The hot temperatures we have been experiencing in the UK so far this summer provide a glimpse of our future. We asked climate, weather, and air pollution scientists at the National Centre for Atmospheric Science about heatwaves, the link to climate change, and what this means for the UK’s air quality.

Heatwaves are periods of abnormally hot weather usually lasting several days. A heatwave is normally measured relative to the usual climate in the area and to normal temperatures for the season. 

According to the Met Office, “a UK heatwave threshold is met when a location records a period of at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold. The threshold varies by UK county.”

Scientists are warning that we have not seen anything close to the most intense heatwaves possible under today’s climate, let alone the ones we expect to see in the coming decades.

They are most common when long-lasting high pressure weather systems develop during the summer. This creates sinking air, which suppresses cloud formation and leads to clear skies and generally settled, dry weather. High pressure systems are typically slow moving and can last for days or weeks. They can occur in the UK due to the location of the jet stream, which is usually to the north of the UK in the summer, allowing high pressure to develop.

Heatwaves cause disruption to travel and infrastructure, and can be a risk to anyone’s health. In particular, heatwaves are hazardous to older people, children, people with pre-existing medical conditions, and those working long hours and in extreme conditions or exertive roles, e.g. factory or construction workers.

People living in cities are more at risk from heatwaves, compared to surrounding rural areas. Tall buildings trap heat, more heat is absorbed by tarmac and concrete and other surfaces, and additional heat is released by the heating and air conditioning of buildings. This urban heat island effect is typically largest at night when heat stored in road surfaces and buildings is released, and has important health impacts for people who are trying to recover from the heat of the day. 

Other regions around the world undergo damaging heatwaves. In India heatwaves are common before the summer monsoon rains – leading to crop damage, power cuts due to increased demand for air conditioning, shortages in water supply and loss of life. Prolonged hot and dry weather exacerbates the conditions underpinning wildfires across southern Europe, leading to widespread property damage and loss of forests. In the tropical hot regions of the world, such as Africa, where 50°C temperatures are being felt, hundreds of millions of people are increasingly exposed to deadly heat. Even in the UK, past heatwaves have been responsible for thousands of excess deaths.  

Human-induced climate change means that summer weather is occurring on top of a warmer background climate. With every degree of warming to the Earth’s climate, the frequency and intensity of heatwaves are projected to get worse.

Whilst it may not seem rapid, Earth is warming at a rate that is unprecedented in the history of human civilisation. Globally, 2024 was the warmest year on record and approximately 1.55°C warmer than the pre-industrial (1850-1900) average. The UK is warming at a rate of approximately 0.25°C per decade, and the last three years have been in the UK’s top five warmest on record. The Intergovernmental Panel on Climate Change has shared compelling evidence that human-induced climate change is affecting weather and climate extremes in every region on Earth.

In the next three decades, record-shattering heatwaves could become two to seven times more frequent across the world than in the last 30 years. Beyond 2050, if global greenhouse gas emissions continue to rise, hazardous hot spells could be three to 21 times more frequent. Heatwaves can be deadly, and we must treat them with enormous seriousness. We can limit future damages by making rapid and substantial cuts to our emissions of greenhouse gases.

While at a national level air pollution continues to reduce, poor air quality is still the greatest global environmental risk to human health. Air pollution levels are closely linked to the weather, and hot spells often arrive hand-in-hand with poor air quality as the sun turns up the heat on a melting pot of airborne chemicals.

With climate change making heat waves more likely, we must consider the different ways that rising temperatures will affect us, including the air we breathe.

Heatwaves are known to trigger ozone pollution events because sunlight reacts with precursor pollutants already in the air – such as nitrogen oxides from vehicle exhausts and volatile organic compounds – to produce ground-layer ozone. 

To make matters worse, slow moving air during hot spells can lead to a build-up of precursor pollutants over the UK, faster evaporation can increase the rate of volatile organic compound emissions, and air arriving to the UK from the south and east can bring ozone and particulate matter pollution across from mainland Europe. 

Unlike many pollutants, ozone is rarely emitted directly by human activities. Instead, it’s formed by a reaction that takes place in the atmosphere. At ground level, ozone can cause shortness of breath, asthma attacks, and increase our risk of respiratory infection and disease.

Dr Grant Forster, research scientist at the National Centre for Atmospheric Science and University of East Anglia, who also operates the Weybourne Atmospheric Observatory on the north Norfolk coast, warns that we need to be aware of the impact of heatwaves in future. “Due to the effects of climate change, we can expect to see heatwaves more often in the future, leading to a higher frequency of dangerous ozone pollution events across the UK,” says Dr Forster. However, it will not be straightforward to predict how ozone levels will respond to other changes – such as emissions of nitrogen oxides and volatile organic compounds.

Particulate matter pollution can be affected by heatwaves, with a few possible explanations. Wildfires can produce small organic material and black carbon particles. The sun’s heat can trigger chemical reactions in the atmosphere leading to new organic particle formation. Larger, coarse pollution particles containing silicon, iron, calcium and aluminium are due to dust from the ground. Dry ground, as well as wind and vehicles, can add dust into the air. 

“Given that we can expect more extreme heat events in the future, it is essential that we understand what the implications are for air quality, as well as the issues caused directly by heat,” says Dr James Allan, air quality research scientist at the National Centre for Atmospheric Science and University of Manchester.

Over the coming decades, climate change will impact air quality in the UK and around the world. “Hotter and drier summers in the UK will lead to a range of effects that could create poor air quality. These could range from increased emissions from wildfires, to calm and persistent weather events that cause stagnated air to accumulate in cities, and increased air conditioning use in buildings that might worsen indoor air quality,” explains Professor Jacqui Hamilton, science director at the National Centre for Atmospheric Science and air quality researcher at the University of York.

At the moment, meteorologists are able to make useful forecasts up to one week in advance. It’s not possible to predict one month ahead whether a particular day will be sunny or rainy, but atmospheric scientists are working on model predictions that could indicate if we are likely to experience a period of very hot or wet or windy weather four weeks in advance.

Weather forecasts a few weeks in advance, which say if heatwave conditions are likely, will be important for preparing and protecting vulnerable people – especially those living in urban areas. Forecasters will need to be able to provide warnings of upcoming high temperatures. Professor Andy Turner, climate scientist at the National Centre for Atmospheric Science and University of Reading sets out: 

“The forecasting of extended periods of hot weather will help health and social care professionals to reduce the harmful impacts on people across the UK, especially those living in cities. Hot weather alerts can help prepare those who are particularly vulnerable, such as older people with multiple health conditions, that they might struggle to cope with the heat and may need to take action to reduce risk to their health.”

Heatwave warnings in the UK are often accompanied with advice about staying out of the sun, drinking plenty of water and understanding the warning signs of heatstroke and heat exhaustion. But in the future we need to get better at finding ways to respond to the extreme heat when it’s predicted, developing infrastructure adaptations, and changing our patterns of work and travel. We will need to adapt our homes and places of work to better cope with a warmer environment – improved insulation and ventilation systems will be key.