Rising seas are a major consequence of global warming, with many implications for low-lying coastal areas. Coral reefs, which are highly sensitive to their environment, are also particularly vulnerable to fluctuations in sea level. When the sea level rises, sunlight may no longer penetrate the water to reach a coral reef that it could reach before. This can lead to coral bleaching.
Changes in tide patterns and increasing coastal erosion can further stress reef ecosystems already bearing the brunt of warmer waters and ocean acidification.
Significant gaps
Monitoring sea-level rise across ocean basins has been an ongoing scientific priority. In the Indian Ocean, long-term efforts began during the Tropical Ocean Global Atmosphere programme in the Western Indian Ocean (1985-1994). These efforts were later incorporated into the Global Sea Level Observing System, which continues to support research in the region.
According to India’s Ministry of Earth Sciences, the Indian Ocean’s levels have been rising at around 3.3 mm/year on average, which is higher than the global average. The Ocean is also experiencing above-average warming, which can amplify changes in ocean dynamics and atmospheric circulation that in turn affect coral bleaching episodes.
This said, there are still significant gaps in sea-level records, especially in the central tropical Indian Ocean. A new study has now extended sea-level records in this region by 90 years, indicating water levels here may have started accelerating as early as the late 1950s, significantly earlier than data collected by conventional tide gauge records indicate.
Painstaking survey
In the study, a team led by Paul Kench, professor at the National University of Singapore, along with researchers from Nanyang Technological University turned to coral microatolls, a natural structure that they found could provide high-resolution, long-term sea-level records.
Coral microatolls are disk-shaped colonies that grow sideways once their upward growth has become constrained by the height of the lowest tide. Because of this limitation, the upper surface of a microatoll closely reflects the lowest water levels in the area over time. These corals can survive for decades or even centuries, growing slowly in response to changing sea levels.
The study was conducted on Mahutigalaa, a reef platform located in the Huvadhoo Atoll in the Maldives. The team studied a Porites microatoll, measuring and sampling its structure to extract a sea-level history from 1930 to 2019.
The researchers painstakingly surveyed the coral’s outer edge and surface elevation. Then they cut a slab from the outer edge to the centre of the microatoll, and X-rayed the slab to reveal annual growth bands — much like tree rings. These bands provided a precise timeline of the coral’s growth, including when it reached sea level and when it died.
The team also used uranium-thorium dating to determine its historical elevation relative to the sea level.
Assumption challenged
The data the team reconstructed in this way showed that sea levels had risen by around 0.3 metres over the 90-year period. The rate of rise increased markedly over time: 1-1.84 mm/year in 1930-1959, 2.76-4.12 mm/year in 1960-1992, and 3.91-4.87 mm/year in 1990-2019.
Also according to the team, sea-level rise in the region began in the late 1950s, decades earlier than previously believed.
This means the Maldives, Lakshadweep, and the Chagos archipelago have been experiencing significant rise for at least 60 years, with a total increase of 30-40 cm over the last half-century. This data challenges the common assumption in climate change and adaptation work that significant sea-level rise only began around 1990.
Since 1959, sea level in these areas has risen by about 3.2 mm/year, and over the last 20 to 30 years at around 4 mm/year.
Historical context
The coral microatoll also preserved environmental signals related to regional climate variability. Periods of slowed or interrupted growth were found to correspond with major El Niño and negative Indian Ocean Dipole (IOD) events — climatic phenomena known to stress corals and lead to bleaching.
The data also revealed the influence of the 18.6-year lunar nodal cycle, where long-term oscillations in the moon’s orbit affect the sizes of tides and sea levels.
The researchers noted that a critical factor in the success of its reconstruction exercise was that the study site was tectonically stable. This stability ensures changes in the microatolls’ elevation can be safely attributed to fluctuations in sea level rather than to vertical land movement.
According to Kench, while coral microatolls are not a substitute for tide gauges or satellite observations, they offer a valuable complementary approach. In remote or data-sparse regions, microatolls can provide historical context and improve understanding of regional variability in sea-level behaviour.
Growing role
The study also highlighted notable differences in sea-level rise patterns across the Indian Ocean basin. While coastal locations have shown more recent acceleration, the central Ocean appeared to have experienced an earlier, more pronounced rise. This variation is thought to be driven by regional oceanic and atmospheric changes, including intensified Southern Hemisphere westerlies, increased ocean heat uptake, and potential shifts in the Intertropical Convergence Zone.
As research continues, coral microatolls are expected to play a growing role in helping scientists rebuild sea-level histories across tropical waters. Their potential to fill critical gaps in observational records is particularly relevant for the central Indian Ocean, “which remains one of the least-monitored basins despite its strategic and ecological importance,” Kench said.
The new findings add to efforts aimed at refining projections of sea-level rise and improving preparedness in regions most at risk. For island nations, where communities and infrastructure are concentrated just above sea level, understanding the timing and magnitude of historic sea-level changes is essential for authorities to develop effective adaptation strategies.
Neelanjana Rai is a freelance journalist who writes about indigenous community, environment, science and health.
Published – September 01, 2025 05:15 am IST