IB Physics HL B.2 Greenhouse effect Questions

SLSL HLHL

Model Earth's energy balance by tracking incoming solar radiation, reflection, surface emission and infrared absorption and re-emission by greenhouse gases.

99 questions in this syllabus scope, with question parts counted together.

Exam points & syllabus

Exam points

Choose a system boundary and balance all incoming and outgoing intensities to find an unknown surface or atmospheric radiation flux.
Explain constant temperature through continuing energy losses at a rate equal to input power, rather than a bare thermal-equilibrium label.
Scale black-body reference emission by emissivity using I = eσT⁴, keeping power per unit area distinct from total radiated power.
For equal emitted intensities, combine the emissivity–temperature relation with Wien’s law; a doubled peak wavelength implies a 16-fold emissivity ratio.
For the stated grey-body enclosure model, use net outward intensity eσ(Tbody⁴ − Twalls⁴), including absorption from the surroundings.
Derive equilibrium temperature by equating eσAT⁴ to stellar incident flux times projected area A/4 and the fraction 1 − albedo.
Infer atmospheric return radiation from surface emission minus the net outgoing intensity, showing all calculation stages where required.
Use albedo α for the reflected fraction and 1 − α for the absorbed fraction; the absorbed/reflected power ratio is (1 − α)/α.
Predict lower albedo and greater solar absorption after loss of reflective ice, and rank usual surface albedos as ocean < desert < snow.
Follow the actual ray path through successive reflections and multiply the reflected fractions, as 0.80 × 0.30 for snow then cloud.
Explain reduced direct surface solar heating through atmospheric scattering or reflection; distinguish it from infrared greenhouse absorption.
For melting under stated no-loss conditions, divide the required thermal energy by incident intensity × area × (1 − albedo).
Give distinct reasons Earth’s albedo is quoted as an average, such as cloud or weather variation and latitude.
Define the solar constant as solar power per unit area at Earth’s mean orbital distance, before atmospheric losses.
Explain solar-constant averaging through variations in solar output and Earth–Sun distance; distinguish these from axial-tilt effects on local sunlight.
Explain the global mean S/4 through a projected disc area πR² intercepting solar power that is averaged over the full spherical area 4πR².
Calculate global mean absorbed intensity as (1 − albedo)S/4, using the incident flux at the relevant planet’s orbit and showing intermediate steps.
Explain why a local ground-level intensity can differ from orbital solar flux, giving two distinct atmospheric or illumination factors.
Identify CH₄, H₂O, CO₂ and N₂O as principal greenhouse gases; distinguish O₂, and supply two correct names when two are requested.
Distinguish natural greenhouse-gas source or sink changes: vegetation loss can increase atmospheric CO₂, while greater ocean uptake removes it.
Contrast shorter-wave incoming solar radiation with longer-wave terrestrial radiation, which greenhouse molecules selectively absorb.
Explain absorption through infrared photon energy matching a molecular energy-level difference, or through the resonance model’s natural frequency.
Complete the radiation pathway with re-emission in all directions and the return of some radiation to the surface, raising its temperature.
Read wavelength-specific absorption dips in an atmospheric spectrum as evidence of selective infrared absorption.
In the stated Earth–Mars comparison, connect low Martian pressure to fewer absorbing molecules and less infrared radiation returned to the surface.
Calculate a molecular vibration frequency from f = 1/T and compare it with the approximate radiation ranges supplied in the model.
Define the enhanced greenhouse effect through human augmentation, linking additional greenhouse gases to a higher Earth surface temperature.
Identify fossil-fuel burning as the primary human cause among the listed options, rather than natural volcanism, solar activity or ice-loss feedback.
Explain fossil-fuel warming through added CO₂ and greater absorption and return of surface infrared radiation.
For reduced greenhouse-gas concentration, link less infrared returned towards Earth to a lower temperature at the new equilibrium.
Compare the listed mitigation changes by their effect on fossil-fuel emissions, including CO₂ capture and replacing gas generation with nuclear power.

Covered syllabus points

B.2.1—Conservation of energy B.2.2—Emissivity B.2.3—Albedo B.2.4—Earth’s albedo variation B.2.5—Solar constant S B.2.6—Mean solar intensity B.2.7—Greenhouse gases B.2.8—Infrared absorption B.2.9—Greenhouse models B.2.10—Enhanced greenhouse effect

Exam points

Balance every incoming and outgoing flux across the chosen surface to find atmospheric back-radiation or the extra loss needed for constant temperature.
Explain a steady temperature by stating that thermal energy is still transferred out, at a rate equal to the input power.

[Maximum number: 1]

The diagram shows a simple model of the energy balance in the Earth surface-atmosphere system. The intensities of the radiations are given.

What is the average intensity radiated by the atmosphere towards the surface?

$100 \mathrm{Wm}^{-2}$

$150 \mathrm{Wm}^{-2}$

$240 \mathrm{Wm}^{-2}$

$390 \mathrm{Wm}^{-2}$

Exam points

For equal emitted surface intensities, combine ε₁T₁⁴ = ε₂T₂⁴ with Wien’s inverse peak-wavelength ratio to determine the emissivity ratio.
Derive a planet’s equilibrium temperature by balancing eσAT⁴ with absorbed stellar flux times projected area A/4 and the factor 1 − albedo.
For a grey body inside a black enclosure, calculate net outward radiative intensity as eσ(Tbody⁴ − Twalls⁴), using absolute temperatures.
Calculate surface emission as eσT⁴, then subtract the net outgoing intensity to infer atmospheric radiation returned towards the surface.

[Maximum number: 3]

The diagram shows a simplified energy-balance model for the Earth surface-atmosphere system.

The following data are given:

\begin{aligned} \text { Average albedo of Earth } & =0.30 \\ \text { Average global temperature of the surface } & =288 \mathrm{~K} \\ \text { Average Earth-Sun distance } & =1.5 \times 10^{11} \mathrm{~m} \end{aligned}

Determine the average intensity re-radiated by the atmosphere towards the surface. Assume that the emissivity of the surface is 0.90 .

Emitted intensity $=5.67 \times 10^{-8} \times 0.90 \times 288^{4}\approx351 \mathrm{~W} \mathrm{~m}^{-2}$

Intensity leaving Earth $=238 \mathrm{~W} \mathrm{~m}^{-2}$

Re-radiated intensity $=351-238=113 \mathrm{~W} \mathrm{~m}^{-2}$

Marking guidance:

Ignore units as they are not required for the answer.

The steps in the calculation must be checked.

Accept outgoing intensity $=240 \mathrm{~W} \mathrm{~m}^{-2}$ for MP2.

Exam points

Use albedo as the reflected fraction and 1 − albedo as the absorbed fraction; form absorbed/reflected power as (1 − α)/α.
Predict that loss of reflective ice lowers albedo and increases absorbed solar energy when incident power is unchanged.
Rank the usual albedos of the listed surfaces as ocean < desert < snow.
Follow the drawn ray path and multiply the reflected fraction at each reflection; snow then cloud gives 0.80 × 0.30 of the incident intensity.
Explain reduced solar heating using the fraction reflected or scattered by the atmosphere, rather than describing reflection as absorption.
In a surface energy budget, add atmospheric back-radiation to solar intensity multiplied by 1 − albedo before equating input and surface emission.
Estimate melting time as required thermal energy divided by (1 − albedo) × incident intensity × illuminated area, using the stated heat-loss model.

[Maximum number: 1]

A planet has an albedo of 0.30 . A simplified energy balance for the planet is shown.

What is the intensity radiated by the surface of the planet?

$70 \mathrm{Wm}^{-2}$

$90 \mathrm{Wm}^{-2}$

$100 \mathrm{Wm}^{-2}$

$130 \mathrm{Wm}^{-2}$

Exam points

Explain why Earth’s albedo is quoted as an average by naming two variable factors, such as cloud cover or weather and latitude.

[Maximum number: 2]

This question is in two parts. Part 1 is about the use of renewable energy sources. Part 2 is about the gravitational potential of the Earth.

Energy resources such as La Rance tidal power station could replace the use of fossil fuels. This may result in an increase in the average albedo of Earth.

State two reasons why the albedo of Earth must be given as an average value.
1.
2.

Exam points

Define the solar constant as solar radiation power per unit area at Earth’s mean orbital distance, before atmospheric losses.
Explain why an average solar constant is used: solar output varies and Earth’s elliptical orbit changes the Earth–Sun distance.
Distinguish axial-tilt effects on local or seasonal illumination from changes in the solar constant at Earth’s orbit.

[Maximum number: 1]

The diagram shows a simplified energy-balance model for the Earth surface-atmosphere system.

The following data are given:

\begin{aligned} \text { Average albedo of Earth } & =0.30 \\ \text { Average global temperature of the surface } & =288 \mathrm{~K} \\ \text { Average Earth-Sun distance } & =1.5 \times 10^{11} \mathrm{~m} \end{aligned}

State what is meant by the solar constant.

The diagram shows a simplified energy-balance model for the Earth surface-atmosphere system.

The following data are given:

Exam points

Explain S/4 by dividing the power intercepted by a planet’s projected disc area πR² by its full surface area 4πR².
Find global mean absorbed solar intensity as (1 − albedo)S/4, using the incident flux at the planet’s own orbit and showing both factors.
In the stated whole-sphere equilibrium model, balance eσT⁴ with (1 − albedo)S/4 and take a fourth root to find the mean surface temperature.

[Maximum number: 2]

The diagram shows a simplified energy-balance model for the Earth surface-atmosphere system.

The following data are given:

\begin{aligned} \text { Average albedo of Earth } & =0.30 \\ \text { Average global temperature of the surface } & =288 \mathrm{~K} \\ \text { Average Earth-Sun distance } & =1.5 \times 10^{11} \mathrm{~m} \end{aligned}

Show that the average global intensity of radiation absorbed by the surface is about $240 \mathrm{Wm}^{-2}$ .

Average incoming intensity $=\frac{S}{4}$ « $=340 \mathrm{~W} \mathrm{~m}^{-2}$ »

Absorbed intensity $=(1-0.30) \times 340$ or $238 \mathrm{~W} \mathrm{~m}^{-2}$

The steps in the calculation must be
shown.

Exam points

Identify CH₄, H₂O, CO₂ and N₂O as greenhouse gases and distinguish them from O₂; supply two correct gases when the question requests two.
Relate reduced vegetation uptake and increased ocean CO₂ absorption to opposite changes in atmospheric CO₂ and warming, including natural vegetation loss.

[Maximum number: 1]

Nuclear fuels, unlike fossil fuels, produce no greenhouse gases.

Identify two greenhouse gases.

1.

2.

methane $/ \mathrm{CH}_{4}$ , water vapour $/ \mathrm{H}_{2} \mathrm{O}$ , carbon dioxide $/ \mathrm{CO}_{2}$ , nitrous oxide $/ \mathrm{N}_{2} \mathrm{O}$ ;
Award [1] for two of the above.

Exam points

Contrast shorter-wave incoming solar radiation with longer-wave radiation emitted by Earth, which greenhouse gases selectively absorb.
Explain infrared absorption when photon energy matches the difference between molecular energy levels.
Explain that absorbed surface infrared energy is re-emitted in all directions, so some radiation returns to the surface and raises its temperature.
Link increased greenhouse-gas concentration to greater absorption of surface infrared radiation and a larger radiation intensity returned to the surface.
In the stated Earth–Mars comparison, connect Mars’s low pressure to fewer absorbing molecules and less infrared radiation returned to its surface.
Use wavelength-specific dips in an atmospheric transmission spectrum as evidence of selective infrared absorption.

[Maximum number: 3]

There is a proposal to place a satellite in orbit around planet Mars.

The ratio $\frac{\text { distance of Mars from the Sun }}{\text { distance of Earth from the Sun }}=1.5$ .

The atmosphere of Mars is composed mainly of carbon dioxide and has a pressure less than 1 % of that on Earth. Outline why the mean temperature of Earth is strongly affected by gases in its atmosphere but that of Mars is not.

Exam points

Use the resonance model to link infrared absorption to a greenhouse molecule’s natural vibration frequency.
Alternatively explain absorption through infrared photon energy matching a difference between molecular energy levels.
Calculate f = 1/T for a molecular vibration and compare it with the radiation ranges supplied in the question’s approximate absorption model.
Complete the greenhouse explanation with Earth’s long-wave emission, selective molecular absorption and re-emission in all directions, partly back to Earth.

[Maximum number: 2]

The diagram shows a simplified energy-balance model for the Earth surface-atmosphere system.

The following data are given:

\begin{aligned} \text { Average albedo of Earth } & =0.30 \\ \text { Average global temperature of the surface } & =288 \mathrm{~K} \\ \text { Average Earth-Sun distance } & =1.5 \times 10^{11} \mathrm{~m} \end{aligned}

Outline the physical mechanism by which some of the radiation emitted by the surface is absorbed by greenhouse gases in the atmosphere and re-radiated towards the surface.

Exam points

Define the enhanced greenhouse effect as the increase due to human activity, linking added greenhouse gases to a higher surface temperature.
Identify fossil-fuel burning as the primary human cause among the listed options, distinguishing it from natural activity or ice-loss feedback.
Explain fossil-fuel warming through increased CO₂, greater absorption of surface infrared radiation and more radiation returned to the surface.
For reduced greenhouse-gas concentration, explain less infrared radiation returned to Earth and a lower temperature at the new equilibrium.
Compare the listed mitigation changes by their effect on fossil-fuel CO₂ emissions, including carbon capture and replacing gas generation with nuclear power.

[Maximum number: 2]

This question is about energy sources.

A small island is situated in the Arctic. The islanders require an electricity supply but have no fossil fuels on the island. It is suggested that wind generators should be used in combination with power stations using either oil or nuclear fuel.

The emissions from the oil-fired power station in (c) are likely to increase global warming by the enhanced greenhouse effect.

State what is meant by the enhanced greenhouse effect.

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