The Case for Action on Tropospheric Ozone

Ozone can mainly be found in two layers of the Earth’s atmosphere. Tropospheric ozone, or ‘bad’ ozone, refers to ozone that is present in the atmosphere up to about 15 kilometers above ground-level. In this layer, ozone acts as a greenhouse gas and super pollutant, contributing significantly to global warming by absorbing infrared radiation and thereby heating the surrounding air. At the bottom of this layer – the Earth’s surface – tropospheric ozone is often referred to as ‘ground-level ozone’. Here, it is responsible for almost half a million premature deaths per year and substantial reductions in agricultural crop yields.

Ozone plays a very different role in the stratosphere – the layer of the atmosphere from approximately 15 to 40 kilometers above the Earth’s surface. Here, ozone protects us by absorbing harmful ultraviolet radiation through the ozone layer. This is the ‘good’ ozone layer that is protected by the internationally negotiated Montreal Protocol.

Stratospheric (‘good’) and tropospheric (‘bad’) ozone are interlinked. The good news is that we can simultaneously protect the beneficial ozone layer while acting to reduce harmful tropospheric ozone levels. There is also growing evidence that increasing levels of tropospheric ozone and other greenhouse gases may erode stratospheric ozone, thereby increasing the importance and urgency of action on tropospheric ozone and climate change more broadly.

Reducing tropospheric ozone levels would make a major contribution to avoiding further global temperature rises in the coming decades. Tropospheric ozone levels have increased significantly over the last century. Since 1995, they have gone up between 2-12% per decade, depending on the region, driven largely by rapid industrialisation and urbanisation. They are estimated to have contributed to 0.23°C of global warming from 1750 to the present. Cutting super pollutants, including tropospheric ozone, can mitigate warming nearly four times faster than decarbonisation alone.

Significant actors in climate action are waking up to this urgent issue. Earlier this year, the US and Chinese governments identified the need for technical cooperation and capacity building to develop abatement solutions for tackling tropospheric ozone as part of efforts to combat climate change.

Tackling tropospheric ozone would bring major health benefits by rapidly improving air quality. Tropospheric ozone poses serious health risks. It contributes to respiratory issues, reduces lung function, and exacerbates chronic conditions like asthma, bronchitis and emphysema. It has also been linked to complications in type 2 diabetes and cardiovascular disease.

Nine out of ten people are exposed to tropospheric ozone levels that exceed WHO guideline levels, contributing to almost half a million premature deaths each year. Meeting the WHO’s guidelines would save hundreds of thousands of lives and provide economic benefits of up to $500 billion per year, through avoided healthcare costs and a healthier, more productive workforce.

Tropospheric ozone severely damages many staple crops, leading to reduced grain size, fewer seeds, slower growth rates and less resilience to environmental stresses. It also accelerates the leaf aging process and causes plants visible injury, like chlorosis or necrosis.

The result is significant losses in global crop yields. For staples such as wheat, soyabean and maize, total economic losses due to ozone damage could amount to $35 billion annually by 2030, if calculated using 2000 prices. Tackling tropospheric ozone would improve food security globally by increasing yields for the crops that feed billions of people.