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
Coal dioxide, a greenhouse gas that causes climate change and global warming. Coal is the largest contributor to the human-made increase of CO in the atmosphere, 40% of the total. In 2016 world gross carbon dioxide emissions from coal usage were 14.5 giga tonnes. For every megawatt-hour generated, coal-fired electric power generation emits around a tonne of carbon dioxide, which is double the approximately 500 kg of carbon dioxide released by a natural gas-fired electric plant. In 2013, the head of the UN climate agency advised that most of the world's coal reserves should be left in the ground to avoid