IB Physics SL B.2 Greenhouse effect Questions

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Practise IB Physics SL B.2 by building surface–atmosphere energy balances from solar input, albedo, thermal emission and greenhouse absorption.

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

Exam points & syllabus

Exam points

Choose the surface or whole-planet boundary before balancing radiation; include atmospheric return only in the balance where that arrow enters.
Use εσT⁴ for emitted surface flux; distinguish lower emissivity from a lower temperature and keep the model’s spectral assumptions explicit.
Calculate albedo with total incident radiation in the denominator; reflected sunlight and thermal emission are different outgoing components.
At constant temperature in a simple model, emitted power equals absorbed power; use α for reflection and 1 − α for absorption.
Explain Earth’s albedo as a spatial and temporal average, giving distinct factors such as cloud cover and latitude when two reasons are requested.
Define the solar constant as incident solar power per area at Earth orbit, and explain its approximation through source output or orbital distance.
Derive global mean input from the projected disk πR² and full sphere 4πR²; explain the area ratio rather than only writing S/4.
Apply reflection and averaging to the correct incident flux once: mean absorbed intensity is the body’s orbital flux × (1 − α)/4.
Separate a locally overhead surface from a global mean, and explain atmospheric reflection or changing illumination when surface flux differs from orbital flux.
For equilibrium temperature, equate absorbed and emitted power with the projected area, albedo and emissivity, then take the fourth root.
In an absorption-limited melt estimate, divide complete warming-plus-fusion energy by non-reflected solar power and retain the stated no-loss assumptions.
Identify the main greenhouse gases and match the requested source–gas pairs; distinguish oxygen from CH₄, H₂O vapour, CO₂ and N₂O.
Explain absorption using frequency matching to resonance or photon energy matching to a molecular-level gap, followed by emission in all directions.
Trace outgoing surface infrared through atmospheric absorption and partial return, distinguishing it from incoming short-wave sunlight or reflection.
Link concentration changes to the amount of surface infrared absorbed and returned; use the particular low-pressure or concentration model stated.
Distinguish human enhancement from the natural effect, identify fossil-fuel burning as a primary cause and explain the added CO₂ radiation chain.
Compare mitigation choices through their emission mechanism: capture/storage and lower-direct-carbon generation differ from higher-carbon fuel swaps.
Connect ice or snow area to reflected sunlight in a conditional greenhouse–albedo chain; fewer emissions do not automatically mean immediate cooling.

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

Choose the surface boundary and balance absorbed sunlight plus atmospheric back radiation against surface emission; subtract reflected sunlight.

[Maximum number: 1]

In an energy-balance climate model, the power of the incoming radiation over an area A is $P_{\mathrm{i}}$ and the power of the outgoing radiation over the same area is $P_{\mathrm{o}}$ . The surface heat capacity is $C_{\mathrm{s}}$ . What is the time taken to increase the temperature of the area by $\theta$ ?

$\frac{\left(P_{\mathrm{i}}-P_{\mathrm{o}}\right)}{C_{\mathrm{s}} \theta}$

$\frac{C_{\mathrm{s}} \theta}{\left(P_{\mathrm{i}}-P_{\mathrm{o}}\right)}$

$\frac{A C_{\mathrm{s}} \theta}{\left(P_{\mathrm{i}}-P_{\mathrm{o}}\right)}$

$\frac{A\left(P_{\mathrm{i}}-P_{\mathrm{o}}\right)}{C_{\mathrm{s}} \theta}$

Exam points

For a grey body at the same temperature, lower emissivity scales emitted intensity down; the model’s peak wavelength remains unchanged.
For equal emitted surface intensities, combine εT⁴ equality with Wien’s temperature ratio; twice the peak wavelength requires 16 times emissivity.
Derive equilibrium temperature by equating εσAT⁴ to stellar flux × projected area × (1 − albedo), then take the fourth root.
Find downward atmospheric radiation from the surface energy balance: first calculate εσT⁴, then subtract the absorbed solar intensity.

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

Exam points

Calculate albedo from reflected solar power divided by total incident solar power; do not include thermally emitted radiation as reflected light.
If absorbed and reflected intensities are given separately, add them to recover incident intensity before calculating albedo.
At constant temperature in the stated simple model, emitted flux equals absorbed flux; reflected flux is the remainder of incoming sunlight.
For equal incident intensity, compare surface albedos through reflected-intensity ratios; distinguish α from reflected-to-absorbed ratio α/(1 − α).
Apply the non-reflected fraction to the specified local sunlight: overhead intensity after atmospheric albedo 0.25 is 0.75S.
Estimate a melt time from total warming-plus-fusion energy divided by non-reflected incident power, retaining the model’s stated loss assumptions.
When reflective ice is replaced by darker ocean, explain the lower regional albedo by comparing the two surfaces’ reflected fractions.

[Maximum number: 1]

The Moon has no atmosphere and orbits the Earth. The diagram shows the Moon with rays of light from the Sun that are incident at $90^{\circ}$ to the axis of rotation of the Moon.

The albedo of the Earth's atmosphere is 0.28 . Outline why the maximum temperature of a black body on the Earth when the Sun is overhead is less than that at point A on the Moon.

Exam points

Explain why Earth’s albedo is an average: it varies with latitude and cloud cover and can change daily; give distinct factors when two are requested.

[Maximum number: 2]

This question is in two parts. Part 1 is about solar radiation and the greenhouse effect. Part 2 is about a mass on a spring.

Part 1 Solar radiation and the greenhouse effect
The following data are available.

State two reasons why the solar power incident per unit area at a point on the surface of the Earth is likely to be different from your answer in (c).

1 :
2:

Exam points

Define the solar constant as solar power per unit area received at Earth’s orbital distance, before atmospheric losses or global surface averaging.
Explain why the solar constant is approximate through changing solar emission or the varying Earth–Sun distance in an elliptical 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.

Exam points

Derive the global mean S/4 by comparing the intercepted disk area πR² with the full spherical area 4πR²; explain intensity as power per area.
Find mean absorbed intensity by multiplying orbital solar intensity by (1 − albedo)/4; show reflection and geometric averaging separately.
Use the globally averaged absorbed intensity in εσT⁴ balance and take the fourth root; keep this model distinct from a local midday 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, water vapour H₂O, CO₂ and N₂O as greenhouse gases, distinguish O₂, and supply two valid examples when two are requested.
Link forest loss, grazing-animal methane and fertiliser-related nitrous oxide to the named atmospheric greenhouse-gas change.

[Maximum number: 1]

This question is in two parts. Part 1 is about Newton's laws and momentum. Part 2 is about the greenhouse effect.

Part 1 Newton's laws and momentum

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 any two of the above.

Exam points

Describe absorption of outgoing terrestrial infrared followed by emission in all directions; some returns to the surface, while some travels elsewhere.
Explain molecular absorption through an infrared photon matching an energy-level gap, or through the resonance model, then state all-direction emission.
Use the wavelength contrast: solar radiation is mainly shorter-wave, while the cooler surface emits longer-wave infrared absorbed by greenhouse gases.
Explain a warmer surface than a no-greenhouse model through absorption and partial return of surface infrared, naming a greenhouse gas when asked.
For the stated concentration increase, link more absorbed surface infrared to a greater downward atmospheric intensity I₂.
In the given low-pressure Mars model, connect fewer greenhouse molecules with less absorbed and returned radiation despite the high CO₂ proportion.

[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

Identify infrared resonance as the relevant molecular property; distinguish it from ultraviolet resonance, molecular size or total electric charge.
Link Earth’s infrared emission to molecular absorption, then all-direction atmospheric emission with partial return to the surface and warming.

[Maximum number: 3]

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.

Outline the mechanism by which greenhouse gases contribute to global warming.

Exam points

Define the enhanced greenhouse effect as human-driven augmentation of the natural effect; identify fossil-fuel burning as a primary cause.
Explain the fossil-fuel warming chain: increased atmospheric CO₂ leads to more absorption of surface infrared and more radiation returned to the surface.
Recognise deforestation as a human warming influence, and compare specified fuel changes through their effect on greenhouse-gas emissions.
Distinguish measures that reduce greenhouse emissions: capture and storage or lower-direct-carbon generation differ from switching gas to oil.
When greenhouse-gas concentration is stated to decrease, connect less infrared returned to the surface with cooling towards a new equilibrium.
Build the suggested ice–albedo chain explicitly: changed greenhouse warming alters ice/snow cover, which changes the reflected fraction of sunlight.

[Maximum number: 2]

Outline why the burning of fossil fuels may lead to an increase in the average surface temperature of the Earth.
Part 2 A mass on a spring
An object is placed on a frictionless surface and attached to a light horizontal spring.

The other end of the spring is attached to a stationary point P . Air resistance is negligible. The equilibrium position is at O . The object is moved to position Y and released.

more $\mathrm{CO}_{2}$ /other named greenhouse gas released into the atmosphere;
hence more energy is absorbed and radiated back to Earth's surface;

Part 2 A mass on a spring

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