This blog post examines whether the ocean can become a solution to global warming through artificial carbon dioxide injection and carbon cycling. We will analyze various methods and their environmental impacts.
The ocean absorbs heat from the atmosphere through ocean circulation and transports that heat to other locations in space and time. This plays a crucial role in Earth’s climate system, moderating extreme temperature fluctuations associated with seasonal changes. By doing so, the ocean strives to prevent abrupt temperature shifts, safeguarding the survival of all life on Earth and providing a habitable environment. For instance, if ocean currents failed to distribute heat globally, polar regions would become even more ice-covered, while equatorial areas would grow hotter, threatening the survival of humans, plants, and animals. However, in recent times, the oceans have also been absorbing and storing vast amounts of carbon dioxide emitted by humanity as a result of our endless desires 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), which is approximately 50 times the amount of carbon dioxide in the atmosphere. This carbon storage capacity demonstrates the ocean ecosystem’s crucial role in regulating atmospheric carbon concentrations. Nevertheless, humanity is seeking ways to artificially inject carbon dioxide into the ocean to address global warming. Let’s explore what methods exist.
First, I will explain how the ocean naturally stores carbon dioxide. The natural flow of carbon dioxide is determined by the difference in partial pressure of carbon dioxide (pCO2) between the atmosphere and the ocean. Specifically, when the pCO2 above the water surface is higher than below it, atmospheric carbon dioxide is absorbed and stored by the ocean. Conversely, when the pCO2 below the water surface is higher than above it, the ocean releases carbon dioxide into the atmosphere. In this process, marine organisms use carbon dioxide through photosynthesis to produce oxygen, which is part of the ocean’s natural carbon cycling process. The pCO2 of surface water varies with water temperature. In the Arctic and Antarctic, the low surface water temperature results in lower pCO2, causing the surface water to absorb atmospheric carbon dioxide. The surface water, having absorbed carbon dioxide, becomes denser and sinks into the deep ocean. This deep water, enriched with carbon dioxide, rises to the surface when it circulates near the equator via ocean currents. Near the equator, relatively high water temperatures cause the partial pressure to rise, releasing the carbon dioxide back into the atmosphere. This process takes approximately 300 to 1,000 years for the carbon dioxide to be re-released into the atmosphere. Thus, the ocean naturally absorbs about 2.0±0.8 GtC of carbon dioxide annually. The amount of carbon dioxide released back into the atmosphere is far too small compared to the amount emitted annually by humanity. To address this issue, humanity has considered methods to artificially inject carbon dioxide into the ocean.
Scientists have proposed numerous methods for artificially injecting carbon dioxide into the ocean. Among these, let’s examine direct injection of gaseous carbon dioxide and direct injection of liquid carbon dioxide. Direct injection of gaseous carbon dioxide involves using diffusers to inject carbon dioxide in its gaseous form into the thermocline layer below the mixed layer. At depths around 500 meters and pressures of approximately 50 atmospheres, the carbon dioxide remains gaseous. Due to buoyancy, the injected bubbles rise. As the gaseous carbon dioxide dissolves into the surrounding seawater, the seawater’s density increases. The density of the solution containing dissolved carbon dioxide becomes higher than that of the surrounding seawater, causing it to sink. Over time, ocean currents will circulate it back to the surface, where the carbon dioxide will be released. To prevent the injected bubbles from reaching the sea surface due to buoyancy, it is crucial to inject them in small sizes so they can all dissolve. Direct injection of gaseous carbon dioxide cannot completely isolate carbon dioxide from the atmosphere, but it can isolate it for a relatively short period.
In contrast, liquid-phase CO₂ direct injection involves injecting CO₂ in liquid form at depths greater than 3,000 meters, enabling complete isolation from the atmosphere. At depths greater than 3,000 meters, the density of liquid CO₂ exceeds that of seawater, causing it to sink to the seafloor. Droplets of liquid carbon dioxide dissolve into the surrounding seawater, much like bubbles of gaseous carbon dioxide. If the droplets are small, they dissolve completely into the surrounding seawater before reaching the seafloor. However, if the droplets are large, a portion that has not fully dissolved reaches the seafloor and accumulates. This accumulated liquid carbon dioxide creates ‘CO2 lakes’ on the seafloor, trapping the carbon dioxide there permanently. This can disrupt natural carbon cycling processes, necessitating long-term research on its impacts on marine ecosystems. Therefore, scientists have proposed a method of injecting carbon dioxide using pipelines connecting the surface to the seafloor. They stated that a 1-meter diameter pipeline could transport and store 70,000 tons of CO2 per day. 70,000 tons of CO2 is equivalent to the amount produced daily by a 3-gigawatt coal-fired power plant.
Humankind has experience constructing pipelines to depths of 1,600 meters for past oil or gas development and currently possesses the technology to build pipelines to depths of 3,000 meters. However, for this method to be successfully applied, it must be accompanied by thorough research on its impact on marine ecosystems, along with environmental regulations and policies. Therefore, there are no technical difficulties for humankind in storing carbon dioxide in the ocean. However, as atmospheric carbon dioxide concentrations rose from 280 ppm in the 19th century to 380 ppm in 2004, the pH of surface seawater also decreased by about 0.1, from 8.2 to 8.1. Consequently, artificially injecting carbon dioxide into the ocean carries a high risk of causing a rapid increase in carbon dioxide concentration, leading to a further decrease in ocean pH or adversely affecting marine ecosystems. Consequently, society and governments oppose ocean carbon storage. Before the implementation of ocean carbon storage technology becomes urgent due to worsening global warming, humanity must strive to reduce carbon dioxide emissions by improving the efficiency of renewable energy. In the long term, increasing the proportion of renewable energy alongside carbon capture and storage technology will be the ultimate solution.
