Three substances provide more than 80 percent of the world's energy and over 90 percent of North America's energy: petroleum, natural gas, and coal. In relatively developed countries, the remainder comes primarily from nuclear, solar, hydroelectric, and geothermal power. Wood provides much of the remaining energy in developing societies. Energy is used in three principal places: businesses, homes, and transportation systems. The most important energy source for businesses is coal, followed by natural gas and petroleum. Some businesses use coal directly in an industrial operation, while others rely on electricity, generated primarily at coal-burning power stations. At home, energy primarily is used to generate heat and hot water. Natural gas is the most common source of home heat and hot water, followed by petroleum. Like businesses, homes also use electricity supplied from coal-burning power plants. Petroleum products operate virtually all transportation systems, including automobiles, trucks, buses, airplanes, and some trains. Only subways, streetcars, and some trains run on coal-generated electricity. Petroleum, natural gas, and coal are known as fossil fuels. A fossil fuel comes from the residue of plants and animals buried beneath the earth's surface millions of years ago. As the earth's crust moved, these buried plants and animals were subject to intense changes in pressure and temperature. The global distribution of fossil fuels raises two problems. First, some regions have relatively abundant reserves of one or more fossil fuels, while other regions have little. Second, fossil fuel reserves are not located in the same regions as their consumers. In general, the relatively developed countries of North America, Europe, and the former Soviet Union possess a disproportionately large share of fossil fuel reserves. They contain approximately one-fourth of the world's population but possess more than half of the world's proven reserves of the three major fossil fuels. By far the largest proven reserves of fossil fuels are located in the former Soviet Union. The former soviet Union possesses more than one-third of the world's proven natural gas reserves and more than one-fourth of the proven coal reserves it also ranks sixth in the world in proven petroleum reserves. The United States has more than one- fourth of the world's proven coal reserves, but its proven petroleum and natural gas reserves are relatively small. European countries possess one-sixth of the world' s proven coal reserves, for the most part in Germany and Poland, but less than 5 percent of the petroleum reserves and less than 10 percent of the natural gas reserves. The region's major sources of petroleum and natural gas lie beneath the North Sea. Japan has virtually no proven fossil fuel reserves. Overall, the developing regions do not have a fair share of energy reserves, but the situation varies by type of fossil fuel. Developing countries possess only one-tenth of the world's proven coal reserves, but nearly one-half of the natural gas and nine-tenths of the petroleum. Energy reserves are not distributed uniformly within the developing world. A handful of the developing countries are well endowed with one of the fossil fuels, but most countries in Africa, Asia, and Latin America have relatively little. China accounts for most of the proven coal reserves in developing countries. Approximately 60 percent of the world's proven petroleum' reserves are concentrated in the five Middle Eastern states of Saudi Arabia, Kuwait, Iran, Iraq and the United Arab Emirates. According to the passage, which is the principal energy source of home heat?