The historical alternative of charcoal cannot be used in modern blast furnaces, and even if it could be used in some form, about 3.5 Gt of dry wood per year would be needed for pig iron smelting alone, which requires plantations that are about two thirds the size of the forests of Brazil. Similarly, there are no plant-based substitutes for hydrocarbon feedstocks (about 100 giga cubic metres (Gm3) of natural gas per year) used in making plastics and synthesizing ammonia for fertilizer production. As a result, any proposal to phase out fossil fuels requires targeted research into alternative industrial processes. For example, owing to its intermittency and need for backup capacity, the potential reduction in greenhouse gas emissions that can be achieved by wind power depends almost entirely upon the existing power system to which it is added.