Ocean acidification of the depths is hard to mitigate

As carbon dioxide levels in the atmosphere increase, so too does absorption of the gas into the oceans, making them more acidic. But what if we manage to decrease atmospheric carbon dioxide concentrations to pre-industrial levels? Would ocean acidification be mitigated or reversed?

In response to this question, Long Cao and colleagues from Zhejiang University found that, even if atmospheric carbon dioxide can be lowered in the future, acidification of the deep ocean will persist for many years.

In the first study of its kind, the group looked at the change in ocean acidity in both the surface and sub-surface ocean in response to a scenario in which atmospheric carbon dioxide initially increases to four times its pre-industrial level at a rate of 1% per year, and then returns to pre-industrial levels at the same rate.

“Earlier studies only looked at the response of ocean acidification to increasing atmospheric carbon dioxide,” Cao told environmentalresearchweb. “We extended previous work by looking at the response to decreasing atmospheric carbon dioxide concentrations. We also looked at the change in chemical conditions of sea water that surrounds deep-sea corals in response to a potential draw-down of atmospheric carbon dioxide.”

The team found that changes in surface ocean acidity largely follow the change in atmospheric carbon dioxide and that, in most parts of the surface ocean, mitigation of atmospheric carbon dioxide will result in mitigation of ocean acidification. There were some exceptions, in particular the Arctic Ocean, where the mitigation of surface acidification will substantially lag behind the mitigation of atmospheric carbon dioxide.

For the deep ocean, changes in acidity would lag a long way behind any changes in atmospheric carbon concentrations, the researchers found. “This happens because it takes a long time for anthropogenic carbon dioxide to penetrate to the deep ocean,” explained Cao. “This means that even if we can lower atmospheric carbon dioxide concentration in the future, some organisms dwelling in the deep ocean, such as those depending on deep-sea coral reefs, would continue to suffer from increased ocean acidity.”

To quantify the effect, Lao and colleagues modelled saturations in the water of aragonite – a form of calcium carbonate vital for shell- and skeleton-building by organisms such as corals and molluscs. The researchers found that at pre-industrial carbon dioxide levels, 9% of deep-sea cold-water corals are surrounded by seawater that is under-saturated with aragonite. When atmospheric carbon dioxide reaches 1120 ppm, 73% of cold-water corals are surrounded by seawater with aragonite under-saturation and when atmospheric carbon dioxide returns to the pre-industrial level, 18% of cold-water corals are surrounded by seawater with aragonite under-saturation.

“These findings are important in that they help us to better understand the environmental effect of mitigation of carbon dioxide concentration and emission,” said Lao, who reported the study in Environmental Research Letters (ERL).

Environmental Research Web, 10 March 2014. Article.

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