‘Unprecedented mass coral bleaching’ expected in 2024, says expert

Record-breaking land and sea temperatures, driven by climate breakdown, will probably cause “unprecedented mass coral bleaching and mortality” throughout 2024, according to a pioneering coral scientist.

The impact of climate change on coral reefs has reached “uncharted territory”, said Prof Ove Hoegh-Guldberg, from the University of Queensland, Australia, leading to concerns that we could be at a “tipping point”.

The upper ocean is undergoing unmatched changes in conditions, ecosystems and communities that can be traced back to the 1980s, when mass coral bleaching first appeared. In a paper published in the journal Science, US and Australian researchers say that historical data on sea surface temperatures, over four decades, suggests that this year’s extreme marine heatwaves may be a precursor to a mass bleaching and coral mortality event across the Indo-Pacific in 2024-25.

Mass coral bleaching happens when delicate corals become stressed due to factors including heat, causing them to lose their brown microbial algae, turning them white. At low stress levels, the algae can return to corals over a few months. But many Caribbean reef areas have recently experienced historically high sea temperatures that began one or two months earlier and lasted longer than usual.

Crucially, 2023 is the first year of a potential pair of El Niño years, with the warmest average global surface sea temperature from February to July on record. Since 1997, every instance of these El Niño pairs has led to a global mass coral bleaching event.

Hoegh-Guldberg, whose work has helped to shape the world’s understanding of the risks to the ocean’s richest ecosystems, said: “The probability is that somewhere in the next 12 to 24 months, we are going see El Niño combine with warming sea temperatures and have a really big impact.

“We are literally in uncharted territory, which we know very little about and don’t know how to respond to and I think we’re dangerously exposed.”

“We don’t know the implications of such a spike in temperature,” said the scientist, speaking from Dubai, where he is attending the Cop28 climate summit. “We may see storms that are even larger than the ones we’ve been seeing. These are the warmest temperatures ever on land and sea.”

Hoegh-Guldberg added: “If it’s going to be ‘summers from hell’ type thing, many of us are fearing that this may be a tipping point that we’ve passed, meaning that we can’t come back. We don’t know the implications of such a spike in temperature.”

Mass bleaching and mortality of coral events in the Indo-Pacific, which will lead to long-term damage to ecosystems and the millions of people in the Earth’s tropical regions who depend on them, could worsen unless greenhouse gas emissions decrease, he said.

The tiny brown algae that live in coral are very sensitive to temperature changes. “Just like us, as humans, there’s a set temperature at which we feel fine. But then one or two temperatures above that, and you’re dead. This is at a planetary scale. It’s a shocker.”

The earth experienced its warmest day since 1910 in July 2023, as well as its warmest month for upper sea temperatures.

Hoegh-Guldberg said: “What we do know, from 40 years of history, is that there’s a very strong, tight connection between this amount of energy in the system and the violence of storms and the impacts on biological systems. That is very well established.”

As coral reefs bleach and die, the habitats on which many associated reef species depend disappears, leading to ecosystem collapse. This could undermine as much as 25% of ocean biodiversity, the researchers say.

He urged policymakers and world leaders to “move faster and with more determination than ever before” to reduce greenhouse gas emissions and to “trust the science” to guide us.

“This is a science-based engineering problem,” said Hoegh-Guldberg. “We need to set the parameters. We need to define the way our planet works and do it in record time. Because we’ve got resources, we can do it. But we’ve got to be smart and involve everyone. And make sure that we get a system that’s going to cool the planet for a while, or at least not increase for a while.”