By Anders Lorenzen

The US government agency, the National Oceanic and Atmospheric Administration’s (NOAA) Coral Reef Watch is the world’s top coral reef monitoring body. NOAA says that at least 54 countries and territories have experienced mass bleaching among their reefs since February 2023, as climate change warms the ocean’s surface waters.

Coastlines from Australia to Kenya to Mexico, and many other of the world’s colourful coral reefs, have turned a ghostly white. This has amounted to what scientists describe as the fourth global bleaching event in the last three decades.

Bleaching

The bleaching occurs, as water temperature anomalies cause corals to expel the colourful algae living in their tissues. Without the algae’s help in delivering nutrients to the corals, the corals cannot survive. According to Coral Reef Watch, more than 54% of the reef areas in the global oceans are experiencing bleaching-level heat stress.

Together with the International Coral Reef Initiative (ICRI), NOAA announced the tragic news of this recent bleaching. It has been judged as global, as it impacts all three ocean basins – the Atlantic, Pacific and Indian – within a 365-day period.

As with this year’s event, the last three global bleachings which occurred in 1998, 2010 and 2014-2017, also collided with the El Nino weather phenomenon which typically creates warmer sea temperatures.  In the past year, global sea surface temperatures have smashed records, kept since 1979, as the combination of El Nino and climate change take effect.

Concerns

Corals are invertebrates that live in colonies. Their calcium carbonate secretions form hard and protective scaffolding, that serves as a home to the single-celled algae.

Scientists are increasingly concerned that many of the world’s reefs may not recover from the intense, prolonged heat stress.

Recurring bleaching events are upending earlier scientific models that forecast that between 70% and 90% of the world’s coral reefs could be lost when global warming reaches 1.5 degrees Celsius above pre-industrial temperatures. Earlier this year, scientific analysis found that across the past 12 months, the world had warmed 1.5 C.

In a 2022 report by the UN Intergovernmental Panel on Climate Change, experts determined that just 1.2 C of warming would be enough to severely impact coral reefs, “with most available evidence suggesting that coral-dominated ecosystems will be non-existent at this temperature.”

This year’s global bleaching event adds further weight to concerns among scientists that corals are in grave danger.

Extensive bleaching

With bleaching surveys ongoing in the Indian Ocean and Pacific, NOAA experts expect that this global bleaching event could turn out to be the most extensive yet.

Caribbean reefs experienced widespread bleaching last August, as coastal sea surface temperatures hovered between 1 C  and 3 C  above normal.  Scientists working in the region then began documenting mass die-offs across the region.

Florida corals subjected to extreme heat shocks did not even have time to bleach.

At the end of the Southern Hemisphere summer in March, tropical reefs in the Pacific and Indian oceans also began to suffer.

A record-breaking number of individual reefs within the Great Barrier Reef have suffered from heat stress in recent months, and many are now draining of colour, said coral biologist Neal Cantin at the Australian Institute of Marine Sciences.  He noted that marine heatwaves were registering some 2.5 C above the normal summertime maximum.

Recent aerial surveys have shown “very high” or “extreme” levels of bleaching in nearly half of surveyed reefs in the Great Barrier Reef Marine Park area.

That makes this the fifth bleaching event in the Great Barrier Reef in just nine years – far more frequent than the twice-per-decade that scientists had expected by the 2030s.

Indian Ocean reefs off Madagascar, Tanzania, Kenya and the Seychelles, have also suffered bleaching, though not as severely as in 2016. this is thanks to an early change in this year’s monsoon leading to cooler conditions.

There are a variety of scientific and technological research projects underway to grow corals artificially, but we are some way off, in knowing how feasible that approach could help in restoring coral reefs. 

Coral reefs are estimated to provide some $2.7 trillion in goods and services every year – with benefits such as attracting tourists, protecting coastal communities from storm surges, and supporting coastal fisheries, according to a 2020 valuation by ICRI’s scientific network.