Most of us are familiar with the “urban warming” effect that can cause temperature stations in and around urban areas to measure warming gradients that aren’t representative of the surrounding areas. Here I present evidence for the existence of an “urban sinking” effect that can cause tide gauge records in and around urban and other populated areas to give similarly non-representative results.
I begin with a plot of three tide gauge records on the east coast of the Malaysian Peninsula, none of them in or close to urban areas. The data aren’t as complete as I would like but I feel confident enough to use them to estimate an average for this particular stretch of coastline (black line):
Figure 1: Three tide gauge records from Malaysia-Thailand (Ko Lak, Ko Sichang, Cendering). Black line shows the average.
Now I superimpose the tide gauge record for Bangkok (Fort Phrachula Chomklao), a couple of hundred kilometers north of Ko Lak at the head of the Gulf of Thailand:
Figure 2: Figure 1 data with Bangkok (Fort Phrachula Chomklao) tide gauge record superimposed in green.
As noted in an earlier post Bangkok is sinking below the waves, but not because of man-made sea level rise. It’s sinking because of excessive man-made groundwater extraction.
Calcutta (Diamond Harbour) has had the same problem, although not quite so extreme:
Figure 3: Calcutta tide gauge record vs. average of eight other Indian Peninsula tide gauge records (Vishakhpatam, Paradip, Madras, Cochin, Bombay, Karachi, Okha, Marmagao)
But Manila is about as bad, although there are no adjacent tide gauge records to compare it with:
Figure 4: Manila tide gauge record
These are of course extreme cases, easy to identify and weed out. But we see the same thing on a smaller scale in other places. An example is Venice:
Figure 5: Venice tide gauge record vs. average of nine other Mediterranean records (Malaga, Ceuta, Alicante, Marseilles, Genoa, Trieste, Bakar, Split, Dubrovnik)
Another example is Chesapeake Bay in the eastern USA. Figure 5 compares Washington D.C. at the head of the Bay with Hampton Roads at the entrance. Since 1960 sea level at Hampton Roads has risen by almost 100mm more than it has in Washington, again as a result of subsidence induced by groundwater extraction:
Figure 6: Washington DC vs. Hampton Roads tide gauge records
And then there’s this example from California. I don’t claim that this is necessarily a case of “urban sinking” because it could be caused by differential tectonic movement. Nevertheless it’s an interesting result:
Figure 7: Mean of four urban tide gauge records in California (San Francisco, Santa Monica, La Jolla, San Diego) vs. mean of four non-urban tide gauge records (Port Reyes, Monterey, Port San Luis, Ensenada)
It would be good to have more examples but there just aren’t that many tide gauge records. However, the above results document the existence of a man-made “urban sinking” effect that impacts only limited stretches of coastline and gives inflated estimates of sea level rise that aren’t regionally representative. The question is, how widespread is it?
Well, a disproportionate number of tide gauge records, and the bulk of the longer and more continuous ones, are located in large port cities where subsidence caused by groundwater extraction might be expected to be occurring or to have occurred. Added to that is the possibility that gauges mounted on piles driven into unconsolidated sediments in busy ports might subside over time because of vibration from ships and heavy machinery, although I have no hard evidence for this. The problem is not so much the easily-identified cases like those shown above but those where the subsidence is on the order of tenths of a millimeter a year, or where it has occurred over limited periods, or where a number of records in the same area are all distorted by about the same amount. These cases would be difficult if not impossible to identify, but enough of them could result in significant overestimation of both relative and absolute sea level rise.
Another question is exactly what level of overestimation we might be talking about. This question is presently impossible to answer, but regardless of how widespread urban sinking is it will result in some overestimation of sea level rise. There are many places along the world’s coastlines where water and other fluids are being pumped out of the ground in large quantities but few if any where they are being injected.