Theme B — The Particulate Nature of Matter

B.2.3 Greenhouse Gases — IB Physics HL Revision Notes & Topical Questions

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B.2.3 Greenhouse Gases — IB PHYSICS SL notes: Identify the main greenhouse gases, explain why they absorb infrared radiation and describe their natural and human sources.Radiation from the Sun — it arrives mostly as visible light, which passes through the atmosphere with little a…

Radiation from the Earth
Not greenhouse gases
Order of contribution
Both origins
Halocarbons
Ozone (O3)

Section 1 of 9What a greenhouse gas is

Radiation from the Sun

Radiation from the Sun it arrives mostly as visible light, which passes through the atmosphere with little absorption.

Radiation from the Earth

Radiation from the Earth the surface re-emits long-wave infrared, with wavelengths mainly between about 4 μm and 50 μm.

Greenhouse gas

Greenhouse gas a gas that absorbs this re-radiated long-wave radiation, so it is not lost to space.

Visible light

Visible light the atmosphere is mostly transparent to incoming visible radiation.

Name

Name greenhouse gases have an effect similar to the glass in a greenhouse.

Not greenhouse gases

Not greenhouse gases nitrogen (78%) and oxygen (21%), whose interaction with infrared is negligible.

Section 2 of 9The main gases

Gas	Natural sources	Human sources
Water vapour (H₂O)	Evaporation from oceans, seas and plants	Controlled by temperature, acts as a feedback
Carbon dioxide (CO₂)	Volcanic eruptions, wildfires, respiration	Fossil fuels, burning wood, deforestation, cement
Methane (CH₄)	Decomposition in oceans and soils, termites, wetlands	Manure, landfill, crops, livestock, gas leaks
Nitrous oxide (N₂O)	Soils and oceans	Artificial fertilisers, burning fossil fuels

Order of contribution

Order of contribution water vapour, carbon dioxide, methane and nitrous oxide.

Both origins

Both origins each greenhouse gas has both natural and human origins.

Ozone

Ozone it is a weaker contributor, and ozone depletion is a separate issue about ultraviolet absorption.

Halocarbons

CFCs and related compounds also contribute.

Section 3 of 9Absorption by individual gases

Ozone (O₃)

Ozone (O₃) absorbs nearly 100% of incoming ultraviolet and strongly absorbs outgoing infrared between 9 μm and 10 μm,
but is not a significant contributor because its concentration is low.

Carbon dioxide

Carbon dioxide a good absorber of infrared between 1.5 and 30 μm, strongest at 15 μm.
Its rising concentration makes it one of the most significant contributors.

Water vapour

Water vapour the best absorber of infrared between 0.8 and 35 μm.
Its concentration increases as the air becomes warmer.

Total atmosphere

Total atmosphere most ultraviolet, infrared and microwave radiation is absorbed, but visible radiation passes through.

Relative importance

Relative importance it depends on the concentration and on how much the gas absorbs specific wavelengths.

Section 4 of 9Why these gases absorb infrared

Resonance

Resonance a molecule absorbs a photon only if the photon frequency matches a natural frequency of the molecule.

Natural frequencies

Natural frequencies atoms vibrate about their equilibrium separations like masses on springs, and molecules also rotate.

Infrared range

Infrared range for greenhouse gases these frequencies lie in the infrared band, so infrared radiation drives them strongly.

Energy gap

E = hf of the absorbed photon matches the gap between two vibrational energy levels.

Dipole needed

Dipole needed the vibration must change the charge distribution so that the molecule has an oscillating electric dipole.
CO₂ does this when bent or stretched unequally, and H₂O and CH₄ have many such modes.

N₂ and O₂

N₂ and O₂ their only vibration is symmetric along the bond, with no oscillating dipole, so they do not absorb infrared.

After absorption

After absorption the molecule re-emits the energy in a random direction or loses it in collisions, warming the air.

Section 5 of 9Absorption and re-emission

Absorption and re-emission

Atmosphere absorbs

Atmosphere absorbs only about 25% of the solar radiation, but about 80% of the re-emitted long-wave radiation.

Downward part

Downward part roughly half of the re-emitted energy goes down, so the surface must supply energy at a higher rate to balance.

Habitable temperature

Habitable temperature absorbed radiation keeps the Earth at a habitable temperature.

Imbalance

Imbalance a change in composition changes the mean surface temperature.

Extra gas

Extra gas increasing the concentration increases absorption, the enhanced greenhouse effect.

Section 6 of 9Concentrations and strength

Gas	Approximate concentration
H₂O	0.1 to 4% (variable)
CO₂	about 0.042%
CH₄	about 1.9 ppm
N₂O	about 0.34 ppm

CO₂ trend

CO₂ trend it has risen from about 280 ppm before industrialisation to more than 420 ppm.

Per molecule

Per molecule methane and nitrous oxide absorb much more strongly than CO₂ but are present in far lower concentrations.

Three factors

Three factors the strength of absorption at wavelengths where Earth radiates, the amount present and how long it persists.

Saturated bands

Saturated bands they respond less to extra gas than spectral windows where the atmosphere is still partly transparent.

Section 7 of 9More from the specification

1Greenhouse gas radiative balance model

Greenhouse gas radiative balance model calculating infrared absorption and terrestrial re-radiation requires an atmospheric radiative equilibrium model.

2Solar Constant and Radiation Intensity

The solar constant represents the average incident radiant power per unit area normal to the Sun's rays received at the top of Earth's atmosphere.
- (S approx 1360 W/m²

3Planetary Albedo and Reflection

Albedo is the fraction of total incident solar radiation reflected back into space;
- surfaces like fresh snow exhibit high albedo (approx 0.8),
while dark ocean waters reflect very little.

4Emissivity and Black-Body Radiation

An ideal black body has an emissivity of 1, radiating maximum power according to P = emissivity * sigma * A * T⁴;
- real planetary surfaces radiate as grey bodies with emissivity less than one.

5Greenhouse Effect and Global Thermal Equilibrium

Earth radiates infrared radiation into space;
- greenhouse gases in the atmosphere absorb and re-emit this terrestrial radiation in all directions, elevating the average global surface temperature.

6Constant

Not changing.

7Intensity

Power per unit area perpendicular to the wave direction.

8Albedo

The ratio of total scattered power to total incident power

9Emissivity

The ratio of power radiated per unit area by a surface to that radiated by a black body at the same temperature

10Distance

Total length of path travelled, a scalar.

11Activity

Number of decays per second of a sample.

12Solar constant

The intensity of the Sun's radiation at the mean Earth–Sun distance on a surface perpendicular to the rays

Section 8 of 9Key terms

Term	Meaning
Greenhouse gas	Gas that absorbs the long-wave infrared radiation re-emitted by the Earth
Infrared radiation	Long-wavelength radiation emitted by the warm surface
Resonance	Strong absorption when the radiation frequency matches a natural frequency of a molecule
Dipole	Separation of positive and negative charge in a molecule
Vibrational energy level	Quantised energy state of the vibration of a molecule
Parts per million (ppm)	Measure of the concentration of a gas in the atmosphere

Section 9 of 9Exam technique

Saying greenhouse gases absorb visible light — they absorb infrared.

Saying nitrogen and oxygen are greenhouse gases — they do not absorb infrared.

Confusing ozone depletion with the greenhouse effect — depletion concerns ultraviolet absorption.

Thinking only human activity produces greenhouse gases — each has natural sources too.

Saying water vapour is controlled by human emissions — it responds to temperature as a feedback.

Last updated: 18 October 2026