Evidence that the Earth is warming at an alarming rate is indisputable, having almost doubled per decade since 1981 (relative to 1880-1981). In many countries, this warming has been accompanied by more frequent and severe heatwaves – prolonged periods of significantly above-average temperatures – especially during summer months.
Heatwaves pose significant threats to human health including discomfort, heatstroke and in extreme cases, death. In the summer of 2003 (one that I am sure many remember for its tropical temperatures), these threats were clear. A European heatwave event killed over 70,000 people across the continent – over 2,000 of these deaths were in England alone. As if these statistics weren’t alarming enough, projections suggest that by 2050, such summers could occur every other year and by 2080, a similar heatwave could kill three times as many people.
Cities face heightened risks
Heat-health risks are not equally distributed. Cities face heightened risks due to the urban heat island (UHI) effect, where urban areas exhibit warmer temperatures than surrounding rural areas. This is primarily due to the concentration of dark, impervious surfaces. In the event of a heatwave, cities are therefore not only threatened by even warmer temperatures, but also by high population densities which creates greater exposure to such extreme heat.
UHIs have been observed and modelled across several of the UK’s largest cities. For example, in Birmingham an UHI intensity (the difference between urban and rural temperatures) of 9°C has been recorded. Some estimates for Manchester and London reach 10°C. However, little research has been conducted into the UK’s smaller cities, including Bristol, despite their rapidly growing populations.
In the UK an ageing population implies that heat vulnerability will increase, especially in light of warming projections. Several other contributors to heat vulnerability are also well-established, including underlying health conditions and income. However, the relative influence of different factors is extremely context specific. What drives heat vulnerability in one city may play an insignificant role in another, making the development of tailored risk mitigation policies particularly difficult without location-specific research.
Climate resilience in Bristol
In 2018, Bristol declared ambitious intentions to be climate resilient by 2030. To achieve this, several specific targets have been put in place, including:
- The adaptation of infrastructure to cope with extreme heat
- The avoidance of heat-related deaths
Yet, the same report that outlines these goals also highlights an insufficient understanding of hotspots and heat risk in Bristol. This poses the question – how will Bristol achieve these targets without knowing where to target resources?
Bristol’s urban heat island
Considering the above, over the summer I worked on my MSc dissertation with two broad aims:
- Quantify Bristol’s urban heat island
- Map heat vulnerability across Bristol wards
Using a cloud-free Landsat image from a heatwave day in June 2018, I produced one of the first high-resolution maps of Bristol’s UHI (see below). The results were alarming, with several hotspots of 7-9°C in the central wards of Lawrence Hill, Easton and Southville. Maximum UHI intensity was almost 12°C, recorded at a warehouse in Avonmouth and Lawrence Weston. Though this magnitude may be amplified by the heatwave event, these findings still suggest Bristol exhibits an UHI similar to that of much larger cities including London, Birmingham and even Paris.
|Image credit: Vicky Norton|
Heat vulnerability in Bristol
Exploratory statistics revealed two principal determinants of an individual’s vulnerability to extreme heat in Bristol:
- Their socioeconomic status
- The combined effects of isolation, minority status and housing type.
These determinants were scored for each ward and compiled to create a heat vulnerability index (HVI). Even more concerning than Bristol’s surprising UHI intensity is that wards exhibiting the greatest heat vulnerability coincide with areas of greatest UHI intensity – Lawrence Hill and Easton (see below).
What’s also interesting about these findings is the composition of heat vulnerability in Bristol. Whilst socioeconomic status is a common determinant in many studies, the influential role of minority status and housing type appears particularly specific to Bristol. Unlike general UK projections, old age was also deemed an insignificant contributor to heat vulnerability in Bristol. Instead, the prevalence of a younger population suggests those under five years of age are of greater concern.
|Image credit: Vicky Norton|
But what do these findings mean for Bristol’s climate resilience endeavours? Firstly, they suggest Bristol’s UHI may be a much greater concern than previously thought, necessitating more immediate, effective mitigation efforts. Secondly, they reiterate the context specific nature of heat vulnerability and the importance of conducting location specific research. Considering UHI intensity and ward-level heat vulnerability, these findings provide a starting point for guiding adaptive and mitigative resource allocation. If Bristol is to achieve climate resilience by 2030, initial action may be best targeted towards areas most at risk – Lawrence Hill and Easton – and tailored to those most vulnerable.