[The Hutchinson River ecosystem, which includes one of the last remaining salt marshes in NYC, has recently come to the notice, interest and scrutiny of scientists and water advocacy groups for various studies, testing and monitoring. HRRP is very pleased that the Hutch is finally getting the attention it deserves. Our hope is that this attention will lead to greater efforts in cleaning up and restoring the river. Our work has taken on even more significance now. At our January Board meeting, we discussed the possibility of HRRP taking on a larger role by assisting in some of these efforts. We’ll let you know what happens.]
Surrounded by landmarks of modernity like Co-op City in the Bronx, a sliver of New York’s ancient past remains relatively untouched.
It is one of the city’s last salt marshes, a coastal ecosystem dominated by dense and sturdy stands of plants and grasses that has been trapping and binding sediments from the flow of the tides for thousands of years.
The sediment there tells a story of the past and, according to a new study, offers a dire warning about the future that corresponds with similar research conducted around the world.
The finding that sea levels are now rising faster than at any other time in 15 centuries is consistent with other measurements made in the western North Atlantic. But in revealing the threat to New York City specifically, the study, which was published online in the scientific journal The Holocene this month, also confirms fears that the region is on a course to realize dire projections set for the next few decades. More than $25 billion worth of infrastructure will be under direct threat from flooding through the coming decades, scientists believe, including seven hospitals, 183 hazardous waste sites and the homes of nearly 100,000 people.
That’s the case in New York, too, where, by some estimates, between 80 and 90 percent of wetlands in and around the city are gone.
But some remnants remain, including a tract that straddles the Hutchinson River Parkway, just west of City Island, around Pelham Bay in the Bronx.
On the part of the wetland area west of the highway, bordering Co-Op City — a 35-building complex with more than 50,000 residents — the degradation of the marsh is evident. Muddy flats have replaced the fields of tall grass, known as Spartina alterniflora. But to the east, on the border of the bay, the marsh is healthier.
Four years ago, a team of scientists from a diverse set of backgrounds set out for that narrow stretch of land to do something never before tried: chronicle sea levels around the city over a 1,500-year stretch.
“We were chasing one of the last little bits of marshes left,” said Troy Hill, a biologist with the federal Environmental Protection Agency who was a graduate student at Yale when the research began.
The lead author of the study, called “Relative Sea-Level Trends in New York City During the Past 1,500 Years,” is Andrew Kemp, a scientist at the Department of Ocean and Earth Sciences at Tufts University.
Mr. Kemp’s previous work, looking at the impact of sea-level increases and hurricane flooding, drew a lot of attention after Hurricane Sandy. But the lack of historical sea-level data for the city was a missing element in any attempt to understand the effects of climate change.
The soil told of local pollution, indicating the use of municipal refuse incinerators, which peaked in 1937, and offering clues of events farther afield, such as evidence of the above ground nuclear weapons tests conducted in the 1950s and 1960s.
Most important, the sediment marked tidal flows. Every day, for thousands of years, the tides would come in and deposit sediment before rolling back out. Mr. Hill likened these layers upon layers of sediment to the growth rings of a tree.
Less than 10 feet of dirt held 1,500 years of history.
Benjamin Horton, a professor at the Department of Marine and Coastal Sciences of Rutgers, who was not involved in the study, said it was valuable because it offered an unusual view of sea-level rise. “Prehistoric sea-level studies have previously been restricted to rural environments with minimal human influence,” he said. “Here Kemp and co-authors reconstruct the first sea-level rise record from an urban environment.”
Because of where the city is positioned geographically, the sea-level rise in the region may be up to 32 percent greater than the global average by the end of this century, scientists have said.
Since 1821, through nearly 200 years, the seas have risen roughly 1.5 feet, he said. But they are expected to rise by the same amount over the next 40.
While New York officials have in the past announced ambitious plans to protect the city, many of the most expansive ideas remain on the drawing board. “The efforts by New York City to adapt the city to flood risk, post-Sandy, have been intense, but are not protecting most of the city,” Mr. Orton said.
He cited two examples: a project known as the Big U, which was initially envisioned as providing protection for all of Lower Manhattan but is currently more focused on the Lower East Side, and another aimed at protecting Hunts Point in the Bronx.
Hunts Point, the food distribution center for the entire region, is a vital part of the city infrastructure. But as things stand, the city does not have the funding to build the protections, he said.
“I’d say the biggest surprise in all this is how expensive it all is — more expensive than expected,” Mr. Orton wrote in an email. “And the finding that protecting 500-plus miles of New York City shorelines from 100-year floods, plus sea level, may prove to be too expensive.”
An earlier version of this article misstated, using information from a researcher, the time it is expected to take for the sea level in the New York area to rise another 1.5 feet, after rising that amount in the last 200 years. It is 40 years, not 85.