Ocean Acidity Could Reach Fourteen-Million-Year High

The world’s oceans are likely to become more acidic than at any time in the past 14 million years, according to scientists from Cardiff University in the U.K.

Ocean acidification occurs when CO2 from the atmosphere is absorbed by seawater, resulting in more acidic water with a lower pH. Around a third of the CO2 released by burning coal, oil and gas gets dissolved into the oceans. Since the beginning of the industrial era, the ocean has absorbed around 525 billion tons of CO2, equivalent to around 22 million tons per day.

The researchers reconstructed levels of ocean acidity and atmospheric CO2 over the past 22 million years by studying the chemistry of fossil shells of tiny marine creatures that once lived near the ocean surface. They were able to put their new records of pH and CO2 levels in context of the range of future carbon emission scenarios that are recognized by the Intergovernmental Panel on Climate Change (IPCC).

The current pH is already probably lower than any time in the last two million years, and under a ‘business-as-usual’ future scenario where CO2 is emitted at the same rate as today, atmospheric CO2 would be near 930 parts per million in the year 2100, compared to around 400 parts per million today.

Similarly, the pH of the oceans would be less than 7.8 in 2100 compared to a pH of around 8.1 today. This is significant as the pH scale is logarithmic, meaning a drop of 0.1 pH units represents a 25 percent increase in acidity.

These levels of atmospheric CO2 and ocean acidity have not been since the Middle Miocene Climatic Optimum period around 14 million years ago, when global temperatures were around 3°C warmer than today as a result of the Earth’s natural geological cycle.

Increasingly scientists are finding evidence of negative impacts. For example, fish are losing their sense of smell because of increasingly acidic oceans, new research from University of Exeter in the U.K. shows. Fish use their sense of smell to find food, safe habitats, avoid predators, recognize each other and find suitable spawning grounds. A reduction in their ability to smell therefore can compromise these essential functions for their survival. The study provides evidence that economically important species, such as sea bass, will be affected by elevated CO2, leaving fish vulnerable because it affects their ability to detect odours.

A study led by Australia's Institute for Marine and Antarctic Studies has highlighted the challenges faced by scientists, governments and communities as ocean acidification continues. These researchers have found that in recent centuries surface-ocean pH has fallen 10 times faster than in the past 300 million years. The economic cost to coral reefs, wild fisheries and aquaculture is projected to reach more than $300 billion per annum.

In some parts of the world, such as Chile and the U.S. West Coast, some fisheries are already adapting to ocean acidification through partnerships between scientists, industry and government. Other global impacts are likely to require similar collaboration and action at an international level, say the researchers. A major question for scientists and policy-makers is whether humans should attempt to mitigate ocean acidification by altering ocean chemistry, or whether communities must simply adapt.