by Vasileios Ntouros (NKUA)
Urban Heat Island (UHI) is known to have significant impacts on the environment, giving rise to ambient temperatures in and around rural areas. At the same time, climate change issues are intensifying the UHI effects. To alleviate the UHI consequences and break this vicious circle, the European Union is financially supporting projects and actions which demonstrate and apply relevant mitigation techniques, as shown in this Overview Article.
Cities tend to be hotter than their surrounding suburban and rural areas, and urban climates differ from rural ones. This microclimatic phenomenon of urban warming, caused primarily by the change in land use and the anthropogenic emissions of waste heat, is known as Urban Heat Island (UHI) and has significant impacts on the environment, threatens human life and affects the economy. Due to its significance, numerous research initiatives in Europe and worldwide are addressing this problem. BUILD UP has also documented this issue and provided useful insights about the status of UHI, its impacts and proposed mitigation techniques.
European capital cities are getting bigger and the correlation between building design, urban planning and microclimate should be considered when planning for sustainable and healthy growing cities. The surface geometry and the thermal properties of the urban built environment’s non‐vegetative surfaces greatly impact the magnitude of the urban heat island and thus give rise to ambient temperatures. Subsequently, the elevated temperatures have a serious impact on electricity consumption in the building sector, considerably increasing the total as well as the peak electricity demand. It is indicative that the UHI phenomenon is at its greatest during dry, clear, and still nights. This is because the surfaces that mostly comprise the urban built environment retain and re‐radiate more heat into the night air than vegetation and non‐urbanised land cover. As a result, higher temperatures are recorded. Furthermore, cities are responsible for no less than 40% of global greenhouse gas emissions, and given current demographic trends, this percentage is likely to grow over time. In response to that, climate change issues an extra challenge to cities by intensifying the urban heat islands. Due to this, climate change experts have expressed concerns about the potential risks associated with the increased number of heat waves in combination with the UHI effects. As it has been observed, climate change has the capacity to modify the climatic potential for urban heat islands, with increases of 30% in some locations.
To ameliorate the UHI consequences and break this vicious circle, the European Union has financially supported several programmes and actions. The UHI project, co-financed by the European Regional Development Fund, aims to develop mitigation, risk prevention and management strategies to counteract the urban heat island phenomenon. Its general objective is to call attention to the man-made risks arising from this microclimatic phenomenon, to explore relevant measures for counteracting UHI and provide deeper insights into the interactions of UHI with climate change. Within the project’s framework, a useful software was developed which allows the end user to address UHI’s relationship to the built environment and identify the interactions between cities, climate and human health. More specifically, in respect to buildings, EU funded projects on nearly Zero Energy Buildings (nZEBs) are key steps towards climate change and UHI mitigation; not only because these buildings require low amounts of energy that is met by renewables but also due to their passive design which enables them to reduce UHI effects. The Horizon 2020–funded project ZERO PLUS goes one step further aiming to achieve positive energy settlements using advanced technology including microclimatic improvements. Also, in the H2020 ABRACADABRA project, special focus is given to the densification and protection of urban permeable surfaces through Nature-Based Solutions such as green roofs.
Implementing Nature Based Solutions
Nature-Based Solutions are practices and actions that harness the power and sophistication of nature in order to tackle environmental and socio-economic challenges. Within this context, the Horizon 2020 financially supported URBAN GreenUP project, aims to contribute to the mitigation of climate change risks in cities, increase the resilience to climate change effects, improve air quality and alleviate UHI by implementing Nature-Based Solutions. The LifeMedGreenRoof and the Quick Urban Forestation, two older LIFE projects, also implemented nature-based approaches against UHI’s adverse effects. LifeMedGreenRoof’s target was to reduce the energy requirements of existing buildings by installing green roofs on them. On the other hand, Quick Urban Forestation’s goal was to promote reforestation in the European cities. These two “green” approaches are also storm water management measures and counteract UHI by cooling the urban environment in three different manners:
- Strategically located shading trees directly reduce building temperatures by reducing the amount of solar energy that reaches a building’s surface
- Vegetation cools the air through evapotranspiration
- More vegetation means less pavement and more soil; the increased water absorption of soil allows more evaporation to take place, thus cooling the surrounding air.
The Stavros Niarchos Foundation Cultural Centre in Athens, Greece (Source: Yorgis Yerolympos)
Other mitigation strategies include the use of cool materials that are characterised by high solar reflectance and infrared emittance values, on buildings and other surfaces of the urban environment. The MAIN (MAteriaux Intelligents) project, financed by the MED Programme, aimed to disseminate the use of "cool materials" on pavements and roofs. Cool pavements with albedo values higher than the traditional asphalt’s value and cool roofs that reduce the absorption of solar radiation during the day by reflecting sunlight, are viable technologies for mitigating the UHI; In addition, reflective and permeable paved surfaces enable greater amounts of evaporation, which in turn cools the air. Permeable, high albedo pavements could potentially also reduce and filter storm water run-off, and decrease street-lighting energy use.
In conclusion, up to now, the European Union has funded several research initiatives aiming to more resilient cities and gained deep knowledge on UHI. The application of green infrastructure and cool materials on buildings has positive impacts and implementing these climate resilient strategies will not only help to reduce the energy demand during peak summer conditions but will also have environmental benefits and will positively contribute to human health, e.g., reduced mortality due to less heat strokes, decreased CO2 emissions, increased thermal comfort, reduced air pollution leading to fewer respiratory diseases. Alleviating UHI is a time consuming task, but as it seems, Europe is dedicated to win this combat.