Coral Microatolls Reveal Early Indian Ocean Sea-Level Rise
Rising sea levels due to global warming threaten coastal and island ecosystems. Coral reefs, sensitive to environmental changes, face stress from reduced sunlight, bleaching, and erosion. Recent research using coral microatolls has extended sea-level records in the central Indian Ocean by nearly a century. This study shows sea-level rise began decades earlier than previously thought, with important implications for climate science and adaptation efforts.
Sea-Level Rise
Sea-level rise reduces sunlight reaching coral reefs. This weakens coral health and causes bleaching. Changing tides and coastal erosion add pressure to reef ecosystems. The Indian Ocean is warming faster than the global average, amplifying these effects.
Historical Monitoring Efforts
Long-term sea-level monitoring in the Indian Ocean started during the Tropical Ocean Global Atmosphere programme (1985-1994). These efforts continue under the Global Sea Level Observing System. India’s Ministry of Earth Sciences reports a rise of about 3.3 mm per year, above the global mean. However, data gaps remain, especially in the central tropical Indian Ocean.
Coral Microatolls as Natural Records
Coral microatolls grow laterally when upward growth is limited by low tide. Their upper surface matches the lowest water level over time. These structures can live for decades or centuries, recording sea-level changes. Researchers studied a Porites microatoll on Mahutigalaa reef in the Maldives to reconstruct sea levels from 1930 to 2019.
Methodology of the Study
The team surveyed the coral’s outer edge and surface elevation. They cut a slab from the coral and X-rayed it to observe annual growth bands, similar to tree rings. These bands provided a precise timeline of growth and sea-level interaction. Uranium-thorium dating helped date the coral’s elevation relative to sea level.
Findings on Sea-Level Rise
The study found a 0.3-metre rise over 90 years. The rate accelerated over time
- 1-1.84 mm/year (1930-1959)
- 2.76-4.12 mm/year (1960-1992)
- 3.91-4.87 mm/year (1990-2019).
Sea-level rise in the region began in the late 1950s, earlier than previously believed. The Maldives, Lakshadweep, and Chagos archipelago saw a 30-40 cm rise over the last 50 years.
Environmental and Climatic Signals
Growth interruptions in the microatoll matched major El Niño and negative Indian Ocean Dipole events. These are known to stress corals and cause bleaching. The coral also recorded the 18.6-year lunar nodal cycle affecting tides and sea levels. The tectonic stability of the site ensured accurate sea-level attribution.
Significance and Future Role
Coral microatolls complement tide gauges and satellites by filling data gaps. They provide long-term regional sea-level histories, especially in remote areas. The study shows different sea-level rise patterns across the Indian Ocean basin. Regional oceanic and atmospheric changes, such as Southern Hemisphere westerlies and shifts in the Intertropical Convergence Zone, influence these patterns. Microatolls will play a growing role in climate research and adaptation planning.
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