The oceans absorb heat and carbon dioxide from the atmosphere, helping to stabilize the Earth’s climate system. Humans are trying to address climate change by artificially injecting carbon dioxide into the oceans, but the long-term effects on marine ecosystems are uncertain and require environmental regulation and further research.
The ocean absorbs heat from the atmosphere and transports it elsewhere in space and time through ocean current circulation. They play an important role in the Earth’s climate system, moderating extreme temperature fluctuations during seasonal changes. By doing so, the oceans work to prevent rapid changes in temperature, helping to ensure the survival of all life on Earth and providing a habitable environment. For example, without ocean currents to transport heat to different parts of the planet, the poles would become more icy and the equatorial regions would become hotter, threatening the survival of humans, plants, and animals. In recent years, however, the oceans have been absorbing and storing much of the carbon dioxide that humans have emitted as a result of our insatiable greed and development. The oceans absorb one-third of the carbon dioxide emitted by humans each year and store a total of 40,000 gigatons of carbon (GtC), about 50 times the amount of carbon dioxide in the atmosphere. This ability to store carbon dioxide shows that marine ecosystems play an important role in regulating atmospheric carbon concentrations. Nevertheless, humans are looking for ways to artificially inject carbon dioxide into the oceans to help combat global warming. Let’s take a look at some of the options.
First, let’s explain how the ocean naturally stores carbon dioxide. In nature, the flow of carbon dioxide is determined by the difference between the partial pressure of carbon dioxide (pCO2) in the atmosphere and the ocean: if the partial pressure of carbon dioxide above the water surface is higher than below the water surface, carbon dioxide from the atmosphere is absorbed into the ocean and stored there. On the other hand, if the partial pressure of carbon dioxide below the water surface is higher than above the water surface, the ocean releases carbon dioxide into the atmosphere. During this process, life in the ocean uses carbon dioxide to produce oxygen through photosynthesis, which is part of the ocean’s natural carbon cycle. The partial pressure of carbon dioxide in surface waters changes with water temperature, and in the Arctic and Antarctic, the lower water temperatures in the surface waters lower the partial pressure of carbon dioxide, causing the surface waters to absorb atmospheric carbon dioxide. As the surface water absorbs carbon dioxide, it becomes denser and sinks to the depths. The carbon dioxide-rich deep water rises to the surface when ocean currents circulate near the equator. Near the equator, the relatively high water temperature causes the partial pressure to rise, releasing carbon dioxide back into the atmosphere. This process can take 300 to 1,000 years before the carbon dioxide is released back into the atmosphere. As such, the ocean naturally absorbs about 2.0±0.8 GtC of carbon dioxide each year. The amount of carbon dioxide that is released back into the atmosphere is too small compared to the amount of carbon dioxide that humans emit each year. To solve this problem, humans have come up with the idea of artificially injecting carbon dioxide into the oceans.
Many methods have been proposed by scientists to artificially inject carbon dioxide into the oceans. There are two main options: direct injection of gaseous carbon dioxide and direct injection of liquid carbon dioxide. Direct injection of gas-phase carbon dioxide is a method that uses a diffuser to inject carbon dioxide as a gas into the water column below the mixed layer. At a depth of 500 meters and a pressure of 50 atm, carbon dioxide remains in the form of a gas, and the injected bubbles rise due to buoyancy. As the carbon dioxide bubbles dissolve in the surrounding seawater, the density of the seawater increases. Since the density of the solution in which the carbon dioxide is dissolved is higher than the surrounding seawater, it will settle and, over time, rise to the surface again due to ocean current circulation, releasing the carbon dioxide. To prevent the injected bubbles from reaching the surface due to buoyancy, it is important to inject bubbles of small size so that they can all dissolve. Direct injection of gas-phase carbon dioxide cannot completely isolate carbon dioxide from the atmosphere, but it can isolate it for a relatively short period of time.
Direct injection of liquid carbon dioxide, on the other hand, involves injecting carbon dioxide in liquid form to depths of 3,000 meters or more, and can completely isolate carbon dioxide from the atmosphere. At depths greater than 3,000 meters, the density of liquid carbon dioxide is higher than that of seawater and sinks to the seafloor. Droplets of liquid carbon dioxide dissolve in the surrounding seawater, as do bubbles of gaseous carbon dioxide. If the droplets of liquid carbon dioxide are small, they are completely dissolved in the surrounding seawater before sinking to the ocean floor. However, if the droplet is large, some of the undissolved portion of the droplet reaches the seafloor and accumulates. This accumulation of liquid carbon dioxide creates a “lake of CO2” on the seafloor, and the carbon dioxide cannot escape from the seafloor forever. This can disrupt the natural carbon cycling process, and long-term studies of the effects on marine ecosystems are needed. Therefore, scientists have proposed injecting carbon dioxide by creating a pipeline that connects the surface to the ocean floor. A 1-meter diameter pipeline could transport and store 70,000 tons of CO2 per day. 70,000 tons of CO2 is the amount of carbon dioxide produced by a 3 GWatt coal-fired power plant per day.
Humans have built pipelines to depths of 1,600 meters for oil or gas development in the past, and currently have the technology to build pipelines to depths of 3,000 meters. However, for these methods to be successfully applied, environmental regulations and policies must be put in place, along with thorough research into the impact on marine ecosystems. So there is no technical difficulty for humans to store carbon dioxide in the ocean. However, as the atmospheric carbon dioxide concentration has increased from 280 ppm in the 19th century to 380 ppm in 2004, the pH of ocean surface water has also decreased by about 0.1, from 8.2 to 8.1. Therefore, artificially injecting carbon dioxide into the ocean is likely to cause a decrease in the pH of the ocean or adverse effects on marine ecosystems due to the rapid increase in carbon dioxide concentration. This is why societies and governments are opposed to storing carbon dioxide in the oceans. Humanity will need to reduce carbon dioxide emissions by increasing the efficiency of renewable energy sources before worsening global warming makes it urgent to implement ocean storage of carbon dioxide. In the long run, increasing the share of renewable energy along with carbon capture and storage technologies will be the ultimate solution.
