<P> These are the proven energy reserves; real reserves may be up to a factor 4 larger . Significant uncertainty exists for these numbers . Estimating the remaining fossil fuels on the planet depends on a detailed understanding of the Earth's crust . While modern drilling technology makes it possible to drill wells in up to 3 km of water to verify the exact composition of the geology, one half of the ocean is deeper than 3 km, leaving about a third of the planet beyond the reach of detailed analysis . </P> <P> In addition to uncertainty in real reserves, there is significant uncertainty in technological and economical factors that impact what percentage of reserves can be recovered gainfully . In general the easiest to reach deposits are the first extracted . Factors affecting the cost of exploiting the remaining reserves include the accessibility of fossil deposits, the level of sulfur and other pollutants in the oil and the coal, transportation costs, and societal instability in producing regions . </P> <P> Coal is the most abundant and burned fossil fuel . This was the fuel that launched the industrial revolution and has continued to grow in use; China, which already has many of the world's most polluted cities, was in 2007 building about two coal - fired power plants every week . Coal is the fastest growing fossil fuel and its large reserves would make it a popular candidate to meet the energy demand of the global community, short of global warming concerns and other pollutants . According to the International Energy Agency the proven reserves of coal are around 909 billion tonnes, which could sustain the current production rate for 155 years, although at a 5% growth per annum this would be reduced to 45 years, or until 2051 . With the Fischer - Tropsch process it is possible to make liquid fuels such as diesel and jet fuel from coal . In the United States, 49% of electricity generation comes from burning coal . </P> <P> Natural gas is a widely available fossil fuel with estimated 850 000 km3 in recoverable reserves and at least that much more using enhanced methods to release shale gas . Improvements in technology and wide exploration led to a major increase in recoverable natural gas reserves as shale fracking methods were developed . At present usage rates, natural gas could supply most of the world's energy needs for between 100 and 250 years, depending on increase in consumption over time . </P>

Where does the world get most of its energy