 1982–1983, 1997–1998, and 2015–2016. The latter event saw the highest ocean temperatures on record.

Though not a “super El Niño,” the 2023–2024 event had intense effects. Scientific literature has shown that these disproportionate impacts may be the result of the association between the phenomenon and global warming during that period. 2024 was the hottest year on record, with a global temperature 1.55°C higher than pre-industrial levels (1850–1900). This was followed by 2023, which saw a temperature increase of +1.45°C, according to the World Meteorological Organization (WMO).

“The 2015–2016 El Niño was strong, but the impact of 2023–2024 was greater. Looking to the future, these extreme events could get even worse. But much will depend on the trajectory of greenhouse gas emissions and how our society uses fossil fuels. The difference between the lowest and highest scenarios is significant. The outlook is grim, but we have options. It’s not so dire that we can’t do anything. We must take action,” Van Passel concludes.

The article “The 2023–2024 El Niño as a preview of future tropical forest climate” can be read at https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2026EF008071.