The global map of 2,213 cities has revealed a fascinating insight into the urban heat trap phenomenon, challenging the conventional assumption that climate alone drives urban heat. This study, led by Siwoo Lee from the Ulsan National Institute of Science and Technology (UNIST) in South Korea, along with collaborators from the U.S. Pacific Northwest National Laboratory (PNNL), has shed light on the intricate relationship between urban form and climate in shaping the heat island effect. What makes this research particularly intriguing is the discovery that cold-climate cities, rather than desert ones, are the ones generating the strongest extra daytime heat due to their built environment. This finding is a game-changer, as it suggests that the way cities are designed and constructed plays a more significant role in urban heat than previously thought.
The researchers employed a sophisticated approach, combining satellite-derived building data, daily near-surface air temperatures, and a six-class typology to categorize city blocks by density and building height. By using machine learning, they were able to link temperature variations at every city pixel to the surrounding structures. This innovative method allowed them to separate the heat island effect caused by climate from that caused by the built environment, providing a comprehensive global picture for the first time.
The study introduced a new metric called TBE (Thermal Impact of the Surrounding Built Environment), which measures the extra warming produced by buildings near a given point beyond what the local climate already contributes. The results were eye-opening, with daytime TBE peaking in cooler, wetter regions, and nighttime TBE peaking in arid ones. This contrast challenged the notion that the hottest climates always result in the hottest cities, indicating that urban form and climate interact in complex ways.
In northeastern North America, parts of Europe, and East Asia, cold-climate cities exhibited the strongest daytime urban heating, with central blocks running nearly 1 degree Fahrenheit hotter than their non-built surroundings. This finding is particularly interesting because it suggests that the urban form in these regions has a more significant impact on daytime temperatures than the local climate. On the other hand, arid cities showed the weakest daytime effect, which can be attributed to the way heat moves in different environments. In wetter regions, rural land releases heat through plants, while cities, sealed under asphalt, cannot, creating a notable difference in heat retention.
The study also revealed that dense and tall buildings consistently produced the largest local warming, while sparse, low-rise blocks produced the smallest. This finding highlights the importance of urban morphology, or the three-dimensional shape of a city, as a major lever alongside climate in shaping urban heat. Furthermore, the researchers projected the numbers forward to mid-century under various future scenarios, finding that climate change dominates the shift in 69% of cities. However, in roughly a third of cities, climate and form combine to push warming higher than either factor would on its own, suggesting that local decisions about density, height, and materials could significantly impact the trajectory of urban heating.
The implications of this research are far-reaching. In the Global South, more than a fifth of cities face daytime warming driven mainly by changing form, as expanding skylines and denser blocks layer structural heat on top of climate change. In contrast, for Global North cities, the balance shifts, with about 60% experiencing climate-dominant heat, where built forms are already established. This hemispheric split echoes earlier studies, indicating that the same global problem can manifest in different structural ways depending on the city's location.
This study has significant implications for urban heat mitigation strategies. For decades, general fixes like more trees, lighter pavement, and cool roofs have been employed, but they have been aimed too broadly. The UNIST and PNNL team argues that these tools should be deployed differently depending on the specific drivers of each city's heat. Rapidly growing Global South cities offer an opportunity to rethink density, height, and materials before the skyline locks in, while wealthier Global North cities, where forms are largely set, can focus on vegetation and street-level cooling.
In conclusion, this research has opened up a new avenue for understanding and addressing urban heat. By recognizing the significant role of urban form in shaping the heat island effect, cities can now be compared on a more nuanced basis, considering which part of the system is doing the trapping and which part can still be redesigned. This study serves as a call to action for urban planners and policymakers to take a more tailored approach to cooling plans, ensuring that resources are allocated effectively to combat the urban heat trap phenomenon.
