Jane Burston, CEO of the Clean Air Fund, discusses the launch of their new roadmap on tropospheric ozone – published during London Climate Action Week
We’ve all heard of the ozone layer. Back in the 80’s, the world woke up to the fact that there was a growing hole in this critical shield of gases that protects people, plants and animals from UV radiation. Countries came together to respond to this threat in the stratosphere: the 1987 Montreal Protocol cut emissions of ozone-depleting chemicals – including aerosols – and the hole is now closing up.
Nearly four decades on we have another, lesser-known ozone problem – and it’s getting worse. When ozone exists in the earth’s lower atmosphere or troposphere, it is harmful to human and planetary health, unlike the ‘good’ ozone in the stratosphere.
Tropospheric ozone is a greenhouse gas, air pollutant and super pollutant, but isn’t emitted directly, like carbon dioxide or methane. Instead, it forms when sunlight interacts with pollutants created from transport, energy, waste and agriculture – including methane, nitrogen oxides, carbon monoxide, and non-methane volatile organic compounds. Heat, humidity and geography can also affect ozone formation. These “precursor” emissions are widespread across sectors and borders. And while ozone may be hard to address, its impact is far-reaching.
Tropospheric ozone is a triple threat. It is a threat to climate: responsible for around 0.23°C of global warming to date. It is a threat to our health: it damages the lungs and shortens lives, causing an estimated 500,000 premature deaths annually. It is a threat to food security and economic development: it harms crops and ecosystems, causing losses of up to 26% in global yields for staple crops, like wheat and soybeans. It has an estimated annual economic cost of over half a trillion dollars.
Yet despite these impacts, tropospheric ozone remains a blind spot in global climate and development agendas. That’s partly because it’s complex: the chemistry of tropospheric ozone formation is non-linear, and reducing a single precursor in isolation might create even more ozone under certain conditions. But it’s also because tropospheric ozone is rarely addressed directly in climate strategies. While methane—as a key ozone precursor—is increasingly targeted in national climate plans, tropospheric ozone reduction typically emerges only as an indirect byproduct of these efforts, with responsibility for direct action often left to city-level air quality strategies.
This fragmented approach doesn’t work. Warmer temperature can elevate ozone, so as the climate warms, tropospheric ozone levels are projected to rise under all business-as-usual scenarios. If we don’t act now, this super pollutant will quietly undermine measures to mitigate climate change, protect food security, and achieve the Sustainable Development Goals.
We have the tools to solve this problem
A new roadmap from Clean Air Fund outlines how to tackle tropospheric ozone through three core principles:
- Multi-pollutant action: Rather than focusing on individual pollutants, we need coordinated strategies that consider how emissions interact with one another in the real world. For example, reducing methane – a major driver of rising temperatures – can help lower ozone levels, but could unintentionally cause increased levels of ozone. A multi-pollutant approach ensures that actions on air quality, super pollutants and greenhouse gases are mutually reinforcing.
- Multi-level governance: From city air quality plans to global climate treaties, ozone action must be integrated across policy levels. That means aligning local monitoring with national emissions targets and global frameworks.
- Cross-sectoral solutions: Ozone precursors come from many sectors, including energy, agriculture, transport, waste and industry. Addressing ozone requires solutions across diverse sectors, while aligning existing policies and integrating ozone where it’s been overlooked.
In the next five years, we need to halve emissions to keep global warming below the threshold of 1.5°C. That means acting fast on carbon dioxide, but also on shorter-lived climate pollutants like tropospheric ozone and black carbon. If taken to its full potential, reductions of tropospheric ozone precursors, alongside reductions of methane, could result in 0.30°C of avoided warming by 2050 – a significant gain in a narrow timeframe.
What’s needed now is not a brand-new campaign or institution, but better integration and coordination. Action on tropospheric ozone should be embedded into climate plans, air quality strategies, and agricultural and ecosystem resilience efforts. We need to raise its visibility not as a technical footnote, but as a major development risk we can address within existing processes.
Countries submitting updated national climate action plans – known as Nationally Determined Contributions – this year need to include tropospheric ozone and other super pollutant targets alongside their carbon commitments. The Climate and Clean Air Coalition provides detailed guidance on how to do this.
As tropospheric ozone levels continue to rise in many regions – even where traditional air pollution is falling – ignoring the ‘other ozone’ problem is no longer an option. We need to recognise that we can solve this problem through deliberate and concerted cooperation to avoid unintended consequences and reap benefits for people and planet.
Leave a Reply