Climate science is all at sea if the ‘experts’ won’t admit what they simply don’t know, argues journalist and Norfolk Island resident Susan Prior.
If you want to gain some perspective on life, the globe and our place in it, there’s no better place to go than a diminutive, remote rock in the South Pacific: Norfolk Island. There is nothing like stepping away from the hustle and broadening one’s horizons to gain some insight into the bigger picture. And the perfect place on Norfolk Island to do this, is from Mount Pitt, 318 metres above mean sea level, at the top of this island. From there you have near 360-degree views of endless ocean. From there it doesn’t take a giant leap of the imagination to truly sense Norfolk’s isolation: nothing out east until you reach Chile; go due south and you skim the tip of New Zealand’s South Island, but then nothing but the purest of air until Antarctica; west lies Australia, 1676km north-east of Sydney as the petrel flies; while due north are a few small islands before bumping into Russia at the other end of the Earth.
This faraway island is adrift in the ceaseless currents of the oceans that batter its shores. Standing on this rock reminds you just how small and insignificant we are, but also how much world there is ‘out there’. Once you have grasped that, it is a good time to pour yourself and a friend a coffee from your Thermos and contemplate the wonderment of that bigger picture. So why do we need this perspective?
In Chapter 17 of the IPA’s recent publication Climate Change: The Facts 2020—a chapter written by Arthur Day and co-author Jennifer Marohasy—the authors talk about “perspectives on sea levels”. One of the tricky things about accurately calculating sea-level rise is obtaining reliable historic data from which to work. There are many variables—including land movements, changes in weather patterns, pauses, and declines or accelerations in sea level, and more besides—with successful modelling heavily dependent on a chain of assumptions. Day and Marohasy outline these in more detail in their chapter.
According to Day and Marohasy, the Intergovernmental Panel on Climate Change (IPCC) has concluded that the oceans have risen about 19cm at an annual rate of 1.7mm between 1901 and 2010 (IPCC 2013). More recently, the CSIRO (2017) position was similar—which was that from 1880 to 2014, the globally-averaged sea level rose about 23cm, with an average annual rate of rise of about 1.6mm. This message is propagated, like the tides, globally, and generally without much question. The average person, like myself, looking out at the vast seething ocean from the top of Mount Pitt, wonders how on Earth such a miniscule average rate rise of 1.6 or 1.7mm a year—about the width of a pinhead—can be calculated. But I’m no scientist, and moreover I’ve been reassured that science is truth. To investigate further, I thought I would take a journey around what we know about sea levels as they pertain to Norfolk Island, which, I’ll be honest, led me to more than a few instances of “not sure”, “possibly” and “we don’t know” from various agencies and government departments.
The alleged annual sea level rise is about the width of a pinhead.
But first, a quick summation of Norfolk Island’s vital stats. About 5km by 8km—a land area of about 37km2—Norfolk Island is the top of an elongated shield volcano that has existed for 2.5 million years. Two other islands are in the group—Phillip and Nepean—plus an array of islets dotted picturesquely around the cliff-ringed coastline. Most of the island sits on a plateau about 100m above sea level, albeit with steep hills and valleys.
As Charles Toogood Downing wrote in 1855, On Norfolk Island, its Character and Productions:
The surface is so irregular that, in the language of a sailor, if correctly laid down in a plan, with all the hills and valleys accurately represented, Norfolk Island would very much resemble the waves of the sea in a gale of wind; for it is composed wholly of long, narrow, and very steep ridges of hills with deep gullies, which are as narrow at the bottom as the hills are at the top.
A relatively small alluvial plain fringed by a coral reef-lined lagoon to the south of the island provides the best access to the sea, and this is where the earliest settlements were constructed. Today, this scenic area, known as Kingston, features on many tourist brochures. An external territory of Australia, Norfolk Island has an Antarctic international phone code (+672), a New South Wales postcode (2899), and, without State-level representation, its citizens vote in the federal seat of Bean, in Canberra. It has had numerous changes to its governance arrangements over the course of its various settlements, which have often led to the island being ‘forgotten’ about, in geoscientific terms, anyway. The charting of the island is a case in point. Until 1996, the BA1110, a marine chart prepared by Captain HM Denham in 1855, was the second-oldest marine chart still in active use in Australia. That led to HMAS Flinders doing a survey in the mid-1990s, creating a new, more accurate chart: AUS609.
The sea-faring Polynesians who first settled on the island circa 1150 AD were there long enough to create a small continuous settlement that lasted about 300 years. First charted by Europeans in 1774 on Captain James Cook’s second voyage of discovery, it was then settled by the British in 1788. Cook had reported on the island’s possibilities for servicing the British naval fleet in the Pacific arena: the tall straight Norfolk Island Pines for masts, sisal for sails and rope, and no inhabitants to object to the intrusion. So it was that Lieutenant Philip Gidley King, the man entrusted by Captain Arthur Phillip to settle the remote outpost, arrived on Norfolk Island on 6 March, just five weeks after the First Fleet sailed into Botany Bay on 26 January.
Long gaps in records make extracting useful data very difficult.
This colonial or first settlement (second, actually, as Polynesians were the first) lasted until 1814, when the island was abandoned, only to be resettled in 1825 in what is known as the penal settlement. Many of Kingston’s historic buildings date from this period and are among the best surviving examples of colonial architecture—and arguably a more intact and accurate representation of convict times than the better-known settlement of Port Arthur in Tasmania. This settlement, associated with the shocking treatment of convicts by some cruel commandants and overseers, ran continuously until 1855.
In 1856, the Pitcairn Islanders—descendants of the infamous Bounty mutineers—arrived, resettled from their Pitcairn hideaway by Queen Victoria. Today, Norfolk Island’s combination of bucolic beauty and fascinating interwoven histories makes for a popular holiday destination.
According to Day and Marohasy, in 1841, at the suggestion of an influential Prussian scientist, Antarctic explorer Captain Sir James Ross organised for a mark to be cut into a cliff face on the Isle of the Dead, an islet off the coast at Port Arthur in Tasmania. From this mark, it has been calculated that sea level has risen 13cm ±3cm relative to the land. (If you’re not mathematically inclined, every time you see ± just say “plus or minus”. In this example, we think it’s risen by 13cm, but there’s some uncertainty around it which means the best we can say is that it’s somewhere between 10cm and 16cm). Similarly, the estimate of sea level rise is that it took place at an average rate of 0.8±0.2mm/year—or 1.0±0.3mm/year if local upward movement of the land surface is taken into account since 1841.
Of course, there are many historical parallels between Port Arthur and Kingston. Recently, Marohasy was contacted by Howard Dewhirst, a reader of Climate Change: The Facts 2020, about a survey marker on the Kingston pier. She asked me to investigate (as I now live on the island), hoping this marker, too, dated back to the 1840s and Norfolk’s penal settlement. Marohasy was hoping it would be able to give an insight into the mean sea level then compared with today’s mean sea level (MSL). After all, you don’t know what is happening unless you are measuring.
Keep uppermost in your mind as I discuss the measuring equipment and practices on Norfolk Island, we are talking about measuring in real terms about a pinhead of sea-level rise a year here. At first blush, the lettering style on the marker, which does have a 19th century look to it (see Figure 1) appears to support Marohasy’s idea. The detail on the survey marker’s concrete gives the latitude and longitude and records that it is 15′.32 (15.32 feet) above MSL. The fact this is written in feet with a decimal point, rather than feet and inches, gives away the fact the marker is not 19th century. MSL is also a modern acronym. The data given on the collar—latitude, longitude and MSL—concurs exactly with that given on a survey chart of the island by Mr R Regnault, Surveyor, prepared in 1953.
Sadly for our quest, I therefore had to tell Jennifer that this marker, known as BM No. 1 (BMs, or benchmarks, are marks that indicate elevation; however this marker was also used for triangulation), more than likely dates to 1957 when the first tide gauge was installed on Kingston Pier. I also investigated this tide gauge as a potential source of data. A wind-up instrument, this was updated over the years to an analogue punch tape affair, then an electronic data log, and finally to an automatic download. It was decommissioned on 15 September 2015. In the late 1980s, a local person whose job it was to monitor the gauge, recalls tearing off the punch tape and sending it in the post each week to CSIRO in Tasmania.
The most recent gauge, before it was decommissioned, was connected to a GPS station installed between the peaks of chimneys on the Pier Store at the pier’s head. This building was the commissariat store until a flood caused by a large tidal wave—some refer to it as a tsunami—on 8 March 1834 rendered it unsuitable for this purpose. I tried to find out why the GPS station was installed on this roof, an old structure with possibly not the steadiest foundations, and not on a solid rock nearby, but without luck.
The GPS station was on an old roof, not on a solid rock nearby.
BM No. 1 has been used as the primary reference for all the tidal gauges installed on the island since 1957. More modern marks along the jetty are well documented, linking BM No. 1 to the tidal gauges. A tsunami warning gauge was installed by the Australian Bureau of Meteorology (BoM) in September 2009. This gauge ran side by side with the tidal gauge until September 2015. Some digging around on-island, led to some excitement, as confirmed the old marker that was in situ before BM No. 1, is still in existence. With some help, I found it behind a shed in a paddock (see Figure 2).
Sadly, though, the markings on the stone are hard to decipher and are meaningless without any accompanying data. The old records could well be sitting in a random cardboard box in some dusty storeroom, who knows where.
Long gaps in the records, and gauge maintenance issues, make it very difficult to extract useful data on the tides. Only the data from 1994 to 2015 is considered reliable enough for inclusion in the Permanent Service for Mean Sea Level (PSMSL) global baseline dataset used for sea-level analysis.
A recent BoM calculation of MSL at Kingston Jetty used only these 1994–2015 tide gauge measurements. The BoM’s calculated MSL lies 4.730m below the BM1 benchmark. At 4.730m this lies around 60mm lower down than the older 4.670m (or 15.32 feet) MSL calculation carved into the cement collar around the old benchmark. However, considering the accumulated measurement uncertainties, these differences are still not ‘significant’ in that they lie ‘within error’. In the greater scheme of things, such as the great swings in sea levels from month to month, the levels are close.
Figure 3 illustrates this point in graphical form. It plots a time series of the mean sea levels for each month between 1994 and 2015 based on the high-quality PSMSL data. A very simple arithmetic adjustment has been made so the data can be plotted relative to the height of the historic BM No. 1 benchmark.
Local effects can dominate sea-level change for decades.
However, focusing on just mean sea level can hide important information about local sea-level change. The Norfolk Island tide gauge has recorded a good example of this. For instance, Figure 3 shows a noticeable difference in the pace of sea-level change between the first and the second decades of the available record. There is virtually no sea-level change in the first decade, but then this is followed by a sudden acceleration in the second decade. Such changes are not unusual and yet they can be smeared out in simple long-term MSL calculations. Decadal changes in sea-level trends can be regional in extent and are often linked to air pressure effects arising from long-term swings in regional weather patterns. Although the Norfolk Island record is relatively short, examples of much longer-term tide gauge records from around the world, such as those in Chapter 17 of Climate Change: The Facts 2020, show that decadal variability in the regional rate of sea-level rise is the norm rather than the exception. Importantly, such changes are never consistently global in extent. The causes are regional, not global. If meteorological effects can stall sea-level rise for long periods, as in the first half of Figure 3, then they are equally capable of accelerating it as well, as in the second half. At Norfolk Island, as in many other places, such local effects can dominate sea-level change for decades. Therefore, perhaps we should ask if a hypothetical rate of ‘global’ sea-level rise, as calculated by the IPCC and the CSIRO, is meaningful, or of any local relevance to Norfolk Island at all?
There are a few other points to consider. As mentioned, the BoM installed a modern tsunami monitoring and warning system in 2009 (see Figure 4). As a result of its installation, 30mm has had to be added to the earlier tidal data to make it consistent with the tide data collected since the tsunami warning system was installed. This serves to illustrate the difficulties around working out the long-term tide and sea-level trends, and how complicated it can get, even when careful records of any changes to equipment or tide gauge positions are kept.
Norfolk Island also has a modern millimetre-precision GPS station for monitoring vertical and horizontal land movements sited near the airport runaway—about 2.2km away from the Kingston marker BM No. 1, and at an elevation of 159m above sea level. Theoretically, this GPS station should record vertical movements of the land, allowing any local sea-level change due to land movement to be separated from sea-level change due to the ocean. These GPS stations should, ideally, be bolted to solid rock. However, on Norfolk Island this doesn’t appear to be the case. This important instrument is only 100m from a major airport runway that serves large passenger and freight jets. It appears from a photograph (Figure 5, opposite page) available online to be sitting on a concrete pad on the island’s brown clay-rich basaltic soil, which is likely to expand and contract between wet and dry periods. This will almost certainly render the vertical movement measurements meaningless as the slab the GPS station is sitting on slowly works its way down into the soil via various up-and-down wobbles along the way. Despite making inquiries, I have been unable to find out more about this instrument’s siting, to confirm if I am correct.
This GPS station was installed in June 2008. Even if the site is perfect, the time frame is probably too short for an accurate measurement of vertical movement. The data suggests the island may be sinking at a rate of 0.12mm per year (±0.39mm/year). However, movement of a tenth of a millimetre is the width of a hair and, considering the short measurement time, and unstable siting, perhaps only the GPS is sinking! The island, in geological terms, is also whizzing along sideways, moving north-north-east with a northerly component of 45.50±0.08mm/year and an easterly component of 14.27±0.14mm/year. Maybe our mango trees will do better as we get closer to the Equator!
A survey by engineers restoring Kingston’s Bounty Street Bridge recently demonstrated the swamp area, on which the northern part of the bridge is built, has shrunk due to the drainage channels being constructed through the area. This probably happened quite early in the penal settlement era. Today, the bridge is submerged to the keystone. At high tide in Emily Bay it is possible for the water to flow up this creek into the Kingston Common area, which has now been protected with a sluice gate. Such localised subsidence might be connected with groundwater loss and lowering of the local water table in response to construction of the drainage channels. So we know the ground is moving not too far away from where BM No. 1 is sited.
More science is needed to figure out what is going on.
Here on Norfolk Island at least, it can be seen that technology is being tested to its limits in terms of calculating, with any degree of certainty, how much the sea level is rising. As I said at the start, this journey around what we know about sea levels as they pertain to Norfolk Island led me to more than a few “not sure”, “possibly” and “we don’t know” responses. In science, saying “we just don’t know” is okay. More—and better—science is needed to figure out exactly what is going on.
The top of Mount Pitt, surrounded by endless horizons and boundless seas, is a great place to gain much-needed perspective and contemplate some of these bigger issues.
Susan is a freelance writer, communications strategist, researcher, and editor living on Norfolk Island. Her current projects include a website guide to marine life in Norfolk Island’s coral reef lagoons: norfolkislandreef.com.au
With thanks to Dr Arthur Day for his invaluable assistance obtaining data for this article and making sense of it all.
To purchase Climate Change: The Facts 2020, go to climatechangethefacts.org.au