Sinking Cities and Sea Level Rise

Guest Essay by Kip Hansen – 21 April 2022

The field of sea level rise, as a scientific enterprise, is rife with all of the elements that make claimed scientific results, as published, unreliable.  The most unfortunate thing is that these unreliable results are then taken as proven fact and translated into activist talking points and treated by policy makers as The Science, despite the tentative statements and caveats included in the original research reports.

The media is complicit, as always.  Activist journalists, with a predetermined narrative given to them by their Editors, latch unto every little study and sift it for bits that match the story they have been told to write.

Sidebar:  You may think this is conspiratorial thinking.  Let me quote an email newsletter I received earlier this week from Covering Climate Now, which gives marching orders to journalists for the week 11 April to 22 April 2022:

This Earth Day, Climate and Democracy Are Both in Trouble  Since Paris, “the world seemed to have swerved sharply away from democracy and toward autocracy — and in the process dramatically limited our ability to fight the climate crisis.” So wrote Bill McKibben in a Guardian article on Monday launching Covering Climate Now’s latest joint reporting project, “Climate & Democracy.”  From April 11 through Earth Day on April 22, CCNow partner outlets including the Guardian, The Nation, Columbia Journalism Review, ABC News, CBS News, NBC News, VICE, Now This, Al Jazeera, Times of India, Die Tageszeitung (taz), and The Daily Maverick [ and many many more – kh] will run stories exploring whether defusing the climate crisis requires also defusing the democracy crisis, and how that can be accomplished. “

Here the latest offering from Katherine Kornei writing in ScienceNews:

“Coastal cities around the globe are sinking — The subsidence renders coastlines even more vulnerable to rising seas”. 

“Coastal cities around the globe are sinking by up to several centimeters per year, on average, satellite observations reveal. The one-two punch of subsiding land and rising seas means that these coastal regions are at greater risk for flooding than previously thought, researchers report in the April 16 Geophysical Research Letters.”

“By measuring the timing and intensity of those reflected waves, the team determined the height of the ground with millimeter accuracy. And because each satellite flies over the same part of the planet every 12 days, the researchers were able to trace how the ground deformed over time.”

“The largest subsidence rates — up to five centimeters per year —are mostly in Asian cities like Tianjin, China; Karachi, Pakistan; and Manila, Philippines, the team found. What’s more, one-third, or 33, of the analyzed cities are sinking in some places by more than a centimeter per year.”

It is valuable information for municipalities to know if they are over-extracting water (or petroleum, oil, etc) from beneath their cities causing serious subsidence issues.  Cities with 30 foot (10 meter) elevation above their local sea level need not worry about flooding due to subsidence – they need to worry about their low lying areas that are already too close to local sea level and liable to be flooded by exceptionally high tides and storm surges from the storms they experience today. Municipalities need to worry about their local Relative Sea Level changes – which represent combined absolute sea level change at their locality and the subsidence of the local landmass.  As you will see further on, many of the areas of the coastal cities studied are not on the shoreline, but inland away from the sea.

Some places have caused themselves quite serious situations due to water pumping from underneath cities.  Jakarta is one, I wrote about it here. I pointed out that of the six cities in the image below (from a different study), “Jakarta leads the pack having sunk over 3.5 meters (almost 12 feet) in the ten years between 1990 and 2000.”   Of course, it was only a particular part of Jakarta that had that much subsidence.  The sinking – subsidence – of Asian cities is nothing new.  It is not news. 

In the new study by Wei et al, they find that coastal cities around the globe are sinking by up to several centimeters per year, on average.  It would be nice to have science journalists who don’t fall into these knowledge black holes in which unrelated things can be averaged into a meaningful single number.  The statement is not even true for the example cities in the original study.  Here’s an example of what is being discussed in the paper:

Here we see Tampa, Florida, in two versions.  The one on the left is the mean velocity of Vertical Land Movement (VLM) according to the authors and on the right a Google Map satellite version.  I’ve moved the white squares representing water wellfields that extract underground water from the satellite to the VLM graphic as black squares and added a red square where the tide gauge for Tampa Bay is located. The red line that cross the northwestern portion of the bay is a causeway and is shown to be subsiding at about 5mm/yr,  or about the thickness of 3 U.S. 1 cent coins stacked or 2 Euro 50 cent coins.   We see that for Tampa/St. Petersberg (St Pete), the area to the north of the Bay is subsiding in the 4-6mm/yr band, but other areas to the East of the bay are rising and the southern tip of the St Pete peninsula is rising as well.  Looking at the area with Google Earth, we see that the areas that are subsiding are riddled with small pot-hole lakes, many of which will have been formed by sinkholes.   Extracting water from the porous rock underlying this area causes the surface to subside.   The area to the north and west of the causeway is Dunedin, which sits atop a rise some 80 feet above sea level.

SONEL  gives VLM for the St Pete/Tampa Bay tide gauge station (red box) at -0.5 to -1.8 mm/yr (depending on GPS solution used), which agrees in general with the study image.

And an “average” for Tampa/St Pete?  Who knows. There is nothing to meaningfully average.  For this one city, it does appear that there is subsidence which might be due to pumping of underground water reserves or it may be subsiding due to the general porous nature of the sub-surface rock of Florida.

So, the data from the study seems sound, there are areas of Tampa/St Pete that are subsiding at ~5 mm/yr.  This however is not and cannot be a “one-two punch of subsiding land and rising seas”. 

The sea level of concern to Tampa/St Pete is the Relative Sea Level measure by the tide gauge there.  It shows that the combination of rising sea and subsiding land at the tide gauge to be about 3 mm/yr – that’s the combined figure.  That actual real-world measurement is about what would be expected from the generally accepted global absolute sea level rise of 1.7/1.8 mm/yr and a 1 to 2 mm/yr subsidence at the tide gauge.  In a century that will add up to less than 1 foot of relative sea level rise. 

One of the authors of the study, Matt Wei, is quoted saying:  “We tried to balance population and geographic location,” he says. While subsidence has been measured in cities previously, earlier research has tended to focus on just one city or region. This investigation is different, Wei says. “It’s one of the first to really use data with global coverage.”

Of course, author’s admit:  “We measured subsidence rates in 99 coastal cities around the world between 2015 and 2020 using satellite data. Subsidence rates are highly variable within cities and from city to city.”    Well, they would have to be, as subsidence is entirely a local problem – local to sub-sections of cities.  Interestingly, they determined the risk of flooding based on:  “To estimate the areas where subsidence has the most immediate potential for increased risk of future flooding, we estimated the total area within each urban area that is elevated less than 10 m from sea level and subsiding faster than 2 mm/yr LOS (Figure 1b). The 2 mm/yr threshold is comparable to the rate of global mean sea level rise. We selected the 10-m elevation cutoff because flooding reached 9.1 m during the 2005 hurricane Katrina (Fritz et al., 2007) and the storm surge will continue to increase as sea level rises (Garner et al., 2017).”

The subsidence data in the study comes from the Sentinel series of satellites.  The latest Sentinel-6 – the most accurate – has a measurement uncertainty, according to its official specifications, has a “combined standard uncertainty is 3.46 cm [ 1.36 in ] for LR ALT-NRT products and 3.35 cm [ 1.32 in ]  for HR ALT-NRT products.”

[NRT = Near Real Time,   ALT = Altimetry,  LR =  Low Resolution,  HR = High resolution   Combined standard uncertainty should be notated as  +/- ]

“Wei and his team relied on observations made from 2015 to 2020 by a pair of European satellites. Instruments onboard beam microwave signals toward Earth and then record the waves that bounce back. By measuring the timing and intensity of those reflected waves, the team determined the height of the ground with millimeter accuracy.”  [ source ]   

The study authors, like many satellite sea level groups, claim that Sentinel series satellites can determine altitude (of the ground or sea surface) to millimeter accuracy.  That is likely not to be true according to the specs of the satellite system, quoted above.    However, combined with ground mounted GNSS stations, they could possibly get a trend of VLM in millimeters over five years.

Overall, the study, which is available in full here, is annoying to read and view.  All of the illustrations use bright red for alarming effect, and the color index changes from image to image, so that they cannot be compared easily – for instance, in some bright red is VLM of -10 mm year, in others -2 mm/yr. 

Bottom Lines:

1.  Some coastal cities have been found to have areas that are subsiding.  This is true of non-coastal cities as well. This was already know from other studies.  Rapidly growing mega-cities in less developed countries suffer greater subsidence problems from ground-water extraction from directly beneath these  large rapidly-growing cities.  While this was previously known, it is important for local governments and planning bodies to know and understand.

2.  None of these groundwater extraction subsidence problems have much, if any, thing to do with sea level rise.

3. Almost every waterfront  property in the world not lying on bedrock,  particularly ports with their docks and piers,  is subsiding at some rate as these are generally built at the confluence of rivers and the sea are situated on man-made filled land over the original shoreline.  This subsidence combines with absolute sea level rise (rising sea surface) to increase risks of flooding. 

4.  Even just the idea of averaging subsidence from unassociated, widely geographically separated cities makes me worry about how science is being done today. 

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Author’s Comment:

In truth, the paper being discussed strikes me as a paper written for the sole purpose of “getting published”.  It is so full of key words meant to attract attention from journals:  Global, (alarm about) sea level, human activity…is likely the main cause, etc. all leading to an alarming sounding single number.

The authors have aggravated my sense of correctness by the idiotic non-scientific action of averaging disparate unrelated data – which is used to create a prime example of the One Number to Rule Them All Fallacy (the tendency to try to reduce complex issues into a single number).

Long-time readers will already know that I have written many times about cities being endangered by sea level rise (and many are) and that most of the threats of the sea are really caused by the already-low-lying land sinking.

Thanks for reading.

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