The world’s oceans might be acidifying 10 times faster than at any time during the last 300m years according to new research. And if geological history is anything to go by, this is bad news for marine species.

Oceans can soak up excess carbon dioxide from the atmosphere. One side effect is that the sea water, which is naturally slightly alkaline, becomes less alkaline and more acidic – a process called ocean acidification.

Scientists are concerned about even quite small shifts in ocean acidity, as it can affect how marine creatures grow their shells, which can be crucial to their survival.

For this new study, published in the journal Science, researchers looked for evidence of ocean acidification in the past, going back through hundreds of existing studies of oceans throughout geological history.

They found that over the last 300m years ocean acidification has never happened faster than it is happening now.

The only period that comes close to present acidification rates is the Palaeo-Eocene Thermal Maximum (PETM), a turbulent period of climate history around 56 million years ago when large amounts of carbon were naturally released into the atmosphere over a few tens of thousands of years. At the time, the changes in the climate and ocean were accompanied by the extinction of many marine species.

Over the past century carbon dioxide levels in the atmosphere have risen by almost a third. Oceans currently take in about a quarter of the carbon dioxide currently released from human activity.

This has led seawater pH (a measure of acidity – lower pH means more acidic) to decrease by 0.1. This is around 10 times faster than acidification during the PETM.

Scientists also suggest that we are releasing carbon much faster than carbon was released during the PETM. Study co-author Professor Andy Ridgwell, University of Bristol, says:

“The geological record suggests that the current acidification is potentially unparalleled in at least the last 300 million years of Earth history, and raises the possibility that we are entering an unknown territory of marine ecosystem change.”

Since climate model projections suggest ocean acidification could become more severe by the end of the century, it is unlikely that any past change in acidification can match what we might see in the future. Ridgewell says:

“Although similarities exist, nothing in the last 300 million years parallels rates of future projections in terms of the disrupting of ocean carbonate chemistry – a consequence of the unprecedented rapidity of CO2 release currently taking place.”

Such an unprecedented rate of change in seawater chemistry could affect some important marine species. Professor Bärbel Hönisch, paleoceanographer at Columbia University’s Lamont-Doherty Earth Observatory and lead author of the study explains:

“What we’re doing today really stands out. We know that life during past ocean acidification events was not wiped out-new species evolved to replace those that died off. But if industrial carbon emissions continue at the current pace, we may lose organisms we care about-coral reefs, oysters, salmon.”

The conclusions reached in this research echo the findings of a recent study combining observations with computer modelling that also labelled man-made ocean acidity changes as ‘unprecedented’.

The video below shows how ocean acidity has changed since the industrial revolution, and how it is projected to change in the future:

The animation shows how aragonite saturation (a measure of ocean acidity) at the ocean’s surface is projected to decrease towards the end of the 21st century as man-made carbon dioxide accumulation in the atmosphere continues to rise. Source:  Tobias Friedrich, SOEST Hawaii.