Examining Urban Climate Resilience Around the World: How Medellin’s Green Corridor Program Has Cooled the City

The Urban Heat Island Effect

Like many urban centers around the world, Medellin, Colombia is susceptible to the Urban Heat Island (UHI) Effect. The UHI Effect occurs when highly paved cities experience higher temperatures than surrounding rural areas primarily due to the low albedo (reflectivity) and high heat-absorbing capacity of paved roads, sidewalks, parking lots, buildings, and other dark-colored, impervious surfaces. The UHI Effect can be felt and quantified; in some cities in the US, daytime temperature differences between urban and rural areas can be around 1-7°F higher. UHIs have many negative consequences for cities, including increased temperatures straining energy systems, impacts to biodiversity, and an increase in heat-related illnesses and death.

Medellin’s “Green Corridors” Strategy

To address rising temperatures and the UHI effect, Medellin launched the internationally acclaimed “Green Corridors” project in 2016. The initiative was aimed at transforming major roads and waterways into shaded green networks that cool the city, filter pollution, and absorb stormwater. The program trained 75 local residents from disadvantaged communities as “urban gardeners,” and together, they planted 2.5 million small plants and over 880,000 trees, creating 30 interconnected corridors across 65-acres of urban space. The city works to incorporate greenery into buildings as well by installing green roofs and vertical gardens on building facades as well as including plants on bridges and other transit infrastructure, helping greenery flow continuously throughout the corridors. 

The strategy may seem straightforward, but there are many ways in which green corridors can alleviate rising temperatures that extend beyond simply providing shade. First, plants absorb less heat than grey infrastructure, preventing the accumulation and re-emission of heat. Additionally, plant evapotranspiration, or the process by which plants release water vapor through their leaves, cools the air. Third, the corridors provide porous soil, which accumulates less heat than impervious surfaces. Lastly, green corridors dissipate heat by creating wind corridors that distribute wind and cool the city and prevent heat-trapping smog. 

In addition to their cooling benefits, green corridors are beneficial for air and water quality. Increasing plants has the potential to filter out air pollutants, while decreasing impervious surfaces allows for water to percolate through the soil and recharge groundwater. The project required an initial investment of $16.3m, and around $600K per year to maintain. 

Results

Since its implementation, the project has already lowered average temperatures by about 2°C, improved air quality, and increased biodiversity. Studies also suggest that one corridor has the potential to sequester around 160,000 kg CO2 daily during early plant growth, a significant contribution to helping the city achieve its carbon neutrality goal. The Medellin mayor’s office  highlights the benefits of the Green Corridors for the city, including: 

1. Mitigating the negative impacts of the transit infrastructure
2. Acting as a noise screen 
3. Improving microclimate by up to 2°C
4. Restoring water buffer zones and protecting the city’s water resources
5. Favoring ecological connectivity that benefits the environmental processes
6. Controlling urban growth in areas adjacent to water networks and high-risk zones
7. Providing green space for passive recreation for communities and to enhance the city’s aesthetic appeal
8. Capturing particulate matter and, consequently, improve air quality

Before the program, Medellín regularly struggled with high levels of fine particulate pollution (PM₂.₅ and PM₁₀). The gardens along busy roads and on building sides provide filtering capacity for these pollutants, improving air quality. Statistics show that between 2016 and 2019, PM2.5 levels were reduced by 8% (1.55 μg/m3), and the mortality rate from acute respiratory infections decreased by over 40% (from 160 to 95, per 1,000 people).

Social Impacts

While the cooling numbers are impressive, the Green Corridors program has also reshaped Medellín’s environmental health, public health, and social life. First, the green corridors provide additional walkways, parks, and biking lanes that allow people to move around without vehicles while keeping cool. The parks and additional greenery are also shown to have beneficial impacts on mental health and stress reduction, meaning the corridors help provide shade, comfort, and mental well-being for the residents of Medellin.

Additionally, many of the locations chosen for these corridors were said to have previously been used as makeshift dumpsters partially because of their nature as unconnected and abandoned locations, according to former mayor Federico Gutierrez. Thus the planting and maintenance of the corridors have provided additional beautification for the city and kept trash from piling up in these unregulated dumpsters, all while connecting these places to the city center. 

One of the most notable positive social impacts these corridors provided goes back to who has been planting and maintaining the greenery. The city has continued training previously unemployed workers from disadvantaged communities, creating ongoing jobs while building and promoting local stewardship. Community involvement has helped sustain the program politically and financially. The project recognized that vulnerability to climate change is often heightened by socioeconomic inequity, and by using nature-based solutions as a vehicle for social change, the project has built community well-being and provided inclusive employment opportunities.

A Model for Nature-Based Solutions

Medellín’s Green Corridors program stands as a powerful testament to the potential of nature-based solutions in addressing urban climate change challenges. The program is an example of how innovative planning and community engagement can allow a city to combat the effects of climate change and enhance quality of life for citizens. It is also a reminder that social justice has to be embedded into these solutions. While the approach may sound simple, the results—cooling temperatures, improved air quality, improved respiratory health, green job creation—are tangible. As global temperatures rise and heat waves become more intense, Medellin stands as a hopeful model city showing that nature can thrive even the densest urban landscapes, and that oftentimes the solution to our climate crisis is as simple as adding some greenery to our concrete jungles.