He has written extensively on environmental issues, and is a particular critic of the practice of hydraulic fracking.

According to research from the University of Pittsburgh, use of hydraulic fracking to extract shale gas is extremely expensive.

The vast expanses of oil in shale formations such as the Bakken were opened up because of advancements in hydraulic fracking technology.

Until recently, there were concerns the world would run out of oil and gas - but the development of hydraulic fracking has diminished those fears.

The US has almost ended its reliance on imported gas after opening up its reserves of shale gas using the process of " hydraulic fracking".

Hydraulic fracking involves releasing gas trapped in rocks by pumping in water mixed with sand and chemicals at high pressure.

Hydraulic fracking has provoked opposition from environmentalists who say the injection of chemicals contaminates water tables, concerns that are vividly depicted in the documentary Gasland.