As the planet warms, we are seeing a steady increase in average global temperatures, the impacts of which we are already experiencing. Projections indicate that the UK will continue to experience increasingly warm, wet winters and hot, dry summers as a result of human-induced climate change.

One of the consequences of rising summer temperatures is an increased risk of exposure to heat stress, and inhabitants of urban environments are at particular risk of suffering from this. In this blog post, we explore why cities are particularly vulnerable to extreme heat and learn about some of the work that is being undertaken to better understand the influence that urban environments have on extreme temperatures. We’ll also explore how this research can inform decision making on the ways that UK cities adapt to future climate change impacts.

What is 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 heat stress include dizziness and headaches, and feeling faint, tired or lethargic. Higher than average temperatures bring with them an increased risk of heat stress, particularly for those people who are vulnerable, for example babies, the elderly and people with underlying health conditions.

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

Urban inhabitants are at particular risk from heat stress due to the ’urban heat island’ (UHI) effect, whereby temperatures are warmer in cities compared to surrounding rural areas. Urban heat islands are caused by a range of factors, including more absorption of heat due to the surface properties of cities, additional trapping of heat due to tall buildings, and additional heat released by human activity such as the heating/cooling of buildings.

There also tends to be less green space in cities which helps to moderate temperature through processes such as evapotranspiration. While the UHI is typically largest at night, it has important health consequences because it prevents urban inhabitants from recovering from heat during the day. This is particularly important during extreme heat events. During the European heatwave in the summer of 2003, it was estimated that 52% of heat-related deaths in the West Midlands were attributable to the urban heat island effect^[1].

As global average temperatures continue to rise, the issue of the UHI effect is likely to worsen, presenting significant risks for people living in urban areas, the number of which is also expected to increase.

How can we respond to extreme heat risks?

To help mitigate the risks posed by climate change and the UHI effect, the Met Office provides climate information to health agencies and urban planners so that they can implement effective adaptation responses for urban areas. These could include increasing the number of trees and green spaces in cities, or a change to the materials and methods with which buildings are constructed. Before beginning to implement such measures however, it is important that decision makers adequately understand the nature and severity of the extreme temperatures and the areas in which they will be most severely felt.

Climate research – improving understanding for resilient cities

To better understand the future 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 in the UK climate projections’, led by Met Office Senior Scientist Will Keat as part of the Strategic Priorities Fund-funded UK Climate Resilience (UKCR) programme, work was undertaken to better understand the urban influence on temperature extremes in UK cities for both present day (1981-2000) and the future (2061-2080). The project used the latest UK Climate Projections (UKCP18), which include 12km resolution Regional Climate Model (RCM) simulations, and state-of-the-art convection-permitting model (CPM) simulations at a higher resolution of 2.2km, which can explicitly represent convective storms and provide 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.

Using the present day as a reference point, the urban influence on temperatures in the RCM was too large, leading in particular to an overestimation of the number of warm nights over urban areas compared to observations. Meanwhile, the CPM more accurately represented both day and night temperatures, and correctly captured the number of warm nights.

This better representation of present-day urban climates in the CPM is a result of both increased resolution and improved representation of the urban environment. This gives us confidence in future projections of urban temperatures and indicates that the use of CPM projections is preferable for the provision of evidence to support urban adaptation strategies.

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

Research such as this is invaluable as it provides policy makers with improved insights into the future risk for urban areas and aids adaptation decision-making, helping build 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 in 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 in the UK Climate Projections, Climate Dynamics, Vol. 57, pp 3583–3597