A CITY COOLED BY TREES

In the face of a rapidly heating planet, one city has found a way to keep its cool. Medellín, Colombia’s second largest city, has cooled by 2°C by planting 2.5 million plants and 880,000 trees. This dramatic change happened in just 3 years.

 With “green corridors” that mimic the natural forest, the Colombian city could become 5 degrees cooler over the next few decades.

Medellín city started its “green corridors” programme in 2016 due to concern about air pollution and rising heat. Composed of more than 30 green corridors, it connects newly-greened road verges, vertical gardens, streams, parks and nearby hills. The groundwork was carried out by 150 citizen-gardeners in a people-led scheme that has helped to sow and maintain hundreds of thousands of trees and plants across the city.

Previously, Medellín had undergone years of rapid urban expansion, which led to a severe urban heat island effect – raising temperatures in the city to significantly higher than in the surrounding suburban and rural areas. Roads and other concrete infrastructure absorb and maintain the sun’s heat for much longer than green infrastructure.

“Medellín grew at the expense of green spaces and vegetation,” says Pilar Vargas, a forest engineer working for City Hall. “We built and built and built. There wasn’t a lot of thought about the impact on the climate. It became obvious that had to change.”

Efforts began in 2016 under Medellín’s then mayor, Federico Gutiérrez (who, after completing one term in 2019, was re-elected at the end of 2023). The city launched a new approach to its urban development – one that focused on people and plants.

The $16.3 million initiative led to the creation of 30 Green Corridors along the city’s roads and waterways, improving or producing more than 70 hectares of green space, which includes 20 kilometers of shaded routes with cycle lanes and pedestrian paths.

These plant and tree-filled spaces – which connect all sorts of green areas such as the curb strips, squares, parks, vertical gardens, sidewalks, and even some of the seven hills that surround the city – produce fresh, cooling air in the face of urban heat. The corridors are also designed to mimic a natural forest with levels of low, medium and high plants, including native and tropical plants, bamboo grasses and palm trees.

Heat-trapping infrastructure like metro stations and bridges has also been greened as part of the project and government buildings have been adorned with green roofs and vertical gardens to beat the heat. The first of those was installed at Medellín’s City Hall, where nearly 100,000 plants and 12 species span the 1,810 square meter surface.

At the launch of the project, 120,000 individual plants and 12,500 trees were added to roads and parks across the city. By 2021, the figure had reached 2.5 million plants and 880,000 trees. Each has been carefully chosen to maximize their impact.

“The technical team thought a lot about the species used. They selected endemic ones that have a functional use,” says Paula Zapata, advisor for Medellín at C40 Cities, a global network of about 100 of the world’s leading mayors.

The 72 species of plants and trees selected provide food for wildlife, help biodiversity to spread and fight air pollution. A study, for example, identified Mangifera indica as the best among 6 plant species found in Medellín at absorbing PM2.5 pollution – particulate matter that can cause asthma, bronchitis and heart disease – and surviving in polluted areas due to its “biochemical and biological mechanisms”.

And the urban planting continues to this day.

The project’s wider impacts are like a breath of fresh air. Medellín’s temperatures fell by 2°C in the first 3 years of the programme, and officials expect a further decrease of 4-5 degrees Celsius over the next few decades, even taking into account climate change. In turn, City Hall says this will minimise the need for energy-intensive air conditioning.

Going forward, preventing and adapting to hotter temperatures will be a major and urgent challenge for cities. The number of cities exposed to “extreme temperatures” is set to triple over the next decades, according to C40 Cities. By 2050, more than 970 cities will experience average summertime temperature highs of 35°C (95°F).

A separate study estimated that in just one of Medellín’s corridors, the new vegetation growth would absorb 160,787 kg of CO2 per year and that over the next century 2,308,505 kg of CO2 will be taken up – roughly the equivalent of taking 500 cars off the road.

In addition, the project has had a significant impact on air pollution. Between 2016 and 2019, the level of PM2.5 fell significantly, and in turn the city’s morbidity rate from acute respiratory infections decreased from 159.8 to 95.3 per 1,000 people.

There’s also been a 34.6% rise in cycling in the city, likely due to the new bike paths built for the project, and biodiversity studies show that wildlife is coming back – one sample of 5 Green Corridors identified 30 different species of butterfly.

Source: reasons to be cheerful (4 March 2024), edited extracts