Over 80% of coral reefs hit by world’s longest, most extensive bleaching: Preservation group

SINGAPORE – The world’s coral reefs – vital to fisheries and tourism – are facing the most prolonged and widespread bleaching event in recorded history, with over 80 per cent affected by marine heatwaves, a global reef preservation initiative said on April 23.

In April 2024, the US’ National Oceanic and Atmospheric Administration (NOAA) and the International Coral Reef Initiative (ICRI) declared the world’s fourth global coral bleaching event – the second in a decade.

One year on, the bleaching event continues to spread, driven by abnormally warm sea surface temperatures.

Bleaching refers to the whitening of corals when they expel the photosynthetic algae that give them their colour and provide them with essential nutrients.

All it takes is for water temperatures to rise 1 deg C to 2 deg C above average for corals to bleach.

From Jan 1, 2023, to March 30, 2025, bleaching-level heat stress impacted 84 per cent of the world’s reefs, with 82 countries, territories and economies suffering damage, ICRI said in a statement.

The initiative comprises 102 members, including dozens of countries, United Nations agencies, conservation groups and companies, and is focused on research and better reef management.

“2023-2024 saw the highest ocean temperatures ever recorded – a surge that shattered previous records,” said Associate Professor Alex Sen Gupta from the Centre for Marine Science and Innovation at the University of New South Wales in Sydney.

“Given the well-established link between ocean heat and coral bleaching, it’s no surprise we’re now witnessing the most widespread coral bleaching event on record,” he said in a statement.

Corals can recover if water temperatures return to normal after a few weeks. But prolonged exposure to elevated temperatures over many weeks makes recovery less likely, and this can lead to widespread death of some coral species.

Singapore’s corals, which suffered a mass bleaching event between May and October in 2024, have mostly recovered, coral scientist Jani Tanzil told The Straits Times recently.

The facility director at St John’s Island National Marine Laboratory said an estimated 5 per cent of corals had died from that event.

This mortality rate is less severe compared with earlier bleaching events. In the aftermath of the 2016 bleaching event, for example, coral mortality rate was 10 per cent in Singapore, according to a 2016 scientific report.

Scientists use satellites to measure changes in sea surface temperatures. This data is for calculating degree heating weeks (DHW) – a measure of accumulated heat stress in an area of reef over the past 12 weeks. It tracks how long and how far temperatures exceed the bleaching threshold, usually 1 deg C above the monthly average. The lower the DHW, the less likely bleaching will result in coral death.

NOAA uses this measure to send out regional and global alerts of the risk of bleaching. Bleaching can occur at 4 DHW, and the longer the period of exposure, the greater the risk of coral death.

ICRI said 84 per cent of coral areas globally have been exposed to 4 DHW or more.

This is the highest percentage of coral areas ever exposed to this level of heat stress, it added.

Global warming has exacted a terrible toll on the world’s coral reefs. The largest reef system, Australia’s Great Barrier Reef, has lost more than half of its coral cover since 1995, mainly because of increasingly frequent and severe bleaching events.

Globally, the area of living coral reefs has declined by half since the 1950s, research has shown, owing to global warming, pollution and damage from mining and destructive fishing practices.

According to ICRI, 21 per cent of reefs globally experienced bleaching-level heat stress during the first coral bleaching event in 1998.

This rose to 37 per cent in the second bleaching event in 2010 and 68 per cent during the third event from 2014 to 2017.

These repeated coral bleaching events mean that reefs have had little time to recover, leading to reduced coral coverage and the loss of the biodiversity for which reefs are famous.

Scientists fear that reefs will continue to deteriorate as the world keeps warming.

Oceans have absorbed about 90 per cent of the heat from man-made climate change, which has been driven mainly by the burning of fossil fuels. This steady warming of the oceans has left corals with little time to adapt.

The decline of coral reefs poses a risk to the livelihoods of millions of people, who rely on them for fish and other seafood, tourism, as well as the coastal protection that the reefs provide by reducing wave damage to coastlines from storms.

Also concerning are the budget cuts to NOAA by the Trump administration, which could hamper the agency’s vital coral monitoring work, said Emeritus Professor Terry Hughes, a leading coral research scientist, from James Cook University in Australia.

“NOAA’s global coverage is essential for forecasting bleaching and mass mortality of corals. Their satellite data gives us the big picture on rising temperatures and heat exposure. The backdrop here is the savage cuts to NOAA’s budget and staff, which threaten to severely damage climate change research around the world,” he said.