Urban Heat Is Delaying Spring in NYC’s Parks

As climate change accelerates and cities grow warmer, researchers assumed spring would arrive earlier each year. Yet, a new study led by scientists at the Yale School of the Environment suggests the opposite is happening in New York City’s parks. Spring is arriving later, especially in the city’s medium-sized green spaces.

“Previously, the assumption was that warmer temperatures would advance the start of spring,” said Juwon Kong, a postdoctoral associate in the Seto Lab at the Yale School of Environment, and lead author of the study, published in Environmental Research Letters.  “However, now we’re seeing that warming, particularly in winter, is actually delaying it.”

Kong and a group of researchers, examined more than two decades of satellite imagery and advanced remote sensing analysis, finding that warming winters and intensified urban heat are delaying the start of the growing season (SOS) for trees in the city’s parks, and the implications could reach beyond New York. 

“What’s happening to trees here could be what happens in natural areas 20 or 30 years from now,” Kong said.

Less chilling out 

In temperate regions such as the Northeast, trees rely on a period of winter chill to prepare for spring growth. “Trees need a certain amount of cold to transition from dormancy. Then, after accumulating enough cold, they also need consistent warmth to leaf out, but not too early to avoid frost," Kong explained.

Researchers found that as NYC winters have warmed, trees no longer experience the chill they need.  “We observed through satellite remote sensing that the start-of-season is being delayed,” Kong said. “It’s counterintuitive, but the warmer winters make trees wait longer for spring.”

This decoupling of spring temperature and tree response has become especially evident in the last decade. “In the early 2000s, warmer spring temperatures clearly advanced the start of season,” Kong said. “However, more recently, that relationship has shifted.”

Medium parks, maximum risk

Among the study’s most surprising findings is how the size of a park affects its vulnerability to warming. Medium-sized parks, between roughly 1 and 18 hectares (or 34 American football fields), show the most significant delays.

“These parks are too small to have a strong interior buffer from the urban heat, but too big to be fully dominated by edge conditions,” Kong said. “They’re caught in the middle. The surrounding heat, buildings, and traffic still influence their interiors.”

Smaller pocket parks (<1 hectare), by contrast, are so small that even their interiors resemble the thermal conditions of surrounding streets, and while larger regional parks have cooler, more stable microclimates, they too are experiencing a rise in urbanization intensity along their edges.

A window into the future

The research team fused data from multiple satellite sources to create a daily, 30-meter resolution dataset stretching back more than 20 years. That long-time series was key, the authors noted.

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“You can’t identify climate change impacts with just four or five years of data,” Kong said. “We needed to look at two decades of trends.”

This is also the first study to investigate long-term phenological shifts within an urban area at the city scale rather than just comparing urban cores to rural surroundings. 

“Most previous studies compare the city to outside the city,” Kong said. “We wanted to know what’s happening within the city, within the urban vegetation itself.”

Kong hopes to apply the methodology to other cities and sees urban parks as early indicators of future ecological shifts elsewhere that will have consequences for how city and forest managers plan for the future. 

As warming temperatures disrupt the timing of natural events, city planners, ecologists, and foresters may need to recalibrate their expectations. Delayed leaf-out could affect carbon uptake, urban cooling, and the services urban trees provide, from biodiversity support to mental health benefits for residents.

“Some ecologists say that earlier springs are good because they mean more carbon uptake,” Kong added. “However, if spring is delayed instead, it could reduce that benefit. We need to rethink those projections.”