Adapting to heatwaves
Heatwaves are becoming more frequent and intense. Vikki Thompson discusses the evidence for extreme heat events, and the need for geoscientists to work with city planners to adapt our urban environment
The climate is changing (Fig. 1). Heatwaves are becoming more intense, more frequent and longer-lasting. There is ever-growing evidence that they are caused by human activity. Several dangerous heatwaves hit Europe during the summer of 2022, with temperatures in the UK reaching 40°C for the first time since records began. It’s not only Europe that is experiencing extreme heat events. This year, China experienced months of record-breaking heat, while large areas of western North America experienced above-average temperatures in June 2021. For example, a national temperature record of 49.6°C was set in Lytton, Canada, breaking previous records by 4.6°C (World Meteorological Organisation, WMO, 2021). Climate scientists have shown that such events are becoming more likely because of climate change.
Adaptation is key to reducing the impacts of extreme heat on society. Given that heat is amplified in cities, it is essential to consider these extremes within urban planning. Many of the solutions proposed to reduce heat in cities will require input from geoscientists and geological engineers.
Modelling extreme heat trends
In my research, I have investigated heatwaves globally. Together with my team, we asked whether there have been previous heatwaves as large as the 2021 western North America event that have gone unnoticed, perhaps due to their location away from heavily populated areas or because they occurred too far in the past to be remembered or recorded (Thompson et al., 2022). Global datasets of daily temperatures are available from 1950 to the present day and comprise observational records supplemented with climate model data. It could be the case that some extreme events are missing in areas where the observed data coverage is particularly poor. Despite these possible data gaps, within the period from 1950 to the present day, we identified several heatwaves that were more intense than the western North America event in June 2021 – the Southeast Asia heatwave of 1998, produced by El Niño conditions, was the most extreme.
We also looked to the future using climate model simulations, which are based on the laws of physics and fluid dynamics. The models incorporate many components of the Earth system to project future climate, in a similar way to a weather forecast but over decadal timescales. Such climate models are very useful when investigating climatic extremes which, by their very nature, are very rare. The models allow us to simulate Earth’s future climate repeatedly, enabling the most extreme events to be sampled multiple times. In model simulations of future scenarios, we found that globally, heatwaves will increase in intensity and frequency at the same rate as the global temperature increases. By 2100, under a high-CO2 emissions scenario, the UK could see temperatures over 40°C every three to four years, although there may be different trends on a local scale – for example, changes in the jet stream may cause European heatwaves to increase faster than the global trend.
Designing cities to cope
Often referred to as a silent killer, extreme heat is deadly – particularly for the young, elderly, or those with pre-existing health conditions. In 2020, about 2,000 excess deaths occurred due to heatwaves in the UK (UK Health Security Agency, 2022). Cities are particularly vulnerable to the hazards caused by extreme heat; tarmac, concrete and brick absorb heat during the daytime, leading to hotter surfaces and higher ambient temperatures than in the countryside. Canyon-like streets alter the low-level airflow, which can lead to more heating. Added to this, vehicles, factories and even air conditioning units generate extra warmth. This collective urban heat island effect makes cities much more likely to experience extreme heat. Seventy-five per cent of Europe’s population currently live in urban regions, and this is expected to increase over future decades (World Bank, 2022).
New buildings can be designed to cope with heat by minimising sun-facing windows, painting outside surfaces white, or adding green living walls or a rooftop garden to give the added benefits of vegetation and shade (Wang et al., 2019). Many buildings could be retrofitted – simply adding external shutters may reduce heat mortality by ~40%, making it the single most effective intervention to UK homes (Taylor et al., 2018). Old types of air conditioning units produce heat and are energy intensive, but modern energy-efficient cooling systems can limit the heat output by industry and offices.
On a larger scale, effective urban planning can help create cooler cities by, for example, including more trees into our city plans. Trees provide shade and store less heat than concrete and tarmac, reducing the ambient temperature (Hesslerová et al., 2022; Fig. 2). At night time, evapotranspiration provides a further cooling effect. Some tree species are more effective than others at lowering temperatures due to their tolerance to heat and drought conditions; silver lime, London plane, and horse chestnut are great choices for hot sites (Appleton et al., 2015). Initiatives to increase the number of trees in cities are springing up across the globe. For example, in July 2022, the Mayor of London, UK, announced a £3.1 million package to fund tree planting over the next two years (www.london.gov.uk). London already contains approximately 8.4 million trees, which cover ~21% of the city’s land area, but the Trees for London programme aims to increase coverage by 10% of current levels by 2050.
In many cities, natural watercourses have been canalised and often culverted, thus reducing their length and diverting them underground, which can increase flood risk as well as the heat hazard. Water requires more energy to heat than air, so the air around water is usually cooler than similar areas without water, while the natural vegetation found in these areas adds further benefits. So, many cities are now working to build or reinstate water features as part of their heat-mitigation strategies. For example, in Athens, Greece, work is underway to restore the city’s water fountains. Another way to add more water into the environment is via water misters along pavements. This approach has been adopted in Vienna, Austria, through the Cool Streets project. When temperatures exceed 25°C, some of the city’s streets are transformed into cool havens, with vehicles banned, benches added and water sprayers in action.
Reducing the number of vehicles in city centres removes the mechanical waste energy, in the form of heat, which they emit. It has been shown that electric cars heat the environment less than conventionally fuelled cars (Li et al., 2015), but no cars at all would lead to the greatest reduction in urban heating.
While many of these interventions seem simple, they are often difficult to implement and several geotechnical, financial and political challenges exist. For example, it can be difficult to identify suitable locations for additional trees in urban environments, adding vegetation to wall and roof spaces can be expensive, and the costs of restoring and renovating urban waterways are often great.
To implement heat mitigation strategies will require coordination between many stakeholders, including technical input from geoscientists and geological engineers. Despite the many challenges, sustainable adaptation and nature-based solutions can lead to cities that are prepared for climate change – and are more pleasant to live in.
Vikki Thompson, climate scientist in the Cabot Institute for the Environment, University of Bristol, UK. @ClimateVikki
- Appleton, B. et al. (2015) Trees for Hot Sites. Virginia Cooperate Extension, Virginia Tech, Virginia State University Technical Note Publication 430-024; https://www.urbanforestry.frec.vt.edu/STREETS/documents/VTexthot.pdf
- Hesslerová, P. et al. (2022) The impacts of greenery on urban climate and the options for use of thermal data in urban areas. Progress in Planning, 159; https://doi.org/10.1016/j.progress.2021.100545
- Li, C. et al. (2015) Hidden Benefits of Electric Vehicles for Addressing Climate Change. Scientific Reports, 5, 9213; https://doi.org/10.1038/srep09213
- Taylor, J. et al. (2018) Estimating the Influence of Housing Energy Efficiency and Overheating Adaptations on Heat-Related Mortality in the West Midlands, UK. Atmosphere, 9(5), 190; https://doi.org/10.3390/atmos9050190
- Thompson, V. et al. (2022) The 2021 western North America heat wave among the most extreme events ever recorded globally. Science Advances, 8 (18); https://doi.org/10.1126/sciadv.abm6860
- Wang, C. et al. (2019) Environmental cooling provided by urban trees under extreme heat and cold waves in U.S. cities. Remote Sensing of Environment, 227, 28-43; https://doi.org/10.1016/j.rse.2019.03.024
- World Meteorological Organisation (2021) June ends with exceptional heat; https://public.wmo.int/en/media/news/june-ends-exceptional-heat
- United Kingdom Health Security Agency (2022); https://www.gov.uk/government/publications/heat-mortality-monitoring-reports
- World Bank (2022) Urban population (% of total population) – European Union; https://data.worldbank.org/indicator/SP.URB.TOTL.IN.ZS?locations=EU
- World Bank (2022) Urban population (% of total population); https://data.worldbank.org/indicator/SP.URB.GROW