CO₂ Capture and Storage

6. Could CO₂ be stored in the deep ocean?

• 6.1 What are the methods of ocean storage?
• 6.2 What are possible environmental impacts and costs of ocean storage

6.1 What are the methods of ocean storage?

Because CO₂ is soluble in water, there are natural exchanges between the atmosphere and the surface of the ocean that occur until a balance is reached. If the atmospheric concentration of CO₂ increases, the oceans are expected to take up additional CO₂ gradually over several centuries until a new equilibrium is reached. The CO₂ would first be dissolved in the upper ocean layer and later be mixed with deep ocean waters. In this way, the oceans have taken up about 500 GtCO₂ of the total 1 300 GtCO₂ released by human activities into the atmosphere over the past 200 years. Oceans are currently taking up about 7 GtCO₂ per year. Most of this carbon dioxide now resides in the upper ocean layer which has become somewhat more acidic as a result (a decrease of 0.1 in pH). To date, however, there has been virtually no change in acidity in the deep ocean.

Captured CO₂ could potentially be injected directly into the deep ocean, where most of it would be isolated from the atmosphere for centuries. This could be achieved by transporting CO₂ via pipelines or ships to an ocean storage site, where it would be injected into the water column of the ocean or at the sea floor. The dissolved and dispersed CO₂ would then become part of the global carbon cycle. Ideas for storing CO₂ away in the deep ocean for even longer periods of time include forming solid CO₂ hydrates and/or liquid CO₂ lakes on the sea floor, and dissolving alkaline minerals such as limestone to neutralize the acidic CO₂. More...

6.2 What are possible environmental impacts and costs of ocean storage

The injection of a few GtCO₂ would produce a measurable change in ocean chemistry in the region of injection, whereas the injection of hundreds of GtCO₂ would eventually produce measurable changes over the entire ocean volume. Over centuries, ocean mixing would result in a gradual release of CO₂ into the atmosphere. Experiments have shown that the addition of CO₂ can harm marine organisms close to injection points or CO₂ lakes. Long term effects of direct CO₂ injection on large ocean areas have not yet been studied. It is, however, expected that impacts on ocean ecosystems would increase with increasing CO₂ concentrations and decreasing pH, and it is unclear how or whether species and ecosystems would adapt to the chemical changes.

The cost of ocean storage has been estimated based on the cost of offshore pipelines or ships, plus any additional energy costs at 6 to 31 US$/tCO₂ net injected. For short distances (100 km offshore), the fixed pipeline option would be cheaper. For larger distances (500 km offshore), injection from a moving ship or transport by ship to a platform with subsequent injection would be more attractive.

The global and regional treaties on the law of the sea and marine environment, such as the OSPAR and the London Convention, also concern ocean storage, but the legal status of intentional ocean storage has not yet been decided.

Storage in oceans would thus be a less environmentally acceptable and a more expensive storage option for CO₂. In the few public perception studies conducted so far, the public has expressed greater reservations about ocean storage than geological storage. More...

Note from the editor: Because of its environmental implications, CO₂ storage in oceans is generally no longer considered as an acceptable option.