How can we reply to the risks of maximum heat in UK cities?


Because the planet warms, we’re seeing a gradual increase in average global temperatures, the impacts of which we’re already experiencing. Projections indicate that the UK will proceed to experience increasingly warm, wet winters and hot, dry summers consequently of human-induced climate change.

Hazy sunset over London. Image: Shutterstock

One among the implications of rising summer temperatures is an increased risk of exposure to heat stress, and inhabitants of urban environments are at particular risk of affected by this. On this blog post, we explore why cities are particularly vulnerable to extreme heat and study a few of the work that’s being undertaken to higher understand the influence that urban environments have on extreme temperatures. We’ll also explore how this research can inform decision making on the ways in which UK cities adapt to future climate change impacts.

What’s heat stress?

Heat stress can in some cases be fatal. It occurs when the human body cannot keep itself cool and maintain a healthy temperature (37°C). Symptoms of warmth stress include dizziness and headaches, and feeling faint, drained or lethargic. Higher than average temperatures bring with them an increased risk of warmth stress, particularly for those people who find themselves vulnerable, for instance babies, the elderly and other people with underlying health conditions.

Graphic showing impacts on the human body from heat-stress. Image: Met Office, UKRI

Why are urban environments more vulnerable to heat? The Urban heat island (UHI) effect

Urban inhabitants are at particular risk from heat stress on account of the ’urban heat island’ (UHI) effect, whereby temperatures are warmer in cities in comparison with surrounding rural areas. Urban heat islands are attributable to a variety of things, including more absorption of warmth on account of the surface properties of cities, additional trapping of warmth on account of tall buildings, and extra heat released by human activity corresponding to the heating/cooling of buildings.

There also tends to be less green space in cities which helps to moderate temperature through processes corresponding to evapotranspiration. While the UHI is usually largest at night, it has necessary health consequences since it prevents urban inhabitants from recovering from heat throughout the day. This is especially necessary during extreme heat events. Throughout the European heatwave in the summertime of 2003, it was estimated that 52% of heat-related deaths within the West Midlands were attributable to the urban heat island effect[1].

As global average temperatures proceed to rise, the difficulty of the UHI effect is prone to worsen, presenting significant risks for people living in urban areas, the variety of which can also be expected to extend.

How can we reply to extreme heat risks?

To assist mitigate the risks posed by climate change and the UHI effect, the Met Office provides climate information to health agencies and concrete planners in order that they will implement effective adaptation responses for urban areas. These could include increasing the variety of trees and green spaces in cities, or a change to the materials and methods with which buildings are constructed. Before starting to implement such measures nevertheless, it can be crucial that call makers adequately understand the character and severity of the intense temperatures and the areas wherein they will probably be most severely felt.

Graphic showing urban adaptation measures. Image: Met Office, UKRI

Climate research – improving understanding for resilient cities

To raised understand the long run exposure of urban inhabitants to heat stress, researchers use climate information produced by computer models to accurately quantify changes to the frequency and severity of temperature extremes.

In a study[2], ’Climate change over UK cities: the urban influence on extreme temperatures within the UK climate projections’, led by Met Office Senior Scientist Will Keat as a part of the Strategic Priorities Fund-funded UK Climate Resilience (UKCR) programme, work was undertaken to higher understand the urban influence on temperature extremes in UK cities for each present day (1981-2000) and the long run (2061-2080). The project used the newest UK Climate Projections (UKCP18), which include 12km resolution Regional Climate Model (RCM) simulations, and state-of-the-art convection-permitting model (CPM) simulations at the next resolution of two.2km, which might explicitly represent convective storms and supply improved estimates of hourly extremes.

What does this research tell us?

The study revealed significant differences in behaviour between the CPM and RCM when examining the influence of urban environments on temperature extremes.

Graphic showing the several outputs of RCM and CPM. Image: Met Office

Using the current day as a reference point, the urban influence on temperatures within the RCM was too large, leading specifically to an overestimation of the variety of warm nights over urban areas in comparison with observations. Meanwhile, the CPM more accurately represented each day and night temperatures, and appropriately captured the variety of warm nights.

This higher representation of present-day urban climates within the CPM is a results of each increased resolution and improved representation of the urban environment. This offers us confidence in future projections of urban temperatures and indicates that using CPM projections is preferable for the availability of evidence to support urban adaptation strategies.

Will Keat said: “These results highlight the importance of considering the brand new UKCP Local (CPM) projections to higher understand future changes in urban temperatures during hot days and warm nights. Without these projections, future daytime extreme temperatures could be underestimated and night-time temperatures overestimated, which could have significant implications for urban resilience planning and public health.”

Research corresponding to that is invaluable because it provides policy makers with improved insights into the long run risk for urban areas and aids adaptation decision-making, helping construct UK resilience to future changes in weather and climate variability.

To learn more about this project, visit the UK Climate Resilience Programme website.

References:1 – Heaviside C, Vardoulakis S, Cai XM (2016) Attribution of mortality to the urban heat island during heatwaves within the west midlands, UK. Environ Health 15(1):49–59

2 – Keat et al., 2021, Climate Change over UK Cities: The Urban Influence on Extreme Temperatures within the UK Climate Projections, Climate Dynamics, Vol. 57, pp 3583–3597

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