− | The main sources of energy at the beginning of the 21st century are divided into two types: nonrenewable ([[Oil as an energy source|oil]], [[Natural-gas liquids|natural gas]], coal, tar sands, oil shales) and renewable (nuclear, hydroelectric, geothermal, solar). | + | The main sources of energy at the beginning of the 21st century are divided into two types: nonrenewable ([[Oil as an energy source|oil]], [[Natural-gas liquids|natural gas]], coal, [[tar sands]], oil shales) and renewable (nuclear, hydroelectric, geothermal, solar). |
| Sources of energy used by mankind have changed widely. Similar changes should be expected in the future. [[:file:AAPGSt54Figure1.JPG|Figure 1]] depicts the changes of energy sources over a 150-year period. | | Sources of energy used by mankind have changed widely. Similar changes should be expected in the future. [[:file:AAPGSt54Figure1.JPG|Figure 1]] depicts the changes of energy sources over a 150-year period. |