Guest Essay by Kip Hansen
The National Oceanic and Atmospheric Administration has some rather useful features on its web site.
One of these is the Tides and Currents which is part of its Center for Operational Oceanographic Products and Services. Having spent most of the current century living at sea, with my wife and occasionally a son or our daughter and granddaughter, on our venerable sailing catamaran, the Golden Dawn, cruising the eastern coast of the United States, the Bahamas, the Turks and Caicos, Hispaniola, and the Spanish, U.S., and British Virgin Islands, I have made extensive use of its features.
For our purposes, the most used features were the predictions of the size and timing of tides in various places which often determined our sailing schedule to allow us to leave or arrive at various states of tides, often necessary to clear the bar to a harbor entrance, pass over a coral reef, or move through a pass between islands with, instead of against, the tide. When sailing up the Hudson River in New York to our oft-times home, we must be aware of the flow of the river, the current, which is tidal, going north on the incoming tide and south on the outgoing tide. The current can be so strong against us that we often just anchor and fire up the barbie and put out a fishing line and wait for more favorable currents.
This is the data (excerpted) for Turkey Point on the Hudson. The Kingston-Rhinecliff Bridge is in the background (looking south). The tide chart shows four feet of tide, 100 miles north of New York City.
I am so familiar with the NOAA Tides and Currents site that I have tended to overlooked many features over the years. In writing about SEA LEVEL: Rise and Fall, I have often used the Sea Level Trends portion of the site, like this one for Montauk, NY.
This graph is marked “Relative Sea level Trend”. So, let’s start with a [hopefully] cute little “Pic-torial” (a picture-based tutorial) on the “what and how” of Relative Sea Level – it should only take 60 seconds or so to view and read:
What Exactly is Relative Sea Level (rise or fall)?
Tide stations measure Local (Relative) Sea Level, which refers to the height of the water as measured along the coast relative to a specific point on land
There are three possible combinations of relative sea level rise under current geological conditions:
This image shows Absolute Sea Level rising and its effect on Relative Sea Level at this Tide Gauge. As time goes by, the surface of the sea rises while the land remains motionless. This results in Relative Sea Level rise.
This image shows Relative Sea Level rising but it is the land moving down, towards the center of the Earth, which is causing the sea surface to rise in relation to the land. Downward Vertical Land Movement causes Relative Sea Level rise. Downward VLM is most often caused by movement of the Earth’s crust, compaction of the soil (which is often created by filling along the water’s edge for docks and piers) and in some cases, water and petroleum extraction.
This third image shows Relative Sea Level rising in the most usual case: the sea surface is a rising a bit and the land is moving down a bit. This is the case for most of the tide gauges on the US Eastern Seaboard, south of Boston. Combined absolute sea level rise and downward VLM make Relative Sea Level rise.
NOAA pegs global absolute sea level rise at about 1.7 mm/yr. VLM, along most of the US Eastern seaboard is of similar magnitude, in that it is measured in single-digit mm/yr, in most cases less than 5 mm/yr.
There is one more case found in the United States. In many places in Alaska, the land is moving up at a greater rate than the sea surface is moving up (at its little 1.7 mm/yr.) This causes the Relative Sea Level to be falling, though in reality it is the land rising faster than the surface of the sea is rising, thus the sea surface falls when measured against the shoreline.
Back to NOAA:
Now, back at the NOAA page for sea level rise in Montauk, NY. At the bottom of that page, and all the other individual tide stations in this section of the site for Relative Sea Level Trends, there is a link to “Comparison of northern Atlantic station trends”.
The three images from that page have been made into the changing image shown below.
What this image shows are the long-term Sea Level trends for various ports along the Eastern Seaboard of the United States. We see that they range from 1.5 mm/yr to almost 6 mm/yr.
Does this mean that the surface of the Atlantic Ocean is rising almost three times as fast in some places along the coast as in others? That the surface of the Atlantic is rising three times as fast at the Chesapeake Bay Tunnel (5.92 mm/yr) as in Southport, N.C. (2.01 mm/yr)?
Short Answer: No.
The fuller answer is supplied by NOAA itself, at the top of the page that displays these charts. NOAA states clearly:
Relative Sea Level Trends for Northern Atlantic
The graphs compare the 95% confidence intervals of relative sea level trends. Trends with the narrowest confidence intervals are based on the longest data sets. Trends with the widest confidence intervals are based on only 30-40 years of data. The graphs give an indication of the differing rates of vertical land motion, given that the absolute global sea level rise is believed to be 1.7 +/- 0.3 millimeters/year during the 20th century.
These charts (and the original graphs of Sea Level change over time at individual tide gauges) do not show how much the surface of the sea is rising (or falling) — they do not show Absolute Sea Level rise or fall. They show Relative Sea Level Trends for individual tide gauge locations and taken together simply demonstrate visually the differing rates of vertical land motion between the stations.
It is a pleasant surprise to find that NOAA, sometimes leaning towards Sea Level Rise catastrophe advocacy, not only explains this clearly, but goes on to reiterate that “global sea level rise is believed to be 1.7 +/- 0.3 millimeters/year during the 20th century” — a figure at great variance from that promulgated by NASA, which is currently claiming to show almost twice that at 3.0 +/- 0.4 mm/yr.
NOAA makes this perfectly clear: Tide Gauges measure Local Relative Sea Level and its changes and reflect mostly the magnitude/rate of vertical land movement.. Thus, Tide Gauges, by themselves, are not fit for the purpose of determining Global Average or Regional Average Sea Level changes, rise or fall.
- Tide Gauges and tide gauge-based data can only be used to measure Local Relative Sea Level and its changes. This data is the only sea level data of importance to localities.
- Comparing Tide Gauge sea level trends from one tide gauge to another, as the NOAA “northern Atlantic station trends” show, only gives us “an indication of the differing rates of vertical land motion”.
- In order for Tide Gauges and Tide Gauge data to be useful for determining actual changes in sea surface height (or rates of change in sea surface height), regionally or globally, the Tide Gauge station must have an associated GPS@TG (GPS at tide gauge) Continuously Operating [GPS] Reference Station mounted on the same structure as the tide gauge that will precisely determine vertical movement (VLM) of the tide gauge itself.
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Author’s Comment Policy:
The topic of this post is pretty narrow: NOAA’s statement of what a comparison of tide station sea level trends shows — “an indication of the differing rates of vertical land motion” and not rates of the rising sea surface.
I have been writing a lot about Sea Level and its changes over the last couple of years, so will be happy to try to answer your questions and refer you to sources of information.
NB: Sea Level Rise is an ongoing Scientific Controversy. This means that great care must be taken in reading and interpreting the past data, new studies and especially media coverage of the topic [including this essay] — bias and advocacy are rampant, opposing forces are firing repeated salvos at one another in the journals and in the press and the consensus may well simply be “an accurate measure of the prevailing bias in the field.” (h/t John Ioannidis)
Address your comments to “Kip…” if you are specifically looking for a response – that way I’ll see it for sure.
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