Of the greenhouse gases that contribute to global warming, carbon dioxide is the most abundant. When carbon dioxide increases in the atmosphere, the oceans absorb it. As previously highlighted, the ocean is absorbing the heat generated by the rise in global temperatures. The fact that the oceans absorb this carbon dioxide underlines the role oceans can play in mitigating global warming. But what will happen if atmospheric carbon dioxide continues to increase?

As the amount of carbon dioxide in the atmosphere increases, the amount of carbon dioxide that can be absorbed by the oceans also increases. Ocean water is naturally slightly alkaline. When carbon dioxide dissolves in water, the water becomes acidic. In other words, as increasing amounts of carbon dioxide from the atmosphere are dissolved into the oceans, the oceans become acidic.

When the oceans become acidic, it spells trouble for the living creatures it supports. For example, shellfish and sea urchins are made of a substance called calcium carbonate, which is formed from the carbon dioxide and calcium contained in seawater. However, as more carbon dioxide is absorbed into the ocean, it reduces the amount of calcium carbonate available in the seawater. Consequently, shellfish and sea urchins are being deprived of the essential compound required to make their shell exteriors.

Coral, is also primarily made of calcium carbonate, and is similarly affected when it becomes difficult for calcium carbonate to form in seawater. Coral reefs are also critical sites of marine biodiversity, as they support a large number and wide variety of sea life. If coral reefs disappear, these creatures will have no place to live.

episode 4
It is Harder for Sea Life to Survive Because the Ocean is Becoming More Acidic

Detailed data and explanations1:
Why does carbon dioxide decrease as carbon dioxide increases?

Why does carbon dioxide decrease as the amount of carbon dioxide dissolved in seawater increases? This explanation is a bit difficult to explain, so only those who are interested should read it. The mechanism is as follows.

1. When carbon dioxide dissolves in water, it bonds with water molecules and splits into hydrogen ions and bicarbonate ions.

2. This increases the number of hydrogen ions in the water. These hydrogen ions combine with carbonate ions to form hydrogen carbonate ions. In other words, carbonate ions are used here to bond with excess hydrogen ions, and results in a reduction in the overall amount of carbonate ions.

3. When carbonate ions are reduced, there are less available to combine with calcium ions to form calcium carbonate. This is the reason why shellfish and sea urchins are less able to produce the calcium carbonate they need to build their shells.

Source: University of Tokyo Ocean Alliance website
https://www.oa.u-tokyo.ac.jp/enjoy-story/010.html

Detailed Data and Explanation 2:
The concentration of carbon dioxide in the ocean is rising.

Source: Japan Meteorological Agency website
https://www.data.jma.go.jp/gmd/kaiyou/shindan/sougou/html_vol2/sum_1_4_1_vol2.html

The above graph shows the concentration of carbon dioxide in the atmosphere and surface seawater in the Pacific Ocean south of the Japanese archipelago (137°E, 7-33°N; red line in the figure below) during winter. It can be seen that the concentration of carbon dioxide in the atmosphere is increasing year by year, while the concentration of carbon dioxide in seawater is also increasing. This indicates that the surface seawater is absorbing carbon dioxide from the atmosphere.