Earth must radiate away as much energy as it absorbs. Viewed from space the planet radiates at −18 °C — that's the "effective temperature". Greenhouse gases (H₂O, CO₂, CH₄, N₂O, O₃) are transparent to visible sunlight but absorb outgoing infrared, re-emitting it in all directions — some straight back down. The surface warms until the escaping flux balances incoming sunlight again. That natural effect is worth +33 °C: without it Earth's average surface would be a frozen −18 °C instead of +15 °C.

Water vapour is the strongest greenhouse gas, but CO₂ is the thermostat: warmer air holds more vapour, so CO₂ (which humans add directly, ~50 Gt CO₂-eq/yr) sets the temperature the vapour amplifies. Methane is ~80× stronger per molecule over 20 years.

This is a simplified single-layer toy model (real radiative transfer is spectral and layer-by-layer). It reproduces Earth's +33 °C well, but "Venus-like" here shows only ~+35 °C where the real Venus sits at +465 °C — its 90-bar CO₂ atmosphere and sulphuric clouds create a runaway greenhouse far beyond any simple model. That gap is itself the lesson about tipping points.