The greenhouse effect is a natural process in which certain atmospheric gases trap outgoing infrared radiation, warming Earth's surface above what it would otherwise be. The four major greenhouse gases are carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and water vapor (H₂O). The natural greenhouse effect keeps Earth's average temperature near a habitable 15°C. The enhanced greenhouse effect is the additional warming caused by human-produced greenhouse gases added on top of this natural baseline. A useful metric for comparing gases is the global warming potential (GWP), which measures a gas's heat-trapping ability relative to CO₂ over a specified period, usually 100 years. Methane, for instance, is about 80 times more potent than CO₂ over a 20-year period, though it has a shorter atmospheric lifetime of approximately 12 years.
Radiative forcing describes the difference between incoming solar energy and outgoing energy radiated back to space; a positive value indicates net warming. Climate sensitivity is the estimated amount of global warming expected from a doubling of atmospheric CO₂, currently projected at 2.5–4°C. Since the Industrial Revolution, human activities — primarily the burning of fossil fuels (coal, oil, and natural gas) for energy, transportation, and industry — have driven a long-term increase in Earth's average surface temperature, known as global warming. As of the early 2020s, average temperatures have risen roughly 1.1–1.2°C above pre-industrial levels, largely because of the large volume of CO₂ emitted by fossil-fuel burning, deforestation, and industrial processes and the gas's long atmospheric lifetime.
Scientists confirm past climate variations using proxies such as ice cores. Trapped air bubbles in glacial ice preserve ancient atmospheric gas concentrations, enabling reconstruction of CO₂ levels and temperature over hundreds of thousands of years. Climate feedback loops amplify or dampen warming; in the ice-albedo feedback, melting ice exposes darker surfaces that absorb more heat, accelerating further melt. Albedo refers to the reflectivity of a surface — ice and snow have high albedo, while dark surfaces like open ocean absorb more solar energy. Together, these mechanisms explain how human emissions have pushed the climate system into a state of long-term warming.