Theme B — The Particulate Nature of Matter
B.2.4 The Greenhouse Effect — IB Physics HL Revision Notes & Topical Questions
5 min read 9 parts Lesson 22 of 53 1 quiz
Lesson summary
B.2.4 The Greenhouse Effect — IB PHYSICS SL notes: Explain the greenhouse effect in terms of absorption and re-emission of infrared radiation, and describe the enhanced greenhouse effect and its evidence.Absorbed solar intensity — (S/4)(1 − a) = 238 W m−2 for Earth.Balance — a pl…
- Black body without atmosphere
- Natural greenhouse effect
- Incoming radiation
- Ultraviolet
- Energy conserved
- Glasshouse
Section 1 of 9Earth without an atmosphere
Earth without an atmosphere
Absorbed solar intensity
- (S/4)(1 − a) = 238 W m−2 for Earth.
Balance
- Balance a planet at constant temperature radiates the same average power.
Black body without atmosphere
- σT4 = 238 W m−2 gives T ≈ 255 K (−18 °C), below freezing.
Observed
- Observed the mean surface temperature is about 288 K (15 °C).
Natural greenhouse effect
- Natural greenhouse effect the extra 33 K makes the planet habitable.
Section 2 of 9Model: resonance and molecular energy levels
Incoming radiation
- Incoming radiation it is mainly ultraviolet and visible, and visible light passes through the atmosphere to be absorbed by the surface.
Ultraviolet
- Ultraviolet it is energetic enough to break bonds and is absorbed by ozone.
Infrared
- Infrared it makes atoms vibrate, and greenhouse gases have natural frequencies in the infrared.
Resonance
- Resonance absorbing infrared makes the molecules resonate and heat up.
Re-emission
- Re-emission they emit it back, some towards the surface.
Section 3 of 9Mechanism step by step
1
Short-wavelength solar radiation passes through the atmosphere; about 30% is reflected and the rest is absorbed by the atmosphere, clouds and mainly the ground and oceans.
2
The warm surface re-emits long-wavelength infrared, with a peak near 10 μm for a body at 288 K.
3
Greenhouse gases absorb part of this infrared and re-radiate it in all directions.
4
Roughly half of the re-radiated energy goes back down; the rest escapes to space.
5
The surface receives sunlight plus this downward infrared and warms until the total outgoing power balances the net incoming solar power.
1Energy conserved
- Energy conserved the surface is warmer because the atmosphere reduces the rate at which energy escapes, not because it creates energy.
2Numbers
- Numbers the power leaving the top of the atmosphere equals 238 W m−2, but the surface radiates about 390 W m−2.
Section 4 of 9Why it is called the greenhouse effect
Glasshouse
- Glasshouse glass is transparent to visible light but largely opaque to infrared, so the warmed interior cannot easily lose energy by radiation.
Limits of the analogy
- Limits of the analogy a real greenhouse also warms because the glass stops warm air leaving by convection.
Atmospheric effect
- Atmospheric effect it depends purely on absorption and re-emission of radiation by gases.
Section 5 of 9Enhanced greenhouse effect
Cause
- Cause human activity increases greenhouse gases.
- CO2 has risen from about 280 ppm to over 420 ppm since the Industrial Revolution.
- Methane and nitrous oxide have also risen.
Natural and enhanced
- Natural and enhanced the greenhouse effect arises from natural causes, but the enhanced effect arises from human activity.
Result
- Result less long-wave radiation escapes, so the planet absorbs more power than it emits until a new balance is reached.
Temperature rise
- Temperature rise the average global temperature has risen by more than 1 °C since pre-industrial times.
Radiative forcing
- Radiative forcing doubling the CO2 concentration changes the energy balance by about 3.7 W m−2.
| Gas | Human sources |
|---|---|
| Carbon dioxide | Burning fossil fuels in power stations and vehicles, burning wood, deforestation |
| Methane | Decay of organic matter in manure, landfill and crops |
| Nitrous oxide | Artificial fertilisers and burning fossil fuels |
Section 6 of 9Feedback and evidence
1
Positive feedback
- Positive feedback warmer air holds more water vapour, and melting ice lowers the albedo, so more sunlight is absorbed.
2
Negative feedback
- Negative feedback changes in cloud cover and more plant growth removing CO2.
3
Net feedback
- Net feedback overall it amplifies the response.
4
Measurements
- Measurements direct CO2 data since 1958.
5
Ice cores
- Ice cores records over 800 000 years show temperature and CO2 varying together.
6
Satellites
- Satellites they show reduced outgoing infrared in the CO2 absorption bands.
7
Other evidence
- Other evidence a global mean temperature about 1.1 °C above pre-industrial, shrinking glaciers and rising sea level.
- (partly thermal expansion and partly melting land ice)
8
Consequences
- Consequences more extreme weather, ecosystem change and risks to coastal communities.
Section 7 of 9More from the specification
1Planetary Radiation Balance Model
- In steady-state thermal equilibrium, the total solar radiation incident on and absorbed by a planet must be exactly equal to the total power emitted as longwave thermal radiation into space.
2Solar Constant and Inverse Square Law
- The solar radiation flux incident on Earth's upper atmosphere depends on solar luminosity and orbital distance via S = L / (4 pi d2, providing an average cross-sectional irradiance of S/4 across Earth's sphere.
3Surface Albedo and Blackbody Emissivity
- Albedo measures the fraction of incident light reflected into space,
- (such as high reflectivity by fresh ice or clouds)
- while surface emissivity e determines radiant power via Stefan-Boltzmann law P = e sigma A T4.
4Greenhouse Effect and Atmospheric Absorption
- Greenhouse gases absorb outgoing infrared radiation emitted by Earth's warmed surface and re-radiate thermal energy in all directions, warming the lower troposphere.
5Emissivity
- The ratio of power radiated per unit area by a surface to that radiated by a black body at the same temperature
6Distance
- Total length of path travelled, a scalar.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Greenhouse effect | Warming of the surface because the atmosphere absorbs and re-emits outgoing infrared |
| Enhanced greenhouse effect | Extra warming from greenhouse gases added by human activity |
| Radiative forcing | Change in the energy balance caused by a change in greenhouse gas concentration |
| Positive feedback | Process that amplifies an initial change |
| Resonance | Strong absorption when radiation frequency matches a molecular natural frequency |
| Equilibrium temperature | Temperature at which absorbed and emitted powers are equal |
Section 9 of 9Exam technique
1
Saying the atmosphere creates energy — it reduces the rate of energy loss.
2
Confusing the greenhouse effect with ozone depletion — they are separate issues.
3
Saying the effect is only human-made — the natural effect keeps the Earth habitable.
4
Saying greenhouse gases reflect infrared — they absorb and re-emit it.
5
Believing the greenhouse analogy is exact — a glasshouse also stops convection.
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