It is well known that warming waters cause corals to expel symbiotic algae that live inside them and produce most of the organisms’ nutrients via photosynthesis. The process turns the corals a ghostly white, a condition called coral bleaching. Corals can survive for a while without the algae — they use tentacles to eat food from the ocean — but when bleaching lasts too long or happens too frequently, the corals eventually die.

UCLA partnered with researchers from global institutions for the study, including Northwestern University, Bremen University, the Indian Institute for Science and University of Cambridge.

The research demonstrates how global warming exacerbates the damage that other factors can cause within complex biological systems. In this case, Eagle said, the temperature stress interfered with coral’s natural ability to regulate its defense against acidification.

And it reinforces the notion that prospects for major reef systems like the Great Barrier — the world’s largest — are grim. Marine heat waves are expected to worsen in the coming decades and are likely to cause more severe bleaching events and kill more corals. Coral species might not necessarily go extinct, Eagle said, but they could possibly shift to different locations and may no longer exist in large, continuous reef systems.

Corals could also develop some ability to adapt to future ocean conditions. But according to Maxence Guillermic, the paper’s lead author, that adaptation could happen too late to save the world’s major reef systems.

“Conservation of corals in a world where heatwaves are becoming more frequent will be challenging,” said Guillermic, a UCLA geochemist. “The story told in our paper is one small piece. It’s important to understand the causes of the interactions between stressors, including climate change-related stressors. Only then can we find ways to conserve corals in a meaningful way.”