change cools the remaining liquid. Because the insulation is very efficient, only a relatively small amount of boil-off is necessary to maintain temperature. This phenomenon is also called auto-refrigeration. Boil-off gas from land based LNG storage tanks is usually compressed and fed to natural gas pipeline networks. Some LNG carriers use boil-off gas for fuel. Environmental concerns Natural gas could be considered the least environmentally harmful fossil fuel because it has the lowest CO2 emissions per unit of energy and is suitable for use in high efficiency combined cycle power stations. For an equivalent amount of heat, burning natural gas produces about 30 percent less carbon dioxide than burning petroleum and about 45 per cent less than burning coal. Biomethane is considered roughly -neutral and avoids most of the -emissions issue. If liquified (as LBM), it serves the same functions as LNG. On a per kilometre transported basis, emissions from LNG are lower than piped natural gas, which is a particular issue in Europe, where significant amounts of gas are piped several thousand kilometres from Russia. However, emissions from natural gas transported as LNG are higher than those of natural gas produced locally to the point of combustion, as emissions associated with transport are lower for the latter. However, on the West Coast of the United States, where up to three new LNG importation terminals were proposed before the U.S. fracking boom, environmental groups, such as Pacific Environment, Ratepayers for Affordable Clean Energy (RACE), and Rising Tide had moved to oppose them. They claimed that, while natural gas power plants emit approximately half the carbon dioxide of an equivalent coal power plant, the natural gas combustion required to produce and transport LNG to the plants adds 20 to 40 percent more carbon dioxide than burning natural gas alone. A 2015 peer reviewed study evaluated the full end-to-end life cycle of LNG produced in the U.S. and consumed in Europe or Asia. It concluded that global production would be reduced due to the resulting reduction in other fossil fuels burned. Some scientists and local residents have raised concerns about the potential effect of Poland's underground LNG storage infrastructure on marine life in the Baltic Sea. Similar concerns were raised in Croatia. Safety and accidents Natural gas is a fuel and a combustible substance. To ensure safe and reliable operation, particular measures are taken in the design, construction and operation of LNG