A cost-benefit analysis of green roofs in Brazil and South Africa

A model developed by a cross-border collaboration including Brazilian researchers could help decision-makers assess the real impact of green roofs on the urban food-water-energy nexus.

By Maria Fernanda Ziegler  |  Agência FAPESP – Often advocated to counterbalance the adverse effects of climate change in cities, green roofs have several benefits, including rainwater collection, food production, temperature modulation and energy saving, among others. 

An international research collaboration created a model to measure the environmental costs and benefits of green roofs in any city worldwide, especially with regard to the urban food-water-energy nexus. The team included researchers affiliated with Getúlio Vargas Foundation (Brazil), Beijing Normal University (China), Wuhan University (China), Xiamen University (China) and Yale University (USA).

The study was supported by FAPESP under the auspices of a call for proposals issued in partnership with the Belmont Forum and JPI Urban Europe as part of the Sustainable Urbanization Global Initiative (SUGI). An article on the study is published in the journal NPJ Urban Sustainability.

“A great deal is said about the impact of green roofs on the sustainability of cities, but what’s the real effect of these structures on important resources, such as food, water and energy? Our study set out to answer this question by creating a model to help decision-makers analyze the costs and benefits of green roofs. We located the sources of food, water and energy in each city, and investigated how the implementation of a series of green roofs could benefit these three systems. Lastly, we tested the model on two cities with very different characteristics: São José dos Campos in Brazil and Johannesburg in South Africa,” said Rodrigo Bellezoni, a co-author of the article and a researcher at FGV. 

According to the article, the authors developed “a generalizable methodology and framework” that “integrates the environmental costs and benefits of green roofs with food-water-energy systems and makes it possible to trace energy-water-carbon footprints across city boundaries”. 

“This is a theoretical study that aims at practical application. Demand for food, water and energy is rising worldwide. These three resources are interconnected and strongly influence greenhouse gas emissions because demand for them intensifies pressures on carbon footprint and water footprint, for example. Green roofs affect the food-water-energy nexus in cities and can contribute to sustainability,” said José Puppim, a professor at FGV and also a co-author of the article.

Local factors

The comparison of the two cities showed that local factors need to be taken into consideration when implementing green roofs and other nature-based solutions, Bellezoni explained, such as whether the raw materials are produced locally or imported. It is also relevant to consider trade-offs between the energy saved by green roofs and the energy used to produce their components, grow vegetables and transport this food to market.

“Our simulations suggested that green roofs would be carbon-neutral in both cities, but the benefits would be greater in São José dos Campos,” he said. Rainwater collection there would be sufficient to offset local irrigation of food crops. This would not be the case in Johannesburg, where demand for water would exceed green roof capacity.

“The results showed that the implementation of a number of green roofs in São José dos Campos would significantly reduce the use of tap water in irrigation, as rainwater collected by the technology would meet most of the demand in this case. That wouldn’t be the case in Johannesburg because of the climate, but green roofs would evidently reduce the amount of tap water used by farmers. The cost-benefit analysis is therefore different for each city,” Bellezoni said.

The researchers also analyzed the differences in the supply of materials for the construction of green roofs. “Brazil is more developed than South Africa, so some of the infrastructure required is locally produced. The closer something is produced, the less energy is consumed for transportation, for example. South Africa has to import a substantial proportion of these materials, so many more units of energy are expended for each unit of end-product,” Bellezoni explained.

Satellite-based remote sensing was used to map each city in order to identify buildings on which green roofs could be implemented. The buildings were then classified as residential, industrial or commercial. “The buildings that could have green roofs were selected using specific criteria, given that the weight of a green roof can reach several tons, for example. As a result, we limited the potential number to only a third of all the buildings identified,” Bellezoni said.

The group also created possible scenarios for expansion of green roofs. “This approach enabled us to study the impact of the infrastructure’s lifecycle, and the amounts of water, energy and emissions involved in producing each of the elements in a green roof. They comprise several materials, all of which must be accounted for. In a second stage, we included maintenance costs and length of working life to draw up a profile of green roofs in São José dos Campos and Johannesburg,” Bellezoni said.

Finally, the researchers calculated the amount of water or energy consumed by a hypothetical food production project designed to supply tomatoes locally instead of from outside the city.

The article “The food-water-energy nexus and green roofs in Sao Jose dos Campos, Brazil, and Johannesburg, South Africa” is at: www.nature.com/articles/s42949-023-00091-3.