Tropical forests in South America, which store approximately 123 billion tonnes of carbon, face a mounting threat from climate-driven extremes that risk turning these vital ecosystems from carbon sinks into carbon sources. Research indicates that during El Niño events, rising temperatures and water stress can disrupt tree growth and increase mortality, stalling the forest’s ability to sequester atmospheric CO₂.
The Impact of El Niño on Carbon Sequestration
Tropical forests act as a global buffer against climate change by absorbing carbon dioxide through photosynthesis. However, this process is highly sensitive to environmental shifts. Prolonged exposure to the high temperatures and drought conditions typical of an El Niño cycle forces trees to close the pores on their leaves to conserve water. While this prevents dehydration, it simultaneously cuts off the tree’s access to the carbon dioxide required for growth and survival.
During the 2015–2016 El Niño, land temperatures surged at least 1°C above historical averages. Scientists observed that these conditions caused some South American tropical forests to effectively stop absorbing carbon. When trees die under such stress, the carbon stored in their biomass is released back into the atmosphere as they decompose, creating a feedback loop that exacerbates global warming.
Vulnerability at the Amazon’s Edge
Not all regions of the Amazon respond to climate stress in the same way. A study tracking over 500,000 trees across six South American countries found that forests located at the fringes of the Amazon basin are particularly susceptible to climate extremes. These areas are already characterized by seasonal droughts, and the added pressure of an El Niño event can push these ecosystems beyond their threshold for recovery.
Data from this research, conducted over three decades, shows that a 0.5°C increase in temperature correlates with a 0.5% loss of aboveground carbon in these drier, peripheral forests. The structural integrity of these regions is increasingly compromised as the frequency of severe climate anomalies leaves little time for forests to recover between major stress events.
Hydraulic Failure and Tree Mortality
The physical toll of extreme weather is most visible in the death of larger trees. During the 2015–2016 El Niño, mortality rates for medium and large trees—those exceeding 20cm in diameter—doubled. Researchers attribute this trend to "hydraulic failure," a process where the atmospheric demand for moisture becomes so intense that the internal water column of the tree snaps.
This mortality pattern is particularly significant because larger trees with less dense wood appear more vulnerable than smaller, high-density counterparts. As these larger trees die, the forest loses its capacity to store massive amounts of carbon, and the resulting debris contributes to a long-term release of greenhouse gases that can last for decades.
Future Risks and Climate Outlook
The current climate outlook raises alarms for the future of these ecosystems. The U.S. National Oceanic and Atmospheric Administration (NOAA) has confirmed that an El Niño event is currently underway. This occurrence is unique because it follows years of record-breaking heat, with 2026 potentially on track to be the warmest year on record.
Because this El Niño began while ocean and air temperatures were already elevated, scientists warn that the cumulative stress on tropical forests could lead to carbon losses at a scale not previously documented. Protecting these regions requires a dual strategy: direct conservation efforts to maintain forest health and a global commitment to limiting overall temperature increases to ensure these ecosystems remain effective carbon sinks.
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