B.2.1—Conservation of energy

Treat the planet as a system

Over a long enough time, a planet at steady average temperature receives and emits radiant energy at equal rates:

$P_{\mathrm{in}}=P_{\mathrm{out}}$

This is conservation of energy applied to the planet–atmosphere system.

Track every pathway

Incoming solar radiation can be reflected by the planet–atmosphere system, absorbed by the atmosphere or surface, and later emitted as infrared radiation. The energy balance concerns the total absorbed input and total emitted output, not just one arrow in the diagram.

Interpret imbalance

If absorbed power exceeds emitted power, the system’s internal energy and average temperature tend to increase. If emitted power exceeds absorbed power, they tend to decrease. Equal rates mean no net long-term energy accumulation.

Boundary check

A steady temperature does not mean radiation stops. It means the net energy change is zero because input and output balance.

How it is assessed

The evidence uses a planetary energy-balance intensity question and a surface-temperature graph question with known albedo and emissivity.

Command terms

Determine / Explain

What earns marks

Write an energy balance before calculating: total absorbed input equals total emitted output at steady average temperature. Keep reflected and radiated intensities as separate terms, and show how each contributes to the net balance.

Watch for

Equating steady temperature with no radiation, or double-counting reflected intensity as absorbed energy.

Representative question

[Maximum number: 1]

The energy balance model of a planet's climate is shown. The reflected and radiated intensities are given in terms of the incident incoming intensity I.

What is the radiated intensity from the surface of the planet?

0.40 I

0.50 I

0.70 I

1.10 I

Start with the energy balance

For a planet at steady average temperature, absorbed incoming radiant power equals emitted outgoing radiant power. Albedo controls the reflected fraction; emissivity controls thermal emission relative to a black body.

Average incoming solar energy

The solar constant S is an intensity on a surface perpendicular to the rays. A spherical planet averages the intercepted power over four times the projected area, giving $S/4$ ; with albedo a, the simple globally averaged absorbed intensity is $(1-a)S/4$ .

Atmospheric mechanism

Earth emits infrared radiation. Greenhouse molecules absorb selected wavelengths through molecular resonance or energy-level transitions, then re-emit in all directions, including back toward the surface.

Human enhancement

The natural greenhouse effect supports a habitable surface temperature. Human-driven increases in greenhouse-gas concentration augment the effect; fossil-fuel burning is a primary cause of this enhanced greenhouse effect.

B.2.1—Conservation of energy

Model Planetary Energy Balance

B.2.1 Exam Analysis

Assessment in practice

Synthesize B.2 Greenhouse Effect

Question 1