Answer: Increased use of wood as a heat source.
Fossil fuels are the fuels that are formed by the decomposition of dead and decayed plant and animal remains. These remains are buried under debris and heavy weighted rocks. The heat and pressure under the debris and rocks produce fossil fuels. These are used as a source of energy. Coal, petroleum and natural gas are important fossil fuels. Use of fossil fuels has reduced the use of wood. Wood was burn to obtain thermal energy in order to perform household services like cooking, keeping the house warm during winter. Fossil fuels has replaced wood in all applications like cooking, transportation and insulation.