Urban heat isn’t just about climate; it’s also shaped by how we build our cities. A recent study of over 2,200 cities reveals something surprising: colder cities create more daytime heat due to their structures. This study, led by Siwoo Lee from the Ulsan National Institute of Science and Technology (UNIST) and in collaboration with the U.S. Pacific Northwest National Laboratory (PNNL), sheds light on urban heat islands in a new way.
The researchers combined satellite data with daily air temperatures and classified city blocks by their density and height. Using machine learning, they linked temperature with nearby buildings. This allows us to differentiate between heat from climate and heat from urban form.
The findings introduced a new metric called TBE—Thermal Impact of the Surrounding Built Environment. This measures how much extra warmth buildings create beyond the local climate. The study provided each city with daytime and nighttime TBE values, revealing a striking difference in heat production.
During the day, urban areas in cooler climates—like northeastern North America, parts of Europe, and East Asia—record the highest temperatures. Specifically, these cold blocks can be nearly 1°F (0.5°C) warmer than surrounding areas. Conversely, dry regions, like deserts, show only about one-fifth the daytime heating.
At night, the pattern flips. Arid cities, such as Phoenix and Dubai, heat up significantly. Dense materials absorb sunlight during the day, then slowly release that heat at night, while the surrounding areas cool off quickly.
Urban density plays a huge role in this phenomenon. High-rise neighborhoods consistently produce more local heating, especially at night when heat radiates from concrete long after sunset. This shows how the shape of a city influences its heat levels.
Looking toward 2070, the study indicates that climate change will drive up temperatures in 69% of cities. However, in about one-third of cities, the combined effects of density and built form push temperatures even higher. This suggests that policy solutions need to be local and tailored. For example, cities in the Global South, undergoing rapid growth, have an opportunity to rethink their urban planning. On the other hand, cities in the Global North are largely built out, where enhancing greenery and cooling techniques is more crucial.
This research reminds us that urban heat is not a one-size-fits-all problem. Different cities require different strategies for cooling. By understanding the sources of urban heat, we can better tackle this growing issue.
For more insights on this topic, check out the study published in Nature Communications.
