Responding to global climate change practice questions.

Injecting sulphate aerosols into the stratosphere has been proposed as a geoengineering technique. What would it do?

• It removes carbon dioxide from the atmosphere and stores it underground
• It increases the rate at which the oceans absorb carbon dioxide
• It traps outgoing longwave radiation more efficiently
• It reduces the shortwave radiation reaching the surface

Answer: It reduces the shortwave radiation reaching the surface

Solar radiation management masks warming by reflecting incoming insolation, but it leaves atmospheric carbon dioxide concentrations - and therefore ocean acidification - untouched, which is why it is not a substitute for emission cuts.

Which pair correctly distinguishes mitigation from adaptation?

• Mitigation reduces emissions or removes carbon; adaptation reduces the harm from changes already occurring
• Mitigation protects communities from impacts; adaptation reduces emissions at source
• Mitigation applies only to governments; adaptation applies only to individuals
• Mitigation is short-term action; adaptation is long-term action

Answer: Mitigation reduces emissions or removes carbon; adaptation reduces the harm from changes already occurring

The two act on different links in the chain: mitigation on the cause, adaptation on the consequence. They are complements rather than alternatives, since some warming is already committed and unlimited adaptation is impossible.

Under the Paris Agreement, states set their own nationally determined contributions rather than receiving assigned targets. What is the main strength and main weakness of this design?

• Near-universal participation, but no guarantee that the pledges add up to the goal
• Legally binding targets, but participation by only a few large emitters
• Automatic penalties for missed targets, but no reporting requirements
• Fixed reductions for every country, but no mechanism for review

Answer: Near-universal participation, but no guarantee that the pledges add up to the goal

Letting states choose their own contributions brought almost every country into the framework, which top-down assignment had failed to do, at the cost of ambition. The ratchet of five-yearly review and the transparency rules are the mechanisms meant to close that gap over time.

A government introduces a carbon price that rises on a published schedule over fifteen years. Why does the schedule matter as much as the level?

• Investment decisions with long lifetimes depend on the expected future price
• A rising price raises more revenue than a constant one in the first year
• Announcing a schedule removes the need to price carbon at all
• Only the current price affects the emissions of existing equipment

Answer: Investment decisions with long lifetimes depend on the expected future price

A power station, a factory or a building lasts for decades, so the decision being influenced is what to build now, and that depends on the price expected over the asset's life. Credibility is therefore part of the instrument, which is why frequent policy reversal weakens carbon pricing even when the current rate is high.

A student sorts four climate responses into mitigation and adaptation. Which one has been placed in the wrong category?

• Planting a mangrove belt along a delta shore to absorb surge energy — adaptation
• Fitting a cement works with carbon capture and storage — adaptation
• Replacing a coal-fired power station with offshore wind — mitigation
• Building a desalination plant because summer river flows have fallen — adaptation

Answer: Fitting a cement works with carbon capture and storage — adaptation

Mitigation acts on the causes by cutting emissions or removing greenhouse gases; adaptation acts on the consequences. Capture and storage keeps carbon dioxide out of the atmosphere, so it is mitigation however industrial it looks, while the mangrove belt only reduces damage from a surge that is coming anyway.

Which situation is a hard limit to adaptation, rather than a limit that more money or better governance could remove?

• A farming district that cannot afford the pumps needed for irrigation
• A city whose planning department lacks the staff to enforce a flood zone
• A low-lying atoll whose land surface would lie beneath the future sea
• A coastal region whose warning messages are not translated into local languages

Answer: A low-lying atoll whose land surface would lie beneath the future sea

Soft limits arise from missing finance, capacity or information and can in principle be lifted. A hard limit remains when no feasible action can avoid the loss, which is the case that low-lying island states put to the negotiations when they argue for loss and damage funding.