From concrete jungles to cooler cities: dealing with the Urban Heat Island effect for a sustainable future

Global temperatures have continued to rise for decades, partly due to human-caused greenhouse gas emissions and subsequent urban heat island (UHI) effects. This current research examines the benefits of urban greenery by studying the impact of green roofs and walls of a building on thermal behavior and heat transfer in a warm and humid climate. This simulation study discusses the importance of greening systems in improving thermal comfort and minimizing the causes of UHI by assessing an integrated green building design. Using the simulation software DesignBuilder, the significance of greening systems, green roofs, and walls in enhancing thermal comfort and reducing the factors that contribute to UHI is investigated. The simulation results are based on the building’s energy usage in hot and humid regions while featuring green roofs and walls. The simulation results indicate a considerable positive impact of greening systems in improving the urban environment in hot and humid tropical climates. Air temperature, radiant temperature, humidity, and solar gain are decreased by urban greening. The total energy consumption and district cooling demand of buildings with green roofs and walls are reduced by 10.5% and 13%, respectively. The greening systems substantially improve air quality and building’s energy efficiency. Thus, the present study‘s findings can benefit urban designers and dwellers in devising strategies for establishing green spaces in congested urban environments by integrating green technologies and systems into built environments.

Urban areas are typically warmer than rural ones. This is mainly due to denser configuration dominated by impermeable surfaces such as buildings and roads, compared to rural areas which are less densely built and mainly dominated by open spaces. Rapid urban expansion in dense cities bares direct impact on surface and air temperature patterns within street canyons; a phenomena which is known as the Urban Heat Island (UHI) effect. Thus, several UK city councils such as Birmingham, Manchester, and London have started to develop strategies aiming at enhancing urban green systems (UGS) through trees, green walls and green roofs. Some of those strategies include considering the green space factor, and increasing green areas within the cities to improve street canyon microclimate and reduce UHI. The Mayor of London has adopted a strategy for London 2050 aspiring to transform it to be the greenest city in the world by increasing the green areas up to 50%. This paper investigates the influence of increasing the UGS percentage which is considered as a key solution to mitigate UHI effect which will, in turn, provide thermally comfortable outdoor environments for pedestrians. The investigation is undertaken by comparing the morphology of precincts and streets in relation to air temperature, mean radiant temperature and surface temperature within Oxford Street canyons in London city centre; being one of the world’s busiest streets. The results from this research demonstrate that different UGS interventions with varying percentage are required depending on particular canyon orientations and geometries. The study found that, in general, more trees would have significant thermal comfort effect followed by living façade, while high albedo pavement (HAP) came last. However, HAP had high influence on improving thermal comfort in North-South orientated streets with minor variance to trees and living facades which, changing their percentage levels was insignificant.

Mitigating urban heat and air pollution considering green and transportation infrastructure

Bio-Facades; An Innovative Design Solution Towards Sustainable Architecture in Hot Arid Zones

Chapter 5: Law and regulation

Chapter 19 - WSUD and Urban Heat Island Effect Mitigation

Evaluation of green infrastructure effects on tropical Sri Lankan urban context as an urban heat island adaptation strategy

Blue-green infrastructure (BGI) is defined as a strategically planned network of natural and semi-natural areas with other environmental features designed and managed to deliver a wide range of ecosystem services, which include microclimate regulation and enhanced human thermal comfort. While green infrastructure is widely known to be capable of mitigating the adverse effects of urban heat island, the effect of blue infrastructure to regulate thermal comfort is still poorly understood. This study investigates several blue-green-infrastructure (BGI) scenarios in the central business district (CBD) of Melbourne, Australia to assess their effects on microclimate and human thermal comfort. Three-dimensional microclimatic modelling software, ENVI-met, was used to simulate the microclimate and human thermal comfort. Physiological equivalent temperature (PET) was used to quantify the level of thermal comfort in selected research areas. Ten different scenarios were simulated, which included those based on green roofs, green walls, trees, ponds and fountains. The simulations suggest that green roofs and green walls in the high-rise building environment have a small temperature reduction in its surrounding area by up to 0.47 °C and 0.27 °C, respectively, and there is no noticeable improvement in the level of thermal perception. The tree-based scenarios decrease temperature by up to 0.93 °C and improve the thermal perception from hot to warm. Scenarios based on water bodies and fountains decrease the temperature by up to 0.51 °C and 1.48 °C, respectively, yet they cannot improve the thermal perception of the area. A deeper water body has a better microclimate improvement as compared to a shallow one. The temperature reduction in the fountain scenario tends to be local and the effect could only be felt within a certain radius from the fountain.

Cities in Australia are experiencing unprecedented levels of urban overheating, which has caused a significant impact on the country’s socioeconomic environment. This article provides a comprehensive review on urban overheating, its impact on health, energy, economy, and the heat mitigation potential of a series of strategies in Australia. Existing studies show that the average urban heat island (UHI) intensity ranges from 1.0 °C to 13.0 °C. The magnitude of urban overheating phenomenon in Australia is determined by a combination of UHI effects and dualistic atmospheric circulation systems (cool sea breeze and hot desert winds). The strong relation between multiple characteristics contribute to dramatic fluctuations and high spatiotemporal variabilities in urban overheating. In addition, urban overheating contributes to serious impacts on human health, energy costs, thermal comfort, labour productivity, and social behaviour. Evidence suggest that cool materials, green roofs, vertical gardens, urban greenery, and water-based technologies can significantly alleviate the UHI effect, cool the ambient air, and create thermally balanced cities. Urban greenery, especially trees, has a high potential for mitigation. Trees and hedges can reduce the average maximum UHI by 1.0 °C. The average maximum mitigation performance values of green roofs and green walls are 0.2 °C and 0.1 °C, respectively. Reflective roofs and pavements can reduce the average maximum UHI by 0.3 °C. In dry areas, water has a high cooling potential. The average maximum cooling potential using only one technology is 0.4 °C. When two or more technologies are used at the same time, the average maximum UHI drop is 1.5 °C. The mitigation strategies identified in this article can help the governments and other stakeholders manage urban heating in the natural and built environment, and save health, energy, and economic costs.

Due to rising urbanisation and climate change, urban areas are increasingly confronted with issues linked to the Urban Heat Island (UHI) effect and stormwater management. These challenges are becoming increasingly apparent. There has been a growing interest in green roofs as a sustainable building strategy due to the fact that they have the potential to alleviate these environmental problems. to what extent green roofs are beneficial in mitigating the urban heat island effect and in managing stormwater in urban environments. The research evaluates the thermal performance of green roofs by using a combination of field measurements, simulation models, and case studies. It also examines the impact that green roofs have on surface and ambient temperatures, as well as their capacity to retain and delay the discharge of rainwater. Green roofs have a substantial impact on lowering temperatures on rooftops, which in turn contributes to a reduction in the overall urban heat island effect. A further benefit of green roofs is that they improve stormwater management by soaking up rainwater, lowering peak flow rates, and delaying the discharge of runoff into urban drainage systems. In addition, the study underlines the fact that performance might vary depending on a variety of parameters, including the species of plant, the depth of the substrate, and the meteorological circumstances.

Climate change impacts increasingly affect human wellbeing worldwide, particularly through consequences of extreme weather events such as heavy precipitation, flooding and heat. Densely populated areas face greater exposure to these hazards than rural areas. The Urban Heat Island (UHI) effect exacerbates the impacts of heat in cities. Nature-based solutions (NbS) such as green and blue infrastructure can be used to adapt to climate change effects and mitigate urban heat stress as well as extreme precipitation impacts. Adaptation measures like urban green spaces (parks, gardens), street trees, vegetation on buildings (facade, roof greenings), water bodies or grass grid paver unsealings have significant cooling effects, enhance water retention and thus can increase thermal comfort and flood resilience of urban dwellers. While Cologne is recognized for its abundance of green spaces like the inner and outer `green belt´, there is still potential to improve and expand existing NbS to more effectively mitigate the challenges of climate change and the UHI effect. The successful implementation of additional NbS to address these challenges is dependent on public acceptance as the establishment and maintenance often rely on public participation. As part of the AKT@HoMe Project aiming to analyse the climate change adaptation potential through citizen participation by assessing the willingness to act and operational empowerment of residents in two socioeconomically contrary districts in the city of Cologne, this research is dedicated to analyze the acceptance of and the activation potential for the implementation of NbS.  To understand residents’ perception of heat stress and the acceptance of NbS in the two districts, we conducted an anonymous survey, achieving over 150 responses. The survey utilized a quantitative, self-administered questionnaire, available in online and paper-based formats. Distribution took place via community events, mailbox inserts, and online platforms between summer 2024 and winter 2024/25. Results show that residents of both districts experienced a high level of heat stress in the past and expect a further increase in future. Preferences for mitigation measures include urban parks and forests, water-permeable pavers, and street trees among other, but also technical, solutions. NbS and hybrid measures are preferred over solely grey measures. Despite a high willingness to act, such as creating and maintaining NbS at home and in the city district, only few of the surveyed residents currently already engage in such activities.  This gap was further examined through workshops with residents from both districts. Throughout three independent sessions citizen were participating in a three hour `future workshop´, working out future scenarios and options to adapt to UHI in their districts. The findings show that residents see the city government as primarily responsible for implementing NbS but also desire a more active role in the process to foster greater engagement, for example through neighbor groups. This highlights the need for a co-creation process between civil society and the public sector, ensuring that residents can actively contribute to the successful implementation and upkeep of NbS. 

BackgroundThis review updates a systematic review published in 2010 (http://www.environmentalevidence.org/completed-reviews/how-effective-is-greening-of-urban-areas-in-reducing-human-exposure-to-ground-level-ozone-concentrations-uv-exposure-and-the-urban-heat-island-effect) which addressed the question: How effective is ‘greening’ of urban areas in reducing human exposure to ground-level ozone concentrations, UV exposure and the ‘urban heat island effect’?MethodsSearches of multiple databases and journals for relevant published articles and grey literature were conducted. Organisational websites were searched for unpublished articles. Eligibility criteria were applied at title, abstract and full text and included studies were critically appraised. Consistency checks of these processes were undertaken. Pre-defined data items were extracted from included studies. Quantitative synthesis was performed through meta-analysis and narrative synthesis was undertaken.Review findings308 studies were included in this review. Studies were spread across all continents and climate zones except polar but were mainly concentrated in Europe and temperate regions. Most studies reported on the impact of urban greening on temperature with fewer studies reporting data on ground-level UV radiation, ozone concentrations (or precursors) or public health indicators. The findings of the original review were confirmed; urban green areas tended to be cooler than urban non-green areas. Air temperature under trees was on average 0.8 °C cooler but treed areas could be warmer at night. Cooling effect showed tree species variation. Tree canopy shading was a significant effect modifier associated with attenuation of solar radiation during the day. Urban forests were on average 1.6 °C cooler than comparator areas. Treed areas and parks and gardens were associated with improved human thermal comfort. Park or garden cooling effect was on average 0.8 °C and trees were a significant influence on this during the day. Park or garden cooling effect extended up to 1.25 kms beyond their boundaries. Grassy areas were cooler than non-green comparators, both during daytime and at night, by on average 0.6 °C. Green roofs and walls showed surface temperature cooling effect (2 and 1.8 °C on average respectively) which was influenced by substrate water content, plant density and cover. Ground-level concentrations of nitrogen oxides were on average lower by 1.0 standard deviation units in green areas, with tree species variation in removal of these pollutants and emission of biogenic volatile organic compounds (precursors of ozone). No clear impact of green areas on ground level ozone concentrations was identified.ConclusionsDesign of urban green areas may need to strike a balance between maximising tree canopy shading for day-time thermal comfort and enabling night-time cooling from open grassy areas. Choice of tree species needs to be guided by evapotranspiration potential, removal of nitrogen oxides and emission of biogenic volatile organic compounds. Choice of plant species and substrate composition for green roofs and walls needs to be tailored to local thermal comfort needs for optimal effect. Future research should, using robust study design, address identified evidence gaps and evaluate optimal design of urban green areas for specific circumstances, such as mitigating day or night-time urban heat island effect, availability of sustainable irrigation or optimal density and distribution of green areas. Future evidence synthesis should focus on optimal design of urban green areas for public health benefit.

In recent decades, urban greening has been conceptualized, and subsequently marketed, as a way of making cities more sustainable. Urban greening has been actualized in large global cities, regional centers, and also in many cities in the Global South, where it has been touted as a potential solution to the urban heat island (UHI) effect and as a way of reducing carbon dioxide (CO2) emissions. This involves planting street trees and installing curbside gardens, bioswales, green walls, green roofs, and the redevelopment of former industrial zones into urban parklands. This paper questions the assumption that this “greening” of the city must necessarily lead to positive environmental impacts. While such infrastructure itself might be constructed with environmental principles in mind, wider questions concerning the production of such landscapes, and the consumption-orientated lifestyles of those who inhabit these urban landscapes, are seldom considered. Moreover, green aesthetics and environmental sustainability are not always as mutually inclusive as the concepts might suggest, as aesthetics are often a dominating influence in the process of planning green urban environments. This review reorients the focus on the way in which the UHI effect and CO2 emissions have been framed by utilizing Foucault's (1980) “regimes of truth,” where environmental issues are contextualized within the “colonised lifeworld” of free-market forces. This review suggests that for sustainability to be achieved in urban contexts, the process of urban greening must move beyond quick techno-fixes through engagement in the co-production of knowledge.

Urban Heat Island (UHI) is a pressing environmental challenge in rapidly urbanizing cities like Bengaluru, where dense built-up areas experience elevated surface temperatures, adversely impacting urban living conditions and public health. The widespread increase in hard surfaces, coupled with the reduction of green spaces, intensifies the Urban Heat Island (UHI) effect by elevating surface temperatures, degrading air quality, and increasing energy demands for cooling. Green roofs, which leverage vegetation to reduce rooftop temperatures and promote urban cooling, offer a sustainable solution to mitigate these effects. This study aims to identify UHI hotspots in Bengaluru and evaluate rooftops suitable for green roof implementation. The methodology employs remote sensing, utilizing Land Surface Temperature (LST) derived from Landsat 8 thermal satellite imagery to pinpoint high-temperature zones. Building footprint data from sources like OpenStreetMap and Google Open Buildings are integrated with thermal data using Geographic Information System (GIS) tools to assess rooftop suitability. Key criteria include roof size (minimum 50 m²), mean temperature, and urban density, ensuring optimal site selection. The analysis identifies 179,587 suitable rooftops spanning 27.25 km², primarily in low- to medium-density areas. These findings provide actionable insights for urban planners and policymakers to strategically deploy green roofs, fostering urban cooling, enhancing air quality, and improving public health. By promoting sustainable urban design, this approach strengthens Bengaluru’s resilience to climate change and supports the creation of liveable, eco-friendly urban landscapes, aligning with global trends toward green infrastructure.

Paved surfaces are quite possibly the most ubiquitous structures created by humans. In the United States alone, pavements and other impervious surfaces cover more than 43,000 square miles—an area nearly the size of Ohio—according to research published in the 15 June 2004 issue of Eos, the newsletter of the American Geophysical Union. Bruce Ferguson, director of the University of Georgia School of Environmental Design and author of the 2005 book Porous Pavements, says that a quarter of a million U.S. acres are either paved or repaved every year. Impervious surfaces can be concrete or asphalt, they can be roofs or parking lots, but they all have at least one thing in common—water runs off of them, not through them. And with that runoff comes a host of problems.