Monday, September 16, 2013

Sinking Cities: Why Rising Seas Threaten Many of The World's Great Destinations

By Terrell Johnson Published: Sep 12, 2013, 5:35 PM EDT weather.com

Shanghai, China

Since the 1920s, China's biggest city has sunk more than 6 feet thanks to a combination of pumping groundwater for drinking and the construction of dozens of densely packed skyscrapers and high-rise buildings. Today, its sinking has slowed to a pace of about half an inch per year. (STR/AFP/Getty Images)

If you walk the streets of many of the world's great coastal cities, there's something going on beneath your feet you'll never notice. But it is happening.
Little by little, the ground underneath cities from Venice to Jakarta is sinking every day. In some places, it's occurring as slowly as a millimeter or two every year, while in others, it's dropping by as much as a few inches every year.
The reasons why vary by location, but scientists who study coastal cities like New Orleans, which has been sinking for decades, point to a familiar script: As people move from the countryside and into urban areas, they bring pressure to extract more and more natural resources from the ground, like water and oil.
Meanwhile, more and bigger buildings are built – sometimes spectacularly big, like China's new Shanghai Tower, which will dwarf the Empire State Building by more than 500 feet when it's completed in 2014 – whose immense weight presses down on the Earth, sometimes even causing cracks on surrounding streets.

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While these are happening, the ground is also naturally sinking in many places, especially in coastal cities built on river deltas. Called land subsidence, this phenomenon accelerates when humans build structures like levees and dams that prevent river floods from replenishing the ground with silt and sand from upstream, and instead wash it out to sea.
And there's a fourth culprit: rising global sea levels, driven by the warming and expanding of the oceans and the melting of glaciers, polar ice, and the Greenland and West Antarctica ice sheets. Depending on the degree ground is subsiding in any given coastal city, sea levels can be expected to rise there from a few inches to a few feet over the next 50 to 100 years.
Asia's coastal megacities reveal how daunting a challenge rising sea levels will bring. As the population of Jakarta, Indonesia's largest city, has grown from about half a million in the 1930s to more than 10 million today, it has experienced some of the most dramatic subsidence of any city in the world, with some heavily populated areas near its shoreline dropping by 2 1/2 to 6 1/2 feet.
To blame is the largely uncurbed pumping of groundwater from hundreds of feet below the surface, leaving hard-to-refill cavities that speed up the city's sinking. "If deep groundwater extraction is not stopped, Jakarta will sink at least another 5 to 6 meters [16 1/2 to 19 feet] by the end of the century," JanJaap Brinkman, a hydrologist with the Dutch research institute Deltares, told The Atlantic.
For cities with resources like New York, this will mean spending many billions of dollars to keep out the sea, by building a system of giant floodwalls, levees and water gates like those used by the Dutch, who have mastered the building of dams and storm surge barriers to protect their population in the Netherlands, much of which lies below sea level.
"If there’s any place in America that we’re probably going to try and defend [from the sea], it’s Manhattan and New York," said Robert S. Young, a professor of geology at Western Carolina University and director of the university's Program for the Study of Developed Shorelines, in an interview with weather.com.
But what happens to places like the Maldives? This tiny island nation, made up of a chain of 26 atolls in the Indian Ocean, faces near-certain inundation by rising seas over the next century.
Surrounded by water on all sides, the Maldives islands lie at an average elevation of just under 5 feet above sea level. If the worst-case predictions for global sea level rise come true, by 2100 the Maldives' entire population of roughly 350,000 will have long abandoned their homeland, leaving behind a place where their culture dates back more than 2,000 years.
Barring a sudden and dramatic reversal in the rise of global temperatures – an extremely unlikely scenario, given that the Intergovernmental Panel on Climate Change reportedly expects temperatures worldwide to rise by 3 to 5 Fahrenheit degrees by the end of this century – the future of this, and other island nations like it, looks grim.
"The Pacific is fighting for its survival. Climate change has already arrived," Christopher Loeak, president of the Marshall Islands, told the Japan Times in a recent interview on the fate of his Pacific island nation, which lies about halfway between Australia and Hawaii and faces a future much like the Maldives' if current climate trends continue.
“We will not stop telling people that it is a real issue for humanity," he added. "We will be the first to feel it, but it will come to them and they should realize it.”

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