How does a city cool down?

The summer of 2022 in Europe, the hottest and driest in memory, has made it clear that climate change is already here and that it requires urban solutions to create ‘islands of cold’ in the world’s large cities and megalopolises – with mini-forests , fountains, rooftop gardens…– that counteract the heat generated by the masses of cement and steel that absorb and retain solar radiation in modern asphalt jungles. By mid-century, 68% of the world’s population will live in them.

The changes will be felt above all in northern areas of the Old Continent where houses have been built since time immemorial to conserve heat, not to dissipate it. In the UK, since 1884, when records began to be kept, the 10 warmest years have occurred this century. In France, two Breton cities, Nantes and Brest, reached 42nd for the first time this year.

In the United States, also for the first time, Newark (New Jersey) had five consecutive days with highs of 40º. In the middle of August in the Financial Times Simon Kuper wondered if in 50 years someone will still be living on the Castilian plateau due to heat and depopulation, which year after year empties the central lands of the peninsula. Many European travel agencies made last-minute changes this summer, with hundreds of families opting for Amsterdam, Copenhagen or Dublin as their destination instead of Rome or Barcelona.

heat stroke

The trend seems irreversible despite the declared intentions of the international community to meet the goals of the Paris Agreement. Due to the revival of its heavy industry, China’s carbon gas emissions exceeded those of all OECD members combined in the past 12 months. In 2019, India became the third largest emitter of CO2, ahead of the EU.

The problem is going to require increasingly active public policies to face a crisis that is ultimately one of public health. When temperatures exceed 35º with 100% relative humidity, the impact on the human body can be deadly because the body dissipates the heat of metabolism through sweat.

Above 35º and high humidity, sweat does not evaporate, preventing body cooling and creating the conditions for heat strokes like the ones that this summer killed some 2,500 people in Spain, most of them over seventy years old, people who with age lose the sensation of thirst. The 2003 heat wave claimed around 70,000 lives across Europe, many of whom were elderly living alone. In Barcelona, ​​with 40º and 50%-60% humidity, there is an alert of severe danger to health. The UN estimates that by 2050 urban heat stress will reduce the working capacity of the global workforce in hot months by 20%.

singapore proving ground

Lee Kwan Yew, former Prime Minister of Singapore, used to say that air conditioning was the most important invention in history because it changed the “nature of civilization” by enabling economic development in the tropics, which, among other things, led to the $60,000 the per capita GDP of his city-state and turned southern China into the world’s factory.

Singapore is located almost on the equator. Its temperatures oscillate throughout the year around an average temperature of 31º with a humidity of 84% and that is increasing at a rate that doubles the world average. This year, the island, which is home to one of the largest petrochemical complexes in the world, reached 37º with a relative humidity of 75%.

Air conditioners are responsible for its largest emissions after the industrial sector. In 2015, an official climate study predicted that the average temperature could rise by four degrees by 2100, which will prevent outdoor activities and make the city practically uninhabitable outside of closed spaces protected by air conditioning systems.

« Air conditioners are responsible for its largest emissions after the industrial sector. »

These conditions make Singapore a prime testing ground for urban innovation in extreme weather conditions. Cooling Singaporea public project launched in 2017 is designing a climate digital twin of the city to assess the effectiveness of heat mitigation measures.

According to his research, post-independence urbanization turned up the heat by replacing the island’s natural forests with skyscrapers made of concrete, steel and glass. Between the center of the city and the forests of the northwest, the differences can exceed 7º. In 2017, Cooling Singapore recommended 86 innovative urban measures, including changing the orientation of buildings to create wind currents and the use of natural cooling systems to cool the air or painting white or reflective colors on various surfaces.

According to Winston Chow, its director of research, the project seeks to elucidate the critical climatic components – wind speed, air temperature, solar radiation… – that most affect citizen well-being and find ways to create “islands of relief” in which can find breezes and refreshing environments.

The Miyawaki Method

Some solutions are more expensive than others. By 2030, Saudi Arabia will use more energy to cool air than it currently exports in the form of oil. On the hottest days, air conditioning consumes 79% of Riyadh’s electricity.

In air conditioning –whose devices use mostly hydrofluorocarbons (HFC), gases that trap heat in the atmosphere 1,000 times more than CO2–, it already consumes 10% of electricity on a global scale.

Other solutions are cheaper and more sustainable. In 2019, the University of Wisconsin estimated that US cities needed 40% more trees in parks, gardens, and streets to significantly reduce urban heat.

“In 2019, the University of Wisconsin estimated that US cities needed 40% more trees.”

One of the options is the miniature forests conceived by Akira Miyawaki (1928-2021), who planted some 1,500 in his native Japan, a model that has been adopted, among many other cities, by Singapore, Paris, Bangalore, Mumbai, Karachi and Lahore. . According to Chow, the presence or absence of shade is a determining factor in exposure to heat, so leafy trees such as the million that have been proposed to be planted in Singapore by 2030 should be privileged. So far it has more than 388,000.

The Miyawaki method grows dense urban forests very quickly on small plots of land and degraded land using native trees and replicating the natural regeneration processes of forests. Miyawaki urban can reduce the surrounding temperature by up to 1.3 degrees, improve air quality and create natural oases for birds and pollinating insects.

Under ideal conditions, the technique can recreate biodiverse habitats in deforested tropical areas. So far, more than 2,000 forests have been successfully created using the methods of the botanist and professor at the University of Yokohama and Blue Planet 2006 Nobel laureate in ecology. Researchers at Wageningen University have found that Dutch Miyawaki forests harbor greater biodiversity than nearby natural forests and absorb up to 30 times more carbon than monocultures such as maize or wheat.

The mini-forests have an artificial aspect because all their trees are of the same age, but the high density of the seeds that are planted makes the shoots grow fast because they compete with each other for sunlight.

Forest Sociology

During his lifetime, Miyawaki restored forests in more than 1,300 places in Japan and in India, Indonesia, and Brazil. In India there are hundreds, including the one in Thiruvananathapuram in New Delhi and the one in Hyderabab. Chennai has promised to create a thousand. The Indian company Afforestt has created 1,348 of these forests in 44 world cities because, among other things, their trees tend to grow 10 times faster, with which the Miyawaki method achieves results in 20 years that in natural conditions would require 200.

To design his forests, Miyawaki drew on phytosociology, which studies the ways in which plants interact with each other and one of whose pioneers was the German biologist Reinhold Tüxen, his teacher and mentor. Trees, for example, in symbiotic alliances can nurture neighboring trees or warn them of danger by sending chemical signals through their roots.

“In 1998, Miyawaki brought together 4,000 people to plant 400,000 trees along the Great Wall of China. In 2000, his method was tested for the first time in a Mediterranean ecosystem in Sardinia. »

The Japanese botanist was also inspired by the chinju-no-morithe sacred groves that surround Shinto temples and shrines, home to indigenous species such as the Japanese oak (quercus glauca) or Japanese chestnut (Castanea creanata) that descend from prehistoric forests and managed to survive by being protected for cultural and religious reasons.

In 1998, Miyawaki brought together 4,000 people to plant 400,000 trees along the Great Wall of China. In 2000, his method was tested for the first time in a Mediterranean ecosystem in Sardinia, in an area where traditional reforestation methods had failed. The clearly favorable results after 11 years moved Paris to adapt it in 2018 to restore an area of ​​400 square meters near the Porte de Montreuil.

The post How does a city cool down? was first published in Foreign Policy.

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