On the present and future changes in heatwave characteristics over the UK using RegCM4

The summer sun has always been a source of life and leisure, but in recent decades, it has also become a source of increasing danger. As the planet warms due to human activity, the heat is not just getting hotter; it is staying longer, arriving more often, and spreading to places that were once cool. This shift is turning brief periods of high temperature into dangerous events known as heatwaves. These are not merely hot days; they are extended spells where the air remains unnaturally warm, often trapped by high-pressure systems that block the usual cooling winds and clouds. When this heat combines with high humidity, the human body struggles to cool itself, leading to severe health risks. Understanding how these events will change in the coming decades is critical for protecting communities, infrastructure, and public health.

A team of researchers from the University of St Andrews, the University of Hertfordshire, and the National Institute of Technology Rourkela has taken a close look at the future of these heatwaves over the United Kingdom. They wanted to know not just if the country will get hotter, but exactly how the nature of these extreme heat events will change. To do this, they used powerful computer simulations. First, they ran a global climate model, which looks at the entire planet, to see how well it could recreate the heatwaves of the past. They found that while this global model captured the general patterns, it missed some of the finer details, particularly in areas with complex, hilly terrain like the Scottish Highlands and inland Wales, where it tended to predict temperatures that were too high. To fix this, the team used a technique called dynamical downscaling. Imagine taking a broad, low-resolution map and zooming in to see the individual streets and hills; this is what their second model, RegCM4, did. It took the broad global data and refined it to create a much sharper, more detailed picture of the UK's climate, correcting the errors of the larger model and providing a clearer view of what is to come.

The researchers then projected these conditions into two future time periods: the near future, spanning from 2030 to 2060, and the far future, from 2070 to 2100. They compared two different paths humanity might take. One path, called SSP2, represents a world where society makes moderate efforts to reduce emissions and adapt to change. The other, SSP5, represents a world where society continues to rely heavily on fossil fuels with little effort to curb emissions. The simulations revealed that while the two paths look somewhat similar in the near future, they diverge sharply by the end of the century. Under the moderate path, the country will see more heat, but under the high-emission path, the increase is dramatic. By the final decades of this century, the most common summer temperature in the UK is projected to rise from the historical average of 14 degrees Celsius to 17.5 degrees under the moderate scenario, and to a scorching 19 degrees under the high-emission scenario.

The impact of this warming is not felt equally across the country. The simulations show that the southern part of the UK, particularly the area around London and the Home Counties, will face the most intense heat stress. This is likely due to the "urban heat island" effect, where concrete and asphalt absorb and retain heat, making cities significantly warmer than the surrounding countryside. In these southern regions, the heat index—a measure that combines temperature and humidity to show how hot it actually feels to a human—is projected to exceed 29 degrees Celsius in parts of London by the end of the century under the high-emission scenario. This level of heat stress poses a serious threat to human health, especially during prolonged exposure. Even the northern regions, which currently enjoy cooler summers, will see significant changes, though the intensity will remain lower than in the south.

Perhaps the most striking finding concerns the frequency and duration of these heat events. The study suggests that under the high-emission path, the UK will experience a much higher number of heatwaves each summer. In the near future, about 33 percent of the country could see five heatwaves in a single summer under the high-emission scenario, a figure that rises to 36 percent by the end of the century. In contrast, under the moderate scenario, the country is more likely to see three heatwaves per summer, covering about 21 to 22 percent of the land. Furthermore, the nights are getting hotter. The researchers tracked "tropical nights," defined as nights where the temperature does not drop below 20 degrees Celsius. These are becoming more frequent and widespread, meaning the body has less time to recover from the daytime heat. In the southeast, these tropical nights could become a regular occurrence, with some areas experiencing up to 15 such nights a year by the end of the century.

The study also looked at the atmospheric mechanics behind these events, specifically the high-pressure systems that trap hot air. The simulations suggest a shift in how these systems behave under high emissions. While the total number of days with these blocking systems might decrease slightly, the ones that do form will last much longer. On average, an established heat-trapping system could persist for about 21 percent longer than it does today. At the same time, summers with no such blocking events at all are becoming more common, suggesting a new pattern of weather where heat events are less frequent in number but far more persistent and intense when they do occur.

The clear message from these simulations is that the future of heat in the UK is not fixed; it depends heavily on the choices made today. The difference between a future with moderate warming and one with extreme warming is stark, with the gap widening significantly after 2060. The research indicates that near-term reductions in greenhouse gas emissions could substantially limit the intensification of these heatwaves and the associated health risks. Without such action, the country faces a future where extreme heat is not just an occasional nuisance, but a frequent, widespread, and persistent reality that challenges the limits of human safety and infrastructure.