Coastal challengesSea Levels Are Rising Fastest in Big Cities – Here’s Why
It is well known that climate-induced sea level rise is a major threat. What is less well know is the threat of sinking land. And in many of the most populated coastal areas, the land is sinking even faster than the sea is rising.
It is well known that climate-induced sea level rise is a major threat. What is less well know is the threat of sinking land. And in many of the most populated coastal areas, the land is sinking even faster than the sea is rising.
Parts of Tokyo for instance sank by 4 meters during the 20th century, with 2 meters or more of sinking reported in Shanghai, Bangkok, and New Orleans. This process is known as subsidence. Slow subsidence happens naturally in river deltas, and it can be accelerated by the extraction of groundwater, oil or gas which causes the soil to consolidate and the surface to lose elevation.
Subsidence leads to relative sea level rise (sea level rise plus land sinking). It turns croplands salty, damages buildings, causes widespread flooding and can even mean the loss of entire coastal areas.
Subsidence can threaten flooding in low-lying coastal areas, much more so than rising sea levels, yet scientists are only just realizing the global implications of the threat with respect to coastal cities.
In fact, while the average coastal area experiences relative sea level rise of less than 3mm per year, the average coastal resident experiences a rise of around 8mm to 10mm per year. This is because so many people live in deltas and especially cities on deltas that are subsiding. That’s the key finding of our new research, where we analyzed how fast cities are sinking across the world and compared them with global subsidence data including less densely populated coastlines.
Our finding reflects that people often choose to live in river deltas, floodplains and other areas that were already prone to sinking, and in doing so will further enhance subsidence. In particular, subsiding cities contain more than 150 million people in the coastal zone – that’s roughly 20% of people in the world who live by the sea. This means relative sea level rise will have a more sudden and more severe impact than scientists had originally thought.
Here are a few of the most affected cities:
Jakarta
The Indonesian capital Jakarta is home to 10 million people, and is built on low-lying land next to the sea. Groundwater extraction caused the city to sink more than three meters from 1947 to 2010 and much of the city is still sinking by 10cm or more each year.
