Abstract:When it comes to energy security and climate change concerns, transportation is the principal culprit. It consumes half the oil used in the world and accounts for almost one fourth of all greenhouse gas (GHG) emissions. In the United States, it plays an even larger role, consuming two thirds of the oil and causing about one third of the GHG emissions in the country. Vehicles, planes, and ships remain almost entirely dependent on petroleum. Efforts to replace petroleum, usually for energy security reasons but also to reduce local air pollution, have continued episodically for years— and largely failed.