Keer Zhang ’25 PhD: Understanding a Warming World Through Urban Expansion

For Keer Zhang PhD ’25 the path from rural China to some of the world’s largest cities didn’t just shape her education —  it set the foundation for a deep, personal curiosity about the environmental impacts of urban life.

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“I think of my life as a process of urbanization,” Zhang said.

Born in a small town in Hunan Province, she moved progressively to larger cities for her schooling — first Changsha, then Guangzhou, followed by Hong Kong — an international hub of approximately 7.4 million people. As she made those moves, she noticed stark environmental differences. “I remember (in some of those places) unbearable summers with no air conditioning and winters where the sky turned red with smog. It was the first time I felt the impact of climate change and urban living.”

Those visceral experiences motivated her to pursue an undergraduate degree in atmospheric science at Sun Yat-sen University — and eventually to the Yale School of the Environment, where her research has focused on the relationship between global urban expansion, surface climate, and human heat stress. “Urban areas are usually hotter than rural ones — that’s the urban heat island effect,” she said. “The story gets more complicated when it comes to heat stress, which also depends on humidity.”

Her dissertation project, which involved two research projects, examined the influences of global urban expansion on surface climate and heat stress.

“Keer’s research challenges the common belief that city air is drier than air in the natural landscape and that this dryness should reduce heat stress on urban residents,” said Xuhui Lee, the Sara Shallenberger Brown Professor of Climate Science at YSE. “She is one of the few young scientists who are proficient in both modern programming languages and the ancient language Fortran, the language of choice for climate modeling.”

One of Zhang’s research projects showed that in tropical cities, high humidity intensifies the effects of high temperature, creating dangerous conditions — especially in informal settlements where access to cooling is limited. The work, published in Nature, adds to a growing body of research highlighting the unique vulnerabilities of rapidly growing cities in warmer climates.

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A second project examined the biophysical climate impact of urban expansion, which is not caused by greenhouse gas emissions but by altering land surfaces, such as replacing vegetation with impervious materials. Zhang’s research found that this warming effect is especially pronounced in dry climate regions and is largely unaccounted for in the global climate models used by the Intergovernmental Panel on Climate Change (IPCC).

“Our goal is to show that these impacts are not negligible. If we want accurate climate predictions, we must represent urban expansion more realistically,” Zhang said.

Now a postdoctoral fellow at the High Meadows Environmental Institute at Princeton University, Zhang is continuing her work by evaluating urban mitigation strategies and integrating better urban processes into global climate models. Eventually, she hopes to join a university faculty, where she can continue to bring scientific rigor and personal insight to the questions that define a warming world.

“For me, urban climate is not just a topic — it’s something I’ve experienced with every step I’ve taken,” Zhang said.