Landscape signatures of surface heat susceptibility in West African coastal cities: A harmonised four-city assessment
This study utilizes harmonized satellite data to assess surface heat susceptibility across four West African coastal cities, revealing that built-up areas consistently drive higher temperatures while vegetation mitigates heat, and establishing a comparative framework for biophysical heat risk screening that excludes socioeconomic vulnerability factors.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Cities are not just collections of buildings; they are active participants in the weather. When a city replaces a forest or a wetland with concrete and asphalt, it changes the way the ground interacts with the sun. Vegetation cools the air by releasing moisture, much like sweat cools a human body, while paved surfaces soak up solar energy and hold onto it, radiating heat long after the sun has set. This phenomenon, known as the urban heat island effect, turns cities into warmer pockets compared to the surrounding countryside. In rapidly growing regions, this warming is not just a matter of comfort; it affects health, energy use, and the very livability of a place. Understanding exactly where and why these hot spots form is the first step toward designing cities that can withstand a warming world.
A team of researchers recently turned their attention to four major coastal cities in West Africa: Accra in Ghana, Lagos in Nigeria, Abidjan in Côte d'Ivoire, and Freetown in Sierra Leone. These cities are expanding quickly, often outpacing the planning and infrastructure needed to manage their growth. The researchers wanted to see how the physical layout of each city—specifically the mix of buildings, bare soil, vegetation, and water—was influencing surface temperatures. They did not look at air temperature, which is what people feel, but rather the temperature of the ground itself, which can be measured from space. By combining satellite images of what the ground looks like with measurements of how hot that ground is, they created a detailed map of surface heat susceptibility for each city.
The team used high-resolution satellite data to sort the landscape of each city into four main categories: built-up areas like roads and rooftops, bare land, vegetation, and water. They then measured the temperature of the ground in these areas using thermal sensors on other satellites. To make sense of the data, they developed a single score for every part of the city that combined three factors: how hot the ground was, how much of it was covered by buildings, and how little vegetation was present. This score allowed them to compare the heat risk across four very different cities on an equal footing. They were careful to note that this score measures the physical potential for the ground to get hot, not the danger to people, which would also depend on how many people live there and how vulnerable they are.
The results revealed a clear story about the relationship between what a city is made of and how hot it gets. In Accra and Freetown, the landscape is dominated by buildings and hard surfaces. In Accra, nearly 57 percent of the mapped area is built-up, and in Freetown, the figure is similar. Consequently, these two cities recorded the highest average ground temperatures, with Freetown reaching an average of 40.03 degrees Celsius. The researchers found that in these cities, the built-up areas and bare soil were consistently the hottest, while the small patches of vegetation and water were significantly cooler. In contrast, Abidjan is a much greener city, with vegetation covering about 67 percent of its area. This abundance of green space kept its average ground temperature lower, at 34.38 degrees Celsius. Lagos, with its unique mix of dense neighborhoods, extensive lagoons, and wetlands, fell in the middle, averaging 34.13 degrees Celsius. The data confirmed that where vegetation and water are present, the ground stays cooler, and where concrete and bare soil dominate, the heat builds up.
When the researchers applied their heat score to the entire region, they found that most of the land in all four cities fell into a moderate category of heat susceptibility. However, the distribution of the most intense heat varied. Freetown stood out as having the highest concentration of areas with high heat risk, although even there, these extreme zones covered only a tiny fraction of the city, about 0.13 percent. No part of any city was classified as having a "very high" risk level. The study showed that while Accra and Freetown are generally hotter due to their dense construction, the specific arrangement of their landscapes matters. For instance, Freetown's compact development on steep hills with very little water coverage made it slightly more susceptible to heat than Accra, despite having a similar amount of building coverage.
The study offers a practical way for city planners to see where cooling is most needed. It suggests that simply planting trees is not enough; the location and connection of green spaces matter just as much. In Accra, where the city is already heavily built up, the focus should be on adding trees to streets and creating small parks to break up the concrete. In Freetown, where space is tight and the terrain is rugged, protecting the vegetation on hillsides and ensuring that drainage corridors remain open could help cool the dense neighborhoods. For Lagos, the priority is to protect its lagoons and wetlands, which act as natural air conditioners, while Abidjan needs to ensure its existing forests are not chopped up by new development. The researchers emphasize that these maps are a starting point. To truly protect people, city officials must combine these heat maps with information about where the most vulnerable populations live, ensuring that cooling efforts reach those who need them most. By understanding the physical fingerprints of heat on the landscape, these cities can begin to plan for a cooler, more resilient future.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.