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At its core, the issue of ocean acidification is simple chemistry. There are two important things to remember about what happens when carbon dioxide dissolves in seawater. First, the pH of seawater water gets lower as it becomes more acidic. Second, this process binds up carbonate ions and makes them less abundant — ions that corals, oysters, mussels, and many other shelled organisms need to build shells and skeletons.


A More Acidic Ocean

This graph shows rising levels of carbon dioxide (CO2) in the atmosphere, rising CO2 levels in the ocean, and decreasing pH in the water off the coast of Hawaii.

Credit:

NOAA PMEL Carbon Program (Link)

Carbon dioxide is naturally in the air: plants need it to grow, and animals exhale it when they breathe. But, thanks to people burning fuels, there is now more carbon dioxide in the atmosphere than anytime in the past 15 million years. Most of this CO2collects in the atmosphere and, because it absorbs heat from the sun, creates a blanket around the planet, warming its temperature. But some 30 percent of this CO2 dissolves into seawater, where it doesn’t remain as floating CO2 molecules. A series of chemical changes break down the CO2 molecules and recombine them with others.

When water (H2O) and CO2 mix, they combine to form carbonic acid (H2CO3). Carbonic acid is weak compared to some of the well-known acids that break down solids, such as hydrochloric acid (the main ingredient in gastric acid, which digests food in your stomach) and sulfuric acid (the main ingredient in car batteries, which can burn your skin with just a drop). The weaker carbonic acid may not act as quickly, but it works the same way as all acids: it releases hydrogen ions (H+), which bond with other molecules in the area.

Seawater that has more hydrogen ions is more acidic by definition, and it also has a lower pH. In fact, the definitions of acidification terms — acidity, H+, pH — are interlinked: acidity describes how many H+ ions are in a solution; an acid is a substance that releases H+ ions; and pH is the scale used to measure the concentration of H+ ions.

The lower the pH, the more acidic the solution. The pH scale goes from extremely basic at 14 (lye has a pH of 13) to extremely acidic at 1 (lemon juice has a pH of 2), with a pH of 7 being neutral (neither acidic or basic). The ocean itself is not actually acidic in the sense of having a pH less than 7, and it won’t become acidic even with all the CO2 that is dissolving into the ocean. But the changes in the direction of increasing acidity are still dramatic.

So far, ocean pH has dropped from 8.2 to 8.1 since the industrial revolution, and is expected by fall another 0.3 to 0.4 pH units by the end of the century. A drop in pH of 0.1 might not seem like a lot, but the pH scale, like the Richter scale for measuring earthquakes, is logarithmic. For example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6. If we continue to add carbon dioxide at current rates, seawater pH may drop another 120 percent by the end of this century, to 7.8 or 7.7, creating an ocean more acidic than any seen for the past 20 million years or more.

Why Acidity Matters

The acidic waters from the CO2 seeps can dissolve shells and also make it harder for shells to grow in the first place.

Credit:

Laetitia Plaisance

Many chemical reactions, including those that are essential for life, are sensitive to small changes in pH. In humans, for example, normal blood pH ranges between 7.35 and 7.45. A drop in blood pH of 0.2–0.3 can cause seizures, comas, and even death. Similarly, a small change in the pH of seawater can have harmful effects on marine life, impacting chemical communication, reproduction, and growth.

The building of skeletons in marine creatures is particularly sensitive to acidity. One of the molecules that hydrogen ions bond with is carbonate (CO3–2), a key component of calcium carbonate (CaCO3) shells. To make calcium carbonate, shell-building marine animals such as corals and oysters combine a calcium ion (Ca+2) with carbonate (CO3–2) from surrounding seawater, releasing carbon dioxide and water in the process.

Like calcium ions, hydrogen ions tend to bond with carbonate — but they have a greater attraction to carbonate than calcium. When two hydrogens bond with carbonate, a bicarbonate ion (2HCO3-) is formed. Shell-building organisms can’t extract the carbonate ion they need from bicarbonate, preventing them from using that carbonate to grow new shell. In this way, the hydrogen essentially binds up the carbonate ions, making it harder for shelled animals to build their homes. Even if animals are able to build skeletons in more acidic water, they may have to spend more energy to do so, taking away resources from other activities like reproduction. If there are too many hydrogen ions around and not enough molecules for them to bond with, they can even begin breaking existing calcium carbonate molecules apart — dissolving shells that already exist.

This is just one process that extra hydrogen ions — caused by dissolving carbon dioxide — may interfere with in the ocean. Organisms in the water, thus, have to learn to survive as the water around them has an increasing concentration of carbonate-hogging hydrogen ions.

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