FUTURE CITIES: How Many Green Roofs Does It Take To Make A Difference To Climate?

Of course, there is no single answer to this question. Many factors need to come into play for our environments to begin to heal and improve. However, while we dream of futuristic, utopian green cities, fully built from sustainable materials, rich with plantlife and inviting wildlife, as well as people, we must deal with the reality of our time, and accept that we will need to start by retrofitting the exsisting built infrastructure. Re-use is always greener than demolishing and building something completely new! However, to know how much intervention it takes to make an actual difference to cities in our climactic situation, we need to consult scientific estimations

Don Davis (American, born 1952). Stanford Torus interior view. 1975. Commissioned by NASA; Space Settlements: A Design Study (Washington, DC: NASA Scientific and Technical Information Office, 1977). Image Courtesy of The Museum of Modern Art. Source: https://www.archdaily.com/1007224/momas-emerging-ecologies-exhibition-explores-the-ecolution-of-environmental-architecture

Researchers are now finding that cities can be designed to be climate-conscious and energy-efficient, contributing to urban sustainability while also addressing global climate issues at the local level (1). Green roofs are a design tool with multiple environmental and social benefits, and due to their hamburger-like structure with multiple layers, they also provide effective thermal insulation to building roofs, cutting building heating/cooling load and electrical energy use (1). Installing green infrastructure such as green roofs may help restore urban ecosystem services, such as retaining water, delaying peak discharges to stormwater sewers, and cooling ambient air temperatures, mitigate damage to natural systems caused by urbanization and also reduce damages to infrastructure and property caused by flooding and combined sewer overflows (2).

One of the biggest problems of cities today is related to heat waves. The urban heat island is a phenomenon describing particularly high temperatures in cities compared to nearby suburban and rural areas (3). This happens mainly because urbanization has reduced the land area covered by natural vegetation, replacing it with hard and impervious surfaces such as buildings, parking lots, and streets, which has also increased stormwater flows and resulted in the contamination of natural water bodies (2). Green roofs can lessen the impact of urban heat islands by providing shade, removing heat from the air, and reducing the temperature of the roof surface and surrounding environment (4). There is a clear correlation between plant cover and land surface temperature, meaning an urban increase in green areas could mitigate the heat island effect. The problem is that modern cities are often very condensed and don’t offer much free space to introduce vegetation – the exception is found in roof surfaces, which are generally available and represent between 20 and 25% of the total area of all city surfaces, which is why introducing urban vegetation to them is an interesting option (3).

Research in Singapore showed a temperature decrease of up to 18 degrees Celsius once the green roof was installed (3). In Chicago, the summer temperatures of the green roof varied from 33 to 48 degrees Celsius, while the conventional roof reached up to 76 degrees (3). Several studies in temperate climactic conditions showed that during the warm and mildly warm seasons, a reduction of heat gain by 92–97% was achieved by installing a green roof, as well as an increase the heat dissipation of 49–20%, in comparison to the traditional roof system (4). Additionally, the air temperature near the surface of the green roof is lower (3). Simulation studies show green roofs can decrease the environmental temperature from 0.3 to 3 degrees Celsius at a city-wide scale, and thus importantly decrease the heat island effect (3).

The impact of GRs protection on buildings from direct solar heat during summer. Source: Hussien et al. (2023)

 

 

So, how many green roofs do we need to combat climate change?

In Seville, Spain, models of climate change forecast a temperature rise from 1.5 to 6 degrees Celsius in summer (3). Researchers Herrera-Gomez et al. (2017) wanted to know precisely how much green roof surface would be needed to make a difference and protect the city against rising temperatures. They found that in Seville, a green roof surface of 740 ha should be implemented for the worst-case climate change scenario, which means covering 40.6% of the existing buildings. In the most optimistic scenario, the forecasted green roof surface required is only 207 ha (11.3% of the roofs) (3).

A similar study was done by researchers Penga and Jim (2015) in Hong Kong, one of the world’s most densely populated cities that faces extremely hot summers, which results in high energy use for cooling and higher emissions of GHGs and other air pollutants, such as SO2, due to fuel combustion at local power stations. In this case, 60% greenery coverage on all the roofs could bring considerable energy savings, both through direct roof thermal insulation and indirect UHI mitigation, fulfilling the CO2 emission reduction goal set by the Hong Kong government of 0.8–1.4 million t/year (1).

Heat is not the only environmental issue we face as a result of climate change. As explained by researchers Andrés-Doménech et al. (2018) managing stormwater in cities through green infrastructure, including green roofs, is one of the challenges included in the European Union’s green infrastructure strategy. The researchers compared the hydrological performance of a green roof and a conventional roof under Mediterranean climatic conditions at two different scales: the plot or building scale, and the city scale. The results showed that the long-term modelling hydrological efficiency of the green roof is high, and predicted runoff volumes are significantly reduced when compared to those in a conventional roof. The analysis at the city scale also provides promising results, with the range of rainfall that can be controlled by the green roof around 15–20 mm, which corresponds with the most frequent rainfall events. The average runoff in Mediterranean conditions can therefore be considerably reduced by introducing green roofs. Andrés-Doménech et al. (2018) found that if half of the current conventional roofs were retrofitted to green roofs, runoff coefficients would be reduced to below 75% of the current ones for frequent rainfall events. These hydrological benefits, when added to other reported benefits of green roofs, make this type of SUDS a promising solution to face urban challenges caused by climate threats (5).

Similarly, Mora-Melià et al. (2018) found that in the last 20 years, Chile’s cities have outgrown most rainwater drainage and evacuation systems. Many cities located in the central region suffer from frequent floods in some of their sectors during winter rainfall events The results of their study show that, for moderate rainfall events, all areas could avoid flooding if at least 50% of the surrounding area had green roofs. In the presence of strong rainfall events, semi-extensive and extensive green roofs covering 60% to 95% of the surrounding area, respectively, could completely prevent flooding (6).

This is just a selection of a growing spectrum of research literature on using green infrastructure to improve climate in cities. Of course, in matters of scientific investigation, guesswork is forbidden. But we challenge you to think of a world where one-half of your city is covered with green roofs, is pleasant and fresh in the summer months and you don't have to worry about flooding!

• Lilliana L.H. Penga, C.Y. Jim (2015) Economic evaluation of green-roof environmental benefits in the context of climate change: The case of Hong Kong, Urban Forestry & Urban Greening 14, 554–561
• Alexander J. Johnson, Cliff I. Davidson, Evan Cibelli, Anna Wojcik (2023) Estimating leaf area index and coverage of dominant vegetation on an extensive green roof in Syracuse, NY, Nature-Based Solutions, Volume 3, 100068
• Sergio S. Herrera-Gomez, Abel Quevedo-Nolasco, Luis Perez-Urrestarazu (2017). The role of green roofs in climate change mitigation. A case study in Seville (Spain), Building and Environment 123 (2017) 575e584
• Aseel Hussien, Nusrat Jannat, Emad Mushtaha, Ahmed Al-Shammaa (2023). A holistic plan of flat roof to green-roof conversion: Towards a sustainable built environment. Ecological Engineering, Volume 190, 106925
• Ignacio Andrés-Doménech, Sara Perales-Momparler, Adrián Morales-Torres and Ignacio Escuder-Bueno (2018). Hydrological Performance of Green Roofs at Building and City Scales under Mediterranean Conditions, Sustainability 2018, 10, 3105; doi:10.3390/su10093105
• Daniel Mora-Melià, Carlos S. López-Aburto, Pablo Ballesteros-Pérez and Pedro Muñoz-Velasco (2018). Viability of Green Roofs as a Flood Mitigation Element in the Central Region of Chile, Sustainability, 10, 1130