Why do heatwaves happen, and how is climate change impacting them?
By Becky Broyd
What is a heatwave?
According to the UK Met Office, a heatwave is ‘an extended period of hot weather relative to the expected conditions of the area at that time of year, which may be accompanied by high humidity’. In the UK, a heatwave is deemed to have occurred when temperatures exceed or meet the regional threshold temperature for three or more consecutive days in a particular location. The thresholds are based on average temperatures from 1991 to 2020 and vary by county, with the highest records being in the southwest of England.
What causes heatwaves?
High pressure at ground level, most common in the summer months, creates heat waves. When high pressure accumulates over an area it results in the sinking of air through the atmosphere. The air compresses and heats up as it sinks. For every 100 metres the air is pushed downward, the temperature increases by 1°C.
High pressure can also result in a heat dome, exacerbating the heatwave. A heat dome is created when an area of high pressure stays over the same area for several days, trapping very warm air underneath. The dome traps air that would otherwise rise and cool before circulating back to the surface, resulting in the continual build-up of heat and reducing the chance of precipitation.
Longer periods of high pressure are more likely in summer when weather patterns are slower moving. When high-pressure systems take longer to leave an area, they have knock-on effects on other weather systems. Cloud cover can be reduced and winds minimised, making the air stuffier and more humid.
Cities and heatwaves
Cities are usually hotter than rural environments, a phenomenon known as the ‘urban heat island’ effect. According to the US Environmental Protection Agency, cities of more than one million people can be between 1-3 degrees Celsius warmer in the day than the countryside, and as much as 12 degrees warmer in the evening.
Many researchers believe that the biggest contributor to the urban heat island effect is the use of concrete and the replacement of natural surfaces such as grass. Natural surfaces have a larger surface albedo than unnatural materials, which means they reflect more of the sun’s radiation back out to the atmosphere, reducing the heat felt near the surface of the Earth. Vegetation also releases water vapour which contributes to cooling. In contrast, man-made surfaces such as concrete are made of water-resistant and non-reflective materials which increase the absorption of solar radiation.
Other factors also contribute to the effect, including heat given off by appliances, people, vehicles and buildings. Heat tends to be stored during the day by man-made structures and released at night.
Heatwaves and climate change
Heatwaves do occur naturally in summer, but man-made climate change is increasing their length, intensity and frequency.
Climate attribution science is the process of determining the extent to which climate change has impacted a particular extreme weather event. To calculate this, scientists run simulations of today’s climate and compare them to simulations of a climate without human-caused greenhouse gas emissions. Attribution science is revealing the significant impact of global warming on heatwaves.
Climate attribution researchers have calculated that the heatwaves in India and Pakistan in May 2022 were made 30 times more likely due to global warming, while the 2019 European heatwave has been deemed one hundred times more likely in France and The Netherlands. That same year, the record heat scorched the Pacific Northwest is considered to have been one hundred and fifty times more likely due to climate change.
Climate change is also impacting heatwaves in the UK. A scientific study by the Met Office into the Summer 2018 heatwave in the UK showed that the likelihood of the UK experiencing another summer as hot or hotter is 30 times more likely now than before the industrial revolution because of the higher concentration of carbon dioxide in the atmosphere (the chance is now one in ten). Heatwaves of similar intensity are projected to become even more frequent, perhaps occurring as regularly as every other year by the 2050s.
This is all taking place because the excess greenhouse gases (such as carbon dioxide), released since the Industrial Revolution, trap heat in the Earth’s atmosphere. Since the late 1800s, this effect has warmed the Earth’s average temperature by around 1°C. This is enough warming of the average temperature to cause a disproportionately large jump in extreme heat.