Giant ocean liners (the new ones are so big that the Panama Canal is being enlarged to accommodate them), are not exactly emission-free vehicles. Still, there has been some movement to make them more efficient, and AutoblogGreen has previously written about an electric-powered oil tanker and using fuel cells in ocean shipping. The news out today is that California Attorney General Jerry Brown, backed by environmental groups like Earthjustice, has asked the federal government to limit emissions on oceangoing ships. Brown et al. also asked the EPA "to adopt standards for carbon dioxide output from the thousands of cargo ships, cruise liners and other large vessels that dock at American ports each year," according to the New York Times. The EPA, which has started looking into ocean emissions regulations by asking the UN International Maritime Organization to set such standards, has six months to respond. The Times says that shipping companies support the international (UN) solution instead of a set of rules that would kick in only when the vessel entered U.S. waters.