Why Is Shanghai Getting Hotter While Its Citizens Are Feeling Less Heat? Major Study by LOUD Published in Scientific Reports

Why Is Shanghai Getting Hotter While Its Citizens Are Feeling Less Heat? Major Study by LOUD Published in Scientific Reports

1. Why Should We Pay More Attention to “Population Heat Exposure” During Extreme Heat Events?

Sweating through the subway in the morning and feeling like you're in a steam room when walking through narrow alleys after work — why are some parts of the city so much hotter than others? Scientists at NYU Shanghai have used satellite data to scan cities around the world and discovered that whether a city feels “hot” depends not only on temperature but also on where people live.

In a study covering the world’s 1,000 largest cities—home to approximately 2 billion people—researchers found that for every 10,000-person per square kilometer drop in population density, exposure to extreme heat decreased by 0.5°C. In other words, urban sprawl—often criticized for being inefficient—might have inadvertently served as a kind of "invisible air conditioner."

This research was conducted by core members of the Shanghai Key Laboratory of Urban Design and Urban Science（LOUD）, including Professors Kangning Huang, Jiayong Liang, and Chenghe Guan, in collaboration with Professor Brian Stone from the Georgia Institute of Technology. The findings were published in the academic journal Scientific Reports, in an article titled "Declining urban density attenuates rising population exposure to surface heat extremes."

Unlike previous studies that focused solely on changes in land surface temperature, this research highlights a more human-centered issue: population heat exposure, or the actual thermal stress experienced by people. Why does this matter? Because even within the same city, temperature and population distribution can vary significantly. For instance, high-rise downtown areas can be several degrees hotter than suburban parks—but if downtown areas are densely populated, more people are exposed to extreme heat.

2. Why Are High-Density Areas Hotter? Can Lower Density Actually Help Cool Things Down?

Think about it: in the height of summer, the narrow alleys of older neighborhoods often feel hotter and more stifling than the open spaces of newer districts. This is because high-density areas typically have tightly packed buildings, limited green space, and poor ventilation—all of which contribute to greater heat accumulation.

Over the past two decades, cities around the world have generally experienced a trend of declining urban density. That is, urban expansion has outpaced population growth, causing residents to gradually disperse toward suburban areas.

Our research reveals that this trend toward lower density has unexpectedly slowed the rise in population heat exposure. The data show that for every 10,000-person per square kilometer drop in urban density over a decade, the increase in population heat exposure is about 0.5°C slower than the rise in average land surface temperature. In other words, more dispersed living patterns may be softening the impact of extreme heat on urban populations.

3. The Shanghai Case: How Urban Redevelopment Is Changing Our Experience of Extreme Heat

In Shanghai, a seemingly paradoxical trend has emerged: between 2003 and 2020, even as average land surface temperatures continued to rise, the actual heat stress experienced by residents has decreased. The key to this lies in the city’s urban renewal process. As part of its development, roughly 3 million residents relocated from high-density, inner-city districts such as Huangpu and Jing’an to medium- and low-density areas like Pudong and Minhang. These older districts are typically 5–6°C hotter than suburban areas; as large numbers of people moved into relatively cooler neighborhoods, overall population heat exposure declined.

At the same time, low-density areas on the urban fringe—such as farmland—have been transformed into mid-density residential zones, which are generally warmer. While this land-use change has contributed to a rise in average surface temperatures, the improved spatial distribution of the population has effectively offset the impact of the heat increase.

This case illustrates a critical insight: through well-designed urban planning that strategically redistributes population, cities can significantly mitigate the health risks associated with extreme heat.

Beyond Shanghai, our global study—covering the world’s 1,000 largest cities and approximately 2 billion people—found that this pattern of “declining density, reduced heat exposure” holds true across diverse urban contexts around the world.

4. Are Low-Density Cities the Perfect Solution? Beware the Hidden Costs

While low-density urban development can help alleviate the urban heat island effect and reduce residents’ exposure to extreme heat, this model comes with significant real-world trade-offs. As populations spread toward the suburbs, private car commuting becomes the dominant mode of transport, leading to a sharp rise in per capita carbon emissions. For example, a commuter living in Songjiang who drives daily to the city center may generate nearly twice the annual carbon emissions of a downtown resident.

Moreover, low-density development undermines the agglomeration benefits that dense urban areas provide. In high-density neighborhoods, amenities such as cafés, convenience stores, and other daily services are tightly clustered, offering both convenience and cost efficiency. In contrast, suburban sprawl often results in greater distances between services and fewer options—driving up living expenses in less visible ways.

Urban expansion into suburban areas also requires substantial investment in supporting infrastructure such as roads, water and sewage systems, and electricity grids. In low-density zones, the per capita demand for infrastructure is significantly higher, and both the initial construction and ongoing maintenance of these systems place a heavy financial burden on public budgets.

These real-world challenges serve as a reminder that an ideal model of urban development must strike a delicate balance between mitigating the impacts of extreme heat and ensuring long-term sustainability.

5. Looking Ahead: Leveraging Science for a Dual Win in Urban Cooling and Carbon Reduction

As cities confront the multifaceted challenges of climate change, it is increasingly clear that no single solution will suffice. Instead, a more integrated and adaptive governance approach is needed.

The LOUD's future research aims to inspire new strategies for urban planners. In high-density urban cores, for example, carefully designed green spaces and shading systems can significantly improve the thermal environment—offering a more sustainable alternative to simply restricting development. In suburban development, innovative mixed-density models can support walkable, transit-accessible neighborhoods that reduce car dependence at the source. Meanwhile, advances in satellite remote sensing and population big data will enable dynamic monitoring and evaluation systems that provide real-time feedback for policy adjustments.

The LOUD remains committed to advancing human-centered urban science, aiming to help cities around the world pursue development pathways that safeguard human comfort, promote social equity, and ensure environmental sustainability.

The lead author of this study, Dr. Kangning Huang, is an Assistant Professor of Environmental Studies at NYU Shanghai and a core member of the LOUD. Dr. Huang received his Ph.D. from the Yale School of the Environment in 2020. Before joining NYU Shanghai, he served as an Advanced Study Program Postdoctoral Fellow at the National Center for Atmospheric Research (NCAR) in the United States.His research and teaching focus on the impacts of urbanization on climate change. Land-cover changes due to urban expansion can alter local hydrometeorological processes, while lifestyle shifts associated with urban living tend to increase fossil fuel consumption—both of which contribute to climate change at different scales. However, the climate impacts of urbanization are not fixed; rather, they depend on where and how future cities are built. By developing global-scale urbanization scenarios, Dr. Huang’s research explores potential pathways for urban climate futures and identifies interventions needed to foster more sustainable outcomes. His work has been supported by NASA (via the NESSF Fellowship), the National Science Foundation (NSF) (via the NCAR Postdoctoral Fellowship), and other research institutions.

This research was supported by NYU Shanghai, the Shanghai Natural and Health Foundation, the Rockefeller Foundation, and Sounds Climate.

Reference:

Huang, K., Stone, B., Guan, C. et al. Declining urban density attenuates rising population exposure to surface heat extremes. Sci Rep 15, 13860 (2025). https://doi.org/10.1038/s41598-025-96045-z