The greenhouse effect is the natural process by which certain gases in Earth's atmosphere trap outgoing infrared radiation (heat) emitted by the surface, warming the planet. Without the natural greenhouse effect, Earth's average surface temperature would be about −18 °C instead of the habitable +15 °C. The principal greenhouse gases are water vapour (the most abundant), CO₂ (carbon dioxide), methane (CH₄), nitrous oxide (N₂O), and ozone (O₃).
The mechanism works as follows: incoming solar radiation (mostly visible light) passes through the atmosphere and warms the Earth's surface. The surface re-emits energy as infrared radiation, but greenhouse gases are opaque to certain infrared wavelengths — they absorb and re-emit this energy in all directions, including back towards the surface. This "double heating" raises the surface temperature above what it would be with only direct solar heating. The effect is analogous to (though physically different from) a glass greenhouse.
The enhanced greenhouse effect — the warming caused by the increase in greenhouse gas concentrations due to human activities — is the primary driver of modern climate change. Atmospheric CO₂ has risen from ~280 ppm (pre-industrial) to over 420 ppm in 2024, a level not seen in at least 800,000 years. Methane has more than doubled. Each molecule of CO₂ added to the atmosphere traps additional heat, causing a radiative forcing of approximately +2.7 W/m² since 1750. Understanding this physics — first described by Fourier (1824), Tyndall (1859), and Arrhenius (1896) — is fundamental to climate science.