Coral reefs have fuelled severe global warming in Earth’s past

The role of coral reefs in Earth’s climate over the past 250 million years has been significant, albeit in a surprising manner.

Contrary to common belief, coral reefs are actually net producers of carbon dioxide due to the emission of the greenhouse gas as a by-product of the formation of calcium carbonate, the primary component of coral skeletons.

In addition to corals, certain types of plankton also utilize calcium carbonate in the construction of their shells. When these organisms perish, the mineral is deposited on the seabed. The presence of extensive coral reefs in shallow marine environments disrupts the balance between calcium and carbonate ions, affecting the availability of these ions for deep-sea plankton.

Tristan Salles and his team at the University of Sydney conducted a study integrating plate tectonics reconstructions, climate simulations, and sediment flow changes to model the interaction between shallow-water corals and deep-sea plankton throughout the last 250 million years.

The research revealed that periods characterized by vast shallow continental shelves, ideal for coral reef development, have disrupted the equilibrium between corals and plankton.

During times when coral reefs are less extensive, the ocean experiences an accumulation of calcium and alkalinity. This leads to increased productivity of plankton, resulting in more carbonate being buried in the deep sea. Consequently, this process aids in lowering CO2 levels and cooling the planet.

The study identified three major events in the past 250 million years where the carbon cycle was severely disrupted, causing significant rises in sea temperatures. These disruptions, occurring in the mid-Triassic, mid-Jurassic, and late Cretaceous periods, involved extensive coral reefs utilizing large quantities of calcium carbonate.

Salles explains that when the delicate balance between shallow-water coral reefs and deep-sea plankton is disturbed, it can take hundreds of thousands to millions of years for equilibrium to be restored.

Moreover, Salles highlights the positive aspect of corals in absorbing excess nutrients to build coral reefs when planktonic nutrient blooms become uncontrollable.

However, the accelerated human-induced CO2 emissions are currently causing unprecedented global warming and ocean acidification, leading to the demise of both corals and plankton. Salles warns that the ecological consequences of these changes are unpredictable but likely to be catastrophic.

Alexander Skeels from the Australian National University underscores the interconnected feedback loop between life and climate revealed in the study. He emphasizes the significant role biological species play in influencing Earth’s climate, creating a co-evolving relationship with the environment.

Skeels points out that corals, along with ancient microbial colonies like stromatolites, have been instrumental in modulating atmospheric carbon levels over deep time scales. This process may have contributed to the alternating warm and cold intervals observed in Earth’s history.