By MARC SANTORA JAN. 19, 2017
In Pelham Bay in the Bronx, an ancient salt marsh has provided a unique laboratory to study historic sea levels and perhaps see what lies ahead. Credit Todd Heisler/The New York Times
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.
Most studies of historic trends in sea levels are conducted in rural areas. Research in urban areas, which are expected to feel the greatest impact of rising oceans, is often difficult or impossible because many of the natural marshes have been lost or deeply disturbed.
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.
Ellen K. Hartig, with the New York City parks, entering the salt marsh. Credit Todd Heisler/The New York Times
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.
Ice left at low tide in the marsh on Jan. 11. Credit Todd Heisler/The New York Times
The team worked in close coordination with Ellen K. Hartig, a scientist at the New York City Department of Parks and Recreation who specializes in wetlands.
In 2012, the researchers began taking core samples from the area, placing segments extracted from the earth into rigid plastic sleeves, labeling them and wrapping them in plastic so they could be refrigerated until they were analyzed in the lab.
Digging was the easy part. The work in the lab was painstaking and would take more than four years.
Every inch of the dirt held clues to what had been occurring in the wider world at the time it was deposited in the marsh. For instance, a small peak in the concentration of lead was found to correspond with its increased production and use during World War I, while a decline corresponded with the Great Depression.
Bottom of Form
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 aboveground 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.”
Troy Hill taking soil samples in the Bronx marshland. Credit Todd Heisler/The New York Times
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.”
Correction: January 19, 2017
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.