6 minute read
Housing and climate change
Arup’s Urban Heat Snapshot, London, UK. Full report can be found here: https://www.arup. com/perspectives/ publications/ research/section/ urban-heat-snapshot © Arup & © 2023 Maxar Technologies
As part of a regular series, Claire Thirlwall explores tools, projects and guidance available to aid understanding of the topic covered by the current edition of Landscape.
With the predicted impact of climate change on heat, flooding, water scarcity and subsidence, the way we design and manage residential areas needs to change. High temperatures are not just an issue of comfort: the UK Office of National Statistic estimates that there were 2,985 deaths over five heat episodes in 2022. That same year saw the first ever UK Level 4 Heat-Health Alert (HHA), a level classed as a national emergency, as well as seven Met Office weather stations showing temperatures above 40°C.
These levels of extreme heat are a new challenge for the UK. Most of our housing stock was not designed to accommodate high temperatures, an issue that can be compounded by the design of the surrounding area. Large areas of glazing, steel and concrete together with low levels of vegetation and permeable surfaces affect heat levels by storing heat and preventing areas from cooling at night.
The levels of heat in housing developments can now be analysed using satellite data. Work by Arup using the UHeat modelling tool looked at the hottest day of the July 2022 heatwave. The variation, detailed in the report ‘Urban Heat Snapshot’, is quite striking, with temperatures fluctuating at a streetby-street level. The AI-based system identified a hotspot in the Kilburn and South Hampstead areas with a 4.5°C night-time heat uplift compared to the rural surroundings. This medium-density housing area had more than 60% hard, impermeable ground cover and 38% vegetation. By comparison during the same time period, Regent’s Park, an area with 89% vegetation and 1% water, was 7°C cooler.
Not all communities are affected equally by heat – as the Arup report explains: ‘The effects of heat are broad, but not everyone is impacted equally by heat. Often, it’s the most vulnerable who are most impacted by these urban heat island hot spots. Income inequality can play a role in the ability for people to be able to cool themselves, as well as being less likely to have greenery and shade from trees to keep their streets cool.’ The report suggests that a quarter of a million vulnerable Londoners within the study area live in heatwave hotspots. Heat stress is an example of the inequality of climate impacts – those on lower incomes contribute least to climate change, but are the most impacted.
Analysis of the existing heat impacts can help address this inequality. By combining heat mapping and socioeconomic data, landscape interventions can be targeted to provide the greatest impact and health benefits. Nature-based solutions, such as SuDS, new areas of vegetation and green roofs are low-cost and scalable techniques that can help create cool islands, as well as improve air quality and biodiversity. SuDS schemes help cool the air through evaporation, in addition to reducing the risk of flooding and managing water scarcity. These cool islands act as refuge from the heat, especially important in areas with low levels of tree cover and public open space.
As Dr Ting Sun, joint author of the Arup report and Lecturer in Climate and Meteorological Hazard Risks at UCL explains, ‘Cities need to fully understand – street to street – the risks being faced by their citizens from extreme heat. Because as this snapshot shows – the risk being faced is not the same for everyone. All cities need to understand their urban heat island hot spots where there is a significant uplift in temperatures –sometimes from one neighbourhood to the next. Only then can they work to mitigate them – particularly by using nature-based solutions.’
Use of SuDS is also a nature-based solution for flooding and subsidence. Well-designed SuDS schemes can accommodate high levels of rainfall and replenish ground water levels, helping to mitigate both flooding and water scarcity. While SuDS schemes can tolerate the anticipated climate extremes, the extensive areas of close-mown grass in gardens and open spaces are a less sustainable option. The RHS report, Gardening in a Changing Climate, predicts that lawns will be replaced by woodland or scrub border in areas where water-logging is an issue, and in areas with limited summer rainfall they will be replaced by gravel beds and cornfield annuals, with species that flower before the hottest months of the year. The report also notes the need for a cultural shift away from verdant green lawns and lush borders. This may be one of the biggest challenges – despite the success of schemes such as Plantlife’s ‘No Mow May’, for many people a green lawn with neat stripes is still regarded as important.
It will be a challenge for landscape architects designing housing schemes to find plant and tree species that tolerate the extreme conditions created by climate change. Few trees can tolerate extreme heat, water scarcity and flooding. Recent research has been published on the topic The Essential Tree Selection Guide includes data on how tolerant trees are to flooding and drought, as well as their tolerance to shade, pollution and nutrient deficiency.
All cities need to understand their urban heat island hot spots where there is a significant uplift in temperatures – sometimes from one neighbourhood to the next. Only then can they work to mitigate them –particularly by using nature-based solutions.
The issue of tree selection and subsidence is an additional concern. The amount of tree cover we need to make our urban areas more liveable in a changing climate requires a change in attitude from developers and insurers. The National House Building Council (NHBC) guidance on building near trees states that ‘the combination of shrinkable soils and trees, hedgerows or shrubs represents a hazard to structures that requires special consideration’. In some situations, such as the shrinkable clays common in much of the south of England, the guidance results in a limited list of tree choices, almost none of which are useful as shade trees.
As landscape architects we are well placed to design schemes that mitigate these impacts. Much of the work we do already increases the resilience of communities, but as the impacts of climate change develop, we will need to question existing guidance and find new, innovative techniques to ensure our urban areas remain comfortable places to live and work.
Claire Thirlwall is director of Thirlwall Associates. From Idea to Site: a project guide to creating better landscapes was published in 2020 by RIBA Books.
