Ozone recovery will amplify near-term global warming

The gas that shields us from ultraviolet radiation is also a heat trapper. That tension sits at the heart of a new analysis that recalculates how much extra warming will come from changes in ozone over the next quarter century.

The headline result is stark, but the mechanics matter even more. The research was led by Professor Bill Collins at the University of Reading.

Ozone recovery and pollution

A multi-model study shows that without strong air pollution controls, ozone will add 0.268 ± 0.084 W/m² of extra heating between 2015 and 2050. This makes ozone the second-largest driver of near-term warming after carbon dioxide.

The team separated two drivers that push ozone upward. One is recovery of the high altitude layer as ozone depleting chemicals keep declining.

The other is the build up of ozone near and above the surface, fed by ozone precursor emissions such as methane and nitrogen oxides from industry, traffic, and agriculture.

Across seven state of the art Earth system and chemistry models, the signal was consistent. Roughly half of the added warming came from stratospheric recovery and half from tropospheric increases.

Those values exceed earlier estimates used by policymakers for the same scenario. The gap arises from the methods used to diagnose forcing and from how clouds, water vapor, and surface reflectivity respond to ozone changes.

Healing ozone also traps heat

Ozone acts as a greenhouse gas by absorbing infrared radiation, particularly near 9.6 micrometers. Higher concentrations in the upper atmosphere trap more outgoing heat, but increases closer to the surface have an even stronger effect because the air there is denser and warmer.

The Montreal Protocol phased out Chlorofluorocarbons (CFCs) and Hydrochlorofluorocarbons (HCFCs), saving the protective layer and cutting dangerous ultraviolet exposure, yet its success has a climate side effect because a thicker ozone layer also holds more heat.

“Countries are doing the right thing by continuing to ban chemicals called CFCs and HCFCs that damage the ozone layer above Earth. However, while this helps repair the protective ozone layer, we have found that this recovery in ozone will warm the planet more than we originally thought,” said Professor Collins.

The study’s message is not to second guess the treaty. It is to count this unavoidable warming correctly while accelerating cuts to the pollutants that create ozone in the lower atmosphere.

Pollution today, warming tomorrow

Near the ground, the troposphere is where we live and where familiar emissions cook up ozone under sunlight. Those reactive gases are short lived, so trimming them yields benefits within years, and it eases a public health burden that hits lungs, hearts, and crops.

Ground level ozone raises risks of coughing, worsened asthma, and reduced lung function, according to the U.S. Environmental Protection Agency (EPA).

“Air pollution from vehicles, factories and power plants also creates ozone near the ground, causing health problems and warming the planet,” said Professor Collins.

Because ozone links air quality and climate, steps that reduce methane, nitrogen oxides, and volatile organic compounds can cool the climate while cleaning the air. The dual payoff is large in cities and rapidly industrializing regions where ozone chemistry is most active.

Metrics change warming numbers

Climate science uses radiative forcing to tally how different agents change Earth’s energy budget. The effective radiative forcing (ERF) for ozone since 1750 is assessed at 0.47 [0.24 to 0.71] W/m², reflecting historical increases in tropospheric ozone as well as stratospheric changes.

ERF is powerful here because it includes fast adjustments in clouds, water vapor, and surface reflectivity that kick in when ozone changes, while leaving out the slower response of surface temperature. That makes ERF more representative of the actual climate push than methods that ignore these rapid adjustments.

By contrast, stratospheric temperature adjusted radiative forcing (SARF) accounts for the quick cooling or warming of the stratosphere but not other rapid adjustments.

The new analysis shows that for ozone changes driven by declining ozone depleting substances, ERF is roughly double SARF, meaning older approaches can undercount the climate push from the healing layer.

This metric choice is not a technical footnote. It influences how we attribute warming to different pollutants, how we design near term mitigation, and how we set expectations for temperature outcomes by mid century.

Ozone recovery needs pollution cuts

According to a NOAA report, stratospheric ozone will continue to recover for decades, meaning the associated warming is effectively locked in regardless of air quality policies on the ground.

That does not weaken the case for the Montreal Protocol’s success or its health protections. It simply means climate planners should pencil a larger ozone line into their near-term forcing budgets and not assume a free climate bonus from the treaty’s chemical ban.

Where we do have leverage is on precursors. Cutting methane curbs a major source of global background ozone and reduces warming on short time scales.

Tightening controls on nitrogen oxides and volatile organic compounds lowers urban and regional ozone peaks, improving health while shaving future climate forcing.

Ozone recovery speeds up warming

Ozone’s protective shield is on the mend, and that is non-negotiable for human and ecosystem health.

At the same time, the climate system will feel extra heat from both the healing layer above and the pollution made ozone below, and counting it correctly helps us choose smarter levers.

Practical steps include updating inventories with ERF-based ozone data, accelerating cuts to methane and ozone precursors, and upholding the Montreal Protocol. These measures protect health gains while aligning energy and air quality policy with the physics that truly drives warming.

The study is published in the journal Atmospheric Chemistry and Physics.

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