Introduction

The introduction presents comprehensive information regarding the sea level patterns observed in Tuvalu as documented in the existing literature.

Sea level rise in Tuvalu has been a significant concern due to its potential impact on this low-lying island nation (see Aung et al. 2009; Chambers and Chambers 2007; Connell 1999; Ede 2003; Lewis 1989; McCubbin et al. 2015; Yamano et al. 2007). Tuvalu is particularly vulnerable to rising sea levels, and the issue is characterized by different key points. Tuvalu has experienced a higher-than-average rate of sea level rise compared to the global average. This increase in sea level poses a direct threat to the low-lying atolls that make up the country. The observed high rate of sea level rise in Tuvalu is influenced by multidecadal oscillations, which are long-term variations in sea level. These oscillations can have a significant impact on short-term sea level records. The subsidence of tide gauges in Tuvalu contributes to the measured sea level rise. This subsidence is the result of various factors, including geological processes and local land movements. Global warming and the associated thermosteric expansion of seawater contribute to rising sea levels worldwide. Overall, the combination of global and local factors makes Tuvalu highly susceptible to the impacts of sea level rise, threatening the livelihoods and existence of the island’s population.

The sea level of Tuvalu has been previously analyzed in the literature.

According to Dickinson (1999), shoreline morphology on multiple islets of Funafuti atoll in central Tuvalu reflects a relative mid-Holocene sea-level high stand of 2.2–2.4 m above modern sea level. When Al Gore’s movie “An Inconvenient Truth” claimed in 2006 that Tuvalu was one of those “low-lying Pacific nations” whose residents have had to “evacuate their homes because of rising seas,” there were not yet data permitting to assert the rate of rise of the sea level in Tuvalu was high and accelerating. The first report of the PSLMP (South Pacific Sea Level Monitoring Project), see Australian Government Bureau of Meteorology (2006), which was published in the same year, was only based on a decade of data, and it was therefore unreliable. Other tide gauges were still providing modest rates of rise of the sea level in the Pacific islands. The average rate of rise of the sea level in the Pacific Island stations managed by the University of Hawaii, the most reliable monitoring project of the time, was in 2002 only 0.67 mm/year if computed as the average of all the 20 stations, and 0.8 mm/year if only the stations with record lengths above 25 years were considered. Table 1, from Australian Government Bureau of Meteorology (2005), presents these data. This was absolutely nothing to worry about.

Table 1 Historical sea level data and their relative sea level trends in Pacific Islands. The University of Hawaii estimations of June, 2002, as presented by Australian Government Bureau of Meteorology (2005)
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Started in 1993, as an alternative monitoring to the established tide gauges, the PSLMP started to propose trends after only a decade of recording, which was well known to be not enough to properly compute trends. One decade is a very short time window to compute sea level trends, when there are important oscillations up to six decades of periodicity. In an area subjected to multi-decadal oscillations of periodicity up to quasi-60 years (Chambers et al. 2012) a minimum of 60 years of data is necessary to infer a trend by linear fitting of the monthly average mean sea level (MSL) time series (Boretti 2020). The new project created a new sea level reality in the Pacific Islands, shadowing every other past or contemporary record. Since the first PSLMP reports (Australian Government Bureau of Meteorology 2005, 2006), much higher rates of rise were proposed in practically all the novel stations, as shown in Table 2. The fact they were based on just one decade of data and thus were unreliable, given the multi-decadal oscillations well correlated in the same basin, and the opportunity the result could have been affected by subsidence was not considered appropriately in the reports. In the Pacific Islands, same as multi-decadal oscillations, proper consideration of subsidence is mandatory to infer realistic sea level rise trends (Boretti 2020).

Table 2 The net relative sea level trend estimates after the inverted barometric pressure effect and vertical movements in the observing platform are taken into account. PSLMP estimations of June, 2005, as presented by the Australian Government Bureau of Meteorology (2005)
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In the Pacific, there are many big volcanoes, many of which reached or exceeded sea level. When eruption ceased they slowly started to sink and corals grew around them, starting as a fringing reef but growing up as the volcano sinks to acquire thick coral, as at Bikini. Darwin’s theory of coral island formation is based on this and has been repeatedly demonstrated (Ollier 1981). There is no reason all volcanoes should sink at the same rate. There can also be tectonic uplift. Niue Island is one example of vertical uplift. Walking around the lagoon, it is possible to study the old atoll rim, just like on a modern coral island by scuba diving. Also on the Trobriand Islands in Papua New Guinea (PNG), there is an east-west line of islands. Vakuta on the west is just above sea level. Then comes Kiriwina which is a normal sort of flat island, but inland there is a steep-sided plateau that shows another uplifted reef. Further east is another island with two steps, with another uplift. The amount of uplift is increasing to the east. At the Huon Peninsula, PNG, coral terraces can be traced to 2500 m (Ollier 1981). The Hawaiian Island Chain presents a special case. It seems the point of eruption is moving south, so the youngest island is in the south, still active on what is the world’s biggest mountain measured from its base. The islands get older to the north, with increasing subsidence.

With all this variation within and around the Pacific, it was (and it is) therefore mandatory to properly assess the subsidence of the tide gauge instrument, as this can be done by global positioning system (GPS) monitoring. In the above table, the rate at the Federated States of Micronesia (FSM) was derived from a comparatively short data record and was therefore flagged. Similarly flagged was the rate at Tonga comparatively larger than its neighbors. Misleading in the table are the very small vertical tide gauge movement contributions, which are not the geocentric subsidence of the tide gauge, but only the subsidence of the tide gauge relative to the instrument. The new PSLMP project was proposing for Tuvalu a rate of rise of 6.4 mm/year computed by linear fitting of the MSL despite only having one decade of data, corrected for barometric pressure and subsidence of the tide gauge but only relative to the benchmark. This was more than six times the previously known rate of rise of the historical tide gauge previously operated (compare Tables 1 and 2 results for Tuvalu). The previously operated tide gauge of Funafuti discontinued in December, 1999, was indeed suggesting a much smaller rate of rise of the relative sea level, at 0.92 mm/year in Table 1, or even less, as claimed by Daly (2001, 2002). Daly (2002) also mentioned one “news release” by the Australian Government National Tidal Facility (NTF), the governmental organization in charge of sea level monitoring before the activity was transferred to the Australian Government Bureau of Meteorology, dismissing accelerating sea level rise claims in Tuvalu as unfounded. The news release title “Sea Level in Tuvalu: It’s Present State” previously available at the link www.ntf.flinders.edu.au/TEXT/NEWS/tuvalu.pdf has been removed. Some other criticism of the claim of accelerating sea level rise in Tuvalu has also been published more recently (Eschenbach 2004; Ollier 2010; Parker 2018; Boretti 2020), building on the removal of one old tide gauge providing a historical perspective to build up a substitutional reality based on a new tide gauge. Table 3 is one latest (September, 2022) estimation of the rate of rise of the sea level in the stations covered by the PSLMP project. The rates of rise are extremely large, with the greatest values now attributed to Samoa, where unaccounted for, a large earthquake occurred in 2009 (Mörner et al. 2018; Parker et al. 2018).

Table 3 Updated overall rates of sea level movement based on SEAFRAME data from installation through September, 2022. PSLMP estimations of September, 2022, as presented by Australian Government Bureau of Meteorology (2022a, b, c)
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This study seeks to provide an explanation for the elevated rates of sea level rise observed in Tuvalu, as per the PSLMP, utilizing MSL and GPS data from Tuvalu, Honolulu, and Sydney Fort Denison. It also incorporates additional information from satellite altimetry and MSL data from other stations in the Pacific Islands. The primary objective is to assess the reliability of the PSLMP and gain insights into the factors contributing to sea level rise in Tuvalu. The focus is on understanding the impact of local elements such as multidecadal oscillations and tide gauge subsidence, as well as the global thermosteric contribution. In addition to these factors, there might be other local influences contributing to the residual sea level rise in Tuvalu, including changes in circulation within the lagoon and specific anthropogenic biases related to the tide gauge location. The goal of this study is to uncover and analyze these additional influences.

The objective of this study is to understand the pattern of sea level rise in the vicinity of Tuvalu by analyzing all available information on the subject.

In the existing literature, there exist divergent evaluations regarding the thermosteric contribution to sea level rise and the notion of land loss in Pacific atolls and islands. This study aims to explore and clarify these perspectives.

Materials and methods

This study employs a narrative review of published information alongside an original analysis of sea level and global positioning system (GPS) data. Trends are determined by fitting time series of observed relative sea levels or absolute GPS dome positions with either linear or parabolic models. These fittings are conducted using Microsoft Excel, although similar results can be obtained with alternative statistical software like Origin. The sea level patterns around Tuvalu are evaluated by examining the relative monthly average mean sea levels (MSL) recorded by tide gauges. Beyond the tide gauges in Tuvalu, data from Pacific islands and atolls, as well as long-term trend tide gauges in Honolulu and Sydney, are analyzed to strengthen and corroborate the findings. Additionally, the analysis is supplemented with information on subsidence derived from GPS monitoring.

Absolute and relative sea level rise in Tuvalu

The MSL data for FUNAFUTI B, a tide gauge with co-located a GPS dome and meteorological instruments managed by the PSLMP, is available from Australian Government Bureau of Meteorology (2022b). They offer monthly mean, maximum, and minimum sea levels. The MSL is also available on the Permanent Service for Mean Sea Levels (PSMSL) website (Permanent Service for Mean Sea Levels 2022b). The tide gauge of FUNAFUTI B has a latitude of − 8.502472 and a longitude of 179.195167. The period of data is 1993 to 2021. Completeness (%) is 96. The PSMSL record has some months missing. Three months is missing in 1993, with no further info. One month is missing in 1994, also with no further info. In 2012, sea level data return during June was not good in Tuvalu, where transmissions of sea level data were not received after June 10, adding to existing problems with ancillary meteorological data being reported from the station (Australian Government Bureau of Meteorology 2012). There were then missing 3 months in this year. Then in 2017, the station was out of service from October 4 to 25 for another missing month (Australian Government Bureau of Meteorology 2017). As noticed in 2019 (Australian Government Bureau of Meteorology 2019) the station battery at Tuvalu continued to deteriorate and eventually failed on August 14 but is scheduled to be replaced as soon as possible. The data returned from Tuvalu were mainly collected during daylight hours when the solar panels provided enough power to charge the battery. There were then other missing 3 months in the year. In the PSLMP database, only two months of data is missing, with therefore some discrepancies between the PSLMP and PSMSL datasets, for a tide gauge which does not seem free of troubles. As better discussed later, the GPS antenna co-located with the tide gauge of Funafuti B does not seem to have provided too much data to other providers of subsidence analyses, such as Nevada Geodetic Laboratory or Système d’Observation du Niveau des Eaux Littorales. From the latest report by the PSLMP (Australian Government Bureau of Meteorology 2022c), the rate of rise of the sea level of Tuvalu based on the MSL measured by the tide gauge, after corrections for subsidence (but only relative to the benchmark) and barometric pressure, is + 4.8 mm/year since 1993. Figure 1 is this relative sea level trend analysis using the PSMSL MSL data (Permanent Service for Mean Sea Levels 2022a).

Fig. 1
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Linear trend analysis of the MSL in Funafuti B, Tuvalu. Data from PSMSL (Permanent Service for Mean Sea Levels 2022a)

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As previously mentioned, very high rates of rise are common to the short-term tide gauges of the area which started recording at times of low water levels such as in the early 1990s. With less than 60 years of data, it is incorrect to compute rates of rise of the sea levels, because of the periodic oscillations of periodicity up to quasi-60 years biasing up the result (Boretti 2020; Chambers et al. 2012). Starting from a period of low water levels, the rates of rise computed by linear fitting of the MSL are largely overrated (Boretti 2020). One possible explanation for the high sea level rise experienced in Tuvalu, at the tide gauge of Funafuti B, is the short time window inadequate to infer a proper trend. Another possible explanation is the unaccounted-for subsidence of the instrument. In addition to its system of tide gauge facilities, the PSLMP also includes a network of earth monitoring stations for geodetic observations, implemented and maintained by Geoscience Australia. The earth monitoring installations provide GPS measurements to allow absolute determination of the vertical height of the tide gauges that measure sea level. Additionally, pressure and temperature are also measured. While the theoretically superior approach should permit a better estimation of the relative and absolute rates of rise of the sea levels, correcting for land subsidence and barometric pressure (Australian Government Bureau of Meteorology 2022a, b, c, 2008), not necessarily the newly established tide gauge was providing more reliable results than the old tide gauge discontinued. Precise leveling of the height of the SEAFRAME sea level sensor relative to an array of land-based benchmarks is undertaken periodically, preferably every 18 months. The precision to which the survey must be performed is dependent on the distance between the SEAFRAME sensor benchmark and the primary tide gauge benchmark and forms part of the project’s design specifications. This correction for the motion of the instrument vs. the benchmark is not a measure of the subsidence of the instrument in an absolute geocentric reference frame, as there is no guarantee the benchmark is not moving. The correction for the motion of the instrument is generally small, about 0.1 mm/year in Tuvalu. While the sea level result should account for the subsidence of the instrument, this subsidence is practically unaccounted for. Based on the vertical position of the GPS dome of the airport (TUVA), the ellipsoidal information from SONEL (Système d’Observation du Niveau des Eaux Littorales 2022) suggests the subsidence at the tide gauge may be above 1.5 mm/year, as better explained later. The tide gauge has a GPS dome co-located, which is TUVT, but PSMSL (Permanent Service for Mean Sea Levels 2022a) and SONEL do not use this co-located GPS dome. They use instead the GPS dome of TUVA at the airport. TUVA has a distance to the tide gauge of 2544 m. NGL (Nevada Geodetic Laboratory 2022) has some data from this co-located dome of TUVT, but not enough to infer any trend (Fig. 2). SONEL, NGL, and JPL (Jet Propulsion Laboratory 2022) all have subsidence information for the GPS dome of TUVA. They consistently attribute subsidence to the TUVA tide gauge at the airport subsidence of about 1.5 mm/year (Fig. 3).

Fig. 2
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Absolute vertical position of the GPS dome of TUVT from NGL (Nevada Geodetic Laboratory 2022). Data are not enough to infer a trend

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Fig. 3
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Absolute vertical position of the GPS dome of TUVA, in Tuvalu, from NGL, JPL, and SONEL. Images from Système d’Observation du Niveau des Eaux Littorales (2022) and Nevada Geodetic Laboratory (2022). Credit Système d’Observation du Niveau des Eaux Littorales and Nevada Geodetic Laboratory

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Thus, the relative sea level rise corrected for subsidence could be + 4.71 mm/year minus the about 1.57 mm/year of subsidence of the TUVA GPS dome, and not the entire + 4.71 mm/year. This still gives a + 3.14 mm/year of absolute sea level rise since 1993 which has to be compared to the absolute sea level rise since 1993 of other more reliable tide gauges. Also important is to understand how the sea level behaves over a time window long enough. To do so, the records of the closest long-term trend tide gauges must be considered. Worth analyzing is also the other tide gauge in Tuvalu, Funafuti (Permanent Service for Mean Sea Levels 2022b) which recorded data since 1977 and was discontinued in 1999, leaving the other tide gauge of Funafuti B alone representing Tuvalu. At latitude − 8.533333 and longitude 179.216667, this other tide gauge was quite close to the new one. With the time of data 1977 to 1999, and completeness (%) 92, the apparent relative rate of rise of this tide gauge was only 0.86 mm/year (Fig. 4), even if Funafuti probably suffered from extra subsidence compared to Funafuti B (see the difference of MSL on the bottom). The assessment of a relative rate of rise in Funafuti B is biased by the low water levels at the end of 1998. Singularly, the rate of rise computed by using the data of Funafuti B was claimed to be unreliable because one low water level event was about the end of the record. Opposite, a low water level event about the start of the record is not considered a bias. PSMSL mentions all data up to and including 1998 received from the University of Hawaii Sea Level Center, which was the organization responsible for the operation of the Funafuti tide gauge. This statement is incorrect. As acknowledged by PSMSL in 2018, the data for 2000 and 2001, which were used to question the analysis by Eschenbach (2004), was wrongly added to the Funafuti record despite having been recorded by the other tide gauge of Funafuti B as apparently requested by an individual and not an organization. These wrong data have finally been removed. Figure 4 also presents the composite tide gauge record of Funafuti + Funafuti B, obtained considering data from Funafuti only from 1977 to 1993, data from both the tide gauges from 1993 to 2000, and data from Funafuti B since 2000. During the period 1993 to 2000, a simple averaging is applied to the two readings. With data from 1977 to 2022, the relative rate of rise reduces to + 3.12 mm/year. To mitigate the issue of the short record, also United States National Oceanic and Atmospheric Administration (2022a) couples the data of Funafuti to the data of Funafuti B. The relative sea level trend is + 3.33 mm/year with a 95% confidence interval of + / − 3.09 mm/year based on the MSL data from 1977 to 2018. A datum offset correction of 0.051 m was applied from November, 1977, to April, 1993, based on the systematic difference in datum between the stations during the period of overlap. An about 1.57 mm/year of subsidence would further reduce the absolute rate of rise to + 1.55 to 1.78 mm/year.

Fig. 4
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Linear trend analysis of the MSL in Funafuti and Funafuti B, Tuvalu, the difference between the MSL of Funafuti B and the MSL of Funafuti during the period of common recording, and the MSL of the composite record Funafuti + Funafuti B. Data from PSMSL (Permanent Service for Mean Sea Levels 2022a, b)

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Absolute and relative sea level rise in Honolulu and Sydney Fort Denison

We know that the focus on the short record is misleading, and the focus on short records with a subsiding tide gauge is even more misleading. So, we may look at the tide gauges of Honolulu, HI, USA, and Sydney Fort Denison, NSW, AU, which span the right amount of years to infer the pattern of sea level rise since the start of the last century, to understand which other explanation may be available for the high rate of rise of the sea level in Tuvalu. Figure 5 presents the linear trend analysis of the MSL in Honolulu, HI, USA, with only the data since 1993, and with the data since 1905.

Fig. 5
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Linear trend analysis of the MSL in Honolulu, HI, USA. Data from PSMSL. Data from PSMSL (Permanent Service for Mean Sea Levels 2022c)

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Honolulu has a relative rate of the sea level based on the MSL data since 1905 of + 1.56 mm/year. Since 1993, the apparent rate of rise is + 2.98 mm/year, which is still much less than the value for Tuvalu. The Honolulu tide gauge is subjected to subsidence (Fig. 6).

Fig. 6
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Absolute vertical position of the GPS dome of HNLC, in Honolulu, HI, USA, from NGL, JPL, and SONEL. Images from Système d’Observation du Niveau des Eaux Littorales (2022) and Nevada Geodetic Laboratory (2022). Credit Système d’Observation du Niveau des Eaux Littorales and Nevada Geodetic Laboratory

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The HNLC GPS dome is co-located with the tide gauge (1 m from the tide gauge). Thus, the apparent relative sea level rise since 1993 corrected for subsidence could be + 2.98 mm/year minus the about 0.5 mm/year of subsidence of the HNLC GPS dome. This gives a + 2.48 mm/year of absolute sea level rise since 1993 which is less than Tuvalu. Figure 7 presents the linear trend analysis of the MSL in Sydney Fort Denison, NSW, AU, with only the data since 1993, and with the data since 1886. Sydney Fort Denison + Fort Denison 2 has a relative rate of the sea level based on the MSL data since 1886 of + 0.80 mm/year. Since 1993, the apparent rate of rise is higher than Honolulu, + 3.42 mm/year, but still much less than the value for Tuvalu. The Sydney Fort Denison tide gauge is also subjected to subsidence (Fig. 8).

Fig. 7
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Linear trend analysis of the MSL in Sydney Fort Denison, NSW, AU, with only the data since 1993, and with the data since 1886. Data from PSMSL (Permanent Service for Mean Sea Levels 2022d, e)

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Fig. 8
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Absolute vertical position of the GPS dome of FTDN, in Sydney Fort Denison, NSW, AU, from NGL (JPL and SONEL do not have coverage for this dome). Images from Système d’Observation du Niveau des Eaux Littorales (2022) and Nevada Geodetic Laboratory (2022). Credit Système d’Observation du Niveau des Eaux Littorales and Nevada Geodetic Laboratory

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The FTDN GPS dome is also co-located, with a distance to the tide gauge 66 m.

Thus, the apparent relative sea level rise since 1993 corrected for subsidence could be + 3.42 mm/year minus the about 1.5 mm/year of subsidence of the FTDN GPS dome. This still gives a + 1.92 mm/year of absolute sea level rise since 1993, which is much less than the value for Tuvalu. So, the rate of sea level rise of Tuvalu assessed at + 4.8 mm/year in Table 3 is (1) an artifact of the short time window and (2) also includes subsidence. The supposed-to-be co-located and continuously operated GPS dome TUVT should have evidenced this subsidence. However, this GPS dome is only used to assess the relative motion vs. a benchmark which is not stationary but likely sinking, as it is sinking the GPS dome of TUVA in the airport. Accounting for the subsidence of TUVA, the absolute rate of rise in Tuvalu is about 3.3 mm/year, which is larger than the values in Honolulu or Sydney. Comparison of the relative MSL in Funafuti B and Honolulu, and Funafuti B and Sydney Fort Denison, clearly shows a much different rate of the sea level, both absolute and relative. The apparent rate of rise in Sydney and Honolulu since 1993 is then about three times the correct sea level trend computed over the full length of the record end of 1800s/beginning of 1900s. The results of Sydney and Honolulu, which are much more reliable than those of Tuvalu, suggest overall a different sea level pattern.

Absolute and relative sea level rise in other Pacific islands and atolls

Satellite altimetry may help in understanding the differential sea level variations across the Pacific, which is everything but uniform. From the map of satellite altimetry global mean sea level (University of Colorado Sea Level Research Group 2022), also based on measurements started in the early 1990s, Tuvalu, which is north of New Zealand and east of Papua New Guinea, has off-coast a rate of rise of the sea level higher than off-coast in Honolulu, but smaller than off-coast in Sydney Fort Denison. Figure 9 is the absolute sea level since 1993. Funafuti and Sydney have experienced elevated sea-surface height trends (relative to the global average) typical of the western tropical Pacific. The reason for these elevated regional trends attributed to developments in low-latitude wind stress affects a dynamic ocean response, partly mediated by Rossby waves, which results in a convergence of heat in the western tropical Pacific (Qiu and Chen 2012; Forget and Ponte 2015). If the values off-coast in front of Tuvalu are expected to be slightly less than off-coast in front of Sydney, and slightly more than off-coast in front of Honolulu, it is expected to have about the same differences between the contemporary absolute rates of rise measured by the tide gauges. As this is not what emerges in the analyses of the two prior sections, there may be other factors to bias the reading of the tide gauge in Tuvalu. Tide gauge results may also be affected by a variety of local and regional phenomena related to ocean variability, tides, storm surges, tsunamis, swells, and other coastal factors producing changes in circulation or filling and emptying processes around the tide gauge.

Fig. 9
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Satellite absolute rate of rise of the sea level from University of Colorado Sea Level Research Group (2022). Credit University of Colorado Sea Level Research Group

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The satellite absolute rate of rise, which is more a sort of computation rather than a real measurement (Parker 2014; Parker and Ollier 2016), is theoretically independent of subsidence. However, University of Colorado Sea Level Research Group (2023) clearly states their global mean sea level (GMSL) “cannot be used to predict relative sea level changes along the coasts” but they “do compare the altimeter sea level measurements against a network tide gauges to discover and monitor drift in the satellite (and sometimes tide gauge) measurements,” which incidentally depend on subsidence. Tide gauges measure relative sea level, which reflects changes in vertical land motion and sea-surface height. Satellite altimetry only computes sea surface height changes. This result should not be affected by vertical land motion if a real direct measurement. The satellite absolute sea level is a mostly computational product introducing many corrections to an almost de-trended noisy signal to give slope and acceleration (Parker 2014; Parker and Ollier 2016). By using the University of Colorado Sea Level Explorer (Colorado Center for Astrodynamics Research 2023), there is some resemblance between the relative sea level rise result measured by the tide gauge of Funafuti and the absolute sea level rise result from the satellite altimeter computations. If recent relative sea-level trends at Funafuti are mostly due to vertical land motion and other local effects, then the trends observed by tide gauge should be different from the trends observed by satellite altimetry, since the former registers vertical land motion but the latter theoretically does not. Thus, if absolute and relative rates of rise are close, this means there is no subsidence. This consistency between different quantities—absolute and relative sea level rises—is however only incidental. A more appropriate analysis such as the GPS monitoring (Figs. 3, 6, and 8) clearly indicates significant subsidence is experienced in Sydney Fort Denison, Honolulu, and Tuvalu. Thus, the satellite altimeter result has a more qualitative than quantitative value. Figure 10 compares the tide-gauge relative sea level record at Funafuti, Sydney, and Honolulu to sea surface height from nearby satellite altimetry. This figure was produced by the University of Colorado Sea Level Explorer (Colorado Center for Astrodynamics Research 2023). The tide-gauge data in this figure were provided by the University of Hawaii Sea Level Center (University of Hawaii Sea Level Center 2023) and the satellite-altimetry data were provided by the NASA Jet Propulsion Laboratory (Jet Propulsion Laboratory 2023). For Funafuti, the satellite gives an absolute rate of rise of 0.47 cm/year vs. 0.43 cm/year relative rate of rise attributed to the tide gauge. We know from Fig. 7 that in Funafuti, since 1993, the relative rate of rise is + 4.71 mm/year, or 0.47 cm/year, with a subsidence contribution of likely about 1.5 mm/year, or 0.15 cm/year (Fig. 3), which makes the absolute rate of rise from the tide gauge and GPS about + 3.21 mm/year, or 0.32 cm/year, indicating small differences. For Sydney, the satellite gives an absolute rate of rise of 0.29 cm/year vs. the 0.34 cm/year relative rate of rise attributed to the tide gauge. We know from Fig. 7 that in Sydney, since 1993, the relative rate of rise is + 3.14 mm/year, or 0.31 cm/year, with a subsidence contribution of likely about 1.5 mm/year, or 0.15 cm/year (Fig. 8), which makes the absolute rate of rise from the tide gauge and GPS about + 1.64 mm/year, or 0.16 cm/year, demonstrating remarkable differences. For Honolulu, the satellite gives an absolute rate of rise of 0.18 cm/year vs. 0.21 cm/year relative rate of rise attributed to the tide gauge. We know from Fig. 5 that in Honolulu, since 1993, the relative rate of rise is + 2.98 mm/year, or 0.30 cm/year, with a subsidence contribution of about 0.5 mm/year, or 0.05 cm/year (Fig. 6), which makes the absolute rate of rise from the tide gauge and GPS about + 2.48 mm/year, or 0.25 cm/year, demonstrating remarkable differences.

Fig. 10
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Comparison of absolute sea level from satellite and relative sea level from tide gauge in Tuvalu (top), Sydney Fort Denison (middle), and Honolulu (bottom). Images from Colorado Center for Astrodynamics Research CCAR (2023)

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It is also worth discussing if the high rates of sea level rise in Table 3 are representative of at least a temporary pattern in the sea levels of the Pacific. We have already discussed Honolulu and Sydney. We may study the other Island locations with tide gauges in the area. Different from Tuvalu, where the other tide gauge has been discontinued, in many other locations where the tide gauges of the PSLMP are operated, there are still other tide gauges recording. In the Marshall Islands, the MSL trend at Kwajalein is + 1.97 mm/year with a 95% confidence interval of ± 0.69 mm/year, based on monthly mean sea level data from 1946/6 to 2019/12 (Sealevel.info 2022). In Majuro C, by adding the data collected previously to the PSLMP in Majuro B, the MSL trend reduces to + 3.93 mm/year with a 95% confidence interval of ± 1.04 mm/year, based on monthly mean sea level data from 1968/10 to 2019/12 (Sealevel.info 2022). In the Federal States of Micronesia, the MSL trend at Kapingamarangi is + 3.17 mm/year with a 95% confidence interval of ± 2.10 mm/year, based on monthly mean sea level data from 1978/9 to 2018/10 (Sealevel.info 2022). The MSL trend at Yap B is − 0.57 mm/year with a 95% confidence interval of ± 3.87 mm/year, based on monthly mean sea level data from 1969/6 to 2005/6 (Sealevel.info 2022). The MSL trend at Pohnpei-B is + 1.88 mm/year with a 95% confidence interval of ± 4.05 mm/year, based on monthly mean sea level data from 1974/5 to 2004/12 (Sealevel.info 2022). In Kiribati, the MSL trend at Kanton Island and Kanton Island B is + 2.19 mm/year with a 95% confidence interval of ± 0.79 mm/year, based on monthly mean sea level data from 1949/6 to 2018/12 (Sealevel.info 2022). The MSL trend at Christmas Island and Christmas Island II is − 0.12 mm/year with a 95% confidence interval of ± 1.25 mm/year, based on monthly mean sea level data from 1956/1 to 2018/12 (Sealevel.info 2022).

In Samoa, the PSLMP considers a single record of the data collected before and after the 2009 earthquake, which affected both the American and Western Samoa. This is openly incorrect (Parker et al. 2018; Mörner et al. 2018; Boretti 2022). Table 3 attributes an MSL trend of + 10.4 mm/year, completely neglecting the jump in the measured sea levels that occurred immediately after the earthquake, which has absolutely nothing to do with global warming. The relative sea levels accelerated in Pago Pago and Apia B after the 2009 earthquake. This acceleration is completely explained by the subsidence induced by the earthquake. The relative sea level patterns are fully consistent with the subsidence pattern. Also United States National Oceanic and Atmospheric Administration (2022b) avoids coupling together the data collected by the tide gauge of Pago Pago in American Samoa. The American Samoa suffered of same tectonic movements as close-by Western Samoa. Following the earthquake, the relative sea level rose about 0.25 m until January, 2020. The cause of this rise is rapid post-seismic land subsidence which will likely continue, although at a slower rate over the next 2–3 decades. Figure 11 presents the linear trend analysis of the MSL in Apia B, Samoa, before and after the earthquake of 2009. Figure 12 then presents the absolute vertical position of the GPS dome of SAMO, in Samoa. The distance to tide gauge is 3515 m.

Fig. 11
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Linear trend analysis of the MSL in Apia B, Samoa, showing the different behaviors before and after the earthquake of 2009. Data from PSMSL (Permanent Service for Mean Sea Levels 2022f)

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Fig. 12
figure 12

Absolute vertical position of the GPS dome of SAMO, in Samoa, from NGL and JPL. Images from Système d’Observation du Niveau des Eaux Littorales (2022)

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United States National Oceanic and Atmospheric Administration (2021) explains how the 2009 earthquake affected and continues to influence the sea levels measured by the tide gauge they manage in American Samoa. Opposite, the PSLMP completely ignores that there has been an earthquake in the Samoas, and suggests a rate of rise increased to 10.4 mm/year coupling data collected before and after the earthquake caused a large vertical displacement of the land where the tide gauge was located. In the Cook Islands, the MSL trend at Penrhyn is + 2.32 mm/year with a 95% confidence interval of ± 1.48 mm/year, based on monthly mean sea level data from 1977/5 to 2018/12 (Sealevel.info 2022). The MSL trend at Rarotonga and Rarotonga B is + 1.75 mm/year with a 95% confidence interval of ± 0.86 mm/year, based on monthly mean sea level data from 1977/5 to 2019/12 (Sealevel.info 2022). Same as Tuvalu, in Papua New Guinea, Solomon Islands, Nauru, and Tonga, there are unfortunately no other tide gauges presently recording, but those previously recording were all suggesting smaller rates of rise. In Vanuatu, where no other tide gauge is recording, Port Vila B shows a small rate of rise also in Table 3, about 1/3 of the value anticipated in Table 2. The MSL trend is + 0.40 mm/year with a 95% confidence interval of ± 2.14 mm/year, based on monthly mean sea level data from 1993/1 to 2019/12 (Sealevel.info 2022). In Fiji, there is another tide gauge recording, Suva, which suggests an even larger rate of rise, but it is likely subjected to subsidence and even shorter than the Lautoka tide gauge used in the PSLMP, with measurements only starting in 1997.

Figure 13 is the sea level trend analysis in Pacific islands monitored by the US National Oceanic and Atmospheric Administration. In some cases, the record of the same tide gauges used by the PSLMP is made longer by coupling the latest recording with prior recordings from neighboring tide gauges to permit a meaningful computation. In other cases, different tide gauges providing longer records are used. The PSLMP average relative rate of rise is more than three times the average rate of rise of the analysis by the US National Oceanic and Atmospheric Administration, which is remarkable.

Fig. 13
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Latest sea level trend analysis in Pacific Islands according to the US National Oceanic and Atmospheric Administration. Images from United States National Oceanic and Atmospheric Administration (2022c, d, e, f, g, h, i, j, k, l, m, n, o, p, q)

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Table 3 is therefore not representative of the sea level rise across the Pacific, as there are other conflicting data of not less value from other or the same locations that must be considered in a proper assessment. It is additionally based on short records and suffers from unaccounted subsidence. From the point of view of emerged surfaces, which is one of the consequences of sea level rise, many Pacific islands have grown rather than shrunk in size in recent years (Kench et al. 2018; Duvat 2019; Tuck et al. 2019; Masselink et al. 2020). These data support the view the threat of sea level rise in Tuvalu is overrated.

Discussion

The high relative rate of rise of the sea level in Tuvalu, Funafuti B, is mostly due to the subsidence of the instrument and the short tide gauge record. There may be other factors pushing up the relative rate of rise, as the closest long-term trend tide gauges of Sydney Fort Denison, NSW, AU, and Honolulu, HI, USA suggest a smaller apparent rate of rise on the short-term tide window starting in 1993, both relative and absolute, and on same time window, satellite altimetry does not indicate higher rates of rise off-coast of Tuvalu than in Sydney. In Honolulu, the long-term trend relative rate of rise from 1915 to 2022 is + 1.56 mm/year, while the short-term trend relative rate of rise from 1993 to 2022 is + 2.98 mm/year. Accounting for 0.49 mm/year of subsidence, the absolute rates of rise reduce to + 1.07 and + 2.49 mm/year, respectively. In Sydney, the long-term trend relative rate of rise from 1886 to 2022 is + 0.80 mm/year, while the short-term trend relative rate of rise from 1993 to 2022 is + 3.42 mm/year. Accounting for 1.53 mm/year of subsidence, the absolute rates of rise reduce to − 0.73 (absolute sea levels reducing) and + 1.89 mm/year, respectively. In the currently operated tide gauge of Tuvalu, Funafuti B, the short-term trend relative rate of rise from 1993 to 2022 is + 4.71 mm/year. Accounting for 1.57 mm/year of subsidence, the absolute rate of rise reduces to + 3.14 mm/year. This is + 0.65 mm/year vs. Honolulu and + 1.25 mm/year vs. Sydney. The PSLMP is biased toward higher rates of rise, in Tuvalu, as well as in the other stations they consider to represent the Pacific islands, compared to prior and contemporary assessments. It uses data from the new tide gauges to compute trends when the length of their record is insufficient and it does not state the correct subsidence of the instruments. It also ignores perturbing events such as the huge effects of the 2009 Samoan earthquake which provides large apparent acceleration of the sea levels. Compared to the latest survey of sea levels for the Pacific Islands by the US National Oceanic and Atmospheric Administration, the average rate of rise is about three times larger. As the surface of the islands is increasing rather than reducing and other tide gauges consistently show relatively stable sea levels, there is nothing unprecedented or dramatic in the rate of rise of the sea level in Tuvalu, which may have a different explanation from the thermosteric effect.

Biases, such as alterations in the tide gauge instrument or sea exposure, along with subsidence, have the potential to impact the time series. Occasionally, straightforward flow-related issues can influence the readings obtained by water level measuring devices (Daneshfaraz et al. 2022a; 2022b).

Building upon the groundwork laid by earlier research efforts (Parker 2018), the current contribution advances the field by conducting a more comprehensive analysis. This involves not only an expansion in the scope of data considered but also the utilization of additional tools to enhance the depth and breadth of the examination. Through these augmentations, the present study aims to provide a richer and more nuanced understanding of the subject matter, contributing valuable insights to the existing body of knowledge.

The work has an immediate impact on the ocean and coastal management, which should be based on proven sea level data, and not on computations failing validation, or subjective views of carefully selected short records in locations likely suffering from subsidence, neglecting the information from the long record that may also have subsidence information. The primary concern in Tuvalu revolves around the unsustainable population growth in Funafuti, relying on external assistance for essential resources like food and water—an outcome inherited from its colonial past. The claimed 4.8-mm/year relative sea level rise is exaggerated; the actual long-term rate of rise is probably less than half of this figure. The impact could have been even more minimal without additional stress from infrastructure development. Given the increasing total emerged surface of the islands, adopting modest water containment measures may be adequate for addressing rising sea levels, once the concurrent challenge of the growing population in Funafuti is appropriately tackled.

Conclusions

The pattern of sea level rise around Tuvalu is inferred from the analysis of the relative monthly average mean sea levels (MSL) recorded by tide gauges. In addition to examining the data from historical and contemporary tide gauges within Tuvalu, the analysis is extended to include MSL data from tide gauges in Pacific islands and atolls, as well as the long-term trend tide gauges in Honolulu and Sydney. To complement this analysis, information on subsidence obtained from global positioning system (GPS) monitoring is also provided. The substantial increase in sea level observed at Tuvalu’s present tide gauge is primarily attributed to multidecadal oscillations, significantly impacting short-term records, and the subsidence of the tide gauge. This emphasis on local factors takes precedence over the global thermosteric contribution. The residual rate of sea level rise is likely influenced by additional factors, such as changes in circulation within the lagoon or other local anthropogenic biases. This contribution suggests a perspective of sea levels characterized by relative stability, derived from a comprehensive analysis of the entire body of evidence, rather than relying on a subset that may filter out undesired information. The proposed analysis corresponds with part of the previous research, supporting the view that sea levels are gradually increasing and that the surfaced area of Pacific islands and atolls remains stable, contrary to inaccuracies present in selective other studies that focus on specific data while neglecting others.