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Theme B — The Particulate Nature of Matter

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

5 min read 10 parts Lesson 18 of 44 1 quiz

📚 IB Physics SL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

B.2 Greenhouse Effect — IB SL Notes — IB PHYSICS SL notes: Model the Earth's energy balance using emissivity, albedo and the solar constant, and explain the greenhouse effect and its enhancement.Equilibrium — energy is conserved, and at equilibrium the Earth radiates as much powe…

  • Simple model
  • Emissivity e
  • Solar constant S
  • Mean intensity
  • Surface emission
  • Absorption

Part 1 of 8Energy balance

Equilibrium
  • Equilibrium energy is conserved, and at equilibrium the Earth radiates as much power as it absorbs.
Simple model
  • Simple model a one-layer atmosphere above the surface.
Imbalance
  • Imbalance if the incoming and outgoing energies differ, the temperature changes.

Part 2 of 8Emissivity and albedo

Emissivity e
  • Emissivity e the ratio of the power radiated per unit area to that of a black body at the same temperature, e = P/σAT4.
  • A black body has e = 1, and real bodies radiate P = eσAT4.
Albedo a
  • Albedo a the ratio of total scattered power to total incident power.
  • Earth's average is about 0.3.
  • It depends on cloud cover, ice, vegetation, latitude and incident angle.
  • Snow has a high albedo and oceans a low one.

Part 3 of 8Solar constant

Solar constant S
  • Solar constant S the intensity of the Sun's radiation at the Earth's mean orbital distance, measured perpendicular to the rays.
  • S ≈ 1.36 × 103 W m−2.
Mean intensity
  • Mean intensity the Earth intercepts SπR2 but has surface area 4πR2, so the mean intensity is S/4 ≈ 340 W m−2.

Part 4 of 8Energy balance without an atmosphere

Equation
  • Equation absorbed intensity (1 − a)S/4 = emitted σT4.
Earth
  • Earth with a = 0.30 this gives about 255 K, much colder than the observed mean of about 288 K.

Part 5 of 8Greenhouse effect

Surface emission
  • Surface emission the warm surface emits infrared radiation.
Absorption
  • Absorption greenhouse-gas molecules have vibrational energy-level differences that match infrared photon energies,
    • (natural frequencies)
  • so they absorb the radiation by resonance.
Re-emission
  • Re-emission they re-emit in all directions, including back to the surface.
Result
  • Result the surface must be warmer to restore balance.
Greenhouse gases
  • Greenhouse gases water vapour, carbon dioxide, methane and nitrous oxide.
  • Natural sources include respiration, volcanoes and wetlands.
  • Human sources include burning fossil fuels, agriculture and deforestation.

Part 6 of 8Enhanced greenhouse effect

FeatureNaturalEnhanced
CauseGases naturally presentExtra gases from human activity
EffectKeeps mean surface temperature near 288 KRaises the mean temperature further
Main gasesH2O, CO2CO2, CH4, N2O
Mechanism
  • Mechanism rising concentrations increase absorption of outgoing infrared and raise the equilibrium temperature.
Feedback
  • Feedback positive feedback includes melting ice lowering the albedo and warmer oceans releasing dissolved CO2.

Part 7 of 8Sequence of energy flows

1
The Sun emits mainly visible and short-wave radiation.
2
About 30% is scattered back to space (albedo) and the rest is absorbed by the surface and atmosphere.
3
The warm surface emits long-wave infrared radiation.
4
Greenhouse gases absorb part of this infrared and re-emit it in all directions.
5
The surface receives extra downward infrared and settles at a higher equilibrium temperature.

Part 8 of 8More from the specification

Albedo
  • Albedo the fraction of incoming radiation reflected by a surface, high for ice and low for ocean, affecting the global average temperature
Greenhouse gases
  • Greenhouse gases absorb infrared of longer wavelength re-emitted by the Earth, trapping energy
Ozone
  • Ozone absorbs ultraviolet light in the stratosphere, separate from the enhanced greenhouse effect
Energy balance
  • Energy balance equilibrium occurs when incoming equals outgoing power,
  • and fresh snow raises reflection, a positive feedback problem to analyse in balance problems
Wavelength
  • Distance between a point on one wave and the same point on the next wave

Key terms

TermMeaning
EmissivityThe ratio of power radiated per unit area by a surface to that radiated by a black body at the same temperature
AlbedoThe ratio of total scattered power to total incident power
Solar constantThe intensity of the Sun's radiation at the mean Earth–Sun distance on a surface perpendicular to the rays
Greenhouse gasA gas that absorbs and re-emits infrared radiation
Greenhouse effectWarming of a planet's surface because its atmosphere absorbs outgoing infrared and re-radiates part of it back
Enhanced greenhouse effectAdditional warming due to greenhouse gases released by human activity

Avoid these mistakes

1
Confusing albedo with emissivity — albedo is about reflection, emissivity about emission.
2
Using S instead of S/4 for the mean intensity — divide by 4.
3
Saying greenhouse gases absorb visible light — they absorb infrared.
4
Thinking the effect creates energy — it reduces the rate at which energy escapes.
5
Using °C in the Stefan–Boltzmann law — use kelvin.

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