Heat islands: what are the latest methods to combat them?

In recent years, hot temperatures have become increasingly frequent and intense, especially in urban centres, which tend to heat up more than the surrounding countryside, giving rise to the phenomenon known as heat islands.

This effect, linked to urbanisation and cementification, transforms metropolitan areas into ‘heat traps’, where tarmac, cement and other dark surfaces absorb and then slowly release solar heat energy, makin temperatures by several degrees higher than in rural areas.

What is a heat island and why does it form?

A heat island forms when the temperatures recorded in an urban area are significantly higher than those of outlying or rural areas.  The main cause is the artificial covering of the ground’s surface, as natural vegetation is replaced by materials such as cement, tarmac or brick, so that evapotranspiration is reduced, while the surface’s capacity for thermal absorption is increased.

By day, these surfaces accumulate solar heat, while slowly releasing it during the night, thereby preventing the atmosphere from cooling adequately.  This is compounded by the effects of air conditioning systems, heavy traffic and poor building ventilation, which further exacerbate the situation.

The consequences?  Higher energy consumption, greater thermal stress, a decline in air quality and a direct impact on public health, especially on vulnerable people such as the elderly, children and those with chronic illness.

Nature based solutions: the key to combatting heat islands

So-called Nature-Based Solutions currently represent the most efficient and sustainable approach to mitigating the effects of heat islands.  They incorporate measures based on natural processes, such as the use of shade, evapotranspiration and photosynthesis, to improve the urban microclimate and increase the city’s resilience to climate change.  The main strategies which are already proving their efficiency include:

1. urban microforests: miniature woods, planted in areas of just a few dozen square metres, featuring a wide range of vegetation species. Thanks to the methods used by the Japanese botanist Akira Miyawaki, such forests can grow up to ten times faster than a traditional forest, creating a flourishing and self-regenerating environment in just a few years.  As well as reducing the temperature and absorbing CO₂, microforests provide a habitat for insects, birds and small mammals, improving urban biodiversity and creating authentic ‘green lungs’ in the middle of densely built-up areas;
2. ‘pocket’ parks and neighbourhood gardens: considering that not all cities have large spaces suitable to accommodate new parks, ‘pocket parks’ have started to appear, consisting of small green spaces nestled between buildings, carparks and city crossroads. Despite their small size, these mini parks have a significant impact on thermal comfort in the local area.  They offer shaded areas, encourage social interaction and provide ‘cool refuges’ on the hottest days.  With relatively low initial costs and simplified maintenance, they are a solution which can be replicated in many urban contexts;
3. green canopies and urban vegetation features: another innovative solution which is becoming increasingly widespread is the installation of green canopies at tram and bus stops. These structures, which are covered with plants and flowers, create shade and lower surrounding temperatures, thanks to the process of evapotranspiration carried out by the vegetation.  Compared to traditional shelters, green ones can reduce the air temperature by several degrees and look much more attractive at the same time.  The same principle can be applied to urban vegetation installations such as benches with flowerpots or strips of vegetation along the edges of footpaths;
4. green rooves and walls: among the most well-known and popular ecological solutions are green rooves and walls, which allow the surfaces of buildings to be transformed into ecological spaces. Thanks to their substrata and vegetation, green rooves can provide heat insulation for buildings, reducing the need for air conditioning and improving rainwater management, as the water is stored and then gradually released.  Green walls reduce the effects of reflected sunlight, filter particulates in the air and improve overall air quality;
5. permeable, cooling pavements: traditional tarmac is one of the main culprits behind the heat island effect, as it absorbs large quantities of heat from the sun’s rays. Permeable pavements provide a valid alternative; they allow rainwater to enter the ground, reducing pooling as well as surface temperature.  Cooling materials now exist which are highly reflective and able to reflect a greater amount of sunlight while remaining cooler (cool pavements).  These surfaces can be used for carparks, paths and courtyards, contributing to thermal mitigation throughout the urban environment;
6. tramways and ecological corridors: another interesting strategy is the use of green tramways, which transform spaces between the tram tracks into green areas of grass or vegetation. As well as reducing the temperature and heat reflection, these areas improve the urban landscape and encourage the filtration of water into the ground.  When linked together these infrastructures can form ecological corridors, facilitating connections between green areas and enhancing the continuity of the city’s natural network;
7. gathering and reuse of rainwater: one aspect which is often overlooked in the battle against heat islands is sustainable water management. The integration of systems for the gathering and reuse of rainwater enables the irrigation of green spaces and green rooves or walls, without wasting hydrological resources.  This approach, which is in line with the circular economy principle, reduces reliance on urban water networks and ensures the resilience of green areas, even during periods of drought;
8. environmental education and training: in order that green solutions for combatting heat islands are effective and durable, it is not sufficient to just plant trees or green rooves; training, management and awareness is also required. Training of staff in local coucils and urban green technicians is essential to allow for planned interventions coherent with the urban ecology specifications of each district;
9. urban planning and green regulations: both technical and natural solutions must be sustained by adequate urban policies. The introduction of urban green planning, building regulations that incentivise green rooves and walls and minimum objectives for vegetation coverage (like the target of 10% by 2030 introduced by the European Union) are essential tools to develop coherent long-term planning.

Translated by Joanne Beckwith