Our planet is surrounded by an insulating blanket of gases. It is a protective layer necessary to maintain a global temperature of about 60° Fahrenheit to support most life on Earth.
The most plentiful gas is water vapor that occurs in nature. Others, the greenhouse gases—resulting from human activities—can be harmful. These are carbon dioxide, CO2 (75 percent of the total volume); methane, CH3 (16 percent); nitrous oxide, N2O (6 percent); and fluorocarbons (2 percent).
The wavelength of incoming solar thermal energy lets 70 percent of it pass through atmospheric gases to the planet’s surface. However, the reradiation from the earth’s surface is at a different wavelength, and greenhouse gases trap most of it. The abundance of the gases in the global blanket determine how much heat passes out to space. If too much heat is retained, global warming occurs. This is the greenhouse effect.
Carbon dioxide can remain in Earth’s atmosphere for centuries, and the amount has steadily increased since the Industrial Revolution, 1760–1840. The cause is humans burning fossil fuels. Called the anthropogenic effect, it is now a major cause of global warming.
Carbon dioxide readily dissolves in seawater, so the atmosphere and surface waters are eventually in equilibrium. As the amount of CO2 increases in the atmosphere, the ocean will slowly absorb more to maintain equilibrium. The total volume of the World Ocean has an almost unlimited capacity to absorb CO2 but it is a long-term process. Carbon capture and storage (CCS) technologies could greatly accelerate this.
That is the good news. Increasing CO2 in the oceans’ surface waters, however, also will make them increasingly acidic and less hospitable to marine organisms such as corals and shellfish.
For injected carbon to remain in place depends on temperature and pressure at the dump site. Pumping CO2 down to a depth of about 3,500–10,000 feet results in most of it changing phase from a liquid to a negatively buoyant solid form.
CO2 collection is done on land, where it is compressed, liquefied, and stored. Then it can be sent through pipelines to the seafloor if deeper water is nearby. Or it can be loaded into tankers for delivery to more distant disposal sites.
Injection into the ocean can be done in several ways. The easiest and lowest cost is through a pipeline from land to a nearby deep-sea site. Where this is not feasible, a tank ship can send the CO2 down a long pipe to the site at the desired depth, where the gas becomes heavier than water and sinks to the seafloor, forming “CO2 lakes.”
A less efficient method is for the ship to discharge the gas through a shorter pipe as the ship moves slowly through the water. Satisfactory mixing can be possible, as most of the CO2 should remain in the ocean when temperature and pressure requirements are met.
In areas near on-land or offshore oil and gas fields, geological formations can be used to store CO2 thousands of feet underground. This is by direct injection into depleted oil and gas reservoirs in which production has stopped. In 2021, Exxon Mobil proposed a $100-billion-plus joint government-industry CCS project in the Gulf of Mexico using former reservoirs. The Department of Energy estimates these sites, when fully operational, could sequester 500 billion tons of CO2—equivalent to 130 years of all domestic generation of this gas.
Other nations such as the United Kingdom, Norway, and China are conducting CCS feasibility studies for large-scale carbon-storage projects. And in the Netherlands, the government has offered a $2.6 billion subsidy for an industry-led project that would annually sequester 2.5 million tons of CO2 under the North Sea.
However, much of this is speculative. Although modeling and small-scale testing over two decades have shown that at-sea injection is feasible, using the World Ocean for carbon sequestration has not yet been done at any significant scale. In addition, there are legal and societal concerns about this type of ocean dumping.
Costs for sea sequestration will be very steep, even though this might be an optimum way to remediate global warming. A major question is whether large-scale CO2 removal from the atmosphere can be done in time to have any near-term effect. Global warming is close to a tipping point, so will burying this gas at sea be too late to help?
