It is clear to many commentators that the world cannot continue along its current energy course. Among the many reasons why it cannot do so, perhaps two stand out. The first is the challenge of supplying enough energy for a world population that is not only growing but also increasing its average per capita energy use. Global energy demand is expected to rise to double or treble its present level by mid-century, while oil production from ‘conventional’ sources, which accounted for about 40 per cent of traded primary energy in the year 2000, may peak within two or three decades. Conventional reserves of gas will last rather longer, but by mid-century they too may be under heavy pressure, especially if gas supplants oil. Unconventional energy resources such as shale oil or methane hydrates may complement the conventional ones, but to what extent is not clear.