Greenhouse effect practice questions.

The solar constant is about 1360 W/m², yet the mean intensity of solar radiation arriving at Earth, averaged over the whole surface, is about 340 W/m². Which statement explains this factor of four?

• Half of Earth is in darkness at any instant, and clouds block half of the remaining radiation
• The atmosphere reflects and absorbs a full three quarters of all the incoming solar radiation before any of it can reach the surface
• Earth intercepts solar radiation over its cross-sectional area πR², but this power is shared over its total surface area 4πR²
• Earth's albedo reduces the incident intensity by a factor of four

Answer: Earth intercepts solar radiation over its cross-sectional area πR², but this power is shared over its total surface area 4πR²

The planet presents a disc of area πR² to the Sun, but the intercepted power is averaged over the full sphere of area 4πR², giving 1360/4 = 340 W/m². Albedo and day–night effects are separate considerations.

A region of Earth receives solar radiation of average intensity 340 W/m² and scatters 102 W/m² of it back into space. Calculate the albedo of this region.

• 0.30
• 0.70
• 0.43
• 3.3

Answer: 0.30

Albedo = scattered intensity / incident intensity = 102/340 = 0.30. Choosing 0.70 confuses albedo with the absorbed fraction.

Which of the following lists contains only greenhouse gases?

• Nitrogen, oxygen and gaseous carbon dioxide
• Methane, water vapour and nitrous oxide
• Argon, methane and oxygen
• Ozone, nitrogen and water vapour

Answer: Methane, water vapour and nitrous oxide

The main greenhouse gases are carbon dioxide, methane, water vapour and nitrous oxide. Nitrogen, oxygen and argon do not absorb infrared radiation significantly despite making up most of the atmosphere.

Which statement best explains why greenhouse gas molecules absorb infrared radiation emitted by Earth's surface?

• They reflect infrared photons back towards the surface
• Their electrons are ionised by infrared photons
• They absorb high-energy ultraviolet radiation from the Sun and then re-emit that energy as lower-energy infrared radiation towards the ground
• The natural frequencies of vibration of the molecules lie in the infrared region, so the photons are absorbed by resonance

Answer: The natural frequencies of vibration of the molecules lie in the infrared region, so the photons are absorbed by resonance

Greenhouse gas molecules have vibrational modes whose natural frequencies match infrared frequencies; absorption occurs by resonance, not by reflection or ionisation.

A planet absorbs solar radiation at an average rate of 240 W per square metre of its surface. Treating the planet as a black body in thermal equilibrium, its mean surface temperature is closest to (σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴):

• 288 K
• 255 K
• 303 K
• 240 K

Answer: 255 K

In equilibrium σT⁴ = 240 W/m², so T = (240 / 5.67 × 10⁻⁸)^(1/4) ≈ 255 K. Earth's actual mean surface temperature of about 288 K is higher because of the greenhouse effect.

Suppose the fraction of Earth covered by ice and snow increases significantly while the solar constant remains unchanged. What is the expected effect on the mean surface temperature at the new equilibrium?

• It decreases, because the higher albedo means more solar radiation is scattered back into space
• It increases, because ice and snow emit much more infrared radiation than open liquid ocean water does
• It is unchanged, because the solar constant has not changed
• It increases, because the albedo of the planet decreases

Answer: It decreases, because the higher albedo means more solar radiation is scattered back into space

Ice and snow have a high albedo, so more incident radiation is scattered and less is absorbed; the equilibrium temperature therefore falls. Cooling then allows more ice to form, which is why the ice–albedo feedback is a positive feedback.