What is carbon storage, and why is it frequently mentioned as a potential way to mitigate global warming? Also known as carbon sequestration, carbon storage is a complex method of capturing carbon dioxide emissions and storing them in coal seams, aquifers, depleted oil and gas reservoirs and other spaces deep under the surface of the Earth. Theoretically, this would prevent those gases from having an effect on climate.

Carbon dioxide gases are captured either at the source of production, such as a power plant, or directly from the air. Carbon dioxide can be separated from other gases either before or after combusting fuel in a plant or industrial facility. Removing carbon dioxide from the atmosphere, a form of geoengineering, is substantially more difficult and expensive; proposals include creating giant sponges that are mounted high in the air like wind turbines near carbon dioxide-producing plants to capture the gases.

Carbon storage undergroundCarbon Storage Methods

The most widely supported way to store captured carbon dioxide is in deep geological formations like oil fields, gas fields, coal seams and saline aquifers. The most common carbon dioxide emitters like power plants are often already located above these naturally occurring underground 'storage tanks', making them an attractive solution. Furthermore, injecting carbon dioxide into these spaces can help utilities recover more of the valuable oil and gas that already exists in the field. The costs of carbon capture and storage can then be offset by the sale or use of these fuels. Similar benefits are seen in coal seams, where pockets of methane can be displaced with carbon dioxide. However, burning that methane would produce more carbon dioxide.

While storing carbon in deep saline formations doesn't produce any value-added by-products, the U.S. Department of Energy, which is currently studying the behavior of carbon dioxide when stored in geologic formations, notes that it has other advantages. Not only are there enough deep saline formations in the United States to potentially store more than 12,000 billion tons of carbon dioxide, but they're already accessible to most sources of carbon dioxide emissions, reducing the cost of transporting the gases.

Some carbon storage proposals involve injecting carbon dioxide into the ocean at depths at least 1,000 meters below the surface. The carbon dioxide would then either dissolve into the water or, when injected under high pressure at depths greater than 3,000 meters, accumulate into 'lakes' on the sea floor, where it could theoretically take millennia to dissolve.

Storing carbon in minerals may also be possible by reacting carbon dioxide with metal oxides like magnesium and cadmium. This process is called mineral sequestration. When it occurs naturally, over thousands of years, this process creates surface limestone; when sped up, it turns the carbon dioxide into stable carbon solids.

Pros and Cons of Carbon Storage

Carbon storage would prevent widespread carbon dioxide emissions from continuing to cause and exacerbate climate change, and advocates say that it's less expensive than switching from fossil fuels to forms of renewable energy like solar power. However, the process increases the amount of energy required by power plants, and most experts agree that carbon storage should only be used as a transitional solution. Carbon capture and storage would require a significant investment in fossil fuel-burning power plants and would allow the environmental destruction caused by coal mining to continue well into the future.

Underwater carbon storageStoring carbon in the ocean has drawbacks of its own. As carbon dioxide reacts with the water it forms carbonic acid. This could worsen ocean acidification, which kills sea life such as coral and edible species of fish that form a major part of the world food supply. Even when the carbon dioxide is pumped to great depths, it may not be long before it is released back into the atmosphere. Strong winds caused by climate change are mixing waters in the oceans, causing carbon dioxide to rise to the surface from the ocean's depths.

Critics of carbon storage are also concerned about the possibility of carbon dioxide leaks from underground storage spaces. Naturally occurring leaks can be extremely destructive, killing humans and animals, and if carbon storage became a common solution, such leaks could increase in frequency and severity. Even when fitted with no-return valves, carbon injection pipes could break down over time, allowing the gases to resurface.

One solution to the problems associated with carbon storage is to find uses for the captured carbon. Carbon capture and utilization could be more economically viable than storage, turning the captured carbon dioxide into valuable new products like bio-oils, fertilizers, chemicals and fuels.

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Image: Lawerence Berkeley National Laboratory/Department of Energy

See also:

What is carbon capture