Tropical rainforests have long been regarded as one of Earth’s most effective natural climate defenses, absorbing billions of tonnes of carbon dioxide (CO₂) from the atmosphere every year. However, new research suggests this critical role is becoming increasingly fragile. Tropical forests may release more carbon during El Niño, particularly when prolonged heat and drought weaken trees and reduce their ability to absorb CO₂. The Amazon rainforest alone stores an estimated 123 billion tonnes of carbon, making it the world’s largest terrestrial carbon reservoir. Yet researchers found that during the powerful 2015-2016 El Niño, some South American tropical forests temporarily stopped functioning as carbon sinks altogether.

With climate scientists warning that strong El Niño events are becoming more frequent and global temperatures continue to rise, the findings raise important concerns about the future of tropical forests and their role in slowing climate change.

Why Tropical Forests Matter in the Global Carbon Cycle

Tropical forests play an essential role in regulating Earth’s climate by removing carbon dioxide from the atmosphere through photosynthesis and storing it in trunks, branches, leaves, roots, and soils.

According to the Food and Agriculture Organization (FAO), forests cover about 31% of the world’s land area, while tropical forests contain roughly 45% of all forest carbon. Their ability to lock away enormous amounts of carbon helps slow the accumulation of greenhouse gases that drive global warming.

If these forests weaken or begin releasing more carbon than they absorb, the pace of climate change could accelerate significantly.

The study shows that tropical forests may release more carbon during El Niño because hotter temperatures and prolonged drought disrupt the delicate balance between photosynthesis and respiration.

• Plants absorb carbon dioxide through tiny pores called stomata.
• During extremely hot and dry conditions, these pores close to conserve water.
• While this helps trees avoid dehydration, it also limits their ability to absorb CO₂, reducing photosynthesis and slowing growth.
• At the same time, stressed trees become more vulnerable to disease, dehydration, and death.
• As dead trees decompose, the carbon they stored over decades is gradually released back into the atmosphere.

Researchers found that during the 2015-2016 El Niño, when average land temperatures were more than 1°C above normal, several South American forests effectively stopped removing carbon from the atmosphere.

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Massive Study Reveals Forest Vulnerability

To understand how tropical forests respond to climate extremes, scientists conducted one of the largest long-term forest monitoring projects ever undertaken.

• Researchers measured over half a million trees across six South American countries over more than 30 years, using repeated field measurements to calculate changes in forest biomass and carbon storage.
• The analysis revealed that forests located along the southern and eastern edges of the Amazon Basin were particularly vulnerable because these regions already experience seasonal drought.
• On average, researchers found that a 0.5°C increase in temperature caused these drier forests to lose about 0.5% of their above-ground carbon, highlighting how even modest warming can reduce long-term carbon storage.

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Larger Trees Are Dying Faster

One of the study’s most striking findings was that the largest trees suffered the greatest losses.

During El Niño years, annual tree mortality across South American tropical forests increased from approximately 1.8% to 3%. However, mortality among medium-sized and large trees increased much more sharply.

Scientists believe this is mainly due to hydraulic failure rather than carbon starvation. Under severe drought, the tension inside a tree’s water transport system becomes so great that water movement breaks down, preventing moisture from reaching the canopy and causing rapid death.

Large trees are particularly important because they store a disproportionately large share of a forest’s carbon.

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Why Stronger El Niño Events Matter

Climate records show that very strong El Niño events have become twice as common over the past 60 years compared with the previous six decades.

At the same time, the Amazon has experienced some of the fastest warming rates in the tropics. Many forests are now facing repeated heatwaves before fully recovering from previous droughts, increasing the likelihood of long-term ecological damage.

Scientists warn that these overlapping climate stresses could push some tropical forests beyond their natural ability to recover.

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Beyond the Amazon: Global Climate Implications

The findings extend well beyond South America.

• As tropical forests may release more carbon during El Niño more frequently, global carbon budgets could become increasingly difficult to manage.
• Forests that once absorbed billions of tonnes of CO₂ each year could instead amplify global warming by releasing stored carbon.
• This creates a dangerous climate feedback loop: rising temperatures weaken forests, weaker forests store less carbon, and additional atmospheric CO₂ drives even more warming.

The Intergovernmental Panel on Climate Change (IPCC) continues to identify forest conservation as one of the most effective natural climate solutions available today.

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Protecting Tropical Forests Is More Important Than Ever

Although tropical forests have demonstrated remarkable resilience over centuries, scientists caution that resilience has limits.

Protecting intact forests, reducing deforestation, restoring degraded landscapes, and rapidly cutting global greenhouse gas emissions remain essential for maintaining their role as long-term carbon sinks.

Without meaningful climate action, future El Niño events may increasingly transform some of the world’s most important forests from climate allies into sources of additional greenhouse gas emissions.

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Frequently Asked Questions

1. What is El Niño?

El Niño is a natural climate pattern caused by unusually warm sea surface temperatures in the central and eastern Pacific Ocean. It often brings hotter and drier conditions to many tropical regions.

2. Why are tropical forests important for the climate?

Tropical forests absorb carbon dioxide through photosynthesis and store enormous amounts of carbon in vegetation and soils, helping slow global warming.

3. Why do forests release more carbon during El Niño?

High temperatures and drought reduce photosynthesis, increase tree stress and mortality, and cause stored carbon to be released as dead trees decompose.

4. Why are large trees especially vulnerable?

Large trees require more water and are more susceptible to hydraulic failure during prolonged drought, making them more likely to die during severe El Niño events.

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