Coral growth records 20^th Century sea-level acceleration and climatic variability in the Indian Ocean

Domain: earth_science
Published: 2025-07-01
Editorial outcome: minor_revision

Abstract
Empirical observations of the rate of sea-level rise (SLR) and the timing of its recent acceleration are critical for validating ensemble methods used to determine global mean sea level trends. Such records are critically important at far-field locations where instrumental datasets are scarce. Here we construct a continuous 90-year sea level record (1930–2019) from the central tropical Indian Ocean derived from the incremental growth of a coral microatoll, which is demonstrated to reflect changes in sea level at annual timescales. Our record, which overlaps with tide gauge observations, extends the instrumental record by six decades and reveals climatic variability and marked changes in sea level behaviour across the twentieth century characterised by: (1) an increase in sea level of 0.30 m between 1930 and 2019; (2) low rates of SLR in the early twentieth century (1.42 ± 0.42 mm.yr−1); (3) a marked acceleration in SLR to ~3.44 ± 0.68 mm.y−1 in the late 1950’s; and (4) a further increase to 4.39 ± 0.48 mm.y−1 over the past three decades. Our results provide empirical evidence for a mid-century SLR acceleration in the central Indian Ocean, which is earlier than instrumental records indicate for coastal sites at the continental margins.

Introduction
Observational records of 20th century global mean sea level (GMSL) change are essential to improve our understanding of climate change and natural climatic variability1. Historical records allow for assessment of the current state of the climate system and changes over time, improving knowledge of regional and global sea level variability. In addition, they enable the evaluation of climate and earth systems models2,3 and play a crucial role in quantifying the socio-economic impacts of sea-level change on vulnerable coastal communities and ecosystems worldwide4.

Observational sea level records have been successfully used to validate synthesised sea level records at both global and regional scales2,3. These studies show good agreement in the period since satellite altimetry data of ocean water level has been available, and confirm that the pace of sea level change has accelerated since 19935,6. However, there are greater disparities between models and tide gauge observations in the twentieth century prior to the availability of satellite data3. Indeed, it has been shown that the recent increase in rate of sea-level rise was not a recent phenomenon that was coincident with satellite altimetry data, but had been underway for several decades4. Furthermore, regional sea level reconstructions based on tide gauge records show sea-level change has substantial regional departures from the GMSL at decadal to multidecadal timescales3. Comparison of ocean reanalysis products that reconstruct patterns and trends in sea surface height over the past 60 years also shows significant disagreement between models prior to the satellite altimeter record7.

Several factors account for such disparities in the pattern of estimated past sea level change. First, there are very few instrumental sea-level records of sufficient length (>70 years) to detect long-term trends8,9. Second, the spatial distribution of these records is mostly concentrated at coastal sites in the northern hemisphere, while large tracts of major ocean basins within far-field locations have few, if any, instrumental observations to resolve long-term sea-level behaviour. This lack of observational data is particularly acute in the Southern Ocean and mid-ocean basins7. Third, coastal records are beset with numerous confounding factors associated with the dynamical response of the ocean to atmospheric forcing, their interaction with shallow coastal shelves, terrestrial runoff and water storage, and vertical land motion3. While advances have been made to resolve a number of the above issues in closure of the 20th century sea level budget10–12, significant gaps in observational data remain.

The Indian Ocean has only two long sea-level records that span the twentieth century, located on the northwestern coastline of India (Mumbai, PMSL Station ID 43, 142 years) and the southwestern coast of Australia (Fremantle, PMSL Station ID 111, 124 years), which are 7300 km apart. In contrast, instrumental observations at central tropical locations in the Indian Ocean are only 3-4 decades in length (Supplementary Table 1 and Supplementary Fig. 1). These short records indicate rates of sea-level rise between 3.37±0.05 and 7.06±0.76 mm.yr−1 over the past three decades, though are not able to shed light on the timing of the acceleration in sea level change, or variations in the pattern of sea level change in the tropical Indian Ocean across the twentieth century. Tide gauge reconstruction and ocean assimilation analysis have also shown equivocal and contrasting results for the central Indian Ocean since 19607 and significant spatial variation13. The lack of observational data within the Indian Ocean inhibits the ability to verify sea-level reconstructions based on statistical and reanalysis techniques. Consequently, the current absence of extended historical records represents a significant gap in the improved understanding of sea-level behaviour, including recent accelerations and rates of rise, in the central Indian Ocean14. Such data is critical in the region given the abundance of low-lying coastal areas, and islands, and heightened vulnerability of coastal communities and coastal ecosystems in the Indian Ocean15–17.

In the absence of long-term instrumental records, reconstructions of relative sea-level (RSL) can be derived using sea-level proxies. Such proxies include sedimentary deposits, geomorphic features, fixed biological indicators, corals, and salt marsh systems18. Each proxy has a systematic and quantifiable relationship to sea-level, and they have been adopted to resolve sea-level behaviour over a range of geological timescales spanning the mid-Pliocene warm period, last interglacial, and across the recent Holocene postglacial sea-level transgression18. The utility of each proxy is constrained by its preservation, post-mortem destruction, and known relationship to sea level.

In tropical coral reef environments, the capacity of coral microatolls to record changes in sea level is well established19–22. Microatolls are long-lived coral colonies that develop a discoidal form in intertidal reef settings due to the interplay of the survival constraints of living coral tissue and water level. Corals are limited in their upward growth by exposure during low tide, and consequently, once maximum vertical growth is attained, continued growth is constrained to the outer sides of a colony and lateral growth dominates23,24. Based on the close association between coral growth and limiting water level, microatolls have been used as proxy sea level indicators in two primary ways. First, the presence of flat discoidal microatolls have been used to signify past sea level positions. In such studies the precise elevation of microatolls and radiometric dating are used to constrain patterns of sea-level behaviour and sea-level highstands during the Holocene25–30, and detect changes in water level resulting from storm rubble rampart destruction31, as well as coseismic activity along tectonically active coasts32–35. Second, over shorter timescales, microatolls have been used to resolve interannual and longer-term sea-level behaviour19,21,36 based on the ability to resolve annual growth properties and observations that short-term changes in water level can be reflected in the surface and skeletal growth response of corals19. Indeed, growth patterns have been associated with rising, stable and falling sea levels over decadal to centennial timescales37. However, few studies have attempted to extend sea level records using the continuous growth chronology of corals21. This in part is attributed to the focus on microatoll specimens from intertidal and emergent reef flats where the vertical growth trajectory is compressed, and samples are diagenetically altered or heavily bioeroded, which can obfuscate the continuous growth signature of corals apart from identification of notable die-down events28,36.

Here, we present an extended and near-continuous 90-year record (1930–2019) of sea-level behaviour for the central Indian Ocean, that spans the early to late 20th century, from the growth records of a pristine fossil coral microatoll in pedestal growth form, and recent living microatoll that colonised the outer edge of the fossil microatoll (Fig. 1, “Methods”). The height of living coral (HLC) at the study site has a close approximation to mean lowest low water (MLLW, −0.41 m below msl) and this close association to known sea level is examined to reconstruct the change in relative sea level. Our record is based on a methodological development where due to the unique sample quality and preservation of the microatoll, we construct a near-continuous number of sea level mini points (47) and sea level index points (SLIPS, 21) along the growth axis of the coral, which tracks surface morphology and produces highly-resolved sea level data at annual scales. Notably, our sea level reconstruction spans the early 20th to 21st centuries and overlaps the installation of tide gauges in the Maldives archipelago, and extends the observational record by a further six decades. The sea level reconstruction is examined to resolve changes in sea-level behaviour and the onset of the acceleration in sea-level rise in the Indian Ocean.

Results
Growth history of fossil and living coral microatolls
Uranium-Thorium ages indicate the slabbed fossil coral colony captures at least 106 years of growth (1888–1995; Fig. 2b and Supplementary Table 2). Skeletal banding signatures suggest the centre of the slab records the initial stages of vertical growth and transitions to lateral growth around 1900 (Fig. 2b). We reconstruct sea level beginning in 1930 when consistent annual skeletal bands are visible. The surface of the microatoll has minimal bioerosion (“Methods”), an undulating surface, and multiple die-down or negative growth trends (Fig. 2b). Coral growth rate varied along the coral slab with constrained skeletal extension evident during die-down events (mean 1.02 cm.yr−1, range 0.5–1.34 cm.yr−1), and wider growth bands (mean 1.34 cm.yr−1, range 0.78–2.1 cm.yr−1) during periods when the microatoll surface increases in elevation (Supplementary Fig. 5). Lateral growth exceeded or equalled vertical growth in all instances indicating a close coupling of water level to the coral growth surface.

The growth record of this long-lived colony has preserved the impact of a number of climate anomalies during the twentieth century, as recorded in stress signatures that alter coral growth, and reduction in elevation of the height of living coral (Fig. 2). Several climate modes are known to modulate both sea surface temperature and sea level in the Indian Ocean that impact coral growth responses, though the climate processes exhibit significant spatial and temporal variability in the Indian Ocean10,38–41, and the precise impact on coral growth also varies between events. At the Maldives study location strong El Niño events combined with positive phases of the Indian Ocean Dipole (IOD) can elevate the seasonal peaks in sea surface temperatures (SSTs, Supplementary Fig. 7a), causing widespread coral bleaching, that may result in marked reduction in coral growth, temporary cessation of growth, or coral death (Fig. 2)42,43. On the other hand, lower sea level anomalies may occur after negative IOD events38–41 (Supplementary Fig. 7b) which can promote reduction in the height of living coral tissue and reduction in elevation of the surface of the coral skeleton. Indeed, our coral record also shows multiple die-down episodes that are associated with negative sea level anomalies (Fig. 2, Supplementary Fig. 7b).

Three notable stress events are apparent in the skeletal growth history depicted by the truncation of annual growth bands, followed by regrowth of the colony downwards toward the reef surface. The first two events occurred in 1967–1968 and 1980–1982, respectively, and are temporally proximate with combined El Niño and IOD events. Of note, the 1982 event was closely followed by a negative IOD event, likely inducing a negative sea level anomaly. The third marked stress event, which forced a marked die-down in the height of living tissue between 1992 and 1993, is coincident with an El Niño episode that elevated sea surface temperature in the Maldives 1991/199242 followed immediately by a negative phase of the IOD that temporarily lowered sea level (Supplementary Fig. 7b). There is a subsequent further marked reduction in height of living tissue between 1993 and 1995 following the negative IOD event. The colony appear to have died in 1995, as there are no further growth bands present (Fig. 2a, c). Notably, the period in which the coral experienced successive die-downs leading to mortality was coincident with three successive years where maximum low water levels were the lowest on record (Fig. 3b). In total, there are 21 downward growth bands in the coral record.

Following a 12-year hiatus, growth of a new Porites lutea colony began on the outer edge and surface of the dead pedestal microatoll (Figs. 1e, 2c). This colony grew vertically between 2006 and 2015 as sea-level rise had opened a depth window across the pedestal surface. In 2015, the new coral colony reached its vertical growth limit and began lateral growth. A significant die-down occurred in 2016, which aligns with the very strong El Niño and global mass coral bleaching event that impacted the Maldives archipelago43. This event was associated with a lower sea level as a consequence of the following negative IOD (Figs. 2c, 3a, b and Supplementary Fig. 7b), and confirms the skeletal growth record is responsive to the sea level signal.

90-year sea level reconstruction
The pristine preservation of the coral, lack of bioerosion, comparison of surface topography with growth capacity, and skeletal growth signature that is restrained within its vertical growth capacity by sea level (“Methods”) are used to reconstruct a high-fidelity relative sea level record at this tectonically stable location (“Methods”, Fig. 3).

Our sea level reconstruction identifies several significant features of sea level behaviour at this locality in the central equatorial Indian Ocean (Fig. 3). First, there is a clear cyclic pattern which has an ~18.6 year frequency, likely driven by the lunar nodal cycle44 with the maximum amplitude of the lunar diurnal K and O constituents corresponding with higher sea level periods aligned around 1932, 1950/51, 1969, and 1988 (Figs. 2, 3). The coral record also shows sequential years of downward growth in the years preceding the minimum amplitude of the lunar constituents at ~1942, 1960, 1978/79. In particular, the growth record shows eight years of successive downward growth before the 1978/79 minima, and our growth record was truncated by marked die-down (followed by mortality in 1995) three years prior to the 1997 minima.

Due to the equatorial location of our study site in the central Indian Ocean, the amplitude of the nodal tide is expected to be low45–49, confirmed through analysis of the nodal cycle in our dataset (Methods) indicating an amplitude of ~15 mm. Where we have continuous sea level data (1930–1992), we calculated rates of sea-level change with the nodal lunar tide removed (NLTR), and we report it alongside estimates based on the raw micro-atoll sea level reconstruction. These results show the nodal cycle has only a small impact on calculations of rates of sea level change.

Second, superimposed on this cyclic pattern, sea level has risen by ~0.174 m between 1930 and 1992, and a further 0.127 m between 1992 and 2019 (total of 0.33 m across the entire record) at rates of 3.06±0.26 mm.y−1 (NLTR rate 3.04±0.14 mm.y−1) between 1930 and 1992 (length of primary coral record) and 3.52±0.19 mm.y−1 (NLTR rate 3.62±0.17 mm.y−1) across the entire synthesised record (1930–2019). Third, it is apparent that the increase in sea level has not been uniform. The first 30 years of the record (1930–1959) in the early to mid 20th century show a lower rate of sea-level change at ~1.42±0.42 mm.y−1 (NLTR rate 1.51±0.33 mm.y−1). From around 1959, the rate of sea-level rise increased to ~3.44±0.68 mm.y−1 (NLTR rate 3.07±0.37 mm.y−1, 1960–1992). Between 1990 and 2019 the rate of sea-level rise is ~4.39±0.48 mm.y−1.

Methods
Field setting
A sea-level record was constructed from the skeletal growth patterns archived within a coral microatoll from a reef platform in Huvadhoo atoll, southern Maldives archipelago (Fig. 1a, b). Huvadhoo is the largest and deepest atoll in the Maldives with a perimeter of 261.4 km and an area of 3279 km2,59. The lagoon is well flushed via numerous deep and wide passages that breach the atoll rim60. The sampled microatoll is from the Mahutigala a lagoonal reef platform located in the southwest sector of the atoll (0°17'22.77“N, 73°12'1.88“E, Fig. 1b, c). Elliptical in shape, the platform is approximately 0.13 km2 in area, and supports a 0.004 km2 vegetated island (Supplementary Fig. 2). The topography of the reef platform surface is broadly basin-shaped with a higher elevation outer rim (~0.4 m below mean sea level (MSL)) and shallow inner platform (0.4–1.5 m below MSL). The reef surface can be divided into four major eco-geomorphological zones, which vary in width around the platform61 (Supplementary Fig. 2b). (1) The outer reef edge (20–80 in width) incorporates the transition to the fore reef slope and comprises a dense and complex in situ reef framework of live coral (~10–30% cover and predominantly of massive morphology) and dead coral (~30–50% cover). (2) The outer reef margin transitions to a narrow inner reef (10–30 m wide) comprising an open framework of living coral (10–30% cover, predominantly of massive morphology) and dead coral (20–40%) that ranges in elevation from −0.5 to −0.8 m MSL. This zone has dense microatoll growth forms (Porites spp. and Heliopora coerulea) and is the location of the sample cut and analysed in this study. Of note, in this zone, there are multiple specimens similar to that analysed in this study. (3) The reef flat surface grades into a patch reef zone (−0.8 m to −1.2 m MSL) that ranges in width from 20 to 90 m around the platform and is characterised by a mixed rubble and sand substrate (60–80%) with isolated coral patches, including microatolls. (4) The inner reef platform is a shallow trough, 20 to 100 m wide, that encircles the vegetated sand cay and is composed of medium-fine sized sand.

Fields of fossil Porites spp. microatolls have previously been described on the central reef platform, occupying the sandy trough and coral patch zones26 (Supplementary Fig. 2a). This study examines the growth record preserved in a modern microatoll (~100 years in age) from the inner reef zone (Supplementary Fig. 2b). The microatoll is 2.7 m diameter and has a characteristic discoidal shape, and near planar surface, consistent with lateral growth morphology under constrained sea-level conditions (Fig. 1e).

The southern Maldives provides a suitable environment that maximises the potential use of microatolls to detect sea level changes37. First, the 850 km long archipelago is located in a tectonically stable setting midway along the Laccadive-Chagos aseismic submarine ridge62,63. Second, the atoll experiences a microtidal regime (mean neap and spring tide range of ~0.4 m and 1.1 m, respectively), with low amplitude annual variations in sea level, which enhances the sensitivity of microatolls to sea level change19,21. Third, as Mahutigalaa is located 11 km inside the atoll periphery, it is protected from direct oceanic swell impact. Consequently, incident wave energy is low to moderate as it can only be impacted by residual ocean swell that propagates through passages in the reef rim, and internal lagoon-generated waves. Due to the low energy character of the wave regime impacting the platform, significant wave-driven differences in water level that can influence variations in microatoll growth in larger and higher energy reef systems are considered negligible at the study site. Fourth, the equatorial setting implies that high-energy storm events, that can physically impact and mobilise coral colonies or induce moating through deposition of rubble tracts, are rare23,26. Fifth, the Mahutigalaa platform is free draining at the current sea level and ponding is not apparent as demonstrated through surveys of the height of living microatolls.

Instrumental sea level records
Sea-level records are available from satellite data, and two tide gauges located 432 km north (Hulhule, Male’ atoll) and 108 km south (Gan, Addu atoll) of the Mahutigala a reef platform (Supplementary Fig. 3). Consistent data from these sources begins around 1990 and reveal an increase in sea-level over the past 3 decades, with rates of 3.37±0.58 mm.y−1 at Gan to 4.53±0.74 mm.y−1 at Male’. The satellite altimeter record for the field location (Supplementary Fig. 3a) also shows an increase in sea level of 3.71 mm.y−1.

Microatoll sampling
We sampled a dead Porites lutea microatoll on the freely draining northeastern side of the Mahutigala a reef platform at the transition of the outer reef flat to inner reef zone (0°17'22.77“N, 73°12'1.88“E, Fig. 1 and Supplementary Fig. 2). The microatoll was 2.7 m diameter and assumed a pedestal growth form in which the colony grew from its core and subsequently transitioned to lateral growth over the reef and sediment surface (Fig. 2). The outer margin of the colony was 0.632 m above the bed with a 0.2 m gap between the sediment surface and underside of the colony (Fig. 2a). A living Porites lutea coral colony was growing on the outer edge and surface of the larger dead microatoll (Figs. 1e, 2a, c). A 10 cm-wide slab was cut using hand saws along the radius of the microatoll beginning at the island-facing outer edge (Fig. 2). A 10 cm wide slab was also cut from the living colony growing on the outer margin of the microatoll.

Prior to slabbing, laser level surveys (error of ±2 cm) were undertaken to determine the elevation of the outer and inner margins of the slab relative to sea level. Surveys of the height of 30 living microatolls were also undertaken to determine the variability in height of living coral (range 9.5 cm ± 2.3 cm SD). All survey data was reduced to sea level based on the known elevation of benchmarks in the island surface using the tidal datums calculated from the Gan sea level records (UHSLC Stations (hawaii.edu)). Analysis of this data indicates the height of living coral is −0.42 m ± 0.03 m below MSL, which is a close approximation to MLLW (−0.41 m msl).

Slab processing and establishment of the growth chronology
The slab was recut into a 0.7 cm slice, using a 0.45 mm diamond wire saw. An X-ray image was captured at 50 kV, 10 mAs with a source-to-object distance of 1 m and an exposure time of 20 msec at the Diagnostic Imaging Laboratory at National University Hospital, Singapore. An annual growth chronology for the slab was constructed based on the converted annual skeletal density growth bands from the X-ray images64 using ImageJ, and temporally constrained by disequilibrium Uranium-Thorium ages (230Th ages).

Nineteen Uranium-Thorium dates (including two replicates) were obtained along the slab in order to determine the time window of coral growth, refine ages of highest and lowest levels of colony growth, and resolve age disparities either side of apparent stress events in the skeletal growth record (Supplementary Table 2). Samples were gently crushed into 1 to 3 mm3 segments. Clean and pristine pieces were selected under a microscope, and subsequently ultrasonically cleaned (three times) in super clean water. For each sample, 30 to 60 mg was used for U-Th chemistry65. All samples were dissolved in ultrapure nitric acids, and a 229Th-233U–236U mixed spike was then added to the solutions. HClO4 was also added to remove organic compounds. Each sample solution was refluxed at 175 °C for 2 h and dried. Samples were re-dissolved by adding 2N ultrapure HCl solution. Co-precipitation was achieved by adding FeCl3 and concentrated NH4OH. Co-precipitates were re-dissolved and put through an anion-exchange resin in columns to separate and purify uranium and thorium. U and Th fractions were then dissolved in 1% HNO3 (+0.1% HF) for instrumental analysis. U and Th isotopic compositions were determined on a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS, a Neptune Plus from Thermo Fisher Scientific) at the Earth Observatory of Singapore and Asian School of the Environment (ASE), Nanyang Technological University, Singapore. The measurements were undertaken using a recently improved SEM peak-jumping technique66,67. Uncertainties in the U and Th isotopic data and 230Th dates are quoted at the 2σ level in this paper.

Microatoll bioerosion
Bioerosion can have a significant impact on the surface and internal structure of coral microatolls and subsequent reconstructions of sea level. We quantified the degree of bioerosion of the coral slab using digital analysis of the proportion of the slab that had undergone boring (erosion holes, boring tunnels) relative to total slab area. The analysis examined both bioerosion signatures on the surface and underside of the coral. Boring holes and tunnels, and sections of the slabs (e.g., surface and underside) where skeletal banding was visibly speckled, most likely by CCA overgrowth (Supplementary Fig. 4), were segmented using the Magnetic Lasso tool in Adobe Photoshop. Segmented pixels were quantified against the total pixel area of each slab and the total area of the coral sample (Supplementary Table 3). The images were not enhanced prior to analysis.

Overall, 20% of the coral slab had been affected by bioerosion. Bioerosion was most prevalent on the underside of the slab (17.8%) and on older central portions of the coral which was continuously in contact with the sediment surface (Supplementary Fig. 4). Notably, only 2.3% of the upper surface was affected by bioerosion, and excluding the central section (which was not used in this study for sea level reconstruction), the remaining surface of the coral had only 0.91% bioerosion. Notably, this portion of the slab was perched above the sediment surface at the time of slabbing. We conclude the surface of the coral used for sea level reconstruction was remarkably well preserved.

Reconstruction of water level
Reconstruction of water level changes were based on the elevation of the coral growth surface relative to sea level as measured from the continuous growth record of the microatoll surface and observed die-downs in skeletal bands. We note the pristine nature of our sample, with negligible bioerosion of the coral surface that shows a continuous record of coral growth and incremental transitions between upward growth and several die-down periods (Fig. 2). To resolve changes in surface elevation of growth bands, the x-ray image was analysed digitally. Field survey data of slab elevation was used as control points to rectify the image to the correct horizontal and vertical orientation, as the in-situ slab. The rectified image was imported into the Matlab function grabit.m68 and calibrated to the known x- and y-scale. Each annual growth band was digitised based on the highest level of coral growth observed within each band. Digitising precision is estimated at 0.5 mm, and error is estimated at ±0.3 mm, calculated as the mean absolute error between repeat digitising of an individual line. Digitised points were saved for each annual increment and transformed to the sea level datum.

Inferring water levels from skeletal growth signals. Microatolls preserve a low-frequency record of the water level that constrains upward coral growth (e.g., MLLW), and therefore is not a record of mean sea level20. Interpretation of water level responses from the skeletal record must be undertaken with great care (Supplementary Fig. 6). When a microatoll experiences an extreme low water level, the coral tissue can be stressed, and those parts of the coral tissue at highest elevation can die. At these times, lower water level limits the highest level of survival (HLS). Such ‘die down’ events are expressed as a reduction in elevation of the coral growth surface (Supplementary Fig. 6). Our complete record contains 21 years when the surface of the coral and skeletal bands signify reduction in elevation from previous years (Fig. 2). These die-down elevations are considered to robustly reflect sea level limiting points and are used to construct sea level index points (SLIPS)37. Conversely, when a microatoll experiences higher than normal low water levels, the coral tissue can grow vertically. The height to which the coral can grow is limited by the colony vertical growth capacity (VGC). Analysis of the annual skeletal growth record in the microatoll sample showed the microatoll had a mean growth rate of 14.0 ± 0.39 mm.y−1 and ranged from 9.1 to 28.3 mm.y−1 (Supplementary Fig. 5). The mean value is similar to that previously identified for Porites corals in the Maldives69. We adopt this mean value as the VGC for the microatoll. In circumstances where the increase in water level is larger than the VGC the coral surface decouples from the low water level signal and there is a temporal lag before the growth of the coral surface catches up to water level (Supplementary Fig. 6). During this lag period the coral surface is submerged beneath the limiting water level (e.g., by ~0–10 cm, Supplementary Fig. 6) and the surface topography does not provide a precise characterisation of water level (TWL>VGC). At best, the microatoll surface presents a conservative lower estimate of water level position. However, where the VGC≥SLR the coral surface can remain coupled to its limiting water level and therefore provides a more continuous record of water level behaviour (Supplementary Fig. 6). Significantly, at no stage in our longer slab record did the vertical growth exceed the lateral growth capacity of the colony suggesting the coral surface is likely to reflect a close association with water level change. We identify 48 years of incremental upward growth that represent the highest level of growth (HLG) of coral tissue and are used to reconstruct sea level minimum points37,70,71. We note that the annual incremental growth in our record falls well within the annual growth rate of Porites lutea from the field site69.

The outer living coral slab began growing in submerged conditions and both the skeletal growth signature and growth rates from 2006-2014 indicate upward growth dominated (Fig. 2c). These years of growth are not reliable indicators of water level as the coral was in catch-up growth mode (e.g., Supplementary Fig. 6 and purple triangles in Fig. 3). These data points are excluded from analysis of sea level change. However, the period from 2015–2020 is characterised by lateral growth as the colony was coupled to its limiting water level (Figs. 2c, 3a).

Evaluating uncertainties
We reconstruct a relative sea level history from the skeletal signatures of the fossil coral (sea level index points and sea level minima points), which in our record terminate at the height of living coral (HLC) in 2019. Total elevation uncertainty is estimated from the expression Ei=(e12+e22+en2)1/2, where e1…en are individual sources of error for sample i. Our sea level reconstruction considers four sources of uncertainty. (1) Variability in the Height of Living Coral (HLC). This biological elevation reference was established based on surveys of the elevation of the living upper surface of 30 live Porites spp. located within 50 m of the pedestal coral location. At each coral, a minimum of five elevation points were recorded around each specimen to account for within colony variability. The elevation range of the upper surface of living Porites colonies was 9.5 cm ± 2.3 cm, and we adopt the standard deviation of the observations (±2.3 cm). The low range of growth elevations is consistent with other studies of microatolls in microtidal settings20,36. (2) Uncertainty from surveying the elevation of the coral microatolls with a laser level (±2.0 cm). (3) Uncertainties in digitising to reconstruct the coral surface (±0.05 cm). (4) Uncertainty associated with surface bioerosion. As analysis of the pedestal slab showed minimal surface bioerosion along the sector of the sample used to reconstruct sea level (Supplementary Fig. 4 and Supplementary Table 3) we adopt a conservative value of ±1.0 cm. This value is lower than that used in previous studies, where the coral surfaces are heavily bioeroded. Consequently, we estimate the total elevation uncertainty of our record at ±3.21 cm.

As the study focusses on sea level behaviour over the past 100 years, we present the relative sea level change as recorded within the skeletal record. While we do not attempt to transfer our record to an absolute sea level datum, we compare our HLC measurements to the known tidal datum at Gan based on fixed benchmarks on the island. We estimate our HLC level equates to a tidal value of −0.42 m ± 0.03 m below MSL. This elevation is a close approximation to MLLW (−0.41 m).

Removal of lunar nodal tide cycle
Following the identification of the 18.6-year nodal lunar tide in the coral sea level reconstruction, we explored its influence on estimated rates of sea-level rise. Our study site is located near the equator in the central tropical Indian Ocean. Although it remains debated the extent to which the 18.6-yr nodal cycle follows the equilibrium law, the amplitude of the nodal tide at the Maldives study location is expected to be less than 20 mm45–48. We removed the lunar nodal cycle signal from our microatoll-based sea level record. First, we performed polynomial regression on the sea level record to estimate the nodal tide cycle (R2=0.98). The obtained cycle peaks at around 1933, 1952, 1970 and 1989, consistent with the relatively high sea level points around 1932, 1950/51, 1969 and 1988 observed in the microatoll (Fig. 2), as well as the calculated timing based on the equilibrium law72. The cycle has an amplitude of ~15 mm on yearly averaged sea level variation, which approximates the theoretical value of ~9 mm derived from the equilibrium law48.

We then removed the nodal tide cycle from the original data and recalculated rates of sea-level rise. Results showed only marginal differences in sea-level rise estimates before and after correcting the nodal cycle. For example, for the period 1930-1992 calculated rates of sea-level rise were 3.06±0.26 mm.y−1 from the raw dataset and 3.04±0.18 mm.y−1 when the nodal lunar tide cycle was removed (a difference of −0.02 mm.y−1). Between 1930 and 2019, the estimated values were 3.52±0.19 mm.y−1 (raw data) and 3.62±0.17 mm.y−1 (nodal lunar tide removed) a difference of 0.1 mm.y−1. These results show the nodal cycle has only a small impact on calculations of rates of sea level change. However, we note that our dataset has a significant gap (1995–2015), which increases the uncertainty of corrected sea level rise estimates in this period. Consequently, where we have continuous sea level data (1930–1992), we calculated rates of sea-level change with the nodal lunar tide removed (NLTR), and we report it alongside estimates based on the raw micro-atoll sea level reconstruction.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This works presents a reconstruction of a sea-level record in the Maldives from a coral microatoll. This product is relevant\ndue to its length (since 1930) and high resolution (annual), and also to the fact that it is located in a data scarce region. It is\ntherefore a valuable contribution to increase the confidence in long-term global mean sea level changes and to provide\naccurate estimates of regional sea level variations in the Indian Ocean. I am not an expert in corals and sea level\nreconstructions from microatolls; although I have gone through the methods section, I am not qualified to comment on the\ntechnical details. Instead, I list below my questions and comments. Overall, I think that the manuscript is well organised and\npresented and the findings are well explained. My recommendation is that this contribution should be published, after some\nrevisions following the indications below.\n- Check where sea-level must be hyphenated.\n- Paragraph 40-47: the disparities discussed here have, to a large extent, been addressed by Frederikse et al 2020 (your\nreference 44). Please, consider including references to the closure of the 20th century sea-level budget discussed in this\npaper.\n- line 147: lunar cycle should be lunar nodal cycle\n- Figure 3: as pointed out by the authors there is a clear cyclic pattern in the sea level curve likely caused by the nodal cycle,\nwith a period of 18.6 yrs. Indeed, it has been shown that the nodal cycle can be reflected in mean sea level curves. Since\nthis oscillation is unrelated to climate variability I suggest removing the cycle from the mean sea level curve. It would be\nuseful to compare with the nodal oscillation in Gan, too, to ensure they are consistent. The removal of the nodal cycle is\nparticularly relevant for the computation of linear trends, that are reported here for periods of roughly 30 years, and which are\ntherefore affected by the presence of these decadal scale mean sea level changes.\n- Lines 153-159, and also fig. 3: the acceleration found in the sea level curve is larger than that reported by Dangendorf et al\n(2019) (reference 4), who found statistically insignificant acceleration in the region using their sea level reconstruction (see\ntheir figure 3). Note that they did not include tide gauges in the Maldives as part of the sources for reconstruction, but they did\nuse them to validate the output (see their fig S3). Please, discuss.\n- line 164: NOAAA should be NOAA\n- line 189-195: it is unclear why some ENSO events drive the three major stress events, while other are captured only as\ndiedowns due to sea level changes. For example, the El Niño 1966-67 and 1957-59 are both moderate but trigger different\nresponses. Any ideas?\n- Lines 210-215: In this sea-level rise trends, has any correction of vertical land motion been applied? If not, please, refer to\nthem as relative sea-level rise, to highlight that differences could be attributed to this non-climatic effect.\n- Figure 4: SODA is clearly different from the observational data in this figure. I wonder which is the added value of including\na reanalysis that is clearly biased in this region. Instead, I suggest including the altimetry record. Also, sea-level time series\nwould be better represented using lines rather than unconnected points (or both, but not only the points which makes the\nreading harder).\n- Lines 231-240: the consistency of the sea level curve with GMSL reconstructions has not been shown. In any case, I think it\nshould be compared to regional mean sea level rather than GMSL, since, as pointed out by the authors, regional differences\nare expected.", "reviewer_2": "Kench et al. use well-preserved coral microatolls from the Maldives in the central Indian Ocean to reconstruct relative sea-\nlevel (RSL) changes from 1930-2020. By measuring the growth of the microatoll surface and diedowns in the coral skeletal\nbands, this study provides a near-continuous record of annual RSL. The closest tide gauge to the study site began in 1987,\nso the new coral record extends the observational record by nearly 5 decades. Ocean reanalysis products typically show the\nmost disagreement before the satellite altimeter record, so proxy-based sea-level measurements provide much-needed\ninformation for data-sparse regions and time intervals.\nThis study finds that the coral record aligns well with instrumental sea-level changes over their common interval of overlap.\nThe study also identifies low rates of sea-level rise in the early 20th century, followed by an acceleration beginning in the\n1950s. The past three decades experienced the highest rate of 4.1 ± 0.1 mm/year. Another key finding is that some RSL\nchanges correspond with Indo-Pacific climate variability, including El Niño events and the Indian Ocean Dipole.\nThis study is clearly and succinctly presented and likely will be of broad interest to readers. The introduction effectively\nemphasizes why the selected coral microatoll is novel and well-suited for the analyses. The main and supplementary figures\nare also clear and easy to interpret. Supplementary Figures 2-3 and 5-6 are particularly helpful because they illustrate the\nmorphology of the coral and facilitate the proxy interpretation.\nOverall, this is a strong manuscript. I suggest minor revisions to clarify the climatic implications and enhance its accessibility\nto a broader audience. To help further strengthen the manuscript, I provide a few points and clarifying questions below.\n1. The manuscript discusses the correspondence between measured coral diedown and strong El Niño and IOD events.\nThe study also identifies three stress events in the skeletal growth history (1967–1968, 1980–1981, and 1991–1992) that are\ntemporally close to historical El Niño events, as well as a diedown in 2016 following the 2015–2016 El Niño. To better\nhighlight the connection between ENSO/IOD and RSL changes in the central Indian Ocean, I suggest adding a few\nsentences earlier in the main text that more explicitly state how RSL typically responds to strong/very strong El Niño events,\nmoderate/weak El Niño events, and positive IOD events. This additional context would enhance the interpretation of the\ncoral record shown in Figure 3a.\nFrom Figure 3, the link between strong El Niño events and coral growth is more apparent compared to moderate/weak El\nNiño events or positive IOD events. It is unclear from the manuscript how the IOD specifically influences sea-level changes\nin the central Indian Ocean. For example, some IOD events in Figure 3 correspond to diedowns, while others do not. This\ncould benefit from further discussion about the distinct impacts of ENSO and the IOD as well as their potential interactions.\nThis addition would help strengthen the climate-related interpretations in this study and broaden its accessibility to readers.\n2. I am curious why the 1980–1981 coral stress event and the multiple successive years of diedowns occurred several years\nearlier than the strong 1982-83 El Niño event. Is there instrumental evidence showing that SSTs in the region were elevated\nwell in advance of peak El Niño conditions, or documented evidence of coral bleaching in the area at this time? Or could the\nobserved timing discrepancies be due to chronological uncertainties in the coral records due to the growth hiatus? Some\nadditional discussion about the relative timing of the coral diedowns relative to historical El Niño events would strengthen\nthe manuscript.\n3. Lines 247-249 mention that previous studies attributed the recent acceleration in the rate of sea-level rise to basin-scale\nequatorial shifts in the Southern Hemispheric westerlies that promote ocean heat uptake. I recommend including citations\nhere that credit this work. I am also curious if there is an opportunity to more explicitly connect the recent acceleration to\nanthropogenic climate change. While a formal, in-depth attribution is beyond the scope of this study, a brief expansion on\nthis topic in the discussion could enhance the study's relevance and significance for a broader audience.\n4. The manuscript lacks a data availability section. I recommend that the authors include details about where the coral data\nwill be publicly archived upon publication.\nMinor notes:\n- How are the relative water levels for the purple triangles in Figure 3a calculated? Lines 520-521 in the methods section\nstate “these years of growth are not reliable indicators of water level as the coral was in catch-up growth mode,” so it is\nunclear how the values in Figure 3a were calculated. It may also be worth noting this uncertainty in the figure caption.\n- It could also be helpful if the authors included a supplementary figure with a map of where the tide gauge records in\nSupplementary Table 1 are located. This could effectively complement Figure 1 and Supplementary Figure 1. This would\nhighlight the lack of observations spatially and further emphasize why proxy-based records are needed to constrain\nuncertainties in RSL rise during the 20th century.\n- The shaded green and pink bars in Figure 2 are somewhat confusing as they mark historical El Niño and IOD events, but\nthe alignment with the curves of the growth bands in the coral is not immediately clear. To improve clarity, consider adjusting\nthe shading to follow the organic shape of the coral growth bands rather than using vertical bars. This approach could better\nillustrate the temporal correspondence between El Niño events and the coral growth record.", "reviewer_3": "This paper represents a significant contribution to the field of sea-level and climate variability studies. It leverages the\nremarkable ability of coral microatolls to record changes in sea level through their growth patterns. The study reconstructs\nsea level changes over a 90-year period (1930–2020) in the Central Indian Ocean, a region where instrumental records are\nscarce. The detailed analysis of annual growth rings on coral slabs offers an invaluable dataset for understanding long-term\nsea-level trends and climatic events in this understudied area. The work is of high quality and importance, showcasing\nrigorous analysis and innovative methodology.\nStrengths:\nThe use of both fossil and present-day microatolls to reconstruct historical sea level changes provides a robust, multi-faceted\napproach.\nThe detailed examination of bioerosion, growth records, and events such as bleaching enriches the dataset and provides a\ncomprehensive perspective.\nThe study highlights a critical period (1959–1992) of significant sea level rise in the Central Indian Ocean and discusses its\nrelationship with climatic and oceanographic phenomena, which is a novel and noteworthy finding.\nSuggestions for Improvement:\nStructure and Clarity:\nThe paper’s methods section incorporates elements of results and discussion, which could hinder readability for some\naudiences. For example, the analysis of fossil corals is introduced abruptly in the results section without sufficient context.\nTo improve flow, it would be beneficial to clarify upfront that the analysis encompasses both fossil and present-day\nmicroatolls.\nFigures and Visualization:\nFigure 2 should be integrated with Supplementary Figures 2 and 6 to provide a more comprehensive visual representation\nof the findings. Additionally, enlarging Figure 1e to align it with the tidal elevation in Supplementary Figure 2 would enhance\nclarity.\nMicroatoll Growth and Regional Patterns:\nThe observed growth of a new microatoll on the rim of the fossil microatoll, corresponding to a sea level rise of 0.13 m after\nthe 1996 event, is a fascinating result. The dense distribution of microatolls in the area (line 398) raises the question: are\nthere other examples with similar features, or are submerged microatolls present? Addressing this could provide deeper\ninsights into regional sea level responses.\nReef Zonation and Topography:\nThe description of reef zonation (lines 390–402) could benefit from additional clarification. Including a topographic cross-\nsection that maps these zones to their locations in Figure 1d would significantly aid understanding.\nDiscussion on Methodology:\nThe methodological details are rigorous, but the evaluation of factors such as bioerosion and bleaching effects could be\ndiscussed further. This would emphasize the robustness of the reconstruction and highlight the challenges addressed in\nthe study.\nTerminology and Sea Level Datum:\nThe terms \"water level\" and \"sea level\" are used interchangeably throughout the text. To avoid confusion, it should be\nexplicitly stated in the main text that the elevation (HLC) is a close approximation of MLLW (-0.41 m). Furthermore, the\nauthors could explore the relationship between tide levels, microatoll growth, and the observed sea level rise during the\nstudy period (approximately 2 cm) to contextualize the reconstruction accuracy (±3.21 cm).\nClimatic Variability and Regional Differences:\nThe significant sea level rise observed in the Central Indian Ocean during 1959–1992, contrasted with the lack of similar\ntrends in India and Australia, is a key finding. The discussion (lines 247–251) suggests heat transport to the equatorial\nregion as a potential cause. A more detailed exploration of this mechanism, supported by relevant references, would greatly\nenhance the discussion and provide further context for the study’s motivation.\nIn summary, this paper makes an outstanding contribution to the understanding of sea-level and climatic variability in the\nIndian Ocean. Addressing the above suggestions will strengthen the paper's structure and clarity, ensuring its impactful\nresults are communicated effectively."}, "author_response": "RESPONSE TO REVIEW COMMENTS\nReviewer #1 (Remarks to the Author):\nThis works presents a reconstruction of a sea-level record in the Maldives from a coral microatoll. This product is\nrelevant due to its length (since 1930) and high resolution (annual), and also to the fact that it is located in a data\nscarce region. It is therefore a valuable contribution to increase the confidence in long-term global mean sea level\nchanges and to provide accurate estimates of regional sea level variations in the Indian Ocean. I am not an expert in\ncorals and sea level reconstructions from microatolls; although I have gone through the methods section, I am not\nqualified to comment on the technical details. Instead, I list below my questions and comments. Overall, I think that the\nmanuscript is well organised and presented and the findings are well explained. My recommendation is that this\ncontribution should be published, after some revisions following the indications below.\nWe thank the reviewer for their positive comments on the valuable contribution of the manuscript and\ntheir constructive comments. We respond to each comment below with associated amendments in the\nmanuscript.\n1. Check where sea-level must be hyphenated. Thank you for the reminder. We believe we have checked the\ndocument and have made appropriate amendments where necessary.\n2. Paragraph 40-47: the disparities discussed here have, to a large extent, been addressed by Frederikse et al 2020\n(your reference 44). Please, consider including references to the closure of the 20th century sea-level budget\ndiscussed in this paper.\nWe have included reference to this research and closure of the 20th Century sea level budget in the\nrevised text.\n3. Line 147: lunar cycle should be lunar nodal cycle. We have amended this sentence.\nFigure 3: as pointed out by the authors there is a clear cyclic pattern in the sea level curve likely caused by the\nnodal cycle, with a period of 18.6 yrs. Indeed, it has been shown that the nodal cycle can be reflected in mean sea\nlevel curves. Since this oscillation is unrelated to climate variability I suggest removing the cycle from the mean\nsea level curve\nThe Reviewer raises an interesting point related to the influence of the lunar nodal cycle, and we have\nexplored this suggestion. Our study site is located near the equator in the central tropical Indian\nOcean. Although it remains debated the extent to which the 18.6-yr nodal cycle follows the\nequilibrium law, the amplitude of the nodal tide at our site should only be ~10-20 mm (Proudman,\n1960; Houston and Dean, 2011; Woodworth, 2012; Bult et al., 2024). This value is small, and\ntherefore, is unlikely to have a significant effect in our calculations on the rate of sea level rise on\ndecadal timescales. Nevertheless, we followed the suggestion and removed the lunar nodal cycle\nsignal from our microatoll-based sea level record.\nWe first performed polynomial regression on the sea level record to estimate the nodal tide cycle (R2 =\n0.98). The obtained cycle peaks at around 1933, 1952, 1970 and 1989, consistent with the relatively\nhigh sea level points around 1932, 1950/51, 1969 and 1988 observed in the microatoll, as well as the\ncalculated timing based on the equilibrium law (Pugh, 1987). The cycle has an amplitude of ~ 15 mm\non yearly averaged sea level variation, which approximates the theoretical value of ~ 9 mm derived\nfrom the equilibrium law (Woodworth, 2012).\nWe then removed the nodal tide cycle from the original data. See the figure below. The grey curve\nrepresents the original data, while the blue curve represents the data after removing the 18.6-year\nnodal cycle. The red sinusoidal curve shows the 18.6-year cycle.\nWe analysed the transformed data and found that the rates of sea-level rise are similar before and\nafter correcting the nodal cycle, for example, 1.42 mm/yr and 1.51 mm/yr, respectively, during the\ntime period 1930-1959, and 3.48 mm/yr and 3.07 mm/yr, respectively, during the time period 1960-\n1992.\nHowever, due the significant data gap in the period 1993-2015 (as a consequence of the death of the\ncoral colony), which spans the majority of a cycle, removing the nodal trend from the start and end of\nthe period of analysis distorts the trend, and we believe is unreliable. A table of the sea-level rise\nestimates from the raw record and the record with the nodal trend removed is shown below.\nBecause of this latter distortion (resulting from the absence of data) we believe it more robust to\nretain our initial set of data. We believe this is robust as:\ni) The main conclusions hold. No matter with the cycle corrected or not, the rate of sea level rise is\nsubstantially higher (e.g., more than doubled) during the time interval of 1960-1992 than the\nperiod 1930-1959.\nii) Removal of the nodal cycle yields unreliable results in the final time-step (1990-2019). This\napproach is further supported by the physical evidence in which we have established 127 mm of\nvertical growth on top of the dead microatoll surface to 2019 at ~4.3 mm/yr.\nHowever, recognising this influence we have amended the text to alert reader that the likely\ndifference in rates of sea level change are on the order of 0.1-0.3 mm/yr. We are comfortable\nincluding the de-nodal rates for the earlier time periods spanning (1932-1992) as there is no\nsubstantive difference in magnitude and changes in rates. We include these in brackets following the\nraw data tends in the amended text. We have also included our approach to this analysis in the\nmethods.\nTable of sea-level rise estimates from the raw micro-atoll sea level reconstruction (Raw Data) and\nthe transformed denodal trends.\nRaw data De-nodal trends\n1930-2019 3.52 ± 0.19 mm/yr 3.62 ± 0.17 mm/yr\n1930-1992 3.06 ± 0.26 mm/yr 3.04 ± 0.18 mm/yr\n1930-1959 1.42 ± 0.42 mm/yr 1.51 ± 0.33 mm/yr\n1960-1992 3.44 ± 0.68 mm/yr 3.07 ± 0.37 mm/yr\n1990-2019 4.39 ± 0.48 mm/yr 5.24 ± 0.44 mm/yr\n4. Lines 153-159, and also fig. 3: the acceleration found in the sea level curve is larger than that reported by\nDangendorf et al (2019) (reference 4), who found statistically insignificant acceleration in the region using their\nsea level reconstruction (see their figure 3). Note that they did not include tide gauges in the Maldives as part of\nthe sources for reconstruction, but they did use them to validate the output (see their fig S3). Please, discuss.\nThank you. We have included a new passage highlighting these differences (new lines 261-263) and\nprovide further discussion on these differences in new lines 299-313.\n5. Line 164: NOAAA should be NOAA. We have amended this in the manuscript.\n6. Line 189-195: it is unclear why some ENSO events drive the three major stress events, while other are captured\nonly as diedowns due to sea level changes. For example, the El Niño 1966-67 and 1957-59 are both moderate but\ntrigger different responses. Any ideas?\nThis is a good observation. However, while the extreme impacts of such climate variability on coral\nhealth and ecosystems is becoming increasingly better understood, it is also apparent that the impact\nof each event on corals exhibits significant spatial and event variability, due to a raft of localized factors,\nsuch as upwelling and circulation that may buffer water temperatures between sites, and that some\nspecies of coral may be more susceptible/resilient to temperature and water level fluctuations.\nFurthermore, there are two driving forces that can influence coral response. First, is sea surface\ntemperature fluctuations, that stress the coral organism and can lead to wide spread bleaching (and\ndeath of the colony). Second, are lower sea level periods which also expose coral tissue and constrain\nthe height of living coral tissue. At our site negative phases of the Indian Ocean Dipole can induce\nlower sea levels which constrain the height of living coral. On two occasions in our record extreme SST\nincreases (associated with El Niño) is followed by lower sea levels (-ve IOD).\nWe believe it is beyond the scope of this study to be able to isolate specific event-by-event\nconsequences that account for such spatial heterogeneity, as we have no information on some of these\nother environmental controls, particularly earlier in the record.\nHowever, we have amended the text to make more explicit how the different climatic drivers can\ninfluence coral growth (new second paragraph in the results). We also have created an additional\nSupplementary Figure (7) that presents the available tide gauge data (from the closest gauge to the\nsite) and how the negative IOD is associated lower with sea level stages, and the record of sea surface\ntemperature and the association to El Niño events.\n7. Lines 210-215: In this sea-level rise trends, has any correction of vertical land motion been applied? If not, please,\nrefer to them as relative sea-level rise, to highlight that differences could be attributed to this non-climatic effect.\nThank you for noting this. Indeed, no correction for vertical land motion has been applied. We have\namended the text to clarify that our rates refer to relative sea-level rise.\n8. Figure 4: SODA is clearly different from the observational data in this figure. I wonder which is the added value of\nincluding a reanalysis that is clearly biased in this region. Instead, I suggest including the altimetry record. Also,\nsea-level time series would be better represented using lines rather than unconnected points (or both, but not only\nthe points which makes the reading harder).\nWe have amended the plot following the Reviewer suggestions. Specifically, we have removed the\nSODA record, added the altimetry record and represented the data with lines.\n9. Lines 231-240: the consistency of the sea level curve with GMSL reconstructions has not been shown. In any case, I\nthink it should be compared to regional mean sea level rather than GMSL, since, as pointed out by the authors,\nregional differences are expected.\nWe agree. Our point here is that our record is broadly similar to GMSL patterns, and believe this is a\npoint worth noting. The remainder of our discussion focusses on regional comparisons and the regional\ndifferences.\nReviewer #2 (Remarks to the Author):\nKench et al. reconstruct relative sea-level (RSL) changes for the central Indian Ocean using the growth and diedowns of\ncoral microatoll surfaces. This study provides a near-continuous record of annual RSL from 1930-2020, which extends the\nobservational record by approximately six decades. There are large disagreements in ocean reanalysis products prior to the\nsatellite altimeter era, so proxy-based sea-level measurements provide crucial information for data-sparse regions and time\nintervals.\nThis study finds that the coral record aligns well with instrumental sea-level changes over their common interval of overlap.\nThe study also identifies low rates of sea-level rise in the early 20th century, followed by an acceleration\nbeginning in the 1950s with the highest rates of change over the past 3 decades. The study also links some of the observed\nRSL changes to broader Indo-Pacific climate variability, including the El Niño-Southern Oscillation and the Indian\nOcean Dipole.\nIn response to the reviewer feedback, the authors expanded upon some methodological details and clarified the influence of\nENSO and the IOD on local coral growth. They also included new supplementary figure showing the influence of Indian\nocean climate modes on temperature and sea level in the Maldives. The revised manuscript also includes updated figures\nand captions that clarify and improve the accessibility of the results. The manuscript now also includes a data availability\nsection noting that all datasets will be made publicly available via Zenodo.\nOverall, the authors thoughtfully addressed all my comments, and I support its publication. This is a strong, well-written\nmanuscript that will be of broad interest to readers.\n1. The manuscript discusses the correspondence between measured coral diedown and strong El Niño and IOD events.\nThe study also identifies three stress events in the skeletal growth history (1967–1968, 1980–1981, and 1991–1992)\nthat are temporally close to historical El Niño events, as well as a diedown in 2016 following the 2015–2016 El Niño.\nTo better highlight the connection between ENSO/IOD and RSL changes in the central Indian Ocean, I suggest\nadding a few sentences earlier in the main text that more explicitly state how RSL typically responds to strong/very\nstrong El Niño events, moderate/weak El Niño events, and positive IOD events. This additional context would\nenhance the interpretation of the coral record shown in Figure 3a.\nFrom Figure 3, the link between strong El Niño events and coral growth is more apparent compared to\nmoderate/weak El Niño events or positive IOD events. It is unclear from the manuscript how the IOD specifically\ninfluences sea-level changes in the central Indian Ocean. For example, some IOD events in Figure 3 correspond to\ndiedowns, while others do not. This could benefit from further discussion about the distinct impacts of ENSO and\nthe IOD as well as their potential interactions. This addition would help strengthen the climate-related\ninterpretations in this study and broaden its accessibility to readers.\nThank you for raising the above point. It has allowed us to consider this in greater detail. The influence of\nENSO and IOD events is different between the two mechanisms. First, the primary influence of ENSO at the\nfield location is on the sea surface temperature variations. We have included a new Supplementary Figure\n(7) that shows the SST record for the field location and highlight the extreme temperatures that align with El\nNiño events, with a 3-6 month lag from the peak expression in the Pacific Ocean to the field site. We also\nnote that when combined with positive IOD events the SST can be elevated well beyond the seasonal\nmaxima. Increased SST’s can stress coral tissue and in major events can cause both severe stress and death\nof coral colonies (expressed as very narrow, or absent, growth bands). Second, the Indian Ocean Dipole\n(IOD) influences sea level in the Indian Ocean. Specifically, it elevates SL in the Western IO during positive\nIOD events and decreases SL during negative IOD events. However, studies show that such differences are\nmost strongly expressed in the southern IO and Bay of Bengal, and the Maldives which is centrally located\nappears to sit between these zones where responses are more subtle (Han et al., 2019). Consequently, it is\nthe negative phase of IOD rather than the positive phase that is likely to lower sea level at the field site. We\nhave now also included the Gan sea level record with timing of the negative IOD events in Supplementary\nFigure 7b.\nNotably, at the time of the major die-off of the fossil coral slab, a major bleaching event (1991/1992,\nextreme SST) was immediately followed by a strong negative IOD (lower sea level). The following 12 months\nwas also had very low sea levels, though these are not coincident with negative IOD phases.\nAs outlined in response to Reviewer 1 (point 6 above) not all extreme SST events impact all corals and reefs\nto the same extent. It is evident that Porites massive corals are more resilient to temperature stresses than\nbranching corals (See Perry and Morgan as opposed to Ryan et al). However, it is clear that our coral\nexperienced three cessations of growth and multiple diedowns, which are described in the results and more\nexplicitly linked to the different climate drivers.\n2. I am curious why the 1980–1981 coral stress event and the multiple successive years of diedowns occurred several\nyears earlier than the strong 1982-83 El Niño event. Is there instrumental evidence showing that SSTs in the region\nwere elevated well in advance of peak El Niño conditions, or documented evidence of coral bleaching in the area at\nthis time? Or could the observed timing discrepancies be due to chronological uncertainties in the coral records due\nto the growth hiatus? Some additional discussion about the relative timing of the coral diedowns relative to\nhistorical El Niño events would strengthen the manuscript.\nWell noted. The SST record (new Sup Fig 2b) shows that the temperature increase occurred coincident with\nthe 1982 El Nino event (with a 3-6 month lag). The initial draft of the Figure took the decision to not include\ndecimal places or the error term, due to space constraints. However, revisiting this issue we believe it\njustifiable to round the ages. The U-Th dates the age for the hiatus is 1981.9 ±1.2. We have amended the\nfigure to restate the 1981 age as 1982. Therefore we believe the temporal disparity is not large, particularly\nin light of the chronological uncertainty.\n3. Lines 247-249 mention that previous studies attributed the recent acceleration in the rate of sea-level rise to basin-\nscale equatorial shifts in the Southern Hemispheric westerlies that promote ocean heat uptake. I recommend\nincluding citations here that credit this work. I am also curious if there is an opportunity to more explicitly connect\nthe recent acceleration to anthropogenic climate change. While a formal, in-depth attribution is beyond the scope\nof this study, a brief expansion on this topic in the discussion could enhance the study's relevance and significance\nfor a broader audience.\nWe have added relevant citations to this section and increased this discussion that attempts to explicitly\nconnect the recent acceleration to anthropogenic climate change (new line No. 275-295).\n4. The manuscript lacks a data availability section. I recommend that the authors include details about where the\ncoral data will be publicly archived upon publication.\nWe have added a data availability statement and intend to make the dataset available prior to the\nmanuscript being published.\nMinor notes:\n• How are the relative water levels for the purple triangles in Figure 3a calculated? Lines 520-521 in the methods\nsection state “these years of growth are not reliable indicators of water level as the coral was in catch-up growth\nmode,” so it is unclear how the values in Figure 3a were calculated. It may also be worth noting this uncertainty in\nthe figure caption.\nThank you for this query. The purple triangles in Fig. 3a represent the elevation of annual growth\nincrements when the coral was dominated by vertical growth under the scenario outlined in Supplementary\nFigure 6. As such they are not accurate reflections of sea level and are excluded from any analyses of sea\nlevel and sea level trends. As they form part of the coral growth record we feel it important to include the\npoints, despite the fact they are not used in establishing rates of sea-level rise. We have updated the\ncaption and methods to highlight this.\n• It could also be helpful if the authors included a supplementary figure with a map of where the tide gauge records\nin Supplementary Table 1 are located. This could effectively complement Figure 1 and Supplementary Figure 1. This\nwould highlight the lack of observations spatially and further emphasize why proxy-based records are needed to\nconstrain uncertainties in RSL rise during the 20th century.\nWe have included a new Supplementary Figure 1 showing the location of the tide gauges.\n• The shaded green and pink bars in Figure 2 are somewhat confusing as they mark historical El Niño and IOD events,\nbut the alignment with the curves of the growth bands in the coral is not immediately clear. To improve clarity,\nconsider adjusting the shading to follow the organic shape of the coral growth bands rather than using vertical\nbars. This approach could better illustrate the temporal correspondence between El Niño events and the coral\ngrowth record.\nIn the revised figure we have highlighted the growth bands that are associated with very strong El Niño\nevents and strong negative Indian Ocean Diploe events. We have removed the vertical shaded lines, but\nhave retained coloured arrows that indicate times of these strong climatic events.\nReviewer #3 (Remarks to the Author):\nThis paper represents a significant contribution to the field of sea-level and climate variability studies. It leverages the\nremarkable ability of coral microatolls to record changes in sea level through their growth patterns. The study\nreconstructs sea level changes over a 90-year period (1930–2020) in the Central Indian Ocean, a region where\ninstrumental records are scarce. The detailed analysis of annual growth rings on coral slabs offers an invaluable\ndataset for understanding long-term sea-level trends and climatic events in this understudied area. The work is of high\nquality and importance, showcasing rigorous analysis and innovative methodology.\nStrengths:\n• The use of both fossil and present-day microatolls to reconstruct historical sea level changes provides a robust,\nmulti-faceted approach.\n• The detailed examination of bioerosion, growth records, and events such as bleaching enriches the dataset and\nprovides a comprehensive perspective.\n• The study highlights a critical period (1959–1992) of significant sea level rise in the Central Indian Ocean and\ndiscusses its relationship with climatic and oceanographic phenomena, which is a novel and noteworthy finding.\nSuggestions for Improvement:\nStructure and Clarity:\nThe paper’s methods section incorporates elements of results and discussion, which could hinder readability for some\naudiences. For example, the analysis of fossil corals is introduced abruptly in the results section without sufficient\ncontext. To improve flow, it would be beneficial to clarify upfront that the analysis encompasses both fossil and present-\nday microatolls.\nIn the amended manuscript we highlight that the analysis is based on both fossil and present-day\nmicroatolls much earlier (lines 94-95) and have amended the heading in the results (line 113).\nFigures and Visualization:\nFigure 2 should be integrated with Supplementary Figures 2 and 6 to provide a more comprehensive visual\nrepresentation of the findings. Additionally, enlarging Figure 1e to align it with the tidal elevation in Supplementary\nFigure 2 would enhance clarity.\nThank you for this suggestion. We have now revised Figure 2 incorporating elements of old Supplementary\nFigures 2 and 6. While this makes the Figure more complex we feel it does a better job of contextualizing\nthe samples and their relationship to sea level as indicated by the Reviewer. We have consequently deleted\nthe earlier Supplementary Figures 2 and 6.\nMicroatoll Growth and Regional Patterns:\nThe observed growth of a new microatoll on the rim of the fossil microatoll, corresponding to a sea level rise of 0.13 m\nafter the 1996 event, is a fascinating result. The dense distribution of microatolls in the area (line 398) raises the\nquestion: are there other examples with similar features, or are submerged microatolls present? Addressing this could\nprovide deeper insights into regional sea level responses.\nYes, there are multiple other examples of the recolonization of older coral surfaces in the study area, which\nare the subject of ongoing work. We have amended this section to highlight there are multiple specimens.\nWe also note that previously we have published work on older fossil microatolls that define variable sea\nlevels ±0.8 m during the Common Era (Kench et al., Nature Geoscience, 2020). This latter point is outlined in\nthe subsequent paragraph.\nReef Zonation and Topography:\nThe description of reef zonation (lines 390–402) could benefit from additional clarification. Including a topographic\ncross-section that maps these zones to their locations in Figure 1d would significantly aid understanding.\nWe have included a new Supplementary Figure 2 that contains an aerial drone image of the platform\nlocating the sample and other fossil microatolls on the central platform. We also present a surveyed cross\nsection from the reef edge through the sample specimen to island beach.\nDiscussion on Methodology:\nThe methodological details are rigorous, but the evaluation of factors such as bioerosion and bleaching effects could be\ndiscussed further. This would emphasize the robustness of the reconstruction and highlight the challenges addressed in\nthe study.\nThank you. We are a little uncertain precisely what additional information the Reviewer is seeking .\nHowever, we have amended the manuscript in two ways in response to this comment.\nFirst, we have added further material to the methods related to procedures to evaluate bioerosion in the\ncoral specimen. Second, to address the comment regarding bleaching we have included greater detail in\nthe manuscript related to the role of elevated sea surface temperatures on coral health and the main\nclimate modes that influence (SSTs) at the study location. This information has been added to the main text\n(line 124-136) and new Supplementary Figure 7 shows the association of climate modes and SST and sea\nlevel.\nTerminology and Sea Level Datum:\nThe terms \"water level\" and \"sea level\" are used interchangeably throughout the text. To avoid confusion, it should be\nexplicitly stated in the main text that the elevation (HLC) is a close approximation of MLLW (-0.41 m). Furthermore, the\nauthors could explore the relationship between tide levels, microatoll growth, and the observed sea level rise during the\nstudy period (approximately 2 cm) to contextualize the reconstruction accuracy (±3.21 cm).\nThank you for this suggestion. We have amended the text in two places to more explicitly state that the HLC\nis a close approximation of MLLW (-0.41 m). First at the end of the introduction on lines 95-97, and we also\nhighlight this in the revised methods section.\nClimatic Variability and Regional Differences:\nThe significant sea level rise observed in the Central Indian Ocean during 1959–1992, contrasted with the lack of\nsimilar trends in India and Australia, is a key finding. The discussion (lines 247–251) suggests heat transport to the\nequatorial region as a potential cause. A more detailed exploration of this mechanism, supported by relevant\nreferences, would greatly enhance the discussion and provide further context for the study’s motivation.\nThis was a similar point to one raised by the second reviewer. We have now added further discussion on the\nmechanism drawing on additional literature (new line No. 299-313).\nIn summary, this paper makes an outstanding contribution to the understanding of sea-level and climatic variability in\nthe Indian Ocean. Addressing the above suggestions will strengthen the paper's structure and clarity, ensuring its\nimpactful results are communicated effectively.\nThank you very much and thank you for your constructive comments."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have satisfactorily addressed all my concerns on the previous version of the manuscript. In particular, I think it is\nimportant to emphasise the differences obtained when the nodal cycle is accounted for in the reconstructed sea level record.\nI also believe that the manuscript benefits from a deeper discussion on the climatic factors affecting the corals, as highlighted\nby the other reviewers too. I thus think the manuscript is ready for publication.", "reviewer_2": "Kench et al. reconstruct relative sea-level (RSL) changes for the central Indian Ocean using the growth and diedowns of\ncoral microatoll surfaces. This study provides a near-continuous record of annual RSL from 1930-2020, which extends the\nobservational record by approximately six decades. There are large disagreements in ocean reanalysis products prior to the\nsatellite altimeter era, so proxy-based sea-level measurements provide crucial information for data-sparse regions and time\nintervals.\nThis study finds that the coral record aligns well with instrumental sea-level changes over their common interval of overlap.\nThe study also identifies low rates of sea-level rise in the early 20th century, followed by an acceleration beginning in the\n1950s with the highest rates of change over the past 3 decades. The study also links some of the observed RSL changes to\nbroader Indo-Pacific climate variability, including the El Niño-Southern Oscillation and the Indian Ocean Dipole.\nIn response to the reviewer feedback, the authors expanded upon some methodological details and clarified the influence of\nENSO and the IOD on local coral growth. They also included new supplementary figure showing the influence of Indian\nocean climate modes on temperature and sea level in the Maldives. The revised manuscript also includes updated figures\nand captions that clarify and improve the accessibility of the results. The manuscript now also includes a data availability\nsection noting that all datasets will be made publicly available via Zenodo.\nOverall, the authors thoughtfully addressed all my comments, and I support its publication. This is a strong, well-written\nmanuscript that will be of broad interest to readers.", "reviewer_3": "The revised manuscript has fully addressed my previous comments. I find the revisions satisfactory and support its\npublication."}, "author_response": ""}]

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