Science1 publisher2 min readPublished
Modelled cooling from the vapours trees emit still spans a factor of five
Whether a forest's biogenic emissions cool the planet or warm it depends on the air they enter, and the aerosol term is loose enough that a carbon-only planting ledger cannot say which way a stand cuts.
The Scientist · Science desk
What happened
- An Eos feature reports that biogenic volatile organic compounds are roughly 90% of the global mass of nonmethane VOCs entering the atmosphere, citing Khan and colleagues in 2025.
- Isoprene is the largest share globally and comes especially from broad-leaved trees, while monoterpenes and sesquiterpenes are released in smaller amounts, often by conifers and stressed vegetation.
- Once airborne, those vapours oxidize quickly and condense into secondary organic aerosols that can collide and grow into the cloud condensation nuclei around which water droplets form.
- Warming generally amplifies these emissions, and a 2C to 3C rise in mean global temperature is expected to raise total BVOC emissions by 30% to 45% worldwide.
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Why it matters
- constraint No single global adjustment can repair a carbon-only forest ledger, because the aerosol term changes direction with the chemistry of the air a stand sits in. The correction has to be made site by site.
- decision Anyone pricing the full climate effect of a planting project has to choose between booking a term whose modelled value varies five-fold and quoting the stored-carbon line while labelling it partial.
- exposure Urban trees emit into air that people breathe, and the same compounds contribute to air pollution, so a city planting programme's air-quality effect is a separate question from its carbon balance.
- contradiction The same feature that calls the mitigation framing incomplete also finds that cleaner air tilts these emissions toward cooling, so the terms left out of the ledger do not all count against forests.
One route runs through methane. The vapours foliage releases extend methane's atmospheric lifetime [4], and methane traps about 80 times as much heat as carbon dioxide [5]. These are the same compounds behind the smell of a pine stand and the bluish haze that sits over dense tropical canopies [21]. The feature calls the widespread view of forests as mitigators, resting on the carbon dioxide they absorb, incomplete [16].
The aerosol side is where the error bars widen. Modelled estimates of the radiative effect of aerosols formed from these vapours run between -2.0 and -0.4 watts per square meter, which Eos attributes to Bellouin and colleagues in 2020 [9]. The strong end is five times the weak end [18]. That cooling is smaller than current carbon dioxide warming, which Eos puts at roughly 2 to 3 watts per square meter, though large enough to shape regional weather and cloud feedbacks [10]. At the weak end, -0.4 against 3 is about a seventh; at the strong end, -2.0 against 2 is comparable [19]. The feature gives no per-hectare number for a planted stand. Both figures are global mean forcings.
Which way the effect points depends on the air doing the receiving. In cleaner air, the aerosols seed more cloud droplets, the clouds brighten, and more sunlight goes back to space [14]. According to Eos, identical molecules released by living plants can cool or warm Earth depending entirely on the conditions in the atmosphere when and where they enter it [15].
The emission factors move with temperature too. Divide the reported ends of the temperature response, 30% over 3 degrees Celsius and 45% over 2, and it works out to roughly 10% to 22% more emission per degree of mean warming [20]. Isoprene emissions from Arctic sedges are exceptionally temperature sensitive, and the feature names high-latitude, nutrient-limited ecosystems as the most responsive [12]. The response has a ceiling: past the range plants tolerate, or when warming comes with prolonged drought, photosynthesis can falter and emissions can be suppressed [13].
Eos ties the urgency of the accounting to land use, citing Wang and colleagues in 2024, as forest habitats and footprints shift worldwide [17].
What to watch
- Whether the next round of aerosol-cloud estimates narrows the -2.0 to -0.4 watts per square meter band; that span is what blocks a defensible per-hectare figure.
- Direct isoprene measurements from high-latitude sedge systems as the Arctic warms, since that is where the reported temperature response is steepest.
- Whether any forest-carbon or offset methodology adds a BVOC line at all, or keeps quoting stored carbon dioxide alone.