Theme B · Particulate Nature of Matter · SL + HL · 6 hours

B.2 Greenhouse Effect

How does human activity alter the Earth–atmosphere energy balance?

The Earth is in energy balance: the power arriving from the Sun must equal the power radiated back to space, or the planet would warm indefinitely. Albedo (reflected fraction) and emissivity (radiated fraction relative to an ideal black body) are the two dials that control this balance. Greenhouse gases shift the balance by absorbing outgoing infrared radiation and re-emitting it in all directions — including back towards the surface.

The physics is precise: incoming solar radiation is spread over the planet's projected circular cross-section, giving a mean intensity of S/4 at the surface. Equilibrium temperature can be calculated from that balance, accounting for albedo and emissivity. The enhanced greenhouse effect is the increase in this warming caused by human activity, primarily the burning of fossil fuels, which raises the concentration of CO₂, CH₄, and N₂O in the atmosphere.

Earth's albedo is approximately 0.30, its emissivity approximately 0.78, and the solar constant S = 1361 W m⁻². Estimate Earth's equilibrium surface temperature using the energy balance equation. How does this compare to the observed mean temperature of 288 K, and what accounts for the difference?

Key equations and concepts

Emissivity: e = (power radiated per unit area) / σT⁴

Albedo: α = (total scattered power) / (total incident power)

Mean incoming intensity: S/4 (cross-section vs surface area factor)

Energy balance: (1 − α) × S/4 = eσT⁴

What students must understand

Emissivity as a ratio comparing a real surface to an ideal black body radiator
Albedo as the fraction of incoming radiation that is reflected — varies with cloud cover and latitude
The solar constant S and why the mean incoming intensity is S/4 (sphere vs disc)
Main greenhouse gases: CH₄, H₂O, CO₂, N₂O — each with both natural and human-activity origins
Greenhouse mechanism: IR absorbed by molecular energy level transitions, re-emitted in all directions
Two models for the greenhouse effect: resonance model and molecular energy levels
The enhanced greenhouse effect: human activity augments the natural warming
Quantitative energy balance problems: equilibrium temperature from incoming and outgoing intensities

Linking questions

How do SHM and resonance connect to climate change? → C.1 SHM, C.4 Standing Waves and Resonance
How do different methods of electricity production affect the atmospheric energy balance? → B.5 Circuits

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