by By Gloria Dickie and Alison Withers
(Reuters) – Along the coasts of Australia to Kenya to Mexico, many of the world’s colorful coral reefs have turned a ghostly white in what scientists said on Monday was the fourth global bleaching event in three decades.
At least 54 countries and territories are predicted to experience mass loss among their reefs as of February 2023 as climate change affects ocean surface waters, according to the US National Oceanic Atmospheric Administration’s (NOAA) Coral Reef Watch ), the world’s leading coral reef monitoring company.
Bleaching is triggered by water temperature anomalies that cause corals to expel the colorful algae that live in their tissues. Without the help of the algae to deliver nutrients to the coral, the corals cannot survive.
“More than 54% of the reef areas in the global ocean are experiencing heat stress bleaching level,” said Coral Reef Watch coordinator Derek Manzello.
NOAA and the International Coral Reef Initiative (ICRI), a global intergovernmental conservation partnership, announced the latest global bleaching event. To be considered a global event, significant bleaching must occur in all three ocean basins – the Atlantic, the Pacific and the Indian Ocean – within a 365-day period.
Like this year’s bleaching event, the last three – in 1998, 2010 and 2014-2017 – also coincided with the El Nino climate pattern, which usually brings warmer sea temperatures. Sea surface temperatures in the past year have broken records held since 1979, as climate change exacerbates the effects of El Nino.
Corals are invertebrates that live in colonies. Their calcium carbonate secretions form a hard and protective scaffold that serves as a home for many colorful species of single-celled algae.
Scientists have expressed concern that many of the world’s reefs will not recover from prolonged, intense heat stress.
“What is happening is new to us, and to science,” said marine ecologist Lorenzo Alvarez-Filip at the National Autonomous University of Mexico.
“We still can’t predict how much stressed corals will do,” even if they survive immediate heat stress, Alvarez-Filip added.
Recurring bleaching events are based on earlier scientific models that predicted that between 70% and 90% of the world’s coral reefs could be lost when global warming reached 1.5 degrees Celsius (2.7 F) above pre-industrial temperatures. So far, the world has warmed about 1.2 C (2.2 F).
In a 2022 report by the United Nations Intergovernmental Panel on Climate Change, experts concluded that 1.2 C of warming would be enough to have a major impact on coral reefs, “and the most available evidence suggests that it will not coral-dominated ecosystems exist at this temperature.”
This year’s global bleaching event further emphasizes scientists’ concerns that corals are in grave danger.
“A realistic interpretation is that we have crossed the tipping point for coral reefs,” said ecologist David Obura, who heads Coastal Ocean Research and Development in the East African Indian Ocean from Mombasa, Kenya.
“They are deteriorating that we cannot stop, unless we really stop carbon dioxide emissions” that are driving climate change, said Obura.
Coral reefs are estimated to provide about $2.7 trillion in goods and services each year – with benefits such as attracting tourists, protecting coastal communities from storm surges, and supporting coastal fisheries, according to a 2020 valuation by the ICRI scientific network.
A GLOBAL PREDICTION MAY STILL BE NECESSARY
With bleaching surveys underway in the Indian and Pacific Oceans, NOAA experts expect this global bleaching event could be the most extensive yet.
Reefs in the Caribbean were widely bleached last August when coastal sea surface temperatures hovered between 1 C (1.8 F) and 3 C (5.4 F) above normal. Scientists working in the region then began documenting mass disconnections throughout the region.
From the staghorns to brain corals, “everything you see while diving was white in some reefs,” said Alvarez-Filip. “I have not seen this level of bleaching.”
Bleached corals can recover if waters cool, but some corals in the Caribbean were stressed as they continued to die even when temperatures dropped during the winter, Alvarez-Filip added.
Florida corals subjected to extreme heat shocks didn’t even have time to predict, Manzello said.
“They put so much stress, they just died and gave up their tissues,” Manzello said.
At the end of the Southern Hemisphere summer in March, tropical reefs in the Pacific and Indian oceans also began to suffer.
A record number of individual reefs within the Great Barrier Reef have suffered from heat stress in recent months, and many are now shedding colour, said coral biologist Neal Cantin at the Australian Institute of Marine Sciences. Cantin noted that marine heat waves were registering about 2.5 C (4.5 F) above the normal summer maximum.
Recent aerial surveys have shown “very high” or “extreme” levels of bleaching in almost half of the reefs surveyed in the Great Barrier Reef Marine Park area.
That is the fifth bleaching event in the Great Barrier Reef in just nine years – far more frequent than the two times per decade that scientists expected by the 2030s.
Indian Ocean reefs off Madagascar, Tanzania, Kenya and the Seychelles also suffered bleaching, although not as much as in 2016 due to an early change in this year’s monsoon leading to cooler conditions , said Obura.
“The stress on corals in the region is probably less than it could be, which is very fortunate,” said Obura.
(Reporting by Gloria Dickie in London and Ali Withers in Copenhagen; Editing by Katy Daigle and Will Dunham)
