Processes or regions that predominately absorb atmospheric carbon dioxide are referred to as sinks. Carbon dioxide may be removed from the atmosphere when it is used by plants and algae for photosynthesis, dissolved in water, or deposited in the sediments on land or in the ocean.
Green plants use water from the soil and CO2 from the atmosphere to make carbohydrates (glucose) and oxygen in the process of photosynthesis. In the ocean, algae carry on the same process. During photosynthesis, plants and algae convert the radiant energy of the sun into chemical energy to make the carbohydrates (glucose) and produce oxygen as a byproduct. Plants and algae make more glucose in photosynthesis than they consume in respiration. The excess glucose produced by these photosynthetic organisms becomes the food consumed by animals.
carbon dioxide + water + energy → glucose + oxygen
Oxygen is then used by the animals and plants to oxidize food. This provides the animals and plants with energy. So when animals consume plants or they consume other animals that eat plants, they use the carbohydrates (glucose) as a source of energy.
Carbon dioxide can also be absorbed in the surface water of the ocean. As the concentration of carbon dioxide in the atmosphere increases, some of this carbon dioxide will be dissolved in the oceans. When the water is cooler than the atmosphere, more carbon dioxide can be dissolved. Gases are more soluble in cooler water than in warmer water due to the Second Law of Thermodynamics. Gases are exchanged through the ocean surface until equilibrium is reached.
When carbon dioxide combines with water, it forms carbonic acid.
dissolved CO2 + water → carbonic acid
The oceans provide a huge reservoir of carbon. Scientists estimate that the oceans hold more than 50 times the total atmospheric carbon dioxide content. However, as the ocean temperatures rise over the next centuries, surface waters will begin to release carbon dioxide and the oceans will become a source instead of a sink.
Ocean acidification is a very alarming problem for Earth’s oceans, and ultimately, life on
Earth. As more carbon dioxide gas is absorbed in the ocean, the pH of the ocean decreases (becomes more acidic), which lowers the availability
of the carbonate ions needed by organisms that build shells. This impact on organisms that build skeletons and shells out of calcium carbonate
will profoundly affect ocean ecosystem and food webs. You will learn more about ocean acidification in the module on impacts of climate change.