More than a fifth of New York City sits on land that was once water, is near water, or will be swallowed by water in the years ahead. A new study published Wednesday puts hard numbers to what many New Yorkers have felt in their bones since the floodwaters rose on their block: this city has a water problem, and it runs deeper than any storm drain.

Researchers at the New York Botanical Garden mapped what they call “Blue Zones” across the five boroughs, identifying more than 500 areas where historical, current and future flood risk overlap. The findings appear in the Annals of the New York Academy of Sciences and represent the first comprehensive analysis of its kind. The zones cover more than 20 percent of the city’s land area.

“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 a co-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 looks back more than 400 years to map the marshes, ponds, creeks and tidal flats that once defined this string of islands before development filled them in and built over them. That buried geography, it turns out, has never stopped being relevant.

About 1.2 million New Yorkers, roughly 12 percent of the city’s population, live in Blue Zones. Eleven percent of the city’s buildings sit within them. Both LaGuardia and JFK airports were built on former salt marshes and marine ecosystems that were filled in. And about a third of the city’s public housing developments, home to many of the poorest New Yorkers, fall within Blue Zones.

Lucinda Royte, the paper’s lead author and manager of urban conservation, data tools and outreach at the Botanical Garden, knows this terrain personally. She lives in Gowanus, Brooklyn, a neighborhood built atop a salt marsh where Gowanus Creek once ran. When Hurricane Ida tore through in September 2021, the water made its case.

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

Royte argues the Blue Zone framework can help city planners and policymakers get ahead of flooding before the next crisis hits. “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,” she 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.”

Alongside the paper, the Botanical Garden launched a digital tool that provides block-by-block information on historical ecology, current flood vulnerability and projected future risk. That kind of granular data is exactly what community boards, city agencies and residents need to push for real infrastructure investment before the next major storm.

The study’s implications stretch beyond flood preparation. The paper acknowledges that some Blue Zones will become uninhabitable as sea levels rise and storms intensify. That finding puts new pressure on the city’s housing crisis in a way that rarely gets acknowledged in the rezoning and development debates at City Hall. If a significant portion of the city’s residential land is heading toward uninhabitable, New York needs to build more housing, transit and services on higher, safer ground, and it needs to start planning that shift now.

This is not an abstract problem for future generations. Neighborhoods from the Rockaways to Red Hook to parts of Lower Manhattan and the South Bronx already flood regularly. The communities bearing the highest risk are also, too often, the ones with the least political power to demand that the infrastructure investment arrive before the water does.

The Botanical Garden’s research gives city planners a clear-eyed map of what New York is actually built on. Whether City Hall uses it, or files it alongside the other climate reports that gather dust between major storms, will say a great deal about whether this city is serious about the future it is already living in.