During this period, between 2016 and 2024, the Great Barrier Reef, the world’s largest coral reef ecosystem and one of the most biodiverse, has been experiencing massive coral bleaching events. This is when water temperatures become too warm and corals expel algae, which provide them with color and food, and sometimes die.
Earlier this year, aerial surveys of more than 300 reefs off Australia’s northeastern coast revealed bleaching in shallow water areas covering two-thirds of the reef, according to NASA.
They observed largely stable temperatures before 1900 and consistent warming from January to March from 1960 to 2024. And during five coral bleaching years in the past decade — in 2016, 2017, 2020, 2022 and 2024 — temperatures in January and March were significantly higher than anything since 1618, the researchers said.
They used climate models to attribute the rate of warming after 1900 to human-caused climate change. The only other year nearly as warm as the mass bleaching years of the last decade was 2004.
Essential reefs
“The reef is in danger. If we do not change course, our generation will likely witness the demise of one of these great natural wonders,” said Benjamin Henley, lead author of the study and professor of sustainable urban management at the University of Melbourne.
“If you put all the evidence together, coral biology and reef ecology tell us that extreme temperatures occur too often for these corals to adapt and evolve effectively.”
Around the world, reefs are essential to seafood production and tourism. Scientists have long said that further coral loss is likely to be one of the impacts of future warming as the world approaches the 1.5-degree Celsius threshold below which countries agreed to try to keep warming in the 2015 Paris Climate Agreement.
Even if global warming remains within the Paris Agreement target, which scientists say the Earth is almost certain to exceed, 70 to 90 percent of the world’s corals could be at risk, the study’s authors say.
Therefore, future coral reefs will likely have less coral species diversity – something that has already happened as oceans have become warmer.
Coral reefs have evolved over the past quarter century in response to bleaching events like those highlighted by the study authors, said Michael McPhaden, a senior climatologist at the National Oceanic and Atmospheric Administration who was not involved in the study.
“The Canary in the Mine”
But even the hardiest corals may soon no longer be able to withstand the high temperatures expected in a warming climate with “the relentless increase in greenhouse gas concentrations in the atmosphere,” he said.
The Great Barrier Reef provides an economic resource for the region and protection from violent tropical storms.
As more heat-tolerant corals replace less heat-tolerant species in the colourful underwater rainbow jungle, McPhaden said there was “real concern” about the expected extreme loss in species numbers and reduction in the area covered by the world’s largest reef.
“This is the canary in the coal mine when it comes to climate change,” McPhaden warned.
