There may be a surprising upside to losing coral reefs as oceans warm

There might be an upside to the loss of coral reefs. Their decline would mean oceans can absorb up to 5 per cent more carbon dioxide by 2100, researchers estimate, slowing the build up of this greenhouse gas in Earth’s atmosphere.

“It is a beneficial effect if you only care about the concentration of CO2 in the atmosphere,” says Lester Kwiatkowski at Sorbonne University in Paris, France. But the decline of corals will also reduce biodiversity, harm fisheries and leave many coasts more exposed to rising seas, he says.

How much the world will warm depends mainly on the level of CO2 in the atmosphere. So far the land and oceans have been soaking up around half of the extra CO2 we have emitted. Any factors that increase or decrease these so-called land or ocean carbon sinks could therefore have a significant impact on future warming.

It is often assumed that corals remove CO2 from seawater as they grow their calcium carbonate skeletons. In fact, the process, also known as calcification, is a net source of CO2.

“You’re taking inorganic carbon in the ocean, generally in the form of carbonate and bicarbonate ions, turning it into calcium carbonate and that process releases CO2 into the seawater, some of which will be lost to the atmosphere,” says Kwiatkowski.

This means that if reef formation around the world slows or even reverses, less CO2 will be released by reefs and the oceans will be able to absorb more of this greenhouse gas from the atmosphere – a factor not currently included in climate models.

Observations suggest coral reef calcification is already declining as rising seawater temperatures cause mass coral bleaching and die-offs. The higher level of CO2 is also making oceans more acidic, which can make it harder to build carbonate skeletons and even lead to their dissolution.

Kwiatkowski and his team took published estimates of how corals will be affected by warming and ocean acidification and used a computer model to work out how this might change the ocean sink in various emission scenarios. They conclude that the oceans could take up between 1 and 5 per cent more carbon by 2100, and up to 13 per cent more by 2300.

This doesn’t take account of other factors that can cause reef decline such as overfishing and the spread of coral diseases, says Kwiatkowski, so might even be an underestimate.

On the other hand, the work assumes that corals aren’t able to adapt or acclimatise, says Chris Jury at the University of Hawai’i at Manoa, who wasn’t involved in the study.

“If the worst-case or even medium-case scenario in this study comes to pass, it means the near-total destruction of coral reefs globally,” says Jury. “I think that with consideration of realistic levels of adaptation and acclimatisation by corals and other reef organisms, the authors might come to different conclusions under a low to moderate level of climate change.”

If Kwiatkowski’s team is correct, it means that the amount of emitted CO2 that will lead to a given level of warming – the so-called carbon budget – is a little larger than currently thought.

“I think we would like our budgets to be as accurate as possible, even if we’re blowing through them,” says Kwiatkowski.