Climate change will worsen air quality

Scenarios show that global warming will increase the formation of ozone in densely populated regions, and forest fires can affect particle levels far away from the fires.

Ground-level ozone is an air pollutant that can harm human health, but also lead to reduced harvests and destroy ecosystems. Levels of ground-level ozone are partly affected by levels of precursor pollutants, through emissions of nitrogen dioxides and VOCs (volatile organic compounds), and partly by changes in the weather/climate. This is because ozone is formed through a photochemical reaction and needs sunlight for the reaction to take place. Nitrogen dioxides and VOCs are formed through combustion, for example, and if combustion increases or decreases as a result of fossil fuels and forest fires, the levels of ground-level ozone also change. They are also affected by the changing climate, as hot dry periods often involve a lot of sunlight, which means that the opportunities for photochemical reactions increase.

A changing climate thus leads to an increase in heatwaves, which increase the formation of ground-level ozone. Heatwaves are also often associated with periods of stagnant weather, which means less wind, and in turn means that other air pollutants (for example, small particles PM₂.₅) increase locally as there is not as much atmospheric mixing of air. As a result, heatwaves are usually combined with higher levels of air pollution, which affects the health of vulnerable groups in particular.

Climate change can also lead to warmer air, which often leads to more water vapour and cloud formation, which could lead to less ozone formation. However, it is believed that this effect will occur mainly over the seas and to a small extent over land, so the effect on human health will be minimal.

In a scenario where we do not manage to limit our emissions and the average temperature increases by 3 degrees during the second part of this century, ozone levels would rise by 20% over areas with poor air quality, mainly in Asia (Pakistan, Northern India, and Bangladesh), and by 10% over eastern China. The main contributor would be increased emissions of fossil fuels but a fifth of this increase would be due to climate change and the increased frequency of weather favouring photochemical reactions. Even if it is only a small part of the world in terms of land mass that will be affected, it must be remembered that a quarter of the world’s population lives in these areas, so the health effects will be great. If, on the other hand, we reduce our emissions from burning fossil fuels, we also reduce the gases (nitrogen dioxides, methane, VOCs) that can affect ozone formation.

Having said that, we can still be affected by the climate changes that have already happened and forest fires can also increase these emissions. Even in a scenario where we act against climate change, the climate will be changed by the emissions we have already released and the risk of forest fires will increase, but we can reduce the risk level by adhering to a low-emission scenario. Forest fires can affect particle levels even far away from the fires. Particles are harmful to health and the dark particles (soot) can affect the local climate by settling on snow. The lighter particles (for example sulphur) can reflect the sunlight and lead to a reduced greenhouse effect. Thus particles can both increase and decrease climate change. One can therefore sometimes say that there is a trade-off between the climatic effects and health effects of reducing particles. Reducing the burning of fossil fuels (and methane from animal production) generally has a positive effect on both decreasing climate and health impact. If you reduce emissions, you also get less deposition of nitrogen and sulphur, both of which have negative effects on ecosystems.

Ebba Malmqvist

The effect of climate on air pollution is summarised in the latest bulletin from WMO: https://public.wmo.int/en/our-mandate/focus-areas/environment/air_qualit...