The Greenhouse Gas That Damages the Carbon Sink
By Daniel Brouse
Ozone (O₃) is one of the most important individual greenhouse gases contributing to global warming, ranking behind carbon dioxide (CO₂) and methane (CH₄) in terms of its direct contribution to human-caused warming.
According to assessments underlying the IPCC Sixth Assessment Report, anthropogenic tropospheric ozone contributes roughly 5–7% of total human-induced radiative forcing, corresponding to approximately 0.2–0.4°C of warming since the pre-industrial era.
But ozone has another, less widely recognized role in the climate system.
Ozone is also a powerful phytotoxin.
That creates an important feedback: the same ozone pollution that contributes directly to warming can damage vegetation, reduce photosynthesis, weaken carbon sequestration, and thereby leave more CO₂ in the atmosphere.
In other words, ozone can act on the climate system from both directions:
More ozone → more warming
and
More ozone → weaker carbon sinks → more CO₂ → more warming
Why Ozone Is Usually Missing From Simple Emissions Charts
Look at a conventional greenhouse-gas inventory and ozone may be nowhere to be found. That does not mean ozone is unimportant. The reason is fundamentally different from the way CO₂ is accounted for.
We emit ozone precursors—not large quantities of ozone itself.
Methane, NOₓ, CO, and VOCs participate in atmospheric chemistry that produces ozone downwind and sometimes thousands of kilometers from the original emissions.
This makes ozone particularly difficult to attribute to individual sources.
Methane is especially important because it participates in the chemistry controlling the global tropospheric ozone burden, while NOₓ, CO, and VOC emissions provide additional ingredients required for ozone production.
Ozone is therefore a secondary pollutant and a greenhouse gas whose climate influence is partly hidden inside precursor emissions.
Ozone Is a Potent Phytotoxin
Ozone’s importance does not end with its direct greenhouse effect.
It is also toxic to plants.
Ozone enters leaves through microscopic openings called stomata. Once inside plant tissue, it can generate oxidative stress and damage cellular processes involved in photosynthesis.
The consequences can include:
- Reduced photosynthetic efficiency
- Slower plant growth
- Reduced crop yields
- Forest productivity losses
- Increased susceptibility to other stresses
- Premature leaf senescence
- Increased tree mortality
- Reduced carbon sequestration
The magnitude varies substantially with plant species, ozone concentration, exposure duration, environmental conditions, and ecosystem type.
Studies have documented productivity losses ranging from modest effects to more than 40% under some conditions, with particularly sensitive species and ecosystems experiencing larger impacts.
Ozone is therefore more than an air-quality problem.
It is an ecological forcing agent.
The Carbon-Sink Feedback
This is where ozone becomes especially important to climate science.
Plants remove atmospheric CO₂ through photosynthesis and store carbon in vegetation and soils.
When ozone damages vegetation, that biological carbon sink can become less effective.
The sequence is straightforward:
Ozone pollution
↓
Plant damage
↓
Reduced photosynthesis
↓
Reduced plant growth
↓
Reduced carbon sequestration
↓
More atmospheric CO₂
↓
More warming
The effect is not necessarily a simple one-for-one increase in atmospheric CO₂. Carbon-cycle responses involve many interacting processes, including plant respiration, decomposition, soil carbon, mortality, regrowth, and changes in ecosystem composition.
Nevertheless, ozone damage represents a mechanism by which air pollution can reduce the capacity of terrestrial ecosystems to remove carbon from the atmosphere.
Tropical Forests: A Global Warning
A 2024 study published in Nature Geoscience, “Reduced productivity and carbon drawdown of tropical forests from ground-level ozone exposure,” found substantial reductions in tropical forest productivity associated with anthropogenic ground-level ozone exposure.
The study estimated that anthropogenic ozone pollution reduced tropical forest net primary productivity by approximately 17% globally since 2000.
That finding is important because tropical forests represent one of Earth’s major biological carbon sinks.
If ozone pollution reduces their productivity, then the climate system loses some of its ability to absorb atmospheric CO₂.
The issue is therefore larger than the direct warming caused by ozone itself.
Ozone can interfere with one of the natural systems that counteracts CO₂ accumulation.
Pennsylvania Forest Observations
Long-term observations of mature forests in Pennsylvania provide another reason to examine the relationship among ozone, climate stress, vegetation health, and carbon sequestration.
Since approximately 2003, observations across multiple monitoring intervals have reported substantial changes in some old-growth forest systems, including:
- Approximately 40% foliage loss
- Significant canopy thinning
- Approximately 33% reductions in canopy height
- Increased premature mortality
The observations are consistent with a broader scientific concern: multiple climate and pollution stressors can interact to reduce forest productivity and resilience.
The larger question is what happens to the terrestrial carbon sink when those stressors increasingly operate simultaneously.
The Ozone–Climate Feedback
Ozone can participate in a reinforcing climate feedback:
Rising temperatures
↓
Changes in ozone chemistry
↓
More tropospheric ozone in some environments
↓
Additional greenhouse warming
↓
More heat and atmospheric stress
At the same time, ozone can act through vegetation:
Higher ozone exposure
↓
Plant damage
↓
Reduced photosynthesis
↓
Weaker biological carbon sinks
↓
More atmospheric CO₂
↓
Additional warming
A pollutant that damages vegetation can also damage part of the climate system’s capacity to remove CO₂.
The Hidden Multiplier
Ozone’s direct greenhouse effect is only part of the story.
The more consequential question may be what happens after ozone enters the biological system.
A greenhouse gas normally enters a climate accounting framework as a source of radiative forcing.
Ozone does something additional.
It can:
- Trap heat directly.
- Damage vegetation.
- Reduce photosynthetic productivity.
- Reduce carbon sequestration.
- Alter the terrestrial carbon cycle.
That makes ozone unusual.
It is simultaneously an atmospheric forcing agent and an ecological stressor.
The climate significance therefore cannot be captured simply by asking how much warming ozone produces directly.
We also have to ask:
How much additional warming results when ozone damages the systems that remove CO₂ from the atmosphere?
Known Ozone
Ozone is not an unknown greenhouse gas.
Its direct warming effect is well established.
Its effects on vegetation are also well established.
What deserves greater attention is the connection between the two.
Climate change, air pollution, atmospheric chemistry, vegetation productivity, and the carbon cycle are not separate systems. They are coupled.
The atmosphere affects the forest.
The forest affects the atmosphere.
And ozone sits directly in the middle of that interaction.
Ozone warms the planet.
Ozone damages plants.
Damaged plants remove less carbon.
And less carbon removal can mean more CO₂ remaining in the atmosphere.
That is the feedback worth watching.
Known ozone.
Largely overlooked multiplier.