The human population is currently larger than ever before, and it continues to grow. Because of the increasing human presence on earth, human behavior and lifestyle will be a major factor driving the extent of global warming. In the industrialized world, energy use contributes more to global warming than any other human activity. This is because most of the energy comes from carbon-based fossil fuels like coal, oil, and natural gas. Transportation (the movement of people and goods) accounts for about one-third of the developed world's CO2 emissions. Transportation's carbon emissions result from a combination of three factors: number of miles traveled, amount of fuel used, and the amount of CO2 released when a particular fuel is consumed.