Countries have agreed to curb global warming to 2°C, and try to limit temperature rises to 1.5°C to avoid the worst impacts of climate change, including more extreme floods, storms, heatwaves and damage to crops.
But scientists led by the University of Leeds warn that the climate risk is even worse for coral reefs than set out in a UN assessment in 2018, which warned that 70 to 90 per cent would be lost at 1.5°C of warming.
That assessment, by the Intergovernmental Panel on Climate Change, also found almost all corals would vanish at 2°C of warming.
The new research has found that more than 90 per cent of tropical coral reefs would suffer frequent “intolerable” heat stress as a result of ocean warming even under the tougher 1.5°C limit.
Places that can maintain suitable temperatures for corals to survive, while the ocean temperatures in surrounding areas rise, are known as “refugia”, and were identified by the researchers as areas projected to suffer severe heat stress less than once in 10 years.
This is about the time it takes for reefs to grow back and be fully functioning.
In recent decades, 84 per cent of tropical reefs have had enough time between heatwaves that cause coral death or bleaching to recover and re-establish, with these refugia found in all 12 coral reef regions across the world.
But at 1.5°C of warming, that will fall to just 0.2 per cent of coral areas, in Polynesia and the “coral triangle” in the western Pacific Ocean, where lower rates of warming and colder water reduce the frequency of ocean heatwaves.
No “refugia” areas will exist with 2°C of warming, the study based on historical data and climate models shows.
“We confirm that warming of 1.5°C relative to pre-industrial levels will be catastrophic for coral reefs,” they wrote in a paper published in the journal PLoS Climate.
Identifying and protecting refugia and reducing other stresses such as fishing and pollution has been a popular recommendation for managing reefs, the researchers said, but might only be a short-term solution.
Alongside creating protected areas, action to help reefs adapt to higher temperatures and help migration of species might be needed to secure coral reef survival.
“Our finding reinforces the stark reality that there is no safe limit of global warming for coral reefs,” said lead author Adele Dixon, a PhD researcher in the University of Leeds’ School of Biology.
Ms Dixon said that after the Cop26 climate summit in Glasgow where countries made some progress towards keeping temperatures to the 1.5°C limit, “our finding shows that 1.5°C is still a substantial amount of warming for the ecosystems on the frontline of climate change”.
“Our work shows that corals worldwide are even more at risk from climate change than we thought at the time of the IPCC special report on 1.5°C," said Piers Foster, professor of climate physics at the university.
“This reinforces the stark reality that there is no safe limit of global warming, and we need to act urgently to save what we can.”