Ocean acidification can cause mass extinctions, fossils reveal

https://www.theguardian.com/environment/2019/oct/21/ocean-acidification-can-cause-mass-extinctions-fossils-reveal

Damian Carrington Environment editor
Mon 21 Oct 2019 15.00 EDT

Ocean acidification can cause the mass extinction of marine life, fossil evidence from 66m years ago has revealed.

A key impact of today’s climate crisis is that seas are again getting more acidic, as they absorb carbon emissions from the burning of coal, oil and gas. Scientists said the latest research is a warning that humanity is risking potential “ecological collapse” in the oceans, which produce half the oxygen we breathe.

The researchers analysed small seashells in sediment laid down shortly after a giant meteorite hit the Earth, wiping out the dinosaurs and three-quarters of marine species. Chemical analysis of the shells showed a sharp drop in the pH of the ocean in the century to the millennium after the strike.

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The researchers found that the pH dropped by 0.25 pH units in the 100-1,000 years after the strike. It is possible that there was an even bigger drop in pH in the decade or two after the strike and the scientists are examining other sediments in even finer detail.

Henehan said: “If 0.25 was enough to precipitate a mass extinction, we should be worried.” Researchers estimate that the pH of the ocean will drop by 0.4 pH units by the end of this century if carbon emissions are not stopped, or by 0.15 units if global temperature rise is limited to 2C.
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Henehan said: “We may think of [acidification] as something to worry about for our grandchildren. But if it truly does get to the same acidification as at the [meteorite strike] boundary, then you are talking about effects that will last for the lifetime of our species. It was hundreds of thousands of years before carbon cycling returned to normal.”

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Phil Williamson, at the University of East Anglia, who was not involved in the research, said: “It is relatively easy to identify mass extinction events in the fossil record, but much harder to know exactly what caused them. Evidence for the role of ocean acidification has generally been weak, until now.”

He said caution was needed in making the comparison between the acidification spike 66m years ago and today: “When the asteroid struck, atmospheric CO2 was naturally already much higher than today, and the pH much lower. Furthermore, large asteroid impacts cause prolonged darkness.”

Williamson added: “Nevertheless, this study provides further warning that the global changes in ocean chemistry that we are currently driving have the potential to cause highly undesirable and effectively irreversible damage to ocean biology.”

Henehan said the generally lower ocean pH 66m years ago might have made shelled organisms more resilient to acidification. “Who knows if our current [marine] system is as well set up to cope with sudden acidification?”