Carbon dioxide (CO2), the main greenhouse gas, is a product of fossil fuel combustion. But instead of being emitted into the atmosphere, CO2 can be captured from fossil-fuel power plants and industrial plants and stored underground. It is first compressed and then transported to the storage site. CO2 can be stored in confined deep saline aquifers, oil and natural gas fields (with or without enhanced oil or gas recovery) and deep unminable coal seams, both on and offshore.

The North Sea contains a large number of both depleted and producing oil and gas fields that could be used to store CO2. To a lesser extent this is true for the Irish Sea, Baltic Sea and Black Sea. As enhanced oil recovery techniques result in higher recovery rates, the costs of carbon capture and storage can be offset by extra income from oil sales (depending on the specific characteristics of emission sources and transport as well as oil prices).

In a POSEIDON future, efficient offshore electricity production close to natural gas fields will allow CO2 to be stored in nearby geological reservoirs. A CO2 infrastructure can also be used to transport carbon dioxide generated in onshore plants to these reservoirs, resulting in significant cost savings.