A study published in Proceedings of the National Academy of Sciences revealed that the extreme urban heat in tropical and subtropical cities will far exceed the projections of conventional climate models.
The research analyzed 104 cities with populations between 300,000 and one million inhabitants. Additionally, it determined that 81% of them will experience accelerated warming compared to their surrounding rural areas.
The urban heat island phenomenon describes the thermal increase that cities experience compared to their adjacent rural areas.
According to the scientific analysis, 16% of the studied cities could record increases in surface temperature between 50% and 112% greater than estimated for their surroundings.
India and China concentrate the most affected cities by this phenomenon.
In particular, cities like Jalandhar, Fuyang, and Kirkuk show additional urban temperature increases between 0.7 and 0.8 °C. However, Asyut, Patiala, and Shangqui show increases between 1.5 and 2 °C.
Climate models underestimate extreme urban heat
The research employed machine learning techniques to obtain more accurate projections than low-resolution global climate models.
This approach allowed identifying regions where extreme urban heat clearly exceeds rural trends. This situation exacerbates the vulnerability of millions of people to heat waves.
Sarah Berk, the lead author of the study, highlighted the importance of analyzing medium-sized cities: “While global models are essential for projecting future temperature changes, they have limitations in capturing trends in smaller cities.”
The team established strict criteria for selecting the analyzed cities. Therefore, they excluded coastal or mountainous cities to control non-climatic factors and chose locations less than 40° latitude and more than 100 km from the sea.
Variables that determine urban warming
The statistical learning model used satellite variables such as the vegetation difference between city and countryside. It also considered air humidity, rainfall amount, and soil albedo.
These allow predicting how much the land surface temperature (LST) increases in urban areas.
The LST directly influences the air temperature near the ground. Therefore, it was used as a reference to compare extreme urban heat with rural conditions under a global warming scenario of 2 °C compared to the pre-industrial era.
The public health consequences of extreme urban heat
The study warned that extreme urban heat will have severe repercussions for health, especially in regions already enduring high temperatures.
The risks are amplified during the warmest months, when the heat island is more pronounced in cities in northeastern China and India.
Manoj Joshi, co-author of the work, stated: “The urban heat stress under climate change is a growing concern, as many cities in the tropics and subtropics can be warmer than their rural surroundings, thus increasing their vulnerability to rising temperatures.”
The research emphasized that conventional models tend to underestimate the actual increase in urban temperatures.
The team stressed the need to design mitigation strategies and adaptation. They noted that the demand for cooling and energy could exceed the existing infrastructure capacity.
The authors pointed out that urban expansion was not included in the analysis, so the additional warming could be even greater.
They highlighted the importance of continuing to study medium-sized cities, where climate risks tend to be underestimated.
