Wednesday, 23 January 2013

The Ups and Downs of Sea-level Recording

Google sometimes comes up with odd results - not the wrong results I hasten to add, but something unexpected, something novel, something revealing. Anyway, one result was a pdf which referred to Anthony Watts (Wattsupwiththat) as a "sea level expert", with a link to a WUWT post which contains the following:
IMHO The idea that a dock (or piling) is a long term stable measurement platform is simply ludicrous. Piles sink, structures decay, boats whack them, pounding wave action loosens their grip. One feature missing from all these old style tide gauges is any way to reference the long term level of the gauge itself. In the era of GPS we can start doing this, but in the years past, how much is from simple sinking of the pilings over time? When you are looking for millimeters per year, such things become significant.
Anthony Watts would hardly describe himself as a "sea level expert", but that's by-the-by. The post was discussing sea-level past and present in the Maldives, and curiously for such a topic, presented no sea-level data whatsoever. However it's Anthony's statement "One feature missing from all these old style tide gauges is any way to reference the long term level of the gauge itself" which I'm concerned with here. I could criticise him for that statement, written in ignorance, but I'll just ask the question "Can you think of any method of establishing the level of land surface, buildings, or anything attached to the land?". if you can't, go to the bottom of the class. If you thought "surveying" or it's more technical term "levelling" you go to the top. There is indeed a "way to reference the long term level of the gauge itself". Here's a photograph of a  surveyor engaged in that very process next to the Funafuti, Tuvalu SEAFRAME station in January, 2009:

Source: SOPAC
He's using a modern state-of-the-art instrument termed a "total station". The model he's using is one of the best. It can measure to 0.5 arc-seconds in elevation and traverse - that's 1/7200 of a degree, and to 1 in 1,000,000 in line-of-sight distance, which is 1 mm in 1000 metres. The latter is achieved using laser-ranging, also used to continuously monitor the height of the instrument above the benchmark below to 0.1 mm.

I can tell you the make, model and serial number of the instrument he's using (LEICA Total Station Model TCA1800, S/N 424936), his name (Andrick Lal) and the organisation he still works for - SOPAC (Applied Geosciences Division). He was there in 2009 with Nick Brown, a  surveyor for Geoscience Australia. The details of the survey (as for all others) are published in reports on the GA ftp server and many are referenced on the GA website.

They were actually establishing the exact location and height of the tide-gauge benchmark (TGBM) wrt to the benchmarks for the CGPS (continuous GPS) pillar which is located some 2.5 km away from the gauge, but Australia's National Tidal Centre surveyors use identical equipment and techniques every year or two - this is the only Tuvalu survey I've been able to find photos for. There's a chain of an additional 12 BMs between the tide-gauge BM and the CGPS pillar BM - 14 in all.
Levelling the Funafuti CGPS pillar:

Source: SOPAC
I read somewhere that these surveys always seem to take longer on the Pacific islands than elsewhere - I can't imagine why. can you?. BTW levelling doesn't involve any adjustment - it's a measuring exercise. Some claim that CGPS is used to "automatically adjust" gauges. Nothing could be further from the truth - those that make such claims generally include a diagram of a SEAFRAME station which shows the CGPS station firmly on land. Apart from that it's not practical to do so. Few are aware of the "crustal tide" caused by the Moon, which pulls the Earth's crust up and down, synchronised with the ocean tides. At the sites of the  Funafuti stations shown above, it amounts to around 10-20 cm at peak; any automatic adjustment would lower the gauge as the ocean tide rises, and vice versa.

The point I'm making is that this process is carried out regularly and with great care and the results made available on the 'net. Not all gauge sites worldwide are levelled as frequently as those in the South Pacific Sea Level Monitoring Programme however. Most aren't the focus of attention that these islands have become. Many are relatively stable and need less frequent checking. Note that even at other sites with CGPS stations on both the gauge and on land, regular levelling takes place. The benchmark which marks the datum (reference level) for measurements is referred to as TGZ - tide gauge zero. Sometimes the nearest benchmark is above lowest water and a fixed positive offset is added to the gauge readings to give an always-positive reading - from a "virtual benchmark" in fact.

Here's a picture (looking SE) of another gauge which is relevant to the secondary theme of this post. It's the Male-B station in the Maldives, northern Indian Ocean.The gauge, or rather gauges, are on Hulule island where the airport is situated, adjacent to Malé itself.

Source: SONEL

Source: University of Hawaii Sea Level Center
The people in the background on the right aren't back-packers but are part of a University of Hawaii team, on-site to check and maintain the equipment. Looks like a helluva job to me. I don't know how they can stand all that sunshine and sea air.
I include the following description because some people (no names, no pack drill) like to give the impression that sea-level data is recorded by a man with a marked stick and a clipboard, when he can be bothered so to do. The Malé station currently has three independent gauges; a float gauge with the small dome atop on the left, a bubbler gauge with the box on top, and a radar gauge on the black bracket over the water. The two large vertical "pipes" are "stilling wells". They aren't simply open at the bottom, but have holes near the lower ends which restrict the rapid influx or efflux of water (hence "stilling") to eliminate the effect of waves, the wash of a passing boat, or a diving horse, perhaps.

The potentiometer at the top of the float gauge. The toothed metal belt carries the float.
Looking down the float gauge stilling-well; the float is centre-right at the water surface.
Source (both photos): University of Hawaii Sea Level Center
A bubbler gauge releases nitrogen gas into the water near the bottom of the stilling well. At the depth of the release valve the pressure varies with the depth of the water above, and a pressure recorder monitors the pressure of the gas as it's released. Pressure varies linearly with depth so it's easy to convert the pressure readings into an accurate depth. A temperature sensor records water temperature so consequent changes in water density and gas pressure can be allowed for.

The workings of the float gauge, and the radar gauge over the water should be fairly obvious. The dome on the central pillar contains the receiver and aerial for the CGPS (continuous GPS) station which constantly records the E-W, N-S and vertical coordinates (position) of the station. Benchmarks at the station, and three sites around up to a kilometre away provide for levelling by surveying instruments. The station collects data and transmits it to a satellite at hourly intervals and it's been recording since 1990. There's another GPS (& laser telemetry) station located some distance to the NE which is part of the DORIS system, a worldwide network used for precise determination of the JASON-1 satellite's orbit. Here's what Male-B recorded between 1990 and 2010. Bear in mind that the CGPS station recorded some millimetre-sized ups-and-downs over the last decade but they averaged out to just -0.1 mm/year fall.

Data source: PSMSL
To the south, at Gan on Addu Atoll, is another float-type gauge, with a slightly longer record but which shows a similar 13-month profile and annual trend.

Data source: PSMSL
I've never seen these charts (or any earlier version from any source) shown on any sceptical blogs. Nils-Axel Mörner has written screeds on the Maldives, but I've never seen them shown therein either. Do they show something inconvenient? What, precisely, is wrong with the unvarnished truth? Mörner has referred to them however in a 2004 paper titled "New perspectives for the future of the Maldives". I don't know when he visited the Maldives, as rather curiously, the paper doesn't give any dates.  He states in his conclusion (multiple authors, but they're his words, I'm sure)
Tide gauge data have been cited in support of an on-going rise of mean sea level (Singh et al., 2001). Tide gauge records, however, do not provide simple and straight-forward measures of regional eustatic sea level. They are often (not to say usually) dominated by the effects of local compaction and local loading subsidence. With this perspective, our multiple mor-phological and sedimentological records appear more reliable and conclusive. Besides, satellite altimetry does not record any significant rise in global sea level in the last decades (Mörner, 2003a, Fig. 2). In order fully to investigate the situation, however, available tide gauge records, now extending from 1990 to 2002, were re-examined. This reveals a total absence of any rising secular trend (Mörner, 2003b).
He's saying that tide gauge records don't provide reliable indications of sea level trends, but uses them to prove a point he's making, because he claims that what they show conveniently proves that point? There's a word (several words in fact) for that kind of thing. It's also difficult to understand how 13 to 14-year-span gauge records could possibly reveal anything about the presence or absence of a "secular trend", as in this context "secular" means long-term. He also says "extending from 1990" - the Gan record starts in 1988. Incidentally, the reference "Mörner, 2003b" was never published. There's a whole story here but it's for a future post.

I take issue with his statement that "Tide gauge records, however, do not provide simple and straight-forward measures of regional eustatic sea level. They are often (not to say usually) dominated by the effects of local compaction and local loading subsidence". They are not so "dominated", but many are affected. Many others are affected by just the opposite, where the land is rising, and sea-level rise is understated, something he fails to mention here. His term "regional eustatic sea level" is confusing and ambiguous  It's a term he's proud to have invented - eustatic means worldwide or global so he's effectively using the term "regional global sea level". It's worse than confusing and ambiguous. It's meaningless, a contradiction in terms.

In my opinion, and in the opinion of others too, the worldwide tide-gauge network provides a more reliable source of environmental data than do thermometers in weather stations. Tide gauges have inherent sources of error, they're subject to vertical movement caused by subsidence or rebound of their mountings (piers, jetties, etc) and the land or seabed beneath, but those errors can be and are identified, quantified, and allowed for by adjustment of the data. What's more. such adjustments are usually very small and full details are published on the 'net. Many gauge stations have multiple gauges as at Malé, and an increasing number use CGPS to monitor stability and record the slightest movement. Most transmit their data at regular intervals by telephone line or satellite link, so any malfunction is soon spotted. All have a linear and uniform response, unlike temperature sensors whose non-linear response has to be allowed for by the recording circuitry, and they're not subject to sensor drift which may occur for some time before it's identified. There's no equivalent of UHI either, and so far no organisation has been able to get their grubby hands on the data and homogenise it or apply suspicious "adjustments".

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