Urban Nature-Based Solutions – Reducing the Cooling Load

The urban environments in which the majority of the world’s population now reside, and will do so into the future^[1], have increased their vulnerability to rising temperatures by creating urban heat islands^[2] through the extensive use of heat absorbing materials, high spatial building densities, HVAC systems and large numbers of transport vehicles with combustion engines. Integrating nature-based cooling solutions into city planning and landscaping will be a key approach in mitigating this effect, thereby reducing the loads on the cooling systems in buildings, as well as the heat impacts on individual citizens living, working and playing in the urban environment.

Implementing strategies that include street tree planting; the creation of parks, lakes and flowing watercourses; establishing connected corridors of vegetation, and the use of green walls and roofs, can all help reduce urban temperatures and the impacts of extreme heat on citizens and wildlife alike^[3]. A recent study by Arup^[4] analysed the urban heat island effect in 6 global cities using their ‘Uheat’ tool, which combined remote sensing data with a climate model, for the hottest day in each location in 2022. They reported up to an 8˚C change in air temperature within a short distance based on a variation in vegetation and the presence of bodies of water (which act as a heat sink). Significantly, it was noted that “In the majority of cities, the hottest spots had less than 6% vegetation cover – while the coolest spots in most cities had over 70%”.

Green infrastructure small-scale to large

Given the constrained space at ground level in many cities, a fundamental component of urban greening is delivered through green roofs^[5] and green walls^[6]. Applying green infrastructure directly onto a building’s surface contributes both to a reduction in the urban heat island effect and direct cooling of the building itself, reducing the need for active systems and thereby energy consumption (the additional insulation provided by a green roof solution also acts to insulate buildings during colder weather, reducing the need for internal heating in the winter months, leading to further energy savings). In this regard, a study by the Environmental Protection Agency estimated that “green roofs can reduce building energy use by 0.7% compared to conventional roofs”^[7]. The use of green roofs isn’t limited to urban locations and can provide a similar level of cooling benefit for buildings located in rural areas, educational or healthcare campuses and industrial facilities. The challenges associated with green roofs are their initial capital and ongoing servicing costs, as well as maintenance issues including potential roof leakage problems.

Sitting at the nexus of cooling-water-air are ‘urban greening’ solutions that, where space allows, create a network of interconnected green corridors for increased shade, enhanced evapotranspiration (which reduces air temperatures), improved ground water recharge and flooding mitigation (by establishing permeable rather than impermeable surfaces), cleaner air and CO2 sequestration, as well as co-benefits such as the creation of wildlife habitat and improved mental health of city dwellers^[8]. The cooling potential of this green infrastructure, in addition to other co-benefits that they provide, needs to be measured and quantified at a city, national, regional, or continental scale in order to encourage greater adoption of this nature-based solution. The Green Wall in Africa^[9] is one example of a continental scale cross-country green corridor project, spanning 8,000 km across 22 nations, and is anticipated to reverse desertification, reduce impacts of extreme heat and drought, sequester 250 million tonnes of CO2 , enhance food security, and create 10 million ‘green jobs’ by 2030. To back such large-scale and coherent national and international policy making, assessments of the potential of inter-city or cross-country green corridor projects and their climatic impacts need to be conducted.

Global adoption

Planning for the incorporation of green and blue infrastructure into urban environments is being increasingly recognised as crucial to adapting cities and large conurbations to higher seasonal temperatures and more frequent, prolonged, and severe heatwaves. It is for this reason that there are now many examples of cities worldwide that are actively encouraging urban greening through public policy, planning regulations and funding, including:

• in the UK, the London plan, which requires all major developments to include urban greening as a fundamental element of site and building design^[10].
• in southern Brazil, where Curitiba initiated an afforestation plan in 2013 that sought to maintain the 1000s of trees which line public roads and encourage the planting of additional native, disease resistant trees (139,000 planted to-date in accordance with the plan)^[11]
• in Singapore, which through the Landscaping for Urban Spaces and High Rises Programme (LUSH) requires up to 100% of a development site area to be landscaped (typically implemented through green walls)^[12].
• in Italy, where Milan and the wider Lombardi Region has an initiative to plant over 3 million trees as “the most effective, economical and engaging way to slow down global warming, reduce energy consumption, cleaning the air we breathe from fine dust, improving the well-being of citizens”^[13]. Milan is also home to the award-winning Porta Nuova building which is considered a “vertical forest”, designed by the architect Stefano Boeria
• in Colombia, where the city of Medellin received the 2019 Cooling Award from Ashden^[14] for pioneering Green Corridors as a coherent solution for cooler, healthier, and more inclusive living spaces. The project involved establishing a network of trees and plants along the verges of roads, waterways and underpasses and focussed on urban heat islands in the city. It successfully reduced average air temperatures from 31.6°C to 27.1°C, as well as decreased average surface temperatures by up to 10°C in targeted areas^[15]. The subsequent reduction in concentration of particulate air pollutants also saw a nearly 40 per cent reduction in the city’s morbidity rate due to acute respiratory infection. The project trained and employed 2,600 persons from disadvantaged groups as community gardeners, thereby generating additional employment. The greener pathways increased cycling journeys by 35 per cent and walking journeys by 4 per cent. The model is being expanded to other Colombian towns like Barranquilla where it is expected to reduce temperatures by up to 8°C.

However, despite these and many other examples of implementation worldwide, given the projected increases in the demand for cooling globally in the coming decades^[16], an acceleration in the adoption of green and blue infrastructure at all scales is essential in helping us reduce future energy needs and greenhouse gas emissions and survive and thrive in a +50°C World.