Greenhouse Effect
The warming of Earth's surface by greenhouse gases that absorb and re-emit infrared radiation, keeping the planet habitable.
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The greenhouse effect is the process by which certain atmospheric gases trap heat near Earth's surface. The Sun's energy arrives mainly as visible and ultraviolet electromagnetic radiation, which passes through the atmosphere and warms the surface. The surface re-emits this energy as infrared radiation; greenhouse gases — water vapor, carbon dioxide, methane, and nitrous oxide — absorb specific infrared wavelengths and re-emit energy in all directions, including back toward the ground. The result is a warmer surface than the planet would have without an atmosphere.
The effect is large and essential: with no greenhouse gases, Earth's average surface temperature would be about −18°C instead of the actual +15°C — roughly 33°C of warming. The name is a loose analogy to a glass greenhouse: the mechanism is different (glass blocks convection as much as radiation), but the trapping of outgoing radiation is similar. Water vapor is the most abundant greenhouse gas, followed by carbon dioxide, which has a longer atmospheric lifetime and is the main driver of long-term change because its concentration is altered by human activity.
Since the industrial revolution, atmospheric CO₂ has risen from about 280 parts per million to over 420 ppm, primarily from burning fossil fuels and deforestation, and methane and nitrous oxide have also increased. Because the absorption bands of CO₂ are not yet saturated, each additional molecule adds warming — the enhanced greenhouse effect. The added energy warms the surface and ocean, and, through the water cycle, a warmer atmosphere carries more water vapor, which amplifies the warming further. Observed consequences include rising global mean temperature, melting ice, sea-level rise, and shifts in precipitation patterns.
The greenhouse effect is often confused with other atmospheric phenomena: the ozone layer filters ultraviolet light (a separate process), and air pollution aerosols generally cool by reflecting sunlight. Climate science distinguishes the natural greenhouse effect, which has operated for billions of years, from the human-driven enhancement of it, which is the subject of active measurement, modeling, and policy.
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atmosphere climate earth science radiation