研究显示纽约正缓慢下沉

New York City is slowly sinking.

With the weight of its tall buildings, streets and people, the city is sinking at an average rate of 1 to 2 millimeters each year, a new study says.

The sinking is called "subsidence."

That natural process happens everywhere as ground is compressed. But the study published in the publication Earth's Future in May sought to estimate how the huge weight of the city itself is hurrying things along.

More than 1 million buildings are spread across the city's five boroughs. The research team found that all those structures add up to about 1.5 trillion metric tons of concrete, metal and glass. That is equal to the mass of 4,700 Empire State buildings pressing down on the Earth.

The rate of compression is different throughout the city. Midtown Manhattan's tall buildings, or skyscrapers, are largely built on rock, which compresses very little. But some parts of Brooklyn, Queens and downtown Manhattan are on looser soil and sinking faster, the study said.

While the process is slow, parts of the city will eventually be under water, said Tom Parsons. He is the lead researcher of the U.S. Geological Survey. Making the problem worse is rising sea level.

"The ground is going down, and the water's coming up. At some point, those two levels will meet," said Parsons.

Parsons and his team of researchers reached their conclusions using satellite imaging, data modeling and mathematical calculations.

It will take hundreds of years before New York becomes Venice. The city in Italy is famously sinking into the Adriatic Sea. But parts of New York are more at risk.

Parsons said that Manhattan is at risk because of the borough's large weight. "The average elevation in the southern part of the island is only 1 or 2 meters above sea level — it is very close to the waterline, and so it is a deep concern."

The ocean is rising at a similar rate that the land is sinking. So the Earth's changing climate could speed up the process for parts of the city to go under water.

"It doesn't mean that we should stop building buildings...," Parsons said. "The purpose was to point this out in advance before it becomes a bigger problem."

Already, New York City is at risk of flooding because of large storms. Storms like Sandy in 2012 caused the ocean to expand inland and flooded neighborhoods after a lot of rain.

Andrew Kruczkiewicz is a senior researcher at Columbia University's Climate School, who was not involved in the research.

Kruczkiewicz said the study's findings could help inform policy makers as they plan ways to slow the rising seas. "We can't sit around and wait..," he added.

New York City is not the only place sinking. San Francisco, California, is also at risk because of pressure on the ground and the area's active earthquakes.

In Indonesia, the government is preparing for a move from Jakarta, which is sinking into the Java Sea. The government is building a new capital being constructed on the higher ground of an entirely different island.

I'm Anna Matteo.

Dan Novak adapted this story for VOA Learning English based on reporting by The Associated Press.

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subsidence — n. movement of the ground to a lower level

compress — v. to press or squeeze so that it is smaller or fills less space

borough — n. a village, town, or part of a large city that has its own government

calculation — n. a process or result of calculating something

elevation — n. the height of a place

advance — n. to move forward