Urban heat is often assumed to follow climate alone: hotter regions produce hotter cities. The climate does the work, and the buildings just sit in it.
That framing holds for some things and breaks badly for others. A new analysis of 2,213 cities finds that cold-climate cities – not desert ones – generate the strongest extra daytime heat from the way they are built.
Mapping urban heat traps
The study comes from a team led by Siwoo Lee at the Ulsan National Institute of Science and Technology (UNIST) in South Korea, working with collaborators at the U.S. Pacific Northwest National Laboratory (PNNL).
The researchers combined satellite-derived building data, daily near-surface air temperatures, and a six-class typology that sorts every city block by density and building height. Machine learning then linked the temperature at every city pixel to the structures around it.
The result is the first global picture that separates two things urban planners have long tangled together: the heat island caused by climate and the heat island caused by built form.
One number per city
Out of the analysis came a new metric the team calls TBE – the Thermal Impact of the Surrounding Built Environment.
In plain terms, TBE measures how much extra warming the buildings near a given point produce beyond what the local climate already brings.
Each of the 2,213 cities received one daytime value and one nighttime value. The two maps that emerged from those numbers look nothing alike.
Daytime TBE peaked in cooler, wetter regions. Nighttime TBE peaked in arid ones. The split challenged the assumption that the hottest climates always produce the hottest cities.
Cold climates lead
Daytime urban heating was strongest in northeastern North America, parts of Europe, and East Asia. A typical cold-climate central block ran nearly 1 degree Fahrenheit (0.5 degrees Celsius) hotter than its non-built surroundings, purely from the urban form around it.
Arid cities showed the weakest daytime effect – roughly one-fifth as strong. The reason traces back to how heat moves. In wetter regions, rural land sweats off heat through plants. The city, sealed under asphalt, cannot.
That gap widens when the rural backdrop is greener. Earlier research had suggested as much, but never across so many cities at once.
Arid nights run hot
After dark, the map flips. Arid cities – across much of the Middle East, western North America, and parts of Australia – show the largest nighttime warming.
Dense city materials soak up sun all day in places like Phoenix or Dubai. After sunset, they release that stored energy slowly while the surrounding desert cools off fast.
Across the dataset, about 11% of cities fell into the high daytime heating tier. Just under 7% landed in the high nighttime bracket, and the two groups barely overlap.
Cold regions held the largest share of daytime hotspots. Arid regions claimed the nighttime ones. Local form shaped the outcome within each climate zone.
Dense and tall
Inside every climate zone, the buildings did the heavy lifting. High-density, high-rise neighborhoods consistently produced the largest local warming, while sparse, low-rise blocks produced the smallest.
Building density and height told that story almost as cleanly as the climate map. What changed was the size of the gap between day and night.
At night, heat stored in concrete and brick continues radiating long after sunset, sharpening the contrast between dense blocks and open land.
Urban morphology – the three-dimensional shape of a city – emerged as a second major lever alongside climate.
The 2070 picture
The team ran the numbers forward to mid-century under three future scenarios for emissions and city growth. One pattern cut across all of them: climate change dominates the shift in 69% of cities.
In roughly a third of cities, climate and form combine to push warming higher than either factor would on its own. An earlier paper had hinted that morphology and climate interact, but this work pins down where that happens, city by city.
The finding suggests that climate policy alone will not solve urban heating everywhere. In cities where built form is a major driver, local decisions about density, height, and materials could meaningfully change the trajectory.
Two different problems
The picture splits sharply by hemisphere. 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.
For Global North cities, the balance flips. About 60% are climate-dominant, with their built forms long since set. What changes now is the atmosphere overhead.
The split echoes earlier studies on temperatures measured at street level rather than from satellites. Together, those results suggest the same global problem looks structurally different depending on where a city stands.
Tailored cooling plans
For decades, urban heat mitigation has relied on a handful of general fixes: more trees, lighter pavement, and cool roofs. These are useful tools, but they have been aimed too broadly.
The team from UNIST and PNNL argues those tools should be deployed differently depending on what is actually driving each city’s heat.
Rapidly growing Global South cities offer a window to rethink density, height, and materials before the skyline locks in. In wealthier Global North cities – where forms are largely set – the greater leverage lies in vegetation and street-level cooling.
For the first time, a city in Mumbai and a city in Munich can be compared on the same axis. The question is no longer just how hot a city will get, but which part of the system is doing the trapping – and which part can still be redesigned.
The study is published in the journal Nature Communications.
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