El Niño can flip Amazon forests from carbon sink to source, 2026 at risk

Tropical forests are supposed to be the planet’s quiet carbon collectors. During photosynthesis, they pull CO2 from the atmosphere and lock it into biomass. But research carried out in 2023 finds that during El Niño events, some South American tropical forests may fail to act as a carbon sink, effectively turning them into carbon sources. And because El Niño conditions are already underway and compounding heat and drought pressures are intensifying, this year could set up a brutal 2026 reckoning for the global climate system.

The concern is not theoretical. During the 2015-2016 El Niño, when land temperatures were at least a degree higher on average than usual, some South American tropical forests effectively stopped absorbing carbon. The mechanism is a delicate balance between photosynthesis and respiration, and it can break under heat and water stress. In hotter, drier conditions, plants close leaf pores to avoid losing water. That decision reduces CO2 intake, starving growth and tipping forests toward reduced forest growth and increased tree mortality. The carbon does not just vanish. When trees die, carbon is released back into the atmosphere as dead biomass decomposes, and the effects can echo for decades.

Zoom out to what that means for decision-makers. Tropical forests are repeatedly framed as a natural climate buffer. If they stop buffering, the burden shifts to everything else: industrial emissions reductions, land-use controls, and the credibility of targets that rely on sinks continuing to function. That is why the source points to a potentially compounding threat, not a one-off ecology story. The article notes that there have been twice as many “very strong” El Niños in the last 60 years as there were in the 60 years before that. Then it adds a fresh escalation: NOAA has confirmed that a such an El Niño is currently underway.

There is also a sharper risk map hidden inside the forests. The 2023 research measured over half a million trees across six South American countries over more than 30 years, using tape measures to track growth across over 4,000 species. From that, the researchers calculated precise estimates of carbon stored as aboveground biomass. The key vulnerability finding: susceptibility to El Niño conditions was closely linked to a forest’s baseline climate. While many people picture rainforests as uniformly hot and wet, the source emphasizes that seasonal drought is common in many tropical forests. Regions near the edges of the Amazon, for example, tend to be particularly hot and dry.

The article’s numbers get specific. On average, a 0.5°C increase in temperature caused drier forests at the edge of the Amazon to lose 0.5% of their aboveground carbon. Larger trees were hit harder. Mortality rates increased from 1.8% to 3% per year across South American tropical forests as a whole during the El Niño, and mortality effectively doubled for medium and large trees. The researchers interpret the pattern as pointing strongly to hydraulic failure, where intense atmospheric moisture demand snaps the tension in a tree’s internal water column, rather than slower carbon starvation. That distinction matters because it implies that incremental adaptation to seasonal drought may not be enough to prevent failures during extreme events.

Now layer on the “2026 could be the worst” framing. Scientists have warned that 2026 may again be the warmest year on record. The article says no El Niño began before when oceans were already so warm and air temperatures so high. It also notes that over the past three decades, the edges of the Amazon have experienced some of the highest temperatures and most rapid warming the tropics have ever seen. The source argues that forest resilience is not infinite: structural integrity gets compromised when a major climate anomaly occurs before forests recover from recent, multi-year stress.

For executives, this is where boardroom and balance sheet meet ecology. If carbon sinks weaken, the world’s carbon budget effectively tightens. That affects everything from internal carbon pricing models to investor expectations around emissions and nature-related risk, even when those risks are not labeled “regulatory” in any single spreadsheet. It also sharpens the strategic importance of what the source calls for: preserving tropical forests, because the ability to keep acting as carbon sinks hinges on protecting them and on limiting global temperature rise. In other words, the future of the Amazon is not just an environmental concern. It is a systems risk for global climate targets, supply chains tied to agricultural stability, and any strategy that assumes forests will keep doing the job they have historically done.

Related context matters too: an El Niño is a natural fluctuation, but the intensity and timing of recent events appear to be changing. The article highlights that “very strong” El Niños have become more frequent, and it links that trend to increasing heat and drought stress. When natural processes start breaking down more often, the second-order consequences show up in places that might not seem connected at first. Climate instability can become a flywheel that reinforces ecological loss, and that ecological loss can feed back into atmospheric conditions. That is the logic behind the alarm: compounding factors mean tree and carbon losses on scales “not yet seen.”