Introduction
If you removed every greenhouse gas from Earth's atmosphere overnight, the planet would cool by approximately 33°C (91°F) — from its current average of +15°C (59°F) to −18°C (0°F). The oceans would begin to freeze. Most of the liquid water on the surface would be locked into ice within decades. Complex life as we know it could not survive. The greenhouse effect is not a problem — it is the condition for life on Earth.
The greenhouse effect works because the atmosphere is not equally transparent to all types of electromagnetic radiation. Visible light from the sun (short wavelength, high energy) passes through the atmosphere with relatively little absorption, warming Earth's surface. The warm surface then emits infrared radiation (long wavelength, lower energy), but greenhouse gases — primarily water vapour, carbon dioxide, methane, and nitrous oxide — absorb this outgoing infrared radiation before it can escape to space. These gases re-emit the absorbed energy in all directions, including back toward the surface. The surface receives this additional downwelling radiation and must reach a higher temperature to achieve energy balance. This is the mechanism: selective transparency to shortwave radiation, selective opacity to longwave radiation, producing a warmer surface than an airless planet at the same distance from the sun would have.
The concept of radiative forcing quantifies how much any given change — in greenhouse gas concentration, solar output, volcanic aerosols, or land use — affects the flow of energy in the climate system, measured in watts per square metre. A positive forcing warms the planet; a negative forcing cools it. It is the common currency of climate science, allowing scientists to compare the climate effect of doubling CO₂ (a radiative forcing of +3.7 W m⁻²) against a volcanic eruption (negative forcing of −1 to −5 W m⁻²) against a change in solar output (small positive or negative forcing depending on the solar cycle). Understanding forcing and feedback — how the climate system responds to forcings — is essential to understanding both past climate changes and projections of future warming.
Key Terms
A gas that absorbs and re-emits infrared radiation, contributing to the greenhouse effect. Major greenhouse gases: H₂O (water vapour), CO₂, CH₄, N₂O, O₃, and synthetic halocarbons. N₂ and O₂ — the bulk of the atmosphere — are NOT greenhouse gases because they do not absorb IR radiation (symmetric diatomic molecules cannot vibrate in ways that interact with IR photons).
The change in energy flux (in W m⁻²) at the top of the atmosphere caused by a perturbation (e.g., doubling CO₂, volcanic eruption, solar changes). Positive forcing = more energy enters than leaves = warming. Negative forcing = more energy leaves = cooling. Allows comparison of different climate drivers on a common scale. Doubling CO₂: +3.7 W m⁻²; 2023 total anthropogenic forcing: ~3.0 W m⁻².
A process that amplifies or dampens a forcing. Positive feedbacks amplify the original change (ice-albedo: warming melts ice → lower albedo → more warming; water vapour: warming increases H₂O → stronger GHE → more warming). Negative feedbacks dampen it (Planck/blackbody feedback: warmer Earth emits more IR → loses energy faster → limits warming). Net feedback determines climate sensitivity.
The equilibrium global mean surface temperature rise resulting from a doubling of CO₂. Estimated at 2.5–4.0°C (IPCC AR6 likely range), with a best estimate of ~3°C (37°F). Most of the uncertainty comes from cloud feedbacks. A 3°C equilibrium warming from CO₂ doubling includes all physical feedbacks (water vapour, ice-albedo, lapse rate) but not slow feedbacks (ice sheet collapse, permafrost carbon release).
Infrared radiation emitted by greenhouse gases and clouds downward toward Earth's surface — commonly called "back-radiation." Amounts to ~340 W m⁻² globally averaged, actually exceeding direct solar absorption at the surface. This downwelling IR is the direct heating mechanism of the enhanced greenhouse effect and is measurably increasing as greenhouse gas concentrations rise.