Eighty meters above the Brazilian rainforest, a tower near Manaus is capturing the Amazon's whispers—and they're frantic. During the savage 2023–2024 El Niño drought, the worst the basin has ever recorded, the forest changed the very chemicals it breathes into the air, a desperate metabolic shift that scientists only recently detected and still barely understand.
Researchers at the Max Planck Institute for Chemistry in Germany installed sensors 23 meters up on the Amazon Tall Tower Observatory (ATTO), positioned directly above the forest canopy 150 kilometers northeast of Manaus. Every one and a half to three hours, they pulled air samples and later analyzed them in laboratories in Mainz using gas chromatography and mass spectrometry. What they found was a forest under siege, mounting a chemical defense never before documented at such scale.
The Amazon did not respond to extreme heat and drought the way scientists expected. Levels of isoprene and monoterpenoids—the volatile compounds vegetation typically releases—stayed relatively steady. Instead, sesquiterpenes surged. These are reactive airborne molecules that trees produce as stress signals and protective substances, and their emissions jumped by 122 percent during the peak of the El Niño event. One familiar example is caryophyllene, the peppery compound found in cloves and black pepper—imagine the entire rainforest flooding the air with such stress-defense chemicals, amplified more than twofold.
The surprises kept coming. After the drought subsided and the wet season returned, the research team detected something even stranger: sesquiterpene alcohols including beta-eudesmol, alpha-eudesmol, and gamma-eudesmol. These are less volatile, more complex compounds that the forest had not visibly emitted before. The timing matters: this shift persisted long after the immediate crisis had passed, suggesting not a temporary panic but a deeper metabolic recalibration. "Our results show that severe drought shifts the atmosphere toward lower-volatility and more reactive compounds," explained Joseph Byron, the study's lead author. "This reflects underlying metabolic changes as the rainforest attempts to mitigate damage from abiotic stress."
The findings matter because the future is likely to bring worse versions of this scenario. El Niño events strike every two to seven years under current climate patterns, and the Amazon's resilience depends on recovering between them. But climate models project that El Niño events will grow more frequent and intense as the century unfolds. If so, these stress-defense emissions could become permanent—a constant feature of the region's atmospheric chemistry rather than an occasional emergency response. The forest's chemical language would shift permanently, with consequences for atmospheric composition and the ecosystem's ability to withstand further pressure.
The research, published in Communications Earth & Environment, expands on the Max Planck team's earlier work identifying molecular stress indicators in the rainforest. This new study shows the forest is not passive but actively adaptive, deploying sophisticated chemical weapons against oxidative damage. What remains unknown is whether this defense mechanism can sustain the Amazon if stressors arrive faster and harder than the forest's metabolic capacity to recover. Project leader Jonathan Williams put it plainly: between El Niño events, the rainforest can revert to normal. But if those events become relentless, adaptation may not be enough.