The study shows that alkalinization – which could be achieved by distributing lime or other alkaline substances in the ocean – could prevent coral dissolving, although it may have harmful side effects.

"We show that the artificial alkalinization of seawater could be one of several strategies that may be needed in conjunction to protect coral reefs from climate change," said Andreas Oschlies of the GEOMAR Helmholtz Centre for Ocean Research, Germany.

Much of the carbon dioxide emitted by humans finds its way into the oceans, where it increases acidity. As a result, corals are less able to absorb calcium carbonate in order to maintain their skeletons, and they begin to dissolve away.

The artificial alkalinization of seawater (AOS) has been considered before as a means both of keeping acidity in check and of allowing more carbon dioxide to be absorbed, so as to reduce the stress on the climate. One possibility is releasing alkaline substances, such as lime, at coasts or throughout the ocean, but the production of lime from limestone is itself very carbon dioxide-intensive. Other methods may involve less carbon dioxide emission, but would still involve the transport of huge masses of material. "Technologically it could be done, but would probably transform large parts of our planet," said Oschlies.

Oschlies and colleagues have been considering an option that is less dramatic than this sort of geoengineering – AOS targeted only at coral reefs. Local AOS may be easier to achieve, the researchers believe, as the transport requirements would be lower and agreements may only have to be made with national governments.

To see how well local AOS would work, the GEOMAR group ran simulations on an Earth system model that contained a simple representation of ocean chemistry and biology. At first they tried adding different constant amounts of alkalinity for a "business as usual" carbon-dioxide emissions scenario. Upon seeing those results, however, they performed simulations in which they added just enough alkalinity to keep the chemistry of the ocean at favourable levels for coral-reef ecosystems.

These simulations suggested that locally targeted AOS would indeed be possible, but there were causes for concern. Temporary spikes in ocean alkalinity – just like widespread acidification – could be detrimental to ocean life: the marine crab is one animal known to be affected by very alkaline conditions. Too little carbon dioxide in the water could also leave some phytoplankton and seagrass struggling to perform photosynthesis.

Finally, AOS does not tackle what is thought to be an even greater problem that coral reefs face from climate change: bleaching due to temperature rises. "AOS has the potential to protect coral reefs against ocean acidification, but the monetary cost is high and it wouldn’t prevent their loss from bleaching," said Oschlies.

The researchers are now investigating what exactly the monetary costs would need to be for AOS to mitigate global atmospheric carbon dioxide increases.

The study is published in Environmental Research Letters (ERL).

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