Ocean acidification explained

Ocean health experts Dr. Ellen Thomas and Dr. Catherine V. Davis walk us through a consequence of climate change impacting marine ecosystems and human livelihoods. 

What is ocean acidification?

“Ocean acidification refers to the whole suite of chemical changes that happen in the ocean when you start decreasing pH in ocean waters,” says Dr. Catherine V. Davis, Yale Institute for Biospheric Studies Gaylord Donnelley Postdoctoral Associate in the Department of Earth and Planetary Sciences.

Davis explains that as you add carbon dioxide to a liquid, it will decrease the pH and start to form acids.

“Think about the difference between carbonated water and tap water,” Davis says. “The carbonated water has carbon dioxide in it and forms acid, which gives you that tingling sensation on your tongue. A similar thing is happening in the ocean, with obviously some larger scale changes associated.”

Dr. Ellen Thomas, Senior Research Scientist in the Department of Earth and Planetary Sciences, further explains that the ocean is not turning into acid, per say. Instead, it is just becoming less basic.

“Remember the pH scale from elementary school,” Thomas says. “There is a number seven in the middle. Everything lower than 7 is acid, and everything higher than 7 up to 14 is basic. Before human activity, the ocean was 8.2 on that logarithmic scale, and we have now moved down by one pH unit to 8.1. The ocean is still basic, as it is still to the right of that value seven, but it has moved towards this direction of more acid – which is very serious.”

What causes ocean acidification?

Our experts explain that ocean acidification is happening as a direct consequence of the amount of carbon dioxide we are emitting into our atmosphere through human activities, like the burning of fossil fuel. Oceans are carbon sinks, meaning that they naturally absorb carbon dioxide from the atmosphere through physico-chemical and biological mechanisms.

“The more carbon dioxide you put into the atmosphere, the more will be taken up by the ocean,” Davis says, adding that scientists currently believe that about 25% of all carbon emissions are being absorbed by the ocean, making it one of the world’s largest carbon sinks.

The science behind the absorption is simple. Atmospheric carbon dioxide is first transferred into the surface seawater, where it dissolves and enters into a series of chemical reactions. The carbon dioxide (CO2) reacts with water molecules (H20) to form carbonic acid (H2CO3). This compound breaks down into a hydrogen ion (H+) and bicarbonate (HCO3).

“The concentration of hydrogen ions in the water determines the pH of the ocean, and adding more hydrogen ions to the ocean water makes it less basic and more acidic,” Thomas says.

How do we know ocean acidification is happening?

Oceanographers have been observing and measuring ocean acidification over the course of the past century.

One technique to obtain extremely precise ocean acidification measurements involves collecting water samples directly from the ocean at marine stations, and then taking the samples to analyze in a lab. Another technique designed for taking rapid measurements of ocean acidification involves setting instruments out in ocean water to capture ocean acidification over time and at a variety of ocean layers, from surface to deeper. Observations can be transferred from floats or marine-mammals to satellites.

What are the impacts of ocean acidification on marine life?

There are two main impacts of ocean acidification on marine life.

“We know that some of the most vulnerable organisms are those that build shells or skeletons out of carbonate minerals, which are directly impacted by acidity,” Davis says. “For animals that rely on carbonates, their shells could be susceptible to dissolution. Before that extreme case happens, it will take these organisms more energy to build their skeletons or shells.”

What are the impacts of ocean acidification to human society?

Around the world, human livelihood depends on ecosystems that are vulnerable to ocean acidification.

Yale Sustainability, 21 September 2021. Full article.

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