Biomass heating systems refers to the various methods used to generate heat from biomass. The systems fall under the categories of direct combustion, gasification, combined heat and power (CHP), anaerobic and aerobic digestion. Historically, before the use of fossil fuels in significant quantities, biomass in the form of wood fuel provided most of humanity's heating, as well as providing our first renewable energy resource. The oil price increases since 2003 and consequent price increases for natural gas and coal have increased the value of biomass for heat generation. Forest renderings, agricultural waste, and crops grown specifically for energy production become competitive as the prices of energy dense fossil fuels rise. Efforts to develop this potential may have the effect of regenerating mismanaged croplands and be a cog in the wheel of a decentralized, multi-dimensional renewable energy industry. Efforts to promote and advance these methods became common throughout the European Union through the 2000s. In other areas of the world, inefficient and polluting means to generate heat from biomass coupled with poor forest practices have significantly added to environmental degradation.