Coral reefs have long been celebrated as biodiversity hotspots – but new research shows they have also played a much deeper role: conducting the rhythm of Earth's carbon and climate cycles for more than 250 million years.
Published this week in the Proceedings of the National Academy of Sciences ( PNAS ), the study reveals that the rise and fall of shallow-water reef habitats have governed how quickly the planet recovered from major carbon dioxide (CO₂) shocks.
Researchers from the University of Sydney and Université Grenoble Alpes combined plate-tectonic reconstructions, global surface processes and climate simulations, with ecological modelling to reconstruct shallow-water carbonate production back to the Triassic Period. They found that the Earth system flips between two distinct modes that determine the pace of climate recovery.
Lead author Associate Professor Tristan Salles from the University of Sydney's School of Geosciences said: "Reefs didn't just respond to climate change – they helped set the tempo of recovery."
Two modes of Earth's carbon cycle
In one mode, when tropical shelves are extensive and reefs thrive, carbonate accumulates in shallow seas, reducing chemical exchange with the deep ocean. This weakens the biological pump – the process by which marine organisms draw down carbon – and slows the planet's recovery from carbon shocks.
In the other mode, when reef space collapses due to tectonic or sea-level change, calcium and alkalinity build up in the ocean. Carbonate burial then shifts to the deep sea, stimulating nannoplankton productivity and accelerating climate recovery.
Reefs as climate regulators
The findings recast reefs and other shallow-water carbonate systems as active modulators of Earth's buffering capacity rather than passive recorders of environmental change. This shifting balance between shallow- and deep-water carbonate burial also influenced the evolution of marine plankton and long-term ocean chemistry.
"These switches profoundly alter the biogeochemical equilibrium," said co-lead author Dr Laurent Husson (CNRS - UGA).
"The big expansion of planktonic life happened exactly when shallow reefs were 'turned down' by the Earth system," he said. Such changes modified the ocean's biological pump and in turn, the climate and the speed at which it recovers from global perturbations.
This study suggests reefs have been central not only to marine biodiversity but also to the planet's ability to stabilise climate.
What this means today
Although this study focuses on Earth's deep past, it offers clear lessons for the future. Modern reef systems are declining rapidly due to warming and ocean acidification. If this trajectory mirrors ancient episodes of reef collapse, carbonate burial may shift from shallow reefs to the deep ocean – a habitability-limited mode. In principle, this could help draw down atmospheric carbon.
However, the very organisms that drive deep-sea carbonate burial – plankton and other calcifying species – are themselves increasingly threatened by acidifying oceans and continued CO₂ emissions. Any potential stabilising effect would therefore come only after severe and irreversible ecological loss.
Associate Professor Salles said: "From our perspective on the past 250 million years, we know the Earth system will eventually recover from the massive carbon disruption we are now entering. But this recovery will not occur on human timescales. Our study shows that geological recovery requires thousands to hundreds of thousands of years."
DOWNLOAD the study, an animation and photos at this link .
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