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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

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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

Part 1 of 7Earth 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.

Part 2 of 7Model: 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.

Part 3 of 7Mechanism 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.

Part 4 of 7Why 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.

Part 5 of 7Enhanced 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.
GasHuman sources
Carbon dioxideBurning fossil fuels in power stations and vehicles, burning wood, deforestation
MethaneDecay of organic matter in manure, landfill and crops
Nitrous oxideArtificial fertilisers and burning fossil fuels

Part 6 of 7Feedback 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.

Part 7 of 7More 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.

Key terms

TermMeaning
Greenhouse effectWarming of the surface because the atmosphere absorbs and re-emits outgoing infrared
Enhanced greenhouse effectExtra warming from greenhouse gases added by human activity
Radiative forcingChange in the energy balance caused by a change in greenhouse gas concentration
Positive feedbackProcess that amplifies an initial change
ResonanceStrong absorption when radiation frequency matches a molecular natural frequency
Equilibrium temperatureTemperature at which absorbed and emitted powers are equal

Avoid these mistakes

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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