The energy system is the largest single source of human greenhouse gas emissions, so transforming it is central to addressing climate change. Energy sources divide into renewable sources, which are naturally replenished on human timescales, including solar, wind, hydropower, geothermal, and biomass, and non-renewable sources, which are finite and include coal, oil, natural gas, and uranium. Solar power comes in two main forms: solar photovoltaic (PV) cells, which convert sunlight directly into electricity, and solar thermal systems, which use sunlight to heat water or other fluids. Wind power generates electricity through turbines, and offshore wind farms, built at sea, typically capture stronger and more consistent winds than onshore sites.
Hydropower, the generation of electricity from moving water, mainly through dams, is the largest established renewable source, and pumped-storage hydropower uses surplus electricity to pump water uphill so it can be released later when demand peaks. Geothermal energy taps heat from Earth's interior for both power and direct heating, while biomass energy and biofuels, such as ethanol and biodiesel, derive energy from organic materials like wood and crop waste. The key climate distinction between biofuels and fossil fuels is timing: the carbon in biofuels was recently part of the atmosphere through plant growth, so releasing it does not add net new carbon on human timescales, whereas fossil carbon was locked underground for millions of years and adds to the active carbon cycle when burned. Nuclear power generates electricity from nuclear fission and is low-carbon but produces long-lived waste, while nuclear fusion, which combines light nuclei to release energy, remains a research frontier without commercial deployment.
The global energy transition refers to the shift from fossil fuels to low-carbon sources, supported by grid storage, the batteries and other systems that store energy for later use, and by smart grids, electricity networks that use digital communication to balance supply and demand efficiently. Energy efficiency, doing more with less energy through better technology and design, is often the cheapest and fastest way to cut emissions, but it can be partially offset by the rebound effect, in which efficiency gains encourage greater overall energy use. Together, these technologies and policies form the technical backbone of decarbonization, but they must be paired with shifts in how energy is used and how economies are organized to reach net-zero emissions.