Switching about 1400 GW of power from coal-fired to natural-gas-fired plants would also provide a wedge, since the amount of carbon from gas per kWh is about half that from coal. Of more long-term importance is the identification of a wedge from increased carbon capture and storage (CCS). One technique first produces hydrogen and carbon dioxide from the fossil fuel. The hydrogen is then burnt to provide power, or could be used in fuel cells, and the carbon dioxide is stored underground. Another technique separates the C02 from the flue gas and then compresses it before pumping it underground. Currently, C02 is injected into oil reservoirs to enhance oil recovery (EOR). An increase of about 100 in the amount of EOR would produce another wedge.
Besides storage in old oil and gas reservoirs, storage in underground aquifers is also possible. In the North Sea off Norway, about one million tons of C02 a year is being stored under the sea in an underground saline aquifer lying above the Sleipner oil field. There is a large number of such formations in the world and the Sleipner project is being used to evaluate the stability and characteristics of storage of C02 in such aquifers. The Utsira aquifer at Sleipner is a massive sandstone aquifer 200—250 m thick capable of storing an estimated 600 Gt of C02, equivalent to 160 Gt of carbon. Carbon dioxide needs to be stored for 1000-10 000 years, by which time the anthropogenic emission of C02 is expected to have essentially ceased. In oil and gas fields the gas has been stored for millions of years and it seems unlikely that leakage of C02 stored underground will be a problem.