A sweeping new study suggests that looking back centuries into New York City’s natural landscape could offer critical insight into its future vulnerability to flooding as climate change intensifies.

Researchers at the New York Botanical Garden have identified more than 500 locations across the five boroughs where water once existed, where flooding persists today, and where it is expected to worsen. The team refers to these areas as “Blue Zones.”

The findings, published Wednesday in the Annals of the New York Academy of Sciences, mark the first citywide analysis of its kind. According to the study, these Blue Zones account for more than one-fifth of New York City’s land.

“Everybody was startled, including us, that it’s more than 20% of the city,” said Eric Sanderson, vice president of urban conservation at the New York Botanical Garden and an author of the paper. “That combination, you can’t really argue with it — places that were wet, are wet and will be wet in the future.”

The research draws connections between historical waterways — including marshes, ponds and streams that shaped the region over 400 years ago — and modern flood patterns. By mapping past and present hydrology alongside future projections, the study aims to guide urban planning decisions.

Lucinda Royte, the paper’s lead author and a manager of urban conservation, data tools and outreach at the Botanical Garden, said the Blue Zones highlight where flood mitigation efforts are most urgent.

“It can be a pretty good guide about where we’ll see flooding in the future as a result of coastal flooding from storm surge and sea level rise, and inland flooding from rainfall events,” Royte said. “It can help us plan a little bit better about where we need to make some infrastructural changes in the city before a flooding crisis happens.”

Roughly 1.2 million residents — about 12% of the city’s population — live within these zones, along with 11% of all buildings. To accompany the report, researchers launched an interactive digital map offering detailed, block-by-block data on historical ecology, current flood exposure and projected risks.

Major infrastructure sits within these vulnerable areas. Both LaGuardia Airport and John F. Kennedy International Airport were built on filled-in salt marshes. About one-third of public housing developments — home to some of the city’s lowest-income residents — are also located in Blue Zones.

Royte said the issue is personal. She lives in Brooklyn’s Gowanus neighborhood, historically a salt marsh, where flooding during Hurricane Ida underscored the study’s conclusions.

“My entire block was underwater,” she said. “I saw ponds and streams and wetlands return.”

The report warns that some Blue Zones could eventually become uninhabitable, reinforcing calls from other studies to expand housing, transportation and services in less vulnerable areas of the city.

Researchers say the concept underscores the scale of the threat posed by climate change, which is expected to bring rising seas, stronger storms and heavier rainfall.

“It shows how large scale this is and it lets you look at the city as a landscape,” Royte said. “We currently view the city through its political boundaries. We care about neighborhoods and zip codes, but water doesn’t care about those boundaries.”

The analysis found that nearly two-thirds of Blue Zone land is at risk from coastal flooding tied to storm surge and sea level rise. Another 5% faces inland flooding from rainfall, while 36% could experience both.

Much of the land identified is publicly owned. Government agencies control about two-thirds of it, with the Department of Parks and Recreation managing roughly half of that share.

“It’s so obvious that investing in parks will save lives and livelihoods,” said Amy Chester, director of Rebuild By Design, who reviewed the study. Her organization previously found that a majority of city parks are vulnerable to flooding.

City agencies acknowledged the importance of the findings. A Parks Department spokesperson said incorporating updated data into planning efforts could help build “a stronger, more equitable park system that protects both people and nature for generations to come.”

The Department of Environmental Protection also described the Blue Zones analysis as “a useful tool in our collaborative stormwater planning efforts,” citing ongoing work to use public land for flood mitigation and to restore natural drainage systems.

Efforts to address flood risks — including relocating residents, upgrading drainage infrastructure and installing green solutions like rain gardens — have accelerated following major storms such as Hurricane Sandy in 2012 and Hurricane Ida in 2021.

“We found, coincidentally, when you look at places that flooded during Hurricane Sandy or Hurricane Ida, they line up pretty closely with historical hydrology,” Royte said.

Sandy’s storm surge inundated former beaches and tidal marshes, while Ida’s intense rainfall overwhelmed areas once occupied by ponds and streams.

Neighborhoods like Hollis in Queens, built on what was once a pond, have faced repeated flooding. The low-lying area known as the Hole, straddling Queens and Brooklyn, regularly retains water for days after rainfall and is now the focus of potential city buyouts.

In the Bronx, officials are working to daylight Tibbetts Brook, a buried waterway, to restore its natural flow and reduce strain on the sewer system.

Despite these initiatives, researchers say broader action is needed.

“I don’t think there’s any choice but to scale up. The climate is going to force our hand, and we’re already seeing that,” Sanderson said.

“You can use planning and ecosystems to help return some of that water to the sky and some to the ground and not assume all has to go through the wastewater treatment plants.”