The specter of climate change presents a number of terrifying feedback loops. Perhaps the scariest of all is the destruction of the oceans.

“Ocean acidification” is one of those phrases that manages to evoke a sense of post-apocalyptic peril. But what exactly does it mean?

Changes in surface temperature pH

On the face of it, it seems straightforward; anthropogenic climate change generates more carbon dioxide annually than has been found in the atmosphere for potentially as long as 200 million years. “This has consequences for the oceans,” said Bernard Boudreau, a professor in Dalhousie’s Department of Oceanography, last week at a seminar titled “Neutralization of Ocean Acidification.”

Although carbon dioxide (CO2) is chemically neutral in the atmosphere, it reacts with seawater to form carbonic acid, a weak acid. This weak acid then releases hydrogen ions, he said; the acidity of the ocean is determined by the concentration of hydrogen ions, so more CO2, and therefore more carbonic acid and hydrogen ions, means more acidic – lower pH – oceans.

The average pH for the world’s oceans has fallen from an average of around 8.12 to roughly 8.08.

“It doesn’t look very large,” said Boudreau, “but this is a big change.”

Historic Highs

“It has been higher,” said Boudreau of the CO2 level, pointing out that hundreds of millions of years ago, atmospheric CO2 concentration reached levels as high as 5,000 parts per million (ppm). By comparison, recent levels have been low: as of January 1, 2015 the atmospheric CO2 concentration was just above 400 ppm; pre-industrial levels, in the 18th century, it was around 280 ppm.

Yet before that revelation is embraced by climate skeptics, Boudreau cautioned that a historically high level of CO2 “is not an excuse.” The big difference is that the prehistoric high levels were natural; they occurred on a reasonable timescale that nature was capable of adjusting to. By contrast, “we’re doing this in a matter of a few decades.”

Throughout the change in CO2 levels, said Boudreau, the pH of the ocean has remained above a threshold of around 7.5. How, then, do oceans remain neutral?

Neutralizing CO2

The answer, said Boudreau, comes from CO2’s interaction with another material: calcium carbonate. When there’s more CO2, and therefore more hydrogen ions floating around, they bond with carbonate ions and dissolve, using up the C02 and establishing equilibrium in one of nature’s astonishing balancing acts.

The problem comes from the fact that those carbonate ions have another function.

It’s important to have seawater saturated with carbonate ions – that is, for some of those ions not to be dissolved and therefore be available for other purposes – for the formation of calcium carbonate, the not-so-secret ingredient in the construction of the shells of mussels, clams and other animals as well as two types of structures found in tiny marine organisms: aragonite and calcite.

Creatures that could be affected by changes in C02 levels include the organisms that build coral reefs, which must deposit calcium carbonate faster than the substance is being worn away.

A decline in building materials caused by the increase in C02 levels would be bad news for the world’s coral, and have devastating knock-on effects on marine biodiversity.

Organisms that use calcium carbonate – such as tiny marine snails called Petropods — are also an essential food source for many species including a variety of fish and whales. Research has shown these organisms are already being affected by ocean acidification.

Methane Gas


“You think that’s bad,” said Boudreau. “Things could get far worse.”

The specter of climate change presents a number of terrifying positive feedback loops – the exposure of dark ocean water caused by melting ice, which absorbs more light, causing faster warming and more melting, for example – and among these are the worries about the role of methane.

Methane hydrates are solid forms of methane embedded in glaciers and in the ocean floor in the waters along the sides of continents. At the cool temperatures on the sea floor, these hydrates are stable. As the floor and the water warm up, though, these hydrates break down, releasing the methane.

Methane is a more potent greenhouse gas than CO2, and higher concentrations of methane in the atmosphere would cause more warming, releasing more methane and contributing to runaway climate change.

Boudreau said this isn’t quite the issue for sea-floor methane; methane released into the oceans transforms into CO2. While this could eventually release CO2 into the atmosphere, it’s the effect on acidification that Boudreau is most concerned with. “If that gets into the atmosphere that’s bad enough,” he said. “But it’s more likely that this is a chronic problem.”

If methane is released into the oceans and becomes CO2, the environment — already bad enough for those calcium carbonate-based creatures — becomes truly hostile. Models forecasting the effect of anthropogenic CO2 increases on those creatures predict decline but ultimately some recovery. But, “with the methane, there’s no recovery,” said Bourdrea.

In other words, he said, “You don’t want to be a little organism if this happens.”

Or a big one that feeds on them. Or the humans that feed on the big ones. Ocean acidification could deplete the fish stocks that much of humanity relies on for food.

Looking Long Term

In the long term there is a modicum of hope: eventually the oceans will adapt and recover.

“It’ll fix everything up, we can’t really screw it up.”

But unlike the speed with which human beings are destroying the planet, Boudreau said not to expect that recovery to happen anytime soon.

Assuming humanity can control the release of carbon into the atmosphere, the ocean “will recover with time, but [that time] isn’t tomorrow.”

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