For over 100 years, petroleum has played a key role in the global energy picture. Sharp increases in the price of petroleum can bring on recession and bankrupt industries in petroleum-importing countries, while at the same time providing huge windfalls for exporting countries. The twenty-first century, however, will see petroleum's share of the global energy portfolio diminish as global production begins to decline. A number of candidates will vie to take over as petroleum’s dominance fades. The renewable candidates include renewable distillates and ethanol. The energy investment required to produce a renewable fuel will be an important measure for determining how sustainable these fuels can be if petroleum is no longer enabling their production.