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".

The world’s most influential climate scientist Rajendra Pachauri speaks

Did I really write that headline above? I'm quoting Crikey "IPCC chief calls for ‘sane voices’ in local climate debate". Can I take it that other climate scientists are less influential, perhaps even less qualified than Rajendra "in 35 years" Pachauri? I should point out here that no climate scientist is less qualified than R35P, I'm certainly not less qualified than R35P, even Barack Obama isn't less qualified on climate than R35P. The latter (I prefer to call him that), is, or rather was, a railway engineer, and he's been active in derailing climate science since he sold his tools on eBay. He might have a view on climate, or sea-level, or why NY subway managers decided to park most of their trains on low-lying land close to the wet stuff just before Sandy did what storms do best. BUT he's not the one to consult on climate, or sea-level, and most certainly not on glaciers. I was going to include Crikey's charming pic of the "world’s most influential climate scientist" but just couldn't force myself to upload it to Picasa. Crikey says

As The Australian claims sea level rise is not linked to global warming, the world’s most influential climate scientist has called on “sane and rational voices” to speak out and correct the record.

More than 250 scientists have gathered in Hobart today for a summit of the Intergovernmental Panel on Climate Change, the UN’s climate science body. The Oz marked the summit’s opening with a front-page “exclusive” story which claimed there was “no link” between sea level rises and global warming.

I used to have a (relatively) high opinion of John Church, despite his employment by Australia's CSIRO but now he's gone full denial on his co-authored paper I quoted from recently.

The Australian has long run a sceptical line on climate change, particularly in its opinion pages. Today’s story, written by environment editor Graham Lloyd, relied on a paper co-authored by Australian scientist Dr John Church. The paper apparently “said it could not link climate change and the rate of sea level rises in the 20th century”.

But Church, a sea level expert with the CSIRO, told a media conference today that was not an accurate description of the paper.

“So sea level clearly is linked to climate change, it is clearly linked to increases in greenhouse gases, and that’s actually in the paper which was quoted by The Australian. So the quote is, I’m sorry, inaccurate,” said Church, a co-ordinating lead author with the IPCC.

(Sigh) I just have to quote the last sentence from the paper's abstract again. This is getting tedious.

Semi-empirical methods for projecting GMSLR depend on the existence of a relationship between global climate change and the rate of GMSLR, but the implication of our closure of the budget is that such a relationship is weak or absent during the 20th century.

There's more -

While The Australian claimed the paper had found no increase in the rate of sea level rise, Church said the paper showed the rate of sea level rise had increased between the 18th and 19th centuries, and research showed a further acceleration of the rate during the 20th century.

OTOH, the abstract plainly states

The reconstructions account for the approximate constancy of the rate of GMSLR during the 20th century, which shows small or no acceleration, despite the increasing anthropogenic forcing.

So we now have "further acceleration" meaning "greater than that in the 18th and 19th centuries", and "small or no acceleration" to reconcile also.

If Church is to be believed, he is therefore co-author of a paper, the abstract of which is at odds with the content on two key findings. Perhaps the abstract (summary) was written well before the paper (IPCC-style). Perhaps the author of the abstract wasn't present at team meetings (IPCC-style). Perhaps the journal editor is a closet sceptic, altered the abstract, and is now gleefully rubbing his hands as he plans his next derailment. Perhaps someone is being economical with the truth.

'Snow joke, this Global Warming

'Snow comment needed.

More Signs Of Accelerating Sea Level Rise - what signs?

Maybe it's me, maybe there's some other meaning of the word "signs" I was previously unaware of. Maybe Forbes writer John McQuaid picked no.243 instead of no.244 from his collection of alarmist headlines. I leave you to judge - here's the entire piece.

More Signs Of Accelerating Sea Level Rise

Evidence continues to mount that recent estimates of sea level rise from global warming were wrong, and that the potential inundation to come may be much worse than previously contemplated, possibly exceeding an average of 3 feet worldwide. This from a new study published in Nature Climate Change focusing on the latest opinions of scientists studying the world’s largest ice sheets in Greenland and Antarctica, where recent, unexpectedly fast melting has raised alarms:

The study found that the ice sheets are likely to contribute a median estimate of 29 centimeters (11.4 inches) of sea level rise by 2100, with a 5 percent chance that it could exceed 84 centimeters (33 inches). When combined with other contributors to sea level rise, such as melting mountain glaciers and thermal expansion of seawater, this implies a “conceivable risk” that sea levels could rise more than 1 meter by 2100.

What’s significant here is that the 1 meter figure, once a highly unlikely, worst-case scenario, has moved decisively into the realm of possibility. It’s still unlikely, according to the scientists surveyed. But current alarming trends mean it has become a lot less so. That said, a lot of uncertainty remains; scientists are unclear on why the melting has accelerated, or if that trend will continue. But given what we’ve seen so far, it’s the official IPCC estimates for sea level rise will likely be revised upward, putting additional pressure on governments to act.

It’s still early in the century, of course, and some urban areas are starting to grapple with changes to infrastructure, coastal settlement, and other issues. But the evidence suggests that the march of disaster will probably continue to outrun attempts to prevent it.

The last sentence of the abstract of the "new study" says

On the critical question of whether recent ice-sheet behaviour is due to variability in the ice sheet–climate system or reflects a long-term trend, expert opinion is shown to be both very uncertain and undecided.

So John McQuaid has decided that "the potential inundation to come may be much worse than previously contemplated" even though the scientists themselves say they don't know whether the trends they identified did indeed reflect a long-tern trend or (natural) variability in the ice-sheet climate system. They updated the ice-sheet component in predictions in AR4 - they didn't validate them. They even say "It's still unlikely" but John (Now working on a book about the science of taste, to be published by Scribner) McQuaid knows better because "current alarming trends mean it has become a lot less so".

What trends (note the "s")? WashPo bleats "2012 hottest year on record in contiguous U.S., NOAA says". Apparently the egg-heads (hard-boiled in the extreme heat perhaps) at NOAA "described the data as part of a longer-term trend of hotter, drier and potentially more extreme weather". Hang on - I was promised "more signs" and I'm served with just one. I was assured of "alarming trends" but have a single dusted off, polished-up old (but somehow confirmed) trend dished up? Aren't short-servings illegal in the good-old US of A?

Those egg-heads seem to think that one warm year "confirms a trend". Don't take any advice on stock-market investments from these guys - they'd have you shivering and hungry on a street-corner in no time flat. Wait - there are still people hungry and shivering on street corners since Sandy struck NY. Sandy confirmed a trend too - a reducing trend in land-falling hurricanes on the eastern seaboard. Short-order writer McQuaid takes this weeks cake for hyperbole. I'd better rephrase that - hyperboles - he wouldn't understand that word if I omitted the "s".

The Nodal Tidal Cycle and a firm belief in the impossible

I've not paid much attention to the prognostications of Prof. Nils-Axel Mörner in the past, but recently his claims concerning global sea-level have been featured on several blogs, most notably Jo Nova. Several things struck me while reading the latter post, so I downloaded his paper entitled, rather uncompromisingly "Sea level is not rising" to give it a once-over. I focus here on this section with an accompanying chart:

French Guyana and Surinam

From this region, there is a very good tide-gauge record covering three 18.6-year tidal cycles (Fig. 14). The cycles vary symmetrically around a stable, horizontal zero-level. Satellite altimetry gives a rise of 3 mm/year in the same area. Facts and fiction seem to clash.

Figure 14. Changes in mean high-water level (cm: left axis) measured by tide gauges at the coast of French Guyana and Surinam (Gratiot et al., 2008; Mörner, 2010b). The record is dominated by the 18.6-year tidal cycle, which swings up and down around a long-term zero trend (the arrowed line), indicating that sea level has been quite stable over the last 50 years. However, satellite altimetry in the same region gives a rise of 3.0 mm/year – another revealing example of the difference between recorded facts and “reprocessed” satellite data.

I'll ignore for the moment that the graphic doesn't represent a tide-gauge record, that it doesn't show mean sea-level (which is what's under discussion here) and that it doesn't cover three tidal cycles. Instead I'll look first at what Gratiot et al., 2008 actually says about the nodal tidal cycle (henceforth NTC) and its manifestations and effects, and whether it affects mean sea-level (MSL) at all. The paper "Signifcant contribution of the 18.6 year tidal cycle to regional coastal changes" can be found here - it concerns itself with the effects of MHWL or mean high water level on erosion and sedimentation along the north-west coast of South America.

MHWL or mean high water level is the average of high water level (maximum tidal height) over the period under consideration. MHWL is accompanied by MLWL, mean low water level, and mean sea level is approximately half way between. The main conclusions of Gratiot et al. aren't disputed here; it seems perfectly logical that higher tidal levels cause greater shoreline erosion and that the nodal tidal cycle, which amplifies and reduces MHWL over an 18.6-year cycle has a significant effect. However, the last paragraph of the paper introduces another topic entirely; a claimed link between the NTC and mean sea level.

What I find amusing is that it isn't even necessary to understand the NTC. what causes the NTC to amplify tidal cycles worldwide, or even the extent of that amplification, to use the accompanying chart in Gratiot et al. to disprove their supplementary (and alarming) claim. However a brief description of the NTC might make things a little clearer. The Moon's orbit is inclined relative to the Earth's orbit around the Sun, which means that the Moon and the Sun pull on the oceans at a slight angle to one another. The lunar nodes are the points where the Moon's orbit crosses the plane of the Earth's orbit. When the Moon is at one of these nodes the Moon and Sun exert their pulls along the plane of Earth's orbit, and the total is greater than at other times. High water is at its highest during that part of the 18.61-year cycle, low water is at its lowest. 9.3 years either side, the effect is opposite, high water is reduced, low water increased. Back to Gratiot et al.

This study confrms the hypothesis that low tidal constituents are a major controlling factor in the evolution of the very gently sloping muddy coastal plain and shoreface of the Guyanas. Although tides have no effect on the long-term sea-level trend, they induce important fluctuations of the MHWL, when considering decadal timescales. As this timescale is particularly important for shoreline management and for policy makers, it is crucial to highlight the shoreline fluctuations associated with the 18.6 year cycle. From now to 2015, the coast of the Guyanas is expected to retreat by about 150m, 60% of this retreat resulting from the effect of the low-frequency tide constituents and 40% from sea-level rise due to global change. The nodal tidal cycle has a predictable effect on the tidal amplitude everywhere. It modulates the tidal amplitude by about 3% so that regions experiencing macro-tidal regimes are particularly concerned. Over the next decade, many coastal areas in Australia, Canada, China, England and France will experience a sea-level rise of several tens of centimetres due to the 18.6 tidal cycle (Fig. 3). This rise will contribute significantly to coastal erosion generated by global sea-level rise.

After having said that "tides have no effect on the long-term sea-level trend", they then say that "many coastal areas in Australia, Canada, China, England and France will experience a sea-level rise of several tens of centimetres due to the 18.6 tidal cycle". They mean of course that the mean sea-level will rise due to the increase from the low phase of the cycle in 2006 (see the chart above) to a high point some 9 years later. Here is their Fig.3, though from the preprint version of the paper - in the published version the title has vanished.

Figure 3 Predicted shifting of the MHWL under the 18.6 year nodal cycle for the next decade. (Adapted from the global map of tidal amplitude proposed by ref. 29 by considering a modulation of signal of 3%.) Grey areas correspond to locations of decrease or negligible rise. The black box (48W-62W-2N-12N) delimits the mud bank system of the Guyanas, South America.

Ref. 29 is Simon, B. La Marée Océanique Côtière (Institut Océanographique, Paris, 2007) if anyone's inclined to follow it up. Babel Fish translates "La Marée Océanique Côtière" as "The coastal ocean tide" which makes sense, if not good grammar, whereas Google Translate mangles it into "Tide Ocean Resort", which has a distinctly commercial flavour to it.

Note however how "Predicted shifting of the MHWL" in the caption becomes "sea-level rise" in the text. Note also that the sign of the "sea-level rise" is always positive; the scale has no negative part. This chart therefore must represent global sea-level rise due to a modification of tidal cycles. It demands the question - where does the water come from to generate this global rise? A second question - where did the water go to generate the implied previous 9-year low? A third question - why do none of the areas claimed as most affected show any part of such a large rise since 2006, nor any similarly large dip of "several tens of centimetres" over the previous 9 years? Their original chart is presumably based on a version of this one

Source: Aviso

.... which shows the lunar component of the global tides.

Their first mistake is assuming that the effect of the NTC is to amplify the tidal range by 3% globally; it does not. Several papers show the effect to be around +-5cm in the English Channel and North Sea, rather than the +-18-30cm their chart shows. The effect along the US Atlantic coast is greater than their chart shows, and the effect along the SW coast of Australia is also greater than their 3%, which I assume they calculated for their area of study, French Guyana and Surinam.

Their second mistake is a simple statistical one; if a range broadens, the difference between the mean of the range and the new maximum increases by only half the broadening. A range of 10-20 has a mean of 15, broadening to 10-30 increases the mean to 15. A range of 10-20 which broadens about its centre, the mean, produces no change in the mean at all. So even if the NTC produces an increase in the MHWL of 10cm, the mean can't increase by any more than 5cm. Even if they are correct in their assumption that an increase in MHWL produces a change in MSL, the latter can't possibly be equal to the former. In fact, tidal cycles expand and contract about their centre as I will show in detail in a later post, and as the NTC produces a simple broadening and shrinking of those cycles, it can't possibly cause any measurable change in MSL. I intend to write to the journal editors on the topic of the logical fallacies in the last paragraph in Gratiot et al., 2008.

A little background information here - It was this very topic, the Nodal Tidal Cycle, which got me interested in studying sea-level change several years ago. Having read of the claimed effect on local MSL, I decided to look for evidence in tide-gauge records. If the effect was as large as claimed, the effects should be obvious. I didn't expect to find that all sites would show it; I'd have been satisfied with a few clear examples. In short, I found none. I did find a few articles where the gauge record appeared to show some correlation for just a part of the record. Adjacent gauge sites I checked didn't show the effect at all, a fact curiously omitted by the authors. I even found a couple of published papers claiming to have found the effect. Their statistical "proof" was, to be candid, laughable. Most authors who refer to and study the effect of the NTC on MHWL, and its effect on erosion or some other phenomenon don't in general mention MSL; if they do, they don't link it with the NTC. Here's the abstract for one paper which does;

Nodal Tidal Cycle of 18.6 Yr.: Its Importance in Sea-Level Curves of the East Coast of the United States and Its Value in Explaining Long-Term Sea-Level Changes

Clifford A. Kaye and Gary W. Stuckey

The 18.6-yr cycle of the Moon's nodes dominates the annual means of high water, low water, and range at Boston and at other East Coast harbors. The maxima and minima of the high-water and range curves agree closely with the 180° and 0° long. yr, respectively, of the Moon's ascending node, and are fairly well accounted for by tide-prediction equations. The curve of annual mean sea level also reflects the cycle, but more weakly. Recognition of the cyclical nature of tidal data both simplifies and clarifies assessments of longer term sea-level trends and points to the need to include only multiples of entire cycles in the computations of these trends. When the curves of mean high water and range are used, it is possible to recognize long-term sea-level trends rapidly and to determine whether these are attributable to tidal or nontidal causes. The data suggest that the secular sea-level rise during the 20th century is tidal in origin and may be caused by vertical movement of the oceanic floor. This has the effect of reducing the volume of ocean basins, and, by changing basin geometry, alters the characteristics of terrestrial tidal constituents (standing waves).

Now I'd say that if the "secular sea-level rise during the 20th century" was caused by "vertical movement of the oceanic floor", then it's clearly not "tidal in origin". They also say that "The curve of annual mean sea level also reflects the cycle, but more weakly", while providing no proof in the text but for Boston, just one of the many sites they analysed. I'll be covering their assessment in a later post.

The realisation that the NTC simply amplified tidal cycles and didn't shift their mid-points soon dawned on me. Put simply, if high water increase due an effect on the gravitational pull of the moon, that water has to come from somewhere. The "somewhere" is in fact two "somewheres", the areas at right angles to the Earth-Moon axis along which the Moon exerts its pull. Those areas see lower tides as a result, and as the Earth rotates though the "tidal bulge" the point experiencing the higher tide experiences the lower tide some 6 hours later. The Gratiot et al. paper hasn't got a "somewhere" to act as a source for their claimed increases, ergo those worldwide increases can't happen, and Mörner's link between MHWL and sea-level is spurious.

I haven't actually provided any actual proof - you know graphs and stuff, that what I claim about the non-effect of the NTC on local mean sea-level is correct. That's for a later post, already in preparation. In the meanwhile, I'll return to my comment on Mörner's reproduced chart. The source is Fig. 1c in the Gratiot et al. paper (my bold in the caption)

c, Nodal cycles of the mhwl in Surinam and French Guiana. From 1958 to 1978, tidal gauge measurements in the mouth of the Surinam river ; from 1979 to the present, data from the tidal model of the Service Hydrographique et Océanographique de la Marine (SHOM, France www.shom.fr/ann_marees) obtained from tidal gauge measurements on Devil’s Islands (French Guiana). The corresponding phases of overall erosion and colonization reported by previous studies and in this work are shown as red and green patches. 

As I said, It's not a tide-gauge record and it doesn't show MSL but MHWL. I've never seen a tide-gauge record which looks like that, with a smooth multi-year cycle - no-one has. I'll overlook the fact that it's stitched-together from two sources. I'll even overlook that it doesn't cover "three 18.6-year tidal cycles". What I can't overlook is misrepresentation. Perhaps we sceptics should be more sceptical of what's claimed as proof - from "friends" as well as the "other side".