The energy transition to meet the world's needs for electricity, heating, cooling, and power for transport in a sustainable way is widely considered to be one of the greatest challenges facing humanity in the 21st century. Production and consumption of energy emits over 70% of the human-caused greenhouse gas emissions that cause climate change. Worldwide, nearly a billion people lack access to electricity, and around 3 billion rely on smoky fuels such as wood, charcoal, or animal dung to cook. These and fossil fuels are major contributors to air pollution, which causes an estimated seven million deaths per year. In general, renewable energy sources such as solar, wind, and hydroelectric energy are widely considered to be sustainable. However, aspects of some renewable energy projects, such as the clearing of forests for the production of biofuels, can lead to similar worse environmental damage than using fossil fuel energy. Nuclear power is a low-carbon source and has a safety record comparable to wind and solar, but radioactive waste and the risk of major accidents are disadvantages of this technology. Moderate amounts of wind and solar energy, which are variable energy sources, can be integrated into the electrical grid without additional infrastructure such as grid energy storage and demand-response measures. These sources generated 8.5% of worldwide electricity in 2019, a share that has grown rapidly. Costs of wind, solar, and batteries are projected to continue falling due to innovation and economies of scale from increased investment. Proposed pathways for limiting, 'global warming' to 1.5 °C describe the rapid implementation of low-emission methods of producing electricity and heat, and a shift towards more use of electricity in sectors such as transport. The pathways also include measures to reduce energy consumption; and the use of low-carbon fuels, such as hydrogen produced by renewable electricity, Vs. fossil fuel subsidies?