Theme B — The Particulate Nature of Matter

B.2 Greenhouse Effect — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 18 of 53 1 quiz

0%read

Lesson summary

B.2 Greenhouse Effect — IB PHYSICS HL notes: Explain emissivity, albedo, the solar constant, greenhouse gases and energy balance models of the Earth.Stars approximate black bodies — planets do not.Emissivity e = power radiated per unit area by a surface divided by that of a black…

For a non-black body P
Earth's albedo is about 0.3
The fraction absorbed
Solar constant S
Define it above the atmosphere
Planets nearer the Sun receive a higher intensity

Part 1 of 8Emissivity

Emissivity

Stars approximate black bodies

Stars approximate black bodies planets do not.

Emissivity e

Emissivity e power radiated per unit area by a surface divided by that of a black body at the same temperature.

The black body has the same temperature and dimensions as the object.
A perfect black body has e = 1 — this must be remembered.

For a non-black body P

For a non-black body P eσAT⁴.

Part 2 of 8Albedo

Albedo

Albedo a

Albedo a the ratio of total scattered (reflected) power to total incident power.

It has no unit, and a value of 1 means all radiation is scattered.

Earth's albedo is about 0.3

30% of the incident radiation is reflected.

It varies with cloud cover and season, latitude, terrain and angle of incidence.

Surface	Albedo
Fresh asphalt	0.04
Bare soil	0.17
Green grass	0.25
Desert sand	0.40
New concrete	0.55
Ocean ice	0.50 – 0.70
Fresh snow	0.85

The fraction absorbed 1 − albedo.

Part 3 of 8Solar constant

Solar constant

Solar constant S

Solar constant S the intensity of the Sun's radiation arriving perpendicularly at the top of the atmosphere at Earth's mean distance.

Average value 1.36 × 10³ W m^-2.

It varies

It varies because the orbit is elliptical, and the Sun's output varies by about 0.1% over the 11-year sunspot cycle.

Define it above the atmosphere

Define it above the atmosphere not at the surface.

Planets nearer the Sun receive a higher intensity

Planets nearer the Sun receive a higher intensity e.g. Venus more than Earth.

Mean incident intensity over the sphere

Mean incident intensity over the sphere a disc πr² intercepts S while the area is 4πr², so mean intensity = S/4.

Part 4 of 8Greenhouse gases

1Greenhouse gas

Greenhouse gas a gas that absorbs the long-wave radiation re-emitted by the Earth, trapping it in the atmosphere.

Main gases in order of contribution.

Water vapour (H₂O) — evaporation from oceans and plants.
Carbon dioxide (CO₂) — volcanoes, wildfires, respiration and fossil fuels.
Methane (CH₄) — decomposition, oceans, soils and termites.
Nitrous oxide (N₂O) — soils and oceans.

Each has natural and human-made sources.

Ozone absorbs nearly 100% of incoming UV, and also absorbs infrared at 9–10 μm, but its concentration is small.

5CO₂ absorbs infrared at 1.5–30 μm

CO₂ absorbs infrared at 1.5–30 μm strongly at 15 μm;
- water vapour absorbs at 0.8–35 μm and increases as air warms.

The atmosphere is mostly transparent to visible radiation.

Part 5 of 8Mechanism of the greenhouse effect

Mechanism of the greenhouse effect

Incoming solar radiation is short-wave (UV and visible)

Incoming solar radiation is short-wave (UV and visible) about 25% is absorbed by the atmosphere.

The surface absorbs visible light

The surface absorbs visible light and re-emits it as long-wave infrared.

About 80% of the re-emitted infrared is absorbed by the atmosphere and re-radiated back to the surface.

The effect keeps Earth at a habitable temperature.

Resonance model

Resonance model greenhouse gas molecules have natural frequencies in the infrared.
Infrared makes the molecules vibrate and heat up.
UV is energetic enough to break bonds instead.

Energy is trapped and accumulates when greenhouse gas concentrations rise.

Part 6 of 8Enhanced greenhouse effect

Enhanced greenhouse effect

The natural greenhouse effect has natural causes

The natural greenhouse effect has natural causes the enhanced effect arises from human activity.

CO₂ has risen by more than 100 ppm to about 420 ppm in 2020.

Less long-wave radiation escapes

Less long-wave radiation escapes average global temperature has risen by over 1 °C since pre-industrial times.

Gas	Human sources
Carbon dioxide	Burning fossil fuels and wood; deforestation
Methane	Decay of manure, landfill waste and crops
Nitrous oxide	Artificial fertilisers; burning fossil fuels

Part 7 of 8Energy balance problems

If incoming and outgoing energy balance, Earth's temperature is constant.

Simple model

Simple model a one-layer atmosphere above the surface.
The surface receives solar radiation and radiation from the atmosphere.
The surface acts as a black body with e = 1 — and temperatures are held constant in each calculation.

Find the solar intensity reaching the surface — I_s = (1 − a) × intensity above the atmosphere.

Find the intensity radiated by the atmosphere — I = eσT⁴.

Add them to get the total intensity absorbed by the surface.

Use I = σT_s⁴ to find the new surface temperature and the temperature rise.

Such models help predict temperature changes for different greenhouse gas concentrations.

Part 8 of 8More from the specification

Wavelength
- Distance between a point on one wave and the same point on the next wave

Key terms

Term	Meaning
Emissivity	Ratio of power radiated by a surface to that of a black body at the same temperature.
Albedo	Ratio of scattered power to incident power.
Solar constant	Intensity of solar radiation at the top of the atmosphere at Earth's mean distance.
Greenhouse gas	Gas absorbing the Earth's re-emitted infrared radiation.
Enhanced greenhouse effect	Warming from human-increased greenhouse gases.
Energy balance	Equality of incoming and outgoing radiation.

Avoid these mistakes

Confusing albedo (reflected fraction) with emissivity (emitted compared with a black body).

Defining the solar constant at the Earth's surface — it is above the atmosphere.

Using ozone depletion as a cause of the greenhouse effect.

Forgetting e = 1 for a perfect black body.

Using temperature in Celsius in the Stefan–Boltzmann law.

Last updated: 18 October 2026