WMO Bulletin: heatwaves worsen air quality and pollution

06 September 2023

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).

Key messages
  • Climate change increases the frequency and intensity of heatwaves
  • Air quality and climate change must be tackled together
  • 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”

The 2023 WMO Air Quality and Climate Bulletin, the third in an annual series, puts the spotlight on heatwaves. This is to draw attention to the fact that it is not just high temperatures which are a hazard, but also the impacts of resulting pollution which are often overlooked but are just as pernicious.

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 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.

A map of the world with a red line on it.
Change in the number of days per year with daily maximum surface temperatures above 35 °C, relative to an 1850–1900 baseline, as predicted by 27 numerical models, in a world that will have experienced 1.5 °C warming
Figure produced using data from the IPCC Working Group I Interactive Atlas

Climate and Air Quality interaction

Climate change caused by heat-trapping greenhouse gases from human activities is a long-term global threat. In contrast, air pollution happens on a timescale of days to weeks and tends to be more localized.

Pollutants include short-lived reactive gases such as nitrogen oxides and biogenic volatile organic compounds which lead to the production of 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, which harm human health.

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

For example, the combustion of fossil fuels emits carbon dioxide (CO2) and nitrogen oxide (NO) into the atmosphere, which can lead to the formation of ozone and nitrate aerosols. Similarly, some agricultural activities are major sources of the greenhouse gas methane and also emit ammonia, which then forms ammonium aerosols which negatively impact air quality.

Air quality in turn affects ecosystem health because air pollutants such as nitrogen, sulfur and ozone are absorbed by plants, harming the environment and reducing crop yields.

A map showing the world's temperature and precipitation.
CAMS EAC4 reanalysis of particulate matter d < 2.5 um (PM2.5): 2022 Anomaly (2003-2022)
Source: European Centre for Medium-Range Weather Forecasts (ECMWF)/CAMS
A map of the world
CAMS EAC4 reanalysis of particulate matter d < 2.5 um (PM2.5): 2003-2022 Mean
Source: European Centre for Medium-Range Weather Forecasts (ECMWF)/CAMS

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.

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.

A map of europe's climate in june and october 2020.
Ozone (O3) exceedances (above WHO guidelines) and temperature anomalies across urban sites in Europe between 10 and 21 July 2022.
Source: University of York and National Centre for Atmospheric Science (Department of Chemistry), United Kingdom of Great Britain and Northern Ireland

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.

Observations of the type suggested by the report were recently collected in São Paulo, Brazil: both temperature and CO2 measurements from two parks indicated that the urban heat island effect is reduced, and CO2 emissions are partly mitigated by incorporating more green spaces within cities, pointing to the benefits of nature-based solutions for climate change.

Notes to Editors

The WMO Air Quality and Climate Bulletin reports annually on the state of air quality and its connections to climate change, reflecting on the geographical distribution of and changes in the levels of traditional pollutants.

The bulletin includes contributions from leading international experts in WMO’s Global Atmosphere Watch network.

Editorial Board in alphabetical order

Chair – Dr Julie M. Nicely (Member, WMO GAW Environmental Pollution and Atmospheric Chemistry Scientific Steering Committee), University of Maryland, USA, and NASA, USA

Members – Greg Carmichael (Chair, WMO GAW Environmental Pollution and Atmospheric Chemistry Scientific Steering Committee), University of Iowa, USA;  

Peter Colarco (member, WMO GAW Scientific Advisory Group on Aerosol), Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, USA;

Owen R. Cooper (Chair, WMO GAW Scientific Advisory Group on Reactive Gases), Cooperative Institute for Research in Environmental Sciences (CIRES,) University of Colorado Boulder and NOAA Chemical Sciences Laboratory (CSL), USA;

Frank Dentener (Co-Chair, WMO GAW Scientific Advisory Group on Applications, member WMO GAW Measurement-Model Fusion for Global Total Atmospheric Deposition Initiative), European Commission Joint Research Centre (JRC) – Institute for Environment and Sustainability, Ispra, Italy;

Lucia Mona (member, WMO GAW Scientific Advisory Group on Aerosol), Institute of Methodologies for Environmental Analysis of the National Research Council of Italy (CNR), Potenza, Italy;

Vincent-Henri Peuch (Chair, WMO GAW Scientific Advisory Group on Applications), Director of Copernicus Atmosphere Monitoring Service (CAMS), European Centre for MediumRange Weather Forecasts (ECMWF) (Reading, UK; Bonn, Germany);  

Ranjeet S. Sokhi (Chair, WMO GAW Scientific Advisory Group on Urban Research Meteorology and Environment), University of Hertfordshire, United Kingdom;  

John Walker (Co-Chair, WMO GAW Scientific Advisory Group on Total Atmospheric Deposition), United States Environmental Protection Agency, Durham, USA. 

The World Meteorological Organization (WMO) is a specialized agency of the United Nations responsible for promoting international cooperation in atmospheric science and meteorology.

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