|Fig. 1: The infamous Doomsday Clock. How near are we to midnight? Wikimedia Commons)|
The International Commission on Stratigraphy, a permanent working subcommittee of the International Union of Geologic Sciences, estimates that fossil fuels are the result of geological processes that began around 300 million years ago.  The date, while perhaps controversial to some, follows from several geochronological dating methods, including radiometric dating. At that time, pressure and bacteria began to compress and transform layers of organic material into fossil fuels. Oil and natural gas are derived from organisms that inhabited bodies of water; they were buried under ocean or river sediments that have long since dried up. For the most part, coal comes from the compressed remnants of vascular plants. 
Industry estimates of the remaining amounts of coal, oil, and natural gas are on display in Table 1. There is much debate over whether these numbers represent true supplies or investment estimates. It is thus it is not not clear when will run out of fossil fuels. It is quite possible that we will shift to alternative energy sources before doing so. Assuming today's level of extraction and the reserve estimates in Table 1, coal will be depleted in 113 years, natural gas in fifty-two years, and oil in fifty. 
Nuclear energy, too, is nonrenewable; uranium - like its carbon-heavy counterparts - is a finite resource. Each year, 2.8 trillion kilowatt-hours of electricity are generated around the world.  Identified uranium resources come out to around 5.7 million metric tons of refined metal. [4,5] Assuming that the 0.7% of this reserve that is fissile U-235 is burned entirely without breeding, the energy content is
|Energy||=||5.72 × 106 tonnes ×
1000 kg/tonne × 0.007 ×
6.022 × 1023 atoms/mole
|×||2.30 × 108 eV/atom ×
1.602 × 10-19 joules/eV
4.2 × 1010 joules/toe
|=||8.89 × 1010 toe|
The IAEA estimates that there are an additional 7.7 million metric tons that remain undiscovered.  The world's uranium reserves, according to most conservative estimates, will be able to sustain current energy consumption level for the next 230 years.  Clearly, though, as fossil fuel reserves dwindle, production will need to ramp up, rolling back the estimate. Competing factors that may extend the life of nuclear energy production are advances in technology and extraction which may double the estimate, according to researchers from Maryland University.  More liberal estimates rely on the advent of breeder reactors that are able to produce more fissile material than they consume. Such reactors could possibly generate energy for 1000 years. 
|Table 1: Fossil fuel reserves as of 2015 according to the BP Statistical Review of World Energy compared with the Uranium reserves as of 2014 according to the IAEA. [2-5] The mass unit is 1 tonne = 103 kg = 1.10 tons. The energy unit is 1 toe (tonne of oil equivalent) = 4.2 × 1010 joules. The natural gas conversion factors are 1 m3 = 0.0090 tonnes and 1 tonne = 1.23 toe. The coal conversion factor is 1 tonne = 0.75 toe.|
© Harrison Caruthers. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for non-commercial purposes only. All other rights, including commercial rights, are reserved to the author.
 K. M. Cohen et al., "The ICS International Chronostratigraphic Chart," Episodes, 36, 199 (2013).
 "BP Statistical Review of World Energy 2016", British Petroleum, June 2016.
 S. Fetter, "How Long Will the World's Uranium Supplies last?", Scientific American, 9 Mar 09.
 "Nuclear Energy Data 2016", Nuclear Energy Agency, NEA No. 7300, 2016.
 N. Chowdhury, "Is Nuclear Energy Renewable Energy?", Physics 241, Stanford University, Winter 2012.
 "Uranium 2014: Resources, Production and Demand", Nuclear Energy Agency, NEA No. 7209, 2014.