Biofuels have significant roles in a variety of international issues, including: mitigation of carbon emissions levels and oil prices, the "food vs fuel" debate, deforestation and soil erosion, impact on water resources, and energy balance and efficiency. The use of biofuels reduces dependence on petroleum and enhances energy security. Also, unlike fossil fuels, which return carbon that was stored beneath the surface for millions of years into the atmosphere, biofuels can produce energy without causing a net increase of atmospheric carbon. This is because as new plants are grown to produce fuel, they remove the same amount of CO2 from the atmosphere as they will release as fuel. Some studies have found that certain crops may produce more harmful greenhouse gases than CO2, indicating that the specific biofuel used is an important factor.