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 at the Ulsan National Institute of Science and Technology (UNIST) in South Korea, and collaborators at the U.S. Pacific Northwest National Laboratory (PNNL), has shed light on the intricate relationship between urban form and heat generation, offering a new perspective on urban planning and climate adaptation strategies.
One of the key findings is that cold-climate cities, rather than desert ones, generate the strongest extra daytime heat due to their built environment. This is particularly intriguing, as it contradicts the common belief that hotter regions produce hotter cities. The researchers developed a new metric called TBE (Thermal Impact of the Surrounding Built Environment) to measure the extra warming caused by buildings near a given point, beyond what the local climate already brings.
Daytime TBE peaked in cooler, wetter regions, while nighttime TBE peaked in arid ones. This split challenged the assumption that the hottest climates always produce the hottest cities. In fact, the study found that dense and tall buildings in cold regions consistently produced the largest local warming, while sparse, low-rise blocks in arid areas showed the weakest daytime effect.
The reason for this discrepancy lies in how heat moves. In wetter regions, rural land sweats off heat through plants, while cities sealed under asphalt cannot. This gap widens when the rural backdrop is greener, as suggested by earlier research. However, this study provides a comprehensive analysis across numerous cities, reinforcing the connection between urban form and heat generation.
At night, the situation reverses. Arid cities, across much of the Middle East, western North America, and parts of Australia, show the largest nighttime warming. Dense city materials absorb sunlight during the day and release that stored energy slowly after sunset, while the surrounding desert cools off rapidly. This creates a sharp contrast between dense blocks and open land at night.
The study also found that climate change dominates the shift in 69% of cities, but in roughly a third of cities, climate and form combine to push warming higher than either factor would on its own. This highlights the importance of considering both climate and urban form in addressing urban heat.
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 flips, with about 60% being climate-dominant, where built forms are largely set, and the greater leverage lies in vegetation and street-level cooling.
This study has significant implications for urban planning and climate adaptation strategies. It suggests that climate policy alone will not solve urban heating everywhere, and local decisions about density, height, and materials could meaningfully change the trajectory in cities where built form is a major driver. By understanding the specific drivers of urban heat in each city, urban planners can tailor cooling plans more effectively, focusing on the most relevant interventions.
In conclusion, this study has provided a comprehensive and insightful analysis of the urban heat trap phenomenon, offering a new perspective on urban planning and climate adaptation strategies. By understanding the intricate relationship between urban form and heat generation, we can develop more effective and targeted solutions to create more sustainable and resilient cities.
