SEA LEVEL: Rise and Fall- Part 2 – Tide Gauges

Guest Essay by Kip Hansen

Why do we even talk about sea level and sea level rise?

There are two important points which readers must be aware of from the first mention of Sea Level Rise (SLR):

1. SLR is a real concern to coastal cities, low-lying islands and coastal and near-coastal densely-populated areas. It can be real problem. See Part 1 of this series.
2. SLR is not a threat to much else — not now, not in a hundred years — probably not in a thousand years — maybe, not ever. While it is a valid concern for some coastal cities and low-lying coastal areas, in a global sense, it is a fake problem. 

In order to talk about Sea Level Rise, we must first nail down Sea Level itself.

What is Sea Level?

In this essay, when I say sea level, I am talking about local, relative sea level — this is the level of the sea where it touches the land at any given point.  If we talk of sea level in New York City, we mean the level of the sea where it touches the land mass of Manhattan Island or Long island, the shores of Brooklyn or Queens.  This is the only sea level of any concern to any locality

There is a second concept also called sea level, which is a global standard from which elevations are measured.  This is a conceptual idea — a standardized geodetic reference point — and has nothing whatever to do with the actual level of the water in any of the Earth’s seas.  (Do not bother with the Wiki page for Sea Level — it is a mishmash of misunderstandings.  There is a 90 minute movie that explains the complexity of determining heights from modern GPS data — information from which will be used in the next part of this essay. Yes, I have watched the entire presentation, twice.)

And there is a third concept called absolute, or global, sea level, which is a generalized idea of the average height of the sea surface from the center of the Earth — you could think of it as the water level in a swimming pool which is in active use, visualizing that while there are lots of splashes and ripples and cannon-ball waves washing back and forth, adding more and more water (with the drains stopped up) would increase the absolute level of the water in the pool.   I will discuss this type of Global Sea Level in another essay in this series.

Since the level of the sea is changing every moment because of the tides, waves and wind, there is not, in reality a single experiential water level we can call local Sea Level.  To describe the actuality, we have names for the differing tidal and water height states such as Low Tide, High Tide, and in the middle, Mean Sea Level.  There are other terms for the state of the sea surface, including wave heights and frequency and the Beaufort Wind Scale which describes both the wind speed and the accompanying sea surface conditions.

This is what tides look like:

Diurnal tide cycle (left). An area has a diurnal tidal cycle if it experiences one high and one low tide every lunar day (24 hours and 50 minutes). Many areas in the Gulf of Mexico experience these types of tides.

Semidiurnal tide cycle (middle). An area has a semidiurnal tidal cycle if it experiences two high and two low tides of approximately equal size every lunar day. Many areas on the eastern coast of North America experience these tidal cycles.

Mixed Semidiurnal tide cycle (right). An area has a mixed semidiurnal tidal cycle if it experiences two high and two low tides of different size every lunar day. Many areas on the western coast of North America experience these tidal cycles.

This image shows where the differing types of tides are experienced:

Tides are caused by the gravitational pull of the Moon and the Sun on the waters of the Earth’s oceans. There are several very good tutorials online explaining the whys and hows of tides:   A short explanation is given at EarthSky here.  A longer tutorial, with several animations, is available from NOAA here (.pdf).

There are quite of number of officially established tidal states (which are just average numerical local relative water levels for each state) — they are called tidal datums and they are set in relation to a set point on the land, usually marked by a brass marker embedded in rock or concrete, a “bench mark” — all tidal datums for a particular tide station are measured in feet above or below this point.  An image of the benchmark for the Battery, NY follows, with an example tidal datums for Mayport, FL (the tidal station associated with Jacksonville, FL, which was recently flooded by Hurricane Irma):

The Australians have slightly different names, as this chart shows  (I have added the U.S. abbreviations):

Grammar Note:  They are collectively correctly referred to as “tidal datums” and not “tidal data”.  Data is the plural form and datum is the singular form, as in “Computer Definition. The singular form of data; for example, one datum. It is rarely used, and data, its plural form, is commonly used for both singular and plural.”  However, in the nomenclature of surveying (and tides), we say “A tidal datum is a standard elevation defined by a certain phase of the tide.“  and call the collective set of these elevations at a  particular place “tidal datums”.

The main points of interest to most people are the major datums, from the top down:

MHHW – Mean High High Water – the mean of the higher of the day’s two high tides.   In most places, this is not much different than Mean High Water. In the Mayport example, the difference is 0.28 feet [8.5 cm or 3.3 inches].  In some cases, where Mixed Semidiurnal Tides are experienced, they can be quite different.

MSL – Mean Sea Level – the mean of the tides, high and low.  If there were no tides at all, this would simply be local sea level.

MLLW – Mean Low Low Water – the mean of the lower of the two daily low tides. In most places, this is not much different than Mean Low Water.  In the Mayport example, the difference is 0.05 feet [1.5 cm or 0.6 inches]).  Again, it can be very different where mixed tides are experienced.

Here’ what this looks like on a beach:

On a beach, Mean Sea Level would be the vertical midpoint between MHW and MLW.

The High Water Mark is clearly visible on these pier pilings where the growth of mussels and barnacles stops.

And Sea Level?  At the moment, local relative sea level is obvious — it is the level of the sea.  There is nothing more complicated than that at any time one can see and touch the sea.   If one can note the high water mark and observe the water at its lowest point during the 12 hour and 25 minutes tide cycle, Mean Sea Level is the midpoint between the two.  Simple!

[Unfortunately, in all other senses, sea level, particularly global sea level, as a concept,  is astonishingly complicated and complex.]

For the moment, we will stay with local Relative Mean Sea Level (the level of the sea where it touches the land).

How is Mean Sea Level measured, or determined, for each location?

The answer is:

Tide Gauges

Tide Gauges used to be pretty simple — a board looking very much like a ruler sticking up out of the water, the water level hitting the board at various heights as the tides came and went, giving passing vessels an idea of how much water they could expect in the bay or harbor.  This would tell them whether or not their ship would pass over the sand bars or become grounded and possibly wrecked.  One name for this type of device is a “tide staff”.

Since that time, tide gauges have advanced and become more sophisticated.

The image above gives a generalized idea of the older style float and stilling well tide gauges and the newer acoustic-sensor gauges with satellite reporting systems and a back-up pressure sensor gauge.  Modern ships and boats retrieve tide data (really, predictions) on their GPS or chart-plotting device which tells them both magnitude and timing of tides for any day and location.  Details on the specs of various types of tide gauges currently in use in the U.S. are available in a NOAA .pdf file, “Sensor Specifications and Measurement Algorithms”.

The newest Acoustic sensor — the “Aquatrak® (Air Acoustic sensor in protective well)” — has a rated accuracy of “Relative to Datum ± 0.02 m  (Individual measurement) ± 0.005 m (monthly means)”.  For the decimal-fraction impaired, that is a rating of plus/minus 2 centimeters for individual measurements and plus/minus 5 millimeters for monthly means.

Being as gentle as possible with my language, let me point out that the rated accuracy of the monthly mean is a mathematical fantasy.  If each measurement is only accurate to ± 2 cm,  then the monthly mean cannot be MORE accurate than that — it must carry the same range of error/uncertainty as the original measurements from which it is made.   Averaging does not increase accuracy or precision.

[There is an exception — if they were averaging 1,000 measurements of the water level measured at the same place and at the same time — then the average would increase in accuracy for that moment at that place, as it would reduce any random errors between measurements but it would not reduce any systematic errors.]

Thus, as a practical matter, Local Mean Sea Levels, with the latest Tide Gauges, give us a measurement accurate to within ± 2 centimeters, or about ¾ of an inch.  This is far more accuracy than is needed for the originally intended purposes of Tide Gauges — which is to determine water levels at various tide states to enable safe movement of ships, barges and boats in harbors and in tidal rivers.   The extra accuracy does contribute to the scientific effort to understand tides and their movements, timing, magnitude and so forth.

But just let me repeat this for emphasis, as this will become important later on when we consider the use of this data to attempt to determine Global Mean Sea Level from Tide Gauge data, although Local Monthly Mean Sea Level figures are claimed to be accurate to ± 5 millimeters, they are in reality limited to the accuracy of ± 2 centimeters of their original measurements.

What constitutes Local Relative Sea Level Change?

Changing Local Relative Mean Sea Level determined by the tide station at the Battery, NY (or any other place) could be a result of the movement of the land and not the rising of the sea.  In reality, at the Battery,  it is both; the sea rises a bit, and the land sinks (or subsides) a bit, the two motions adding up to a perceived rise in local mean sea level.  I use the Battery, NY as an example as I have written about it several times here at WUWT. (see the important corrigendum at the beginning of the essay there – kh)  In summary, the land mass at the Battery is sinking at about 1.3 mm/year, about 2.6 inches over the last 50 years.  The sea has actually risen, during that same time, at that location, about 3.34 inches — the two figures adding up to the 6 inches of apparent Local Mean Sea Level Rise experienced at the Battery between 1963 and 2013 reported in the New York State Sea Level Rise Task Force Report to the Legislature — Dec 31, 2010.

This is true of every tide gauge in the world that is attached directly to a land mass (not ARGO floats, for instance) — the apparent change in local relative MSL is the arithmetic combination of change in the actual level of the sea plus the change resulting from the vertical movement of the land mass. Sinking/subsiding land mass increases apparent SLR, rising land mass reduces apparent SLR.

We know from NOAA’s careful work that the sea is not rising equally everywhere:

[Note: image shows satellite derived rates of sea level change]

nor are the seas flat:

This image shows a maximum difference of over 66 inches/2 meters in sea surface heights — very high near Japan and very low near Antarctica, with quite a bit of lumpiness in the Atlantic.

The NGS CORS project is a network of Continuously Operating Reference Stations (CORS), all on land, that provide Global Navigation Satellite System (GNSS) data in support of three dimensional positioning.  It represents the gold standard for geodetic positioning, including the vertical movement of land masses at each station.

In order for tide gauge data to be useful in determining absolute SLR (not relative local SLR) — actual rising of the surface of the sea in reference to the center of the Earth — tide gauge data must be coupled to reliable data on vertical land movement at the same site.

As we have seen in the example of the Battery, in New York City, which is associated with a coupled CORS station, the vertical land movement is of the same magnitude as the actual change in sea surface height —  2.6 inches of downward land movement and 3.34 inches of rising sea surface.  In some locations of serious land subsidence, such as the Chesapeake Bay region of the United States, downward vertical land movement exceeds rising water. (See The Chesapeake Bay Bolide Impact: A New View of Coastal Plain Evolution and Land Subsidence and Relative Sea-Level Rise in the Southern Chesapeake Bay Region )  In some parts of the Alaskan coast, sea level appears to be falling due to the uplifting of the land resulting from 6,000 years of glacial melt.

Who tracks Global Sea Level with Tide Gauges?

The Permanent Service for Mean Sea Level (PSMSL) has been responsible for the collection, publication, analysis and interpretation of sea level data from the global network of tide gauges since 1933. In 1985, they established the Global Sea Level Observing System (GLOSS), a well-designed, high-quality in situ sea level observing network to support a broad research and operational user base. Nearly every study published about Global Sea Level from tide gauge data uses PSMSL databases.   Note that this data is pre-satellite era technology — the measurements in the PSMSL data base are in situ measurements — measurements made in place at the location — they are not derived from satellite altimetry products.

This feature of the PSMSL data has positive and negative implications.  On the upside, as it is directly measured, it is not prone to satellite drift, instrument drift and error due to aging, and a host of other issues that we face with satellite-derived surface temperature, for instance.  It gives very reliable and accurate (to ± 2 cm) data on Relative Sea Levels — the only sea level data of real concern for localities.

On the other hand, those tide gauges attached to land masses are known to move up and down (as well as north, south, east and west) with the land mass itself, which is in constant, if slow, motion.  The causes of this movement include glacial isostatic adjustment, settling of land-filled areas, subsidence due to the pumping of water out of aquafers,  gas and oil pumping, and the natural processes of settling and compacting of soils in delta areas.  Upward movement of land masses results from isostatic rebound and other general movements of the Earth’s tectonic plates.

For PSMSL data to be useful at all for determining absolute (as opposed to relative) SLR, it obviously must be first corrected for vertical land movement.  However, search as I may, I was unable to determine from the PSMSL site that this was the case.  The question in my mind?  — Is it possible that the world’s premier gold-standard sea level data repository contains data not corrected for the most common confounder of the data? — I email the PSMSL directly and asked this simple question:  Are PSMSL records explicitly corrected for vertical land movement?

The answer:

“The PSMSL data is supplied/downloaded from many data suppliers so the short answer to your question is no. However, where possible we do request that the authorities supply the PSMSL with relevant levelling information so we can monitor the stability of the tide gauge.”

Note: “Leveling” does not relate to vertical land movement but to the attempt to ensure that the tide gauge remains vertically constant in regards to its associated geodetic benchmark.

If PSMSL data were corrected for at-site vertical land movement, then we could  determine changes in actual or absolute local sea surface level changes which could be then be used to determine something that might be considered a scientific rendering of Global Sea Level change.  Such a process would be complicated by the reality of geographically uneven sea surface heights, geographic areas with opposite signs of change and uneven rates-of-change. Unfortunately, PSMSL data is currently uncorrected, and very few (a relative handful) of sites are associated with continuously operating GPS stations.

What this all means

The points made in this essay add up to a couple of simple facts:

1. Tide Gauge data is invaluable for localities in determining tide states, sea surface levels relative to the land, and the rate of change of those levels — the only Sea Level data of concern for local governments and populations. However, Tide Gauge data, even the best station data from the GLOSS network, is only accurate to ±2 centimeters. All derived averages/means of tide gauge data including daily, weekly, monthly and annual means are also only accurate to ±2 centimeters.  Claims of millimetric accuracy of means are unscientific and insupportable.
2. Tide gauge data is worthless for determining Global Sea Level and/or its change unless it has been explicitly corrected by on-site CORS-like GPS reference station data capable of correcting for vertical land movement. Since the current standard for Tide Gauge data, the PSMSL GLOSS, is not corrected for vertical land movement, all studies based on this uncorrected PSMSL data producing Global Sea Level Rise findings of any kind — magnitude or rate-of-change — are based on data not suited for the purpose, are not scientifically sound and do not, cannot, inform us reliably about Global Sea Levels or Global Sea Level Change.

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Author’s Comment Policy:

I am always eager to read your comments and to try and answer your on-topic questions.

Try not to jump ahead of the series in comments — this essay covers only the issues of Tide Gauges, the accuracy of their data and the implications of these details.

I will cover, in future parts of the series: How is sea level measured by satellites?  How accurate are satellite sea level measurements anyway?  Do we know that sea level is really rising? If so, how fast is it rising?  Is it accelerating? How can we know?  Should I sell my sea front property?

Please remember, Sea Level Rise is an ongoing Scientific Controversy.  This means that great care must be taken in reading and interpreting new studies and especially media coverage of the topic — bias and advocacy are rampant, opposing forces are firing repeated salvos at one another in the journals and in the press.  In the end, the current consensus — both the alarmist consensus and the skeptical consensus — may well simply be an accurate measure of the prevailing bias in the field from each perspective.  (h/t John Ioannidis)

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