Extraction of conventional oil and natural gas is done primarily through well drilling, which can cause localized erosion and loss of soil productivity, and carries a risk of spills which can cause serious environmental contamination. However, biofuel production and other uses of biomass require a large input of “raw” biomass feedstock, the production and harvesting of which can have environmental impacts (depending on the biomass source). Electricity production based on combustion of other fossil fuels is also associated with significant environmental impacts, including greenhouse gas and other air emissions.