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Cities in low- and middle-income countries are experiencing a surge in urban heat island effects, driven by unplanned urban expansion, shrinking green spaces, and poor infrastructure, according to new analysis.

Researchers say the phenomenon is worsening the impacts of climate change and compounding social inequalities in urban areas, where more than half the global population currently resides—a figure projected to increase to around 70% by 2050.

The urban heat island effect occurs when dense urban areas, dominated by heat-retaining materials such as concrete and asphalt, trap more heat than their rural surroundings.

A published in the journal npj Urban Sustainability analyzed surface urban heat island intensity trends across more than 5 million urban grids globally between 2003 and 2018.

It found that urban areas are heating up at an average rate of 0.021°C per year, with low- and middle-income countries seeing the most pronounced net increase.

Chuan Liao, assistant professor at the department of global development at Cornell University, New York, and co-author of the study, told SciDev.Net: "Urban heat islands are not just about temperature—they reflect deeper inequities in how cities are built and who gets to benefit from green, climate-resilient infrastructure."

Rapid urbanization

In regions such as South Asia, Sub-Saharan Africa, and the Middle East, urbanization is occurring at unprecedented rates. This typically means replacing vegetation with impermeable surfaces, which affects the water cycle and can contribute to warmer temperatures.

In most cases this is happening without adequate heat mitigation measures.

According to Lotfy Azaz, professor of urban planning and geographic information systems and head of the geography department at Menoufia University, Egypt, the formation of heat islands is closely tied to the "urban fabric."

"This includes factories, high-density housing, paved roads, and complex transportation networks," Azaz told SciDev.Net.

"Low-income countries are undergoing rapid urban development, while cities in high-income nations have largely stabilized and adopted planning approaches that help manage urban heat," said Azaz, who was not involved in the study.

Cities such as Cairo, Karachi, Dhaka, and Nairobi are especially hard-hit, where informal settlements often lack green areas, shade, or effective cooling systems.

The study also found that the surface urban heat island intensity effect is most intense during summer days—when urban materials absorb maximum solar radiation—and winter nights, when built-up areas retain and re-emit heat while rural zones cool more rapidly.

"During summer days, cities heat up faster due to solar exposure, and at night, they stay warmer due to retained heat and emissions from vehicles and heating systems," explained Liao.

"Low- and middle-income countries show especially strong nighttime increases, linked to rapid population growth and energy use."

The consequences of this phenomenon are profound, at a time when extreme heat events are becoming more frequent and intense with climate change.

Poorer urban residents—often living in overcrowded, poorly ventilated housing—face higher risks of heat-related illness, energy poverty, and even death.

The urban poor frequently lack access to air conditioning, reliable electricity, or shaded public spaces.

"Climate injustice is very clear here," added Liao.

"The people contributing least to global emissions are suffering the most, while also having the fewest resources to adapt."

Green solutions

Simple, locally adapted interventions can be the most effective for tackling urban heat, according to Azaz. "Any green cover helps cool urban areas," he said.

"Rooftop gardens are a promising solution, and painting building facades and roofs white can reflect heat and reduce absorption."

The researchers emphasize the importance of community engagement and local data in shaping effective heat mitigation policies.

Reda Ali, assistant professor of environmental engineering and a climate policy expert at the Coastal Research Institute in Alexandria, Egypt, also highlighted the potential of nature-based solutions to combat urban heat.

"Nature-based solutions can provide multiple benefits, including reducing temperatures, improving urban biodiversity, and enhancing residents' quality of life," Ali told SciDev.Net.

"However, these solutions require political commitment and significant investment, especially in the global South."

Ali, who was not involved in the study, stressed the importance of integrating climate adaptation into public policy, calling for heat action plans, sustainable infrastructure, and inclusive development to protect low-income communities.

"There's no one-size-fits-all solution," said Liao. "Each city must tailor its strategy to its climate, resources, and stage of development."

The study's authors call for stronger international cooperation and financial support—particularly through mechanisms such as the Green Climate Fund—to help cities in low- and middle-income countries develop climate-resilient infrastructure.

"Cities need global solidarity," Ali added. "Investment must be channeled toward urban areas that are growing rapidly but lack the means to adapt to extreme heat."