Causes of global climate change practice questions.

Country X emits 900 million t CO₂e per year and has 300 million inhabitants; Country Y emits 200 million t CO₂e and has 20 million inhabitants (hypothetical data). Which conclusion is supported by these figures?

• Country Y's emissions per person are higher than Country X's
• Country X's emissions per person are more than four times Country Y's
• Country Y contributes more to global emissions in total than Country X
• Country X must have the more energy-intensive economy of the two

Answer: Country Y's emissions per person are higher than Country X's

900/300 gives 3 t CO₂e per person against 200/20 giving 10 t, so the smaller emitter has the higher per-capita figure. Totals answer questions about global responsibility, per-capita figures answer questions about lifestyles.

Which statement correctly explains how greenhouse gases raise the temperature of the lower atmosphere?

• They absorb incoming shortwave radiation before it reaches the ground surface
• They absorb outgoing longwave radiation and re-emit some back to the surface
• They thin the ozone layer so more ultraviolet radiation reaches the surface
• They stop warm surface air from rising and mixing with the upper atmosphere

Answer: They absorb outgoing longwave radiation and re-emit some back to the surface

Greenhouse gases are largely transparent to incoming shortwave radiation but absorb the longwave radiation the warmed surface emits, re-radiating part of it downwards. The typical error is thinking they block incoming sunlight, or that the mechanism works through the ozone layer.

Which of these surfaces reflects the highest proportion of the solar radiation that strikes it?

• Open ocean
• Coniferous forest
• Dry asphalt
• Fresh snow

Answer: Fresh snow

Fresh snow has an albedo of about 0.8–0.9, reflecting most insolation, while open ocean absorbs almost all of it (albedo below 0.1). The usual error is assuming water is highly reflective because of surface glare — glare is a narrow-angle effect, not a high albedo.

A student writes: 'Ice cores show that in past glacial cycles carbon dioxide rose after temperature rose, so temperature drives carbon dioxide and today's emissions cannot be warming the planet.' Which single fault makes this reasoning unsafe?

• Ice cores cannot measure past carbon dioxide, so the stated evidence does not exist
• Carbon dioxide is not a greenhouse gas at the concentrations found in ice cores
• Temperature and carbon dioxide were never measured over the same period in cores
• The past lag shows carbon dioxide amplifying an orbital trigger, not that it cannot force warming

Answer: The past lag shows carbon dioxide amplifying an orbital trigger, not that it cannot force warming

In a glacial cycle the trigger is orbital, and the carbon dioxide released by a warming ocean then amplifies it as a positive feedback. A gas that works as a feedback in one situation still works as a forcing when humans add it, and in the modern record the concentration rise clearly precedes the warming.

In the national greenhouse gas inventories that countries report to the UNFCCC, how are emissions from fuel loaded for international shipping and aviation treated?

• They are added to the national total of the country where the fuel was loaded
• They are reported separately as international bunkers, outside every national total
• They are added to the national total of the operator's country of registration
• They are divided equally between the country of departure and the country of arrival

Answer: They are reported separately as international bunkers, outside every national total

Because no state's inventory carries them, these emissions are negotiated through the IMO and ICAO rather than through national pledges. The usual error is to assume that every tonne emitted must appear inside somebody's target.

Cutting methane emissions slows warming within about two decades, while an equal cut in tonnes of carbon dioxide takes far longer to show. Which property of methane explains the difference?

• Methane absorbs outgoing radiation at wavelengths carbon dioxide leaves untouched
• Methane is emitted in far smaller quantities than carbon dioxide is
• Methane is removed from the air within about a decade of being emitted
• Methane is heavier than air and settles out of the atmosphere quickly

Answer: Methane is removed from the air within about a decade of being emitted

The strength of the effect per tonne (global warming potential) and the speed of the response (atmospheric lifetime) are different properties. Methane's short lifetime means concentrations fall soon after emissions do, whereas a large share of carbon dioxide stays in circulation for centuries.