6.2 Greenhouse Effect
Greenhouse gases absorb and re-emit infrared radiation, trapping heat and warming Earth's surface by ~33 K above the effective temperature.
Major Greenhouse Gases
H₂O (Water Vapor)
~60% of natural greenhouse effect. Strong positive feedback.
CO₂ (Carbon Dioxide)
~420 ppm. Radiative forcing: ~2.2 W/m² since pre-industrial.
CH₄ (Methane)
~1.9 ppm. 28× stronger than CO₂ per molecule over 100 years.
N₂O (Nitrous Oxide)
~330 ppb. 265× stronger than CO₂.
Radiative Forcing
$$\Delta F = 5.35 \ln\left(\frac{C}{C_0}\right) \text{ W/m}^2$$
Logarithmic forcing from CO₂ (doubling ≈ 3.7 W/m²)