Global coral bleaching crisis: Indian Ocean reefs severely affected by record heat, says NOAA

El Niño (a climate pattern characterised by the periodic warming of sea surface temperatures in the central and eastern Pacific Ocean) causes elevated sea temperatures that stress and bleach corals, while ENSO neutral conditions (a climate cycle involving periodic fluctuations in sea surface temperatures and atmospheric conditions) bring more stable temperatures, potentially allowing corals to recover and reducing the likelihood of bleaching.

The monitoring of thermal anomalies in the oceans using satellite-derived sea surface temperatures by NOAA’s Coral Reef Watch (CRW) has been successful at predicting where and when coral bleaching is likely to occur. NOAA Coral Reef Watch has predicted all large-scale bleaching events since the late 1990s by observing thermal anomalies during the warmest parts of the year.

The primary algorithm used, Degree Heating Weeks (DHWs), is based on observations of mass coral bleaching and mortality during the 1982/83 El Niño and subsequent laboratory experiments linking elevated temperatures to bleaching.

In this context, India’s major coral areas—the Gulf of Mannar, Lakshadweep, Andaman, and Gujarat—have also been affected by this global bleaching event.

In an exclusive interview, Dr Derek Manzello, Coordinator of NOAA Coral Reef Watch, provides insights into the current status, impacts, and future of coral reefs in the Indian Ocean. While sharing details of the global bleaching event up until June, he elaborates on the influence and impact of changing climate patterns, and the urgent need for conservation measures. While predicting bleaching is highly effective, mitigating its impacts remains a significant challenge.

Can you provide an overview of the current status of coral reefs in the Indian Ocean?

We have received reports of severe bleaching taking place in the Gulf of Mannar. The NOAA has issued a red alert for this region, indicating a high likelihood of mass coral mortality. The Gulf of Mannar coral reefs have already begun to show significant signs of bleaching due to the intense heat stress experienced from March to May this year. Rapid assessment measures are being undertaken to evaluate the extent of the damage. In Lakshadweep, coral reefs are also facing substantial bleaching threats as a result of elevated sea temperatures. While we have yet to receive specific reports from the Andaman or the coast of Gujarat, the heat stress data from NOAA Coral Reef Watch suggests that bleaching has likely occurred in these regions as well. The exact severity of the bleaching in these areas remains unclear, and ongoing monitoring efforts are essential to gain a comprehensive understanding of the impacts on coral reefs throughout the Indian Ocean.

How have the high sea surface temperatures during March, April, and May 2024 impacted these coral reefs?

Bleaching-level heat stress has impacted all of these regions. The accumulated heat stress reached bleaching levels at the end of April for the Gulf of Mannar and Lakshadweep; the heat stress peaked in mid-May at both these locations. In Andaman, bleaching level heat stress started at the beginning of May and peaked at the end of May. For Gujarat, bleaching-level heat stress started in mid-May and the heat stress is still accumulating at this location. In summary, the heat stress has ended and temperatures are declining for the Gulf of Mannar, Lakshadweep, and Andaman; however, the heat stress is still increasing in Gujarat. The heat stress in Lakshadweep was record-setting this year.

Are there any specific coral species in these regions that are more susceptible to bleaching?

Corals in the genus Acropora (small polyp stony corals known for their branching growth patterns, forming complex and diverse reef structures essential to marine ecosystems) tend to be the most sensitive to elevated temperatures and generally suffer the most mortality during bleaching events. This is bad because these species are the dominant shallow-water reef-building corals throughout the Indo-Pacific because they grow and calcify the fastest. There can be interspecific variability in the response of these corals, as there are approximately 169 species of Acropora in the Indo-Pacific, but overall, they are the most detrimentally affected and die at the lowest levels of heat stress relative to most coral species.

How has the El Niño event influenced coral bleaching events in the Indian Ocean and the broader Indo-Pacific region?

Strong El Niño events have historically been the harbinger of severe bleaching events, but recently, large-scale and severe events have occurred during La Nina periods (Great Barrier Reef in 2022; Fiji and Vanuatu in early 2023). This suggests that ocean temperatures may have increased to the point where large-scale bleaching events may now occur during any phase of the ENSO. This global-scale event is due to the combination of ocean warming from climate change plus the added heat associated with El Niño.

A nugget of hope is that El Niño ended, as it was announced by NOAA on June 13, lasting for 11 months. We are predicted to move to ENSO neutral now, and then La Nina sometime between June and September. La Nina will drive cooling in the eastern tropical Pacific initially, and hopefully across the wider Indo-Pacific. Once La Nina conditions develop, the percentage of reef areas experiencing bleaching-level heat stress should begin to decline. The emergence of La Nina will more than likely bring respite to many different areas and lead to declines in the percentage of reef areas being impacted.

Can you draw comparisons between the current bleaching events and previous events, such as the Great Barrier Reef bleaching?

As of June 10, more than two-thirds of the world's coral reef areas (67%) have experienced bleaching-level heat stress just in the past year. The previous record occurred during the 2014-17 global bleaching event, when 56% of the world's reef areas experienced bleaching-level heat stress within a 365-day period. The percentages of reef areas that experienced bleaching-level heat stress within a 365-day period during the past global bleaching events (GBE) are as follows:

GBE1, 1998: 20%;

GBE2, 2010: 35%;

GBE3, 2014-17: 56%; and GBE4, 2023-present: 67% and increasing.

The recent bleaching of the Great Barrier Reef is part of the ongoing Global Coral Bleaching Event, marking the fifth mass coral bleaching event on the Great Barrier Reef in the past eight years.

What immediate conservation measures can be implemented to protect the most vulnerable coral reefs in the Indian Ocean?

Resource managers make use of NOAA Coral Reef Watch’s bleaching predictions to understand the threat and risk of ecological impacts associated with bleaching. There have been two recent actions taken to mitigate the impacts of the ongoing, global coral bleaching event. Pling Island in Phuket, Thailand, was recently closed to tourists in an effort to limit secondary stressors on the corals during this bleaching event. Similarly, the Maldives shut down coastal development projects to minimise stress on the bleaching corals. (Although we just found out that these projects have now resumed in the Maldives).

Other efforts that can be done include things like shading of corals and moving corals to deeper, cooler water. The bleaching response is due to the interaction of high temperature with high light, so active shading or placing corals at depths where light levels are less can help ameliorate bleaching. In the worst-case scenario, corals can be pulled from the ocean and brought to land-based aquaria facilities. This was done in Florida in 2023 and much of the genetic diversity within elkhorn (large, branching coral with thick branches resembling elk antlers, crucial for reef-building in the Caribbean) and staghorn (fast-growing, branching coral with thin branches resembling stag antlers) corals were saved as a result.

Can you provide recommendations for long-term planning and policy initiatives to enhance coral reef resilience in the face of climate change and recurring bleaching events?

Where resources permit, aggressive intervention and restoration techniques are crucial to ensuring that coral reefs are able to continue to provide the ecosystem services that directly benefit so many people. It is imperative that governments do all they can to reduce other stressors impacting coral reefs. This includes improving water quality, limiting sedimentation and land-based sources of pollution, and ensuring sustainable fishing that is not destructive.

All that being said, all the actions that can be taken will only serve as a band-aid unless the primary driver of large-scale coral reef decline is addressed: climate change and ocean warming. Until the international community can come together and figure out how to break humanity’s dependence on fossil fuels, coral bleaching events will continue to get worse and more frequent.