Guest Essay by Kip Hansen — 19 December 2023 — 1300 words/10 minutes
An article in another of America’s once-great-newspapers, The Washington Post, titled “These yard signs offer an inconvenient truth about sea level rise”, published on 13 December 2023 in the “Climate Solutions” section, is a narrative journalism piece about the efforts of a local artist and Miami-Dade County’s artist-in-residence Xavier Cortada to raise awareness of Miami residents about the elevation of their residential properties relative to local sea level.
Why? Miami has thousands of homes perilously close to local Mean Sea Level. That link goes to my 7-year old piece that concluded, in part: “Miami Beach is at such grave risk of sea water flooding today that it should preemptively be declared a disaster zone – not because of global-warming-driven sea level rise but due to a seeming total lack of sensible civil engineering standards and sensible building codes.”
That statement holds true for the thousands of homes built along sea water canals, just one or two feet above the local high tide line. If building those homes sounds crazy to you, then you are thinking correctly.
It sounds crazy to Miami-Dade’s artist-in-residence Xavier Cortada too. But, you see, Cortada has been an environmental and climate change activist his entire career. And he is absolutely right to be worried about those homes and businesses built so very close to Mean Sea Level, some of them at or below the:
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The NOAA Tide Gauge for Miami is NOAA ID 8723214, Virginia Key, Biscayne Bay FL:
It is obvious from the Tide Gauge data that unless there are substantial changes in natural Earth-processes, SLR in Miami will rise another 8-10 inches by 2100. Much of that amount of that Local Relative Mean SLR will be due to local land subsidence, Vertical Land Movement (VLM). Shimon Wdowinski, at Florida International University, found that VLM in the Miami Beach area runs 1-2 mm/yr and upwards to 2-3 mm/yr in localized pockets. Along waterways, neighborhoods and islands built on the fill from canal digging and channel dredging have greater subsidence.
Shimon Wdowinski provides a pretty good illustration of the constituents of Local Relative Sea Level Rise:
Subsidence is downward Vertical Land Movement (VLM) and in Miami is a major component of Local Relative SLR (as reported by tide gauges). Tide gauges are scarce in southern Florida – which is an oddity for a place so intimate with the sea – but there are six of them that report Sea Level Trends:
In the upper left corner above is the previously shown Virginia Key Tide Gauge record which is the tide gauge closest to Miami. The Lake Fort Worth Tide Gauge record (West Palm Beach, lower right) is useless, but agrees with the other five that SLR in Southern Florida is linear – not accelerating – not increasing, but just going up at the same rate across the entire length of each record. The Linear Relative Sea Level Trends at each are different, due to differences in local Vertical Land Movement, which adds to Relative SLR if the land is moving downward – subsiding — (and subtracts if the land is itself rising) — but all are linear – they are all linear trends
If this is the case, and it is, then what has gotten into our campaigning artist, Xavier Cortada?
He seems to believe that “By 2100,[according to the Southeast Florida Regional Climate Compact project] … local sea levels will rise somewhere between two and eight feet.” So, he has gone a bit overboard and established the Underwater Homeowners Association in expectation of that event. His project is to have neighbors place signs in their yards showing how many feet above sea level the property is. The home shown with an 8 on the sign is his home in Pinecrest, Florida. (I have doubts, the yard seems to slope several feet up to the house, but at least he is playing his own game.)
And what is the possibility of mean sea level rising by two to eight feet in the next 77 years?
Vanishing small – Miami has seen almost exactly 1 foot of SLR (including subsidence) in the last 100 years – a long a steady rise. And what that means is that, if the past is any indicator, and it is, that Relative SLR in Southern Florida, including the Miami area, is not going to be suddenly doubling or tripling – thus:
Miami will not be seeing 2 to 8 feet of Relative Sea Level Rise in the next 77 years – but rather 8 to 10 inches, maybe as much as 12 inches.
But wait!….does this mean Miami is off the hook? That it is not in danger from rising seas? Not in danger from hurricane storm surge?
Absolutely not – Miami is a disaster zone pre-made and just waiting to happen. Billions of dollars of built infrastructure sits on Miami Beach which is built on a sand-based/ancient-reef barrier island. Some of Miami Beach is built below Mean High High Water (highest high tides). Much of the infrastructure is underground out of necessity and below Mean Sea Level (MSL), requiring pumps to move water up and out – sewage too. And this means, when the storms come and knock out electrical power, the pumps stop working….
But not from the gently rising seas. The real problem is how close to Mean Sea Level the built environment is already. The tidal range in Biscayne Bay and Miami is only about 2 feet, low to high. The highest tide ever at Virginia Key was just over 3 feet. With many homes built on canals with only a foot or two of freeboard above high tides:
The high tide mark is easily seen on the sea walls as the dark water mark shifts to grey concrete. That’s not much freeboard: a foot, maybe 18 inches. If you somehow think that just a few Southern Florida homes are built on canals like this one, use Google Earth and get a good close look.
If Miami was to experience a Major Hurricane [a hurricane that is classified as Category 3 or higher] arriving from just the right (or wrong) direction at the same time as high tide – with the wind relentlessly pushing water up into Biscayne Bay – those lovely homes in the photo above are going to be flooded and all those boats in the canals will end up on people’s lawns or inside their houses. The effects of a Cat 5 hurricane on the City of Miami Beach would be horrific.
Cortada is right to be worried, but as with the majority of climate activists, he has been misinformed and blindly accepted exaggerated claims of disaster created out of over-heated climate models.
Miami and its surrounds are at risk from the sea – because they have built too close to the sea and too close to the local Mean Sea Level, with intentionally dug canals letting the sea reach far into the interior. Storm surge is the enemy. Almost any unusually high tides flood roads and infrastructure – a major storm with surge combining with high tides would be a flooding disaster.
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Cortada is clever and talented – but pushing misinformation to “inform the public” is not a good idea. The reality is worrisome enough – it needs no exaggeration.
Re-writing building codes in Miami-Dade County would go a long way to improving the situation (and create a building trades boom).
Not one more building should be allowed to be built with less than 8 feet of freeboard above existing Mean Higher High Water.
Thanks for reading.
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