The rising seas represent a threat to coastal cities across the globe. Increasing that threat is the fact that most global cities are slowly sinking as the earth beneath them settles and groundwater is removed. Another factor that has seldom been considered is that in major metropolises, the weight of large, concrete-and-steel skyscrapers may be hastening the sinking.
A new study by the U.S. Geological Survey published in the journal Earth’s Future estimated the weight of every building in New York City – 1.085 million of them – which they determined to add up to 1.68 trillion pounds – and estimated the downward force of those structures across the city.
The study found that buildings have a greater effect in areas that are rich in clay compared with those areas where sand or bedrock predominate. The softer the soil, the more compression there is from buildings. It wasn’t a mistake to build large buildings in New York, but it is important to understand that doing so pushes down the ground more and more.
The study determined that New York is sinking by around 1 to 2 millimeters each year, although some areas are sinking much faster. The researchers say that cities must plan for future sinking, which will exacerbate the impact of rising seas. Sea levels are rising 1 to 2 millimeter each year, so the subsidence caused by the weight of buildings is equivalent to moving a year ahead in time with regard to rising ocean levels. This is not a cause for immediate panic, but it is important to understand that this ongoing process only increases the risk of inundation from flooding.
Photo, posted January 29, 2016, courtesy of Always Shooting via Flickr.