In 2008, over 85% of the energy consumed in the world came from fossil fuels. But fossil fuels are finite resources – eventually they will run out.
If current rates of production and consumption were to remain constant, it is estimated that the world's known oil reserves could be exhausted in the 2050s, known natural gas reserves in the 2070s, and known coal reserves in the 2120s. Accelerating demand for energy from developing economies could lead to these resources being depleted at an increasing rate.
Technological advances may allow previously inaccessible 'unconventional' fossil fuel resources to be recovered, increasing the global quantity of supply – but there are concerns about the costs of accessing these resources.
Uranium, the fuel used in nuclear power stations, is also a finite resource. However it is available in abundant supplies and there are sufficient known supplies to power a new generation of nuclear power plants. In addition there are a range of technical advances, and further sources of uranium expected to be discovered that would mean supplies would last for centuries.
Renewable energy sources like wind, sunlight, tides and flowing water are theoretically inexhaustible, but the quantity of this energy that can be converted into electricity is limited by the efficiency of current technology. In 2008, about seven percent of the energy consumed in the world was electricity generated using renewables.