At the beginning of the 21st century, about 80 percent of the world's energy supply was derived from fossil fuels (fossil fuel) such as coal, petroleum, and natural gas. Fossil fuels are finite resources; most estimates suggest that the proven reserves of oil are large enough to meet global demand at least until the middle of the 21st century. Fossil fuel combustion has a number of negative environmental consequences. Fossil-fueled power plants emit air pollutants such as sulfur dioxide (sulfur oxide), particulate matter, nitrogen oxides, and toxic chemicals (heavy metals: mercury, chromium, and arsenic), and mobile sources, such as fossil-fueled vehicles, emit nitrogen oxides, carbon monoxide, and particulate matter. Exposure to these pollutants can cause heart disease, asthma, and other human health problems. In addition, emissions from fossil fuel combustion are responsible for acid rain, which has led to the acidification of many lakes and consequent damage to aquatic life, leaf damage in many forests, and the production of smog in or near many urban areas. Furthermore, the burning of fossil fuels releases carbon dioxide (CO₂), one of the main greenhouse gases that cause global warming.

In contrast, renewable energy sources accounted for nearly 20 percent of global energy consumption at the beginning of the 21st century, largely from traditional uses of biomass such as wood for heating and cooking. About 15 percent of the world's total electricity comes from large hydroelectric power plants, whereas other types of renewable energy (such as solar, wind, and geothermal) account for 3.4 percent of total electricity generation.

Growth in wind power exceeded 20 percent and photovoltaics grew at 30 percent annually in the 1990s, and renewable energy technologies continue to expand. By 2007 more than 60 countries had adopted policy targets to increase the proportion of energy they derive from renewable sources. The European Union (EU), which produced an estimated 6.38 percent of its energy from renewable sources in 2005, adopted a goal in 2007 to raise that figure to 20 percent by 2020. The goal includes plans to cut emissions of carbon dioxide by 20 percent and to ensure that 10 percent of all fuel consumption come from biofuels (biofuel). In the United States, numerous states have responded to concerns over climate change and reliance on imported fossil fuels by setting goals to increase renewable energy over time. For example, California has required its major utility companies to produce 20 percent of their electricity from renewable sources by 2010.