Caterpillar Outbreak Rewrites Arctic Cloud Chemistry in Unexpected Ways
A massive outbreak of geometrid moth caterpillars in Sweden's Abisko region in 2023 has revealed a previously unmeasured climate feedback loop. The caterpillars, which defoliated millions of birch trees, triggered a significant shift in the chemical composition of the forest's airborne emissions. A study published in Communications Earth & Environment on October 5, 2026, documented this change through three years of field measurements on 256 trees. The research confirmed that moth defoliation increased the complexity of plant-defense compounds released into the atmosphere, altering the raw ingredients for cloud formation in the Arctic.
The study, led by Jolanta Rieksta and senior author Riikka Rinnan of the University of Copenhagen, found that undamaged birch trees emit a relatively simple mix of monoterpenes and sesquiterpenes. However, when caterpillars feed on the leaves, the trees release a more complex blend of chemicals, including green leaf volatiles and reactive sesquiterpenes. These compounds react with atmospheric oxidants to form secondary organic aerosols, which seed clouds. The shift in chemical composition can change the efficiency of these aerosols as cloud seeds, a factor not currently accounted for in climate models.
The researchers also discovered that the forest floor responded differently to the defoliation. As the birch canopy thinned, more sunlight reached the understory, suppressing total BVOC emissions from ground-cover plants like crowberry by up to 72%. This divergence highlights the complexity of scaling field measurements to landscape predictions. The study's findings suggest that current Earth-system models, such as MEGAN, miss this critical biotic feedback loop, which could influence cloud radiative forcing and Arctic warming patterns.
The broader implications of this research were demonstrated in a 2023 modeling study that showed herbivory-induced emissions tripled monthly terpene levels over Fennoscandian birch forests. This had measurable effects on cloud formation and solar radiation, suggesting that Arctic warming could indirectly accelerate moth outbreaks, further altering atmospheric chemistry and potentially moderating some of the warming through increased cloud cover.