renewable energy, usableenergyderived from replenishable sources such as theSun (solar energy),wind (wind power),rivers (hydroelectric power), hot springs (geothermal energy), tides (tidal power), andbiomass (biofuels).

At the beginning of the 21st century, about 80 percent of the world’s energy supply was derived fromfossil fuels such ascoal,petroleum, andnatural 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 fuelcombustion has a number of negative environmental consequences. Fossil-fueled power plants emit air pollutants such assulfur dioxide,particulate matter,nitrogen oxides, and toxic chemicals (heavy metals:mercury,chromium, andarsenic), and mobile sources, such as fossil-fueled vehicles,emit nitrogen oxides,carbon monoxide, and particulate matter. Exposure to these pollutants can causeheart disease,asthma, and other human health problems. In addition, emissions from fossil fuel combustion are responsible foracid rain, which has led to the acidification of many lakes and consequent damage to aquatic life, leaf damage in many forests, and the production ofsmog in or near many urban areas. Furthermore, the burning of fossil fuels releasescarbon dioxide (CO₂), one of the maingreenhouse gases that causeglobal warming.

In contrast, renewable energy sources accounted for nearly 20 percent of global energyconsumption at the beginning of the 21st century, largely from traditional uses of biomass such as wood for heating andcooking. By 2015 about 16 percent of the world’s totalelectricity came from large hydroelectric power plants, whereas other types of renewable energy (such as solar, wind, and geothermal) accounted for 6 percent of total electricity generation. Some energy analysts considernuclear power to be a form of renewable energy because of its lowcarbon emissions; nuclear power generated 10.6 percent of the world’s electricity in 2015.

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Growth in wind powerexceeded 20 percent and photovoltaics grew at 30 percent annually in the 1990s, and renewable energy technologies continued to expand throughout the early 21st century. Between 2001 and 2017 world total installed wind power capacity increased by a factor of 22, growing from 23,900 to 539,581 megawatts. Photovoltaic capacity also expanded, increasing by 50 percent in 2016 alone. TheEuropean 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. By 2016 some 17 percent of the EU’s energy came from renewable sources. The goal also included plans to cut emissions of carbon dioxide by 20 percent and to ensure that 10 percent of all fuelconsumption comes frombiofuels. The EU was well on its way to achieving those targets by 2017. Between 1990 and 2016 the countries of the EU reduced carbon emissions by 23 percent and increasedbiofuel production to 5.5 percent of all fuels consumed in the region. In theUnited States numerous states have responded to concerns overclimate change and reliance on importedfossil fuels by setting goals to increase renewable energy over time. For example,California required its major utility companies to produce 20 percent of their electricity from renewable sources by 2010, and by the end of that year Californiautilities were within 1 percent of the goal. In 2008 California increased this requirement to 33 percent by 2020, and in 2017 the state further increased its renewable-use target to 50 percent by 2030.