heat in India
Individual protection against the heat in Kolkata, India

The Netflix drama Narcos, about the drug lord Pablo Escobar, made it clear to the whole world: In the 1980s, a bloody drug war raged in the middle of Medellín. At the beginning of the 1990s, Colombia's second largest city was still considered the most dangerous metropolis in the world. However, years of rapid development followed and Medellín acquired a new image. A few years ago, the Wall Street Journal described the city as the "most innovative city in the world", not least because of great efforts in the area of ​​sustainability.

Because of the mild climate, locals call Medellín “the city of eternal spring”. But as in many big cities, the eternal spring is increasingly threatening to turn into a dog-day summer. In addition to climate change, several specific urban factors contribute to the phenomenon: the relatively dark building materials of asphalt, Eternit and concrete store solar energy and release it again at night, due to a lack of vegetation there is a lack of evaporative cooling and due to the dense development there are no ventilation options. Waste heat from households, businesses and traffic creates additional heat, and air conditioning systems blow warm air outside. Today it is six degrees warmer in the center of Medellín than in the surrounding area - in technical jargon this is referred to as an urban heat island effect.

Heat islands around the world

Not only Medellín sees itself threatened by unwanted temperature increases. A study published last year in the journal PNAS shows that heat stress almost tripled in more than 13.000 cities between 1983 and 2016. The statement can be illustrated with a close-up example. In Vienna, for example, between 1960 and 1979 there were an average of nine days a year when the thermometer exceeded 30 degrees, between 2000 and 2016 it was 21 days, and in the hot summer of 2018 it was even 42. That this is a specific In our part of the world, winter is a major problem in urban areas: At this time of year, the difference between city and country at night can be ten degrees.

heat city country
In these cities, the phenomenon of urban heat islands becomes clear – reflected in the average maximum surface temperatures.

Current data from the United Nations Environment Program confirm the urban-rural divide: According to this, cities will heat up by an average of 4,4 degrees by the end of the century, which is around twice as much as the global average. This means that death from heat is primarily an urban phenomenon: the World Health Organization assumes that if climate change continues unchecked in 2050, around ten times more older people will die from heat than in 1990.

Fighting urban heat: Pioneers in Colombia

In 2015, Medellín battled the urban heat. With the project Corredores Verdes, green corridors, the city attracted attention worldwide and received several best practice awards such as the Ashden Award 2019.

Green Corridors
Green corridors: It's better to drive in the shade.

The 36 green corridors, created between 2016 and 2019, form a 20 km long, connected network of sidewalks, river banks and also busy roads. They were all shaded and supplied with fresh air with the help of tens of thousands of trees and smaller plants as well as other near-natural solutions. In the newly designed areas, the measures have already led to temperature reductions of two to three degrees on average. And as the newly planted trees grow, the difference will only get bigger in the years to come. 

The project was designed and managed by the landscape architect Marcela Norena Restrepo, among others, on behalf of the city's environmental secretariat. “We didn't just want to plant trees, we also wanted to create places where the population could feel the change towards green infrastructure, for example by cleaning up formerly polluted river banks and planting community managed gardens there,” says Restrepo. 

green corridors in Medellín
The green corridors in Medellín invite you to take a walk.
Medellin thermal image
Thermal images show how effective trees are at cooling down.

Various strategies against urban heat

If the climatic conditions in a city change, the response must also be to change the urban infrastructure. Anyone who lives in Vienna can attest to how action is being taken to combat the heat in every nook and cranny here. Most of the measures are not too difficult to implement: different forms of greening - facade greening, roof gardens or lawn tracks are also recommended - the creation of fresh air corridors, shading, cooling with water or the use of alternative building materials and lighter colors for surfaces (see also corporAID article on "A few degrees cooler").

However, an international team of researchers from ETH Zurich, Princeton and Duke University showed in a study in 2019 that there are no patent solutions. The researchers simulated the development of heat in 520 cities worldwide and came to the conclusion, among other things, that cities in the northern hemisphere will on average shift about a thousand kilometers to the warmer south by 2050, while cities in the tropics will primarily continue to do so drier. 

heat city
Hot Spots: The redder, the hotter these cities are compared to their surrounding areas. (Source: Manoli et al. 2019)

This requires different approaches. The author of the study and environmental researcher Gabriele Manoli illustrates this with two extreme examples: In the US city of Phoenix, a city of over a million inhabitants surrounded by desert, where temperatures regularly exceed 40 degrees in summer, significantly cooler temperatures can be achieved relatively easily through targeted greening measures. Because trees act as if they were natural air conditioning systems: the water that they take deep underground evaporates on their leaves. A tree can evaporate hundreds of liters of water per day - and thus achieve a cooling capacity of two average household air conditioners. In the megacities of Southeast Asia, on the other hand, additional green areas hardly bring the desired effect due to the high humidity. The focus here should be more on wind circulation, more shade and new heat-resistant materials. According to Manoli, “a bundle of strategies” is required, especially in hot, humid places.

Interview with Elie Bou-Zeid, Princeton University

Elie Bou Zeid

Build better cities

Elie Bou-Zeid, Princeton Professor and expert on sustainable infrastructure, recommends differentiated measures.

Fighting heat: impulses from India

With its efforts to not just take individual measures to combat the heat, but to involve the entire population, the metropolis of Ahmedabad, which has eight million inhabitants, has done pioneering work in India, which is particularly badly affected by the heat. The impetus for this was a heat wave in 2010 with peak temperatures of almost 50 degrees, which claimed almost 4.500 lives. As a result, the city administration drew up a heat action plan that contains three objectives: sensitize the population, coordinate the authorities and improve training in the health sector. In addition, an early warning system was installed for extremely hot and therefore dangerous days. Measures were also taken to quickly provide additional water and other cooling options for residents in the slums, who are particularly exposed to the heat. In addition, the Ahmedabad Municipality is in the process of introducing cool roof technologies on a large scale. The planting of 500.000 trees annually should also help reduce the heat island effect. 

heat Bangladesh
In Bangladesh, the Red Cross provides information about the dangers of the heat via megaphone.

A study confirms that Ahmedabad is seeing around a thousand fewer heat-related deaths annually thanks to its far-reaching heat plan. And so the city in the western Indian state of Gujarat became a role model for the whole country. More than a hundred Indian cities have now developed similar strategies. 

Support also comes from international development cooperation: The German Society for International Cooperation GIZ is implementing a Climate Smart Cities project in the three Indian cities of Kochi, Coimbatore and Bhubaneshwar. In cooperation with the TU Berlin, urban design thinking is used to initiate projects in the areas of ecological buildings, urban green spaces and rainwater drainage. At the same time, a framework for climate adaptation and climate change mitigation in cities was developed, which includes measures in the area of ​​local public transport and street lighting. 126 Indian cities use this guideline to review their progress and identify where there is room for improvement. 

Initiative against heat in Africa

In western Africa, Sierra Leone's capital, Freetown, is demonstratively bracing itself against the deadly heat. The term tropical nights, as used in this country for nights when the temperature does not drop below 20 degrees, is not common in the tropics themselves. Probably also because every night is a tropical night - in Freetown the temperatures never drop below 20 degrees. In addition, it is very muggy with a humidity of up to 90 percent. That means: The moist air prevents sweat from evaporating and cools the body – and feels even hotter than it is anyway. And as in India, it is the people in the slums in West Africa who feel the heat the most. Huts with corrugated iron roofs and plastic sheeting walls act like greenhouses and store the heat in the blazing sun. In addition, the narrowness leaves little room for wind movements. More than half of Freetown's approximately one million inhabitants live in one of the dozens of slums.

In October 2021, the city installed a full-time Chief Heat Officer, the first in Africa. This post had already been created in Athens and Miami with the support of the US Rockefeller Foundation. Many other cities around the world are to follow.

Heat Freetown
In the "pressure cooker": In Magazine Wharf - one of the largest slums in
Sierra Leone's capital, Freetown - there's no escaping the heat.

In Freetown, the newly appointed heat officer Eugenia Kargbo faces a double challenge: in the dry season there is a shortage of water and forest fires, in the rainy season there is a risk of landslides and floods. According to Mayor Yvonne Aki-Sawyerr, the city's population explosion is making it "feel like living in a pressure cooker." 

Kargbo wants to counter rising temperatures with a mix of infrastructural and political changes. She wants to create a heat wave database, promote recycling and sanitation, and push ahead with the city's ambitious tree planting program - under the title #FreetownTheTreetown, one million trees are to be planted in the next few years, above all to prevent landslides. It also works with telecommunications companies to send out weather alerts, builds cooling centers with shade and water in slums and is currently examining how disaster insurance could be set up for slum dwellers. 

Kargbo is clear about her role: “Climate change is already here. The heat is already here and it is unbearable. What we are witnessing now in Freetown is unprecedented. We need to adapt, not just soften. I need to make my city a safer, cooler place," she told Bloomberg CityLab.

More nature for less heat

The situation in Colombia's Medellín can hardly be compared. After all, the city enjoys a much more comfortable starting position due to its location at almost 1.500 meters above sea level and the large number of streams that criss-cross it. However, the vehemence of the efforts of both cities to counter the overheating of their living space is comparable. Currently, Medellín is the only Latin American city participating in the European Union-funded URBAN GreenUP project. This aims to increase sustainability in cities through a variety of innovative nature-based solutions. 

concrete pillar
If you let nature, then it wants to go high. Pictured: Medellin

"We are a lively and creative experience laboratory, since a lot of background work has already been done here and, thanks to the positive response worldwide, various projects are currently being implemented," says Marcela Norena Restrepo. The lively landscape planner invites city planners and interested parties from all over the world to take a closer look at Medellín in this respect. Her next project is also set to be her biggest: she is currently working on a comprehensive renaturation plan for Medellín, which should have a particularly favorable effect on the urban climate.

Images: Ashden, Secretaria de Medio Ambiente Medellin, Suprabhat, Princeton University, Manoli/Trinidad, Simon Davis/DFID, BDRCS