IB Physics SL B 2 Greenhouse Effect Questions

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

Exam points

Apply energy conservation to track solar energy absorbed, radiation emitted and energy re-emitted by the atmosphere.
Use the solar constant, albedo and planetary geometry to calculate mean incoming and absorbed intensity.
Apply emissivity and the Stefan–Boltzmann law to relate temperature to emitted intensity and energy balance.
Explain how greenhouse gases absorb infrared radiation at molecular resonances and re-emit it in all directions.
Use energy-balance models to explain the natural greenhouse effect and Earth's warmer surface.
Explain how changes in ice, cloud or surface albedo alter reflected and absorbed solar energy.
Explain how human-driven increases in greenhouse-gas concentrations enhance the greenhouse effect.

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

[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 $\theta$ ?

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

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

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

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

[Maximum number: 8]

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}

(a)

State what is meant by the solar constant.

[ 1 ]

(b)

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.

[ 2 ]

(c)

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

[ 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.

(d)

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

[ 3 ]

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.

[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.

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Covered syllabus points

Question 1

Question 2

Question (a)

Question (b)

Question (c)

Question (d)

Question 3