Human societies use energy in many different ways, in transportation, and in industrial and domestic applications. Energy can be sourced from solar, wind, wave, hydroelectric, geothermal and nuclear sources. Traditionally humans have used plants as a direct source of biomass for heating and cooking. However, more recently, fossil fuels (coal, gas and oil) have been used as energy sources. These reservoirs of carbon compounds produced from CO2 by plants growing over long periods of time have been used extensively to produce electricity and liquid fuels for transportation. These resources are also used as chemical feedstocks for the manufacture of a wide range of carbon compounds from plastics to pharmaceuticals. The burning of fossil fuels results in an increase in greenhouse gases, especially CO2 and the associated threat of global warming. Human societies have a great dependence on oil and for transportation. Oil stocks are declining and prices of products derived from oil have been increasing sharply in recent years. It has been estimated that humans have consumed around 875 billion barrels of oil since we started using oil, that about 1.7 trillion barrels remain in established reserves and that we might eventually find another 900 billion barrels (Ahmann and Dorgan, 2007). Two main factors encourage the search for alternative sources of energy. Firstly, the growing cost of oil and in the long term the ultimately limited nature of the resource makes alternatives attractive. Secondly, the risks of global warming associated with the consumption of fossil fuels provides further incentive to develop new technologies. Options that replace oil for high volume uses such as transportation may also help conserve oil stocks for more critical or less substitutable uses such as chemical feedstocks. Photosynthesis by plants captures light energy by using it to combine CO2 to form carboncontaining compounds mainly in the form of carbohydrates. Simple calculations indicate that the amount of energy captured by photosynthesis
each day is far greater than the amount of energy used by human societies. In 2008, for the first time, the International Energy Agency (IEA, World Energy Outlook, 2008) called for an urgent effort to move away from oil to a more sustainable energy supply system to avoid catastrophic climate change.