…nothing such as a free lunch…

The atmosphere - ocean interface, where gas exchange takes place to establish and maintain equilibrium. (c) C Braungardt 2022

For many decades, we’ve released too much carbon dioxide and other greenhouse gases into the atmosphere and for some decades, scientists have developed and tested geoengineering solutions for the consequences of increased concentrations of greenhouse gases in our atmosphere.

Today, the online version of Science reported yet another approach…that once more shows that there is nothing such as a free lunch.

Ocean acidification, or more accurately, the lowering of seawater alkalinity, is the direct consequence of increased atmospheric carbon dioxide concentrations, as the atmosphere and surface ocean are constantly ‘engaged’ in gas exchange in nature’s drive towards equilibrium.

With more carbon dioxide in the atmosphere, more dissolves in the surface ocean, and in that process, more carbonic acid is produced and hydrogen ions are released into the water. This in turn lowers the alkalinity and increases the acidity of the water.

Carbon dioxide equilibration at the interface between atmosphere and the surface ocean. (c) C Braungardt
Carbon dioxide equilibration at the interface between atmosphere and the surface ocean. As carbon dioxide concentrations in the atmosphere increase, more gets pushed into the surface ocean, where it dissolves, forms carbonic acid that dissociates into bicarbonate and carbonate ions, thereby releasing hydrogen ions that cause ocean acidification. Photosynthesis utilises dissolved carbon dioxide, while respiration releases it. Calcification, biologically or abiotic, removes carbonate from the water and if carbonate solids are incorporated into the sediment, they are removed (relatively) permanently. The dissolution of calcium carbonate releases carbonate it back into the water, and at lower pH, that happens more readily.

Ocean acidification is important: the process draws down CO2 from the atmosphere, reducing the climate change potential of the gas, but it also makes the process of calcification more difficult and hence spells trouble for organisms, such as phytoplankton at the base of the oceanic food web and commercially important animals, such as oysters.

This is where ocean geoengineering could help. From the Iron Enrichment Experiments (IRONEX) of the 1990s and early 2000s (SOFex) that added iron to the surface ocean to stimulate phytoplankton growth, to the recently reported release of alkaline lime powder near imperilled oyster farms in Florida (Science) with the aim to counteract ocean acidification, ocean geoengineering trials show some success.

Given that iron has to be mined, processed and shipped and that the production of lime releases carbon dioxide, I am not the only one to wonder about the net benefit a full life cycle account of such geoengineering approaches would reveal.

Don’t get me wrong – I’d welcome the silver bullet without any unintended consequences just as the next person – but meanwhile, I’ll try to reduce my own footprint and continue to support habitat conservation and the planting of more trees.

Featured Image: Atmosphere – ocean interface where gas exchange takes place to maintain equilibrium. (Southern Ocean, (c) C Braungardt 2022)

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