Chemical Conversations Between Forests and the Sky Affect Air Quality, Clouds, and More

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By Dennis Clark, Eos/AGU. 

Excerpt: Do forests cool the planet or warm it? It’s a deceptively simple question with no clear answer. The catch is that the same forest can push climate in opposite directions at once. Forests are widely viewed as climate change mitigators—and planting trees as a partial climate solution—because they absorb carbon dioxide. Yet that framing is incomplete. Trees also release reactive gases called biogenic volatile organic compounds (BVOCs) that can influence cloud formation, contribute to air pollution, and extend the atmospheric lifetime of methane, a greenhouse gas that can trap about 80 times as much heat as carbon dioxide. Although largely invisible to the public, BVOC emissions are significant. They account for roughly 90% of the global mass of nonmethane volatile organic compounds entering the atmosphere [Khan et al., 2025]. In natural habitats, forest trees are the dominant sources of BVOCs. They’re responsible for the familiar scent of pine stands and the bluish haze often seen above dense tropical canopies. Agricultural crops and wetlands are major sources in rural regions. In cities, BVOC emissions come primarily from parks, street trees, and remnant woodlands. Accounting for the competing effects of BVOCs has become increasingly important for climate scientists, especially as forest habitats and footprints shift and land use patterns continue to change worldwide [Wang et al., 2024]. ...[In cities] Reducing nitrogen oxide pollution decreases ground-level ozone formation, yet expanding urban tree cover may increase emissions of reactive BVOCs that offset these decreases depending on species choice and local conditions.... 

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