The use of energy is considered sustainable if it meets the needs of the present without compromising the needs of future generations. Definitions of sustainable energy typically include environmental aspects such as greenhouse gas emissions, and social and economic aspects such as energy poverty.
Meeting the world's need for energy a sustainable way is one of the greatest challenges facing humanity in the 21st century. The global energy system, which is 85% based on fossil fuels, is responsible for over 70% of the greenhouse gas emissions that cause climate change. The burning of fossil fuels and biomass is a major contributor to air pollution, which causes an estimated 7 million deaths each year. More than 750 million people lack access to electricity and over 2.6 billion rely on polluting fuels such as wood or charcoal to cook.
Renewable energy sources such as wind, hydroelectric power, solar, and geothermal energy are generally far more sustainable than fossil fuel sources. However, some renewable energy projects, such as the clearing of forests for the production of biofuels, can cause severe environmental damage. The role of non-renewable energy sources has been controversial. For example, nuclear power is a low-carbon source and has a safety record comparable to wind and solar, but its sustainability has been debated due to concerns about nuclear proliferation, radioactive waste and accidents. Switching from coal to natural gas has environmental benefits, but may lead to a delay in switching to more sustainable options. Carbon capture and storage technology can be built into power plants to remove their carbon dioxide emissions, but is expensive and has seldom been implemented.
Limiting global warming to levels consistent with the Paris Agreement will require system-wide transformation of the way energy is produced, distributed, stored, and consumed. To accommodate larger shares of variable renewable energy, electrical grids require flexibility through infrastructure such as energy storage. A sustainable energy system is likely to see a shift towards far more use of electricity in sectors such as transport and heating, energy conservation, the use of hydrogen produced by renewables and from fossil fuels with carbon capture and storage. Some technologies that are critical for eliminating energy-related greenhouse gas emissions are still immature.
Wind and solar energy sources generated 8.5% of worldwide electricity in 2019, a share that has grown rapidly. Costs of these energy sources, and of batteries, have fallen rapidly and are projected to continue falling due to innovation and economies of scale. Pathways exist to provide universal access to electricity and to clean cooking technologies in ways that are compatible with climate goals, while bringing major health and economic benefits to developing countries. Well-designed government policies that promote energy system transformation can lower greenhouse gas emissions and improve air quality simultaneously, and in many cases can also increase energy security. Policy approaches can include carbon-pricing and energy-specific policies such as renewable portfolio standards and phase-outs of fossil fuel subsidies.