Petroleum was the source of less than 1% of U.S. electricity generation in 2019. Residual fuel oil and petroleum coke are used in steam turbines. Distillate or diesel fuel oil is used in diesel engine generators. Residual fuel oil and distillates can also be burned in gas turbines. The power generation industry can be split into three areas: power generation, transmission and distribution networks, and metering and sales. Large energy companies tend to operate in all three areas, as it is more cost-effective, but smaller companies often only work in one of these areas. Fossil fuel power stations provide most of the electrical energy used in the world. Some fossil-fired power stations are designed for continuous operation as baseload power plants, while others are used as Peaker plants. Power generation, primarily fossil fuel-based, accounts for approximately one-quarter of total emissions of CO2, the primary contributor to global warming. The Intergovernmental Panel on Climate Change provides the most detailed analyses and data developed to date on the sources and impacts of CO2 concerning global warming. The full repercussions and impacts of increases in CO2 are not fully understood, but research is ongoing by IPCC and others. Liquid fuels may also be used by gas turbine power plants, especially for peaking or emergency service. Of the three fossil fuel sources, oil has the advantages of easier transportation and handling than solid coal, and easier on-site storage than natural gas.