Coastal flooding from sea level rise could be much worse than expected because models neglect to consider that the big coastal cities are sinking, scientists in the United States warned Wednesday.
Researchers from the universities of Arizona and California-Berkeley make their claim in Science Advances, based on a study in San Francisco Bay Area.
Land subsidence in San Francisco is highly variable, this is true. Most of the land by the bay is subsiding relative to sea level by less than 2 millimeters (0.08 inches) a year. But some areas built on landfill and mud deposits are descending by more than a centimeter a year, says the paper by Arizona’s Manoochehr Shirzaei and Berkeley’s Roland Bürgmann.
So, flood risk maps that fail to consider subsidence are badly underestimating the problem, the scientists state.
Note that the Earth is a dynamic place and that whole continents come and go, over millions and hundreds of millions of years, rising above the waves and sinking back. But for a whole city to sink by a meter or two over a few decades because its underlying groundwater is disappearing and because heavy buildings are compressing the land has to be called anthropogenic land subsidence.
For San Francisco, maps estimating 100-year inundation hazards based on projected sea level rise and nothing else underestimate the flood risk area by around 4 percent to 91 percent, compared with maps that do factor in local subsidence, Shirzaei and Bürgmann estimate.
The importance of predicting coastal flooding cannot be overstated. Almost 2 billion people – or more than 40 percent of the world’s population – live within 100 kilometers (62 miles) of the coast, according to a recent United Nations report.
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Mega-dry in the city
A key cause of land subsidence in the megacities is water depletion. To slake the need for water by growing populations, groundwater is being overutilized. Indonesia’s Jakarta, for instance, is sinking faster than the sea is rising because of groundwater pumping. This is a metropolis of nearly 30 million people. “Jakarta is sinking so fast, it could end up underwater,” the New York Times reported last December.
South Africa’s Cape Town is notoriously teetering on the brink of drying out because of water mismanagement, and it’s far from being the only one. Cape Town’s 4 million residents have been reduced to living on 50 liters (13 gallons) of water a day – about 15 percent of what the average American consumes – and Mexico City is approaching that situation at light speed. And it's been a while now that subsidence under Alexandria, Egypt has been recognized as a problem, albeit less of one than had been feared.
Putting aside aquifer salinization and other problems that overpumping causes, when the groundwater disappears, the ground actually shrinks because of the lack of moisture.
Land subsidence can also be caused by compaction of sedimentary layers that often underlie the cities.
Another cause, not as rare as one might think, is mining activities.
Yet another cause of subsidence in megacities may be “settlement” – the sheer weight of buildings causing weak or inadequately compacted ground beneath them to consolidate and contract.
This can all add up significantly. A 2016 Chinese study published in the Remote Sensing journal found that Beijing, for example, has been sinking by 11 centimeters a year, starting in 1935, thanks in vast part to overexploitation of groundwater. Tokyo and Shanghai, meanwhile, have both sunk by more than 2 meters since 1921.
While Beijing and Cape Town are classics in groundwater abuse, Shanghai could be a poster child for the dangers of building skyward: The city’s Geological Research Institute estimates that physical weight of skyscrapers accounts for 30 percent of its surface subsidence.
Now factor all that into the scenario of rising seas (and higher storm surges) thanks to global warming, and you get much worse flooding risk than had been realized.
Waves crashing on the land
Global sea level rise is caused by two factors, both due to global warming: Added water from melting ice sheets and glaciers; and the water’s expansion as it warms.
U.S. President Donald Trump and his Republican administration may be in deep denial, but NASA isn’t. Based on satellite data, the space agency says the seas are rising by about 3.2 millimeters a year. A paper NASA published with the Cooperative Institute for Research in Environmental Sciences in February 2018 found that sea level rise isn’t “increasing steadily” – it’s accelerating.
Accelerating. Getting faster and faster. The forecasts based on assumed steady sea rise were too tame, even before factoring in land subsidence. But faster-than-expected melting of Greenland and Antarctic ice could double the sea level rise projected by 2100 versus projections that assume a constant rate of sea level rise, NASA explains. Now remember that on top of everything, the cities are sinking.
Not included in flood hazard calculations are extreme events such as volcanoes and earthquakes that can cause ocean-level fluctuations (aka tsunamis and lesser waves) or so-called hundred-year storm surges. Which are becoming more frequent as global warming exacerbates extreme weather.
Back in the Bay Area, the analysis by Shirzaei and Bürgmann estimates that the area vulnerable to flooding over the next century isn’t 51 to 413 square kilometers as had been thought (sea rise alone), but between 125 and 429 square kilometers (sea rise and subsidence).
So, they conclude that maps projecting coastal flooding in the Bay Area underestimate the area at risk from flooding by as much as 90.9 percent, compared with revised versions that account for the contribution of local land subsidence.
Though their analysis only covered the Bay Area, the researchers say their methods can be easily applied to other coastal cities.
Could people reach their own conclusions en masse and move to the continental hinterlands? Probably not: Whether because they like the sea view, hope for “Baywatch” scenes or appreciate the possibilities of maritime trade, the fact is that people have been heading coastward for millennia, and certainly in recent decades.
Even the Silicon Valley giants seem to cherry-pick their facts. The great 430,000-square-foot complex Facebook built in Menlo Park, is at risk of flooding and that's even according to the optimistic scenarios, and that The Guardian reported two years ago. Apparently the company is thinking in the direction of building a sea-wall.
Maybe they can, but others can't, and a 2015 paper published in PLOS One by scientists from Kiel University, Germany, predicts even more coastal urbanization in the decades to come, notably in Asia and Africa. As the paper points out, the coastal populations of China and Bangladesh, for instance, grew at roughly double the rate as the population inland between 1990 and 2000.
And even though people know the situation perfectly well and insurance companies are starting to seriously balk at covering flood risks, real estate by the water continues to sell like popcorn at a disaster movie, coming soon to a city near you.