Mitigation of peak oil is another alternative, and combined cycle power generation using natural gas is the cleanest source of power available using fossil fuels, producing about 30% less carbon dioxide than burning petroleum and about 45% less than burning coal. The major difficulty in the use of natural gas is transportation and storage because of its low density. Natural gas pipelines are economical, but are impractical across oceans. Due to its high energy density and ease of handling, oil has a unique role as a transportation fuel. There are, however, a number of possible alternatives. Among the biofuels the use of bioethanol and
Carbon capture and storage (timeline) Sleipner field produces natural gas and light oil from the Heimdal sandstones, which are about 2,500 m below sea level. The natural gas produced at Sleipner contains unusually high levels (about 9%) of carbon dioxide, but the customers require less than 2.5%. To encourage companies to cut their carbon emissions, the Norwegian government imposes a carbon tax equivalent to about $50 per ton of CO released into the atmosphere. To avoid paying the tax, and as a test of alternative technology, all of the CO extracted since 1996, when gas production started at Sleipner, has been pumped back deep underground.