Heatwaves worsen air quality and pollution

• Climate change increases the frequency and intensity of heatwaves
• The European heatwave of 2022 led to more ozone pollution
• Wildfire smoke harms human, ecosystem and crop health
• Parks and trees can ease “urban heat islands”

Climate change is increasing the intensity and frequency of heatwaves. This extreme heat, compounded by wildfires and desert dust, is having a measurable impact on air quality, human health and the environment, according to a new report from the World Meteorological Organization (WMO).

The 2023 WMO Air Quality and Climate Bulletin, the third in an annual series, puts the spotlight on heatwaves. This emphasizes that high temperatures are not solely a hazard; instead, it highlights the often underestimated but equally harmful consequences of pollution resulting from them.

Heatwaves, wildfires, air quality, parks

It shows how heatwaves triggered wildfires in the northwestern United States and heatwaves accompanied by desert dust intrusions across Europe both led to dangerous air quality in 2022. It also includes case studies from Brazil on how parks and tree-covered areas within cities can improve air quality, absorb carbon dioxide and lower temperatures, thus benefiting inhabitants.

“Heatwaves worsen air quality, with knock-on effects on human health, ecosystems, agriculture and indeed our daily lives,” said WMO Secretary-General Prof. Petteri Taalas. “Climate change and air quality cannot be treated separately. They go hand-in-hand and must be tackled together to break this vicious cycle,” he said.

“This Air Quality and Climate Bulletin relates to 2022. What we are witnessing in 2023 is even more extreme. July was the hottest ever month on record, with intense heat in many parts of the northern hemisphere and this continued through August,” he said.

“Wildfires have roared through huge swathes of Canada, caused tragic devastation and death in Hawaii, and also inflicted major damage and casualties in the Mediterranean region. This has caused dangerous air quality levels for many millions of people, and sent plumes of smoke across the Atlantic and into the Arctic,” said Prof. Taalas.

Climate change worsens heatwaves, wildfires

Climate change is increasing the frequency and intensity of heatwaves, and this trend is expected to continue in the future. There is growing scientific consensus that heatwaves will increase the risk and severity of wildfires.

“Heatwaves and wildfires are closely linked. Smoke from wildfires contains a witch’s brew of chemicals that affects not only air quality and health, but also damages plants, ecosystems and crops – and leads to more carbon emissions and so more greenhouse gases in the atmosphere,” says Dr Lorenzo Labrador, a WMO scientific officer in the Global Atmosphere Watch network which compiled the Bulletin.

WMO released the report to coincide with the International Day of Clean Air for blue skies on 7 September. The theme this year is Together for Clean Air, focusing on the need for strong partnerships, increased investment and shared responsibility to overcome air pollution

Climate and Air Quality interaction

Heat-trapping greenhouse gases from human activities cause long-term global threat to climate change. In contrast, air pollution occurs on a timescale of days to weeks and tends to be more localized.

Short-lived reactive gases, such as nitrogen oxides and biogenic volatile organic compounds, produce ozone - a trace gas that is both a common air pollutant and a greenhouse gas - and particulate matter (PM) – a wide range of tiny particles often called aerosols suspended in the atmosphere that harm human health.

The chemical species that affect both air quality and climate are interconnected because they are linked, and substances responsible for climate change and degradation of air quality are often emitted by the same sources. Changes in one inevitably cause changes in the other.

For example, the combustion of fossil fuels releases carbon dioxide (CO2) and nitrogen oxide (NO) into the atmosphere, leading to the formation of ozone and nitrate aerosols. Similarly, agricultural activities emit methane - a greenhouse gas - and ammonia, which then forms ammonium aerosols that negatively impact air quality.

Air quality, in turn, impacts ecosystem health because plants absorb air pollutants such as nitrogen, sulfur, and ozone, which harm the environment and reduce crop yields.

2022 events

The summer of 2022 was the hottest on record in Europe. The long-running heatwave led to increased concentrations of both PM and ground-level ozone.

Hundreds of air quality monitoring sites exceeded the World Health Organization’s ozone air quality guideline level of 100 μg m–3 for an 8-hour exposure. This first occurred in the south-west of Europe, later moving to central Europe and finally reaching the north-east, following the spread of the heatwave across the continent.

During the second half of August 2022, there was an unusually high intrusion of desert dust over the Mediterranean and Europe. The coincidence of high temperature and high aerosol amounts, and therefore PM content, affected human health and well-being.

Whilst high-altitude (stratospheric) ozone protects us from the harmful ultra-violet rays of the sun, ozone close to the Earth’s surface is harmful to human health. It also can reduce both the quantity and quality of yield of staple food crops.

Ozone harms global staple crop yields

Globally, ozone-induced crop losses average 4.4%–12.4% for staple food crops, with wheat and soybean losses as high as 15%–30% in key agricultural areas of India and China.

Heatwaves and dry conditions are conducive to wildfires which, once started, grow rapidly as they encounter dry, easily combustible vegetation. Such situations can lead to an increase in aerosol emissions.

Thus, a lengthy heatwave in September 2022 correlated with anomalously high levels of biomass burning across the north-western United States, leading to unhealthy air quality across much of the region, as reported by the United States Environmental Protection Agency (EPA).

Atmospheric deposition of nitrogen-containing compounds downwind of fires also impacts ecosystems - a phenomenon that will increase with warming climate and heatwaves. In California and the north-west United States, fires were found to contribute large proportions of N deposition in several natural ecosystems, often exceeding critical load thresholds and negatively impacting biodiversity, clean drinking water, and even air quality via emissions that lead to further air pollution.

Urban heat islands

Urban areas often consist of buildings and infrastructure reaching heights of 100 m or more, which influence wind and temperature patterns compared to surrounding rural areas. This effect is usually referred to as the urban heat island (UHI). The magnitude of differences varies with many factors but may reach up to 9 °C at night.

This effect combines with climate change and has many impacts including additional heat stress at night, which would otherwise be a time for recovery from day-time temperatures.

This is important because large portions of the population live and/or work in cities, and exposure to high temperatures can increase morbidity and mortality, especially during heatwaves and at night.

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