Although coal and petroleum are produced by degradation of biomass, yet we need to conserve them because they are non-renewable sources of energy. It takes millions of years for this biomass to form coal and petroleum. It is not easily replenished. If we continue over-exploiting them, we may run out of fossil fuels in the near future. We must use them in a sustainable manner because these resources will last only for a few hundred years and will be exhausted.