Geoengineering is the deliberate, large-scale alteration of Earth’s climate system. Some of it is sensible and already happening. Some of it exists only on paper and alarms the scientists who study it most closely. The examiner does not want you to pick a side — they want you to see clearly why the arguments on each side are strong.
📚 What you need to know
Climate mitigation technology splits into carbon reduction (less emitted) and carbon removal (taken back out).
Key areas: renewable energy, carbon capture and storage, energy efficiency, smart cities and digital technology, and agriculture and forestry innovations.
Geoengineering intentionally alters climate systems on a large scale, treating the symptoms of climate change rather than its causes.
Solar Radiation Management (SRM) reflects sunlight away: stratospheric aerosol injection, space mirrors, cloud brightening.
Carbon Dioxide Removal (CDR) takes carbon dioxide out: ocean fertilisation, direct air capture, BECCS.
Arguments for: rapid cooling, a backup if mitigation fails, offsetting impacts, driving innovation.
Arguments against: high uncertainty, unintended consequences, geopolitical conflict, moral hazard and high costs.
Mitigation technology first
Before the exotic ideas, the ordinary ones. Renewable energy technologies — solar, wind, hydro and geothermal — keep getting cheaper and more efficient, which is what makes replacing fossil fuels at large scale realistic rather than aspirational. Carbon capture and storage catches emissions from power stations and industry and stores them underground, with direct air capture pulling carbon dioxide straight from the air. Energy efficiency technologies cut consumption: smart buildings use sensors and software to manage lighting, heating and cooling, and electric vehicles replace petrol and diesel.
Smart cities use technology to reduce urban emissions — apps and sensors that help people find charging points or recycling, and public transport apps that shift journeys away from private cars. Agriculture and forestry innovations increase carbon storage and cut land-use emissions: precision agriculture uses sensors and drones to monitor crops and minimise fertiliser use, which reduces nitrous oxide release, and sustainable forestry manages forests scientifically to raise their carbon storage.
The two families of geoengineering
Every geoengineering proposal you will meet does one of two things. It either reduces the energy arriving, or it reduces the gas trapping it. That single split organises the entire topic.
The asymmetry matters. CDR fixes the actual problem, slowly and expensively. SRM masks the symptom, quickly and cheaply, while carbon dioxide keeps building up behind the mask.
Solar radiation management
Stratospheric aerosol injection. Releasing reflective particles such as sulphates high in the stratosphere so less sunlight reaches the surface. This is the most-discussed SRM idea because large volcanic eruptions do something similar and measurably cool the planet for a year or two.
Space mirrors. Large mirrors positioned in space to reflect sunlight away from Earth. Nothing is in operation; there have only been proposals and experiments. It would be enormously expensive and technologically complex, though in theory it could provide global cooling.
Cloud brightening. Spraying sea salt into clouds over the ocean to make them whiter and more reflective, intended to cool the sea surface below — which could, for example, slow coral bleaching on a reef.
Carbon dioxide removal
Ocean fertilisation. Adding nutrients such as iron to the ocean to boost algal growth, since photosynthesising algae absorb large amounts of carbon dioxide. The concerns are serious: disruption of marine ecosystems and unknown long-term effects.
Direct air capture (DAC). Machines that pull carbon dioxide straight from the air, which is then stored underground or used. Currently high-cost and energy-intensive, but developing.
BECCS — bioenergy with carbon capture and storage. Burning biomass for energy and capturing the carbon dioxide released. Because the plants absorbed carbon while growing and that carbon is then buried, BECCS can in principle produce negative emissions.
Why BECCS is the clever one on paper. Plants take carbon out of the air. Burn them and you would put it straight back — unless you capture it and store it underground. Do that and the net movement of carbon is out of the atmosphere. The catch is land: growing enough biomass to matter would compete with food production and with the forests we already need.
The problem with masking a symptom
SRM has a specific failure mode that is worth understanding properly, because it is the strongest single argument against it. Reflecting sunlight lowers the temperature but does nothing to the carbon dioxide, which keeps accumulating. If the programme were ever stopped — because of cost, war, or political disagreement — the masked warming would arrive all at once.
This is why SRM is described as a commitment rather than a solution: once started at scale, stopping becomes dangerous, so a future generation inherits an obligation it never agreed to.
Bring this into any evaluation of SRM and it will lift your answer. The technical objection is that SRM does not touch carbon dioxide, so ocean acidification carries on regardless. The deeper objection is about governance: who decides the global thermostat setting, and what happens when one country’s ideal temperature causes another country’s monsoon to fail? These questions have no technical answer at all.
Techniques at a glance
Technique
Family
How it works
Main concern
Stratospheric aerosol injection
SRM
Reflective particles released high in the atmosphere reduce incoming sunlight
May shift rainfall patterns and cause drought in some regions
Space mirrors
SRM
Orbiting mirrors reflect sunlight away before it reaches the atmosphere
Only proposals and experiments so far; extremely costly and complex
Cloud brightening
SRM
Sea salt sprayed into ocean clouds makes them whiter and more reflective
Local effects on weather are poorly understood
Ocean fertilisation
CDR
Added nutrients such as iron boost algal growth, which absorbs carbon dioxide
Could seriously disrupt marine ecosystems; long-term effects unknown
Direct air capture
CDR
Machines extract carbon dioxide from ambient air for storage or use
High cost and very high energy demand
BECCS
CDR
Biomass is burned for energy and the carbon dioxide is captured and stored
Requires large areas of land that compete with food and forest
The argument, both sides
For
Rapid cooling potential. SRM could in theory cool the planet quickly, helping avoid an immediate crisis.
A backup strategy. If emissions reduction fails to control warming, something else exists.
Offsetting impacts. Reducing extreme heat could slow polar ice melt and sea level rise.
Innovation. Research stimulates technological advance and forces serious discussion of global climate strategy.
Against
High uncertainty. Effects on weather, ecosystems and human health are not fully understood.
Unintended consequences. Stratospheric aerosols may alter rainfall patterns and cause drought in some regions.
Geopolitical conflict. Different countries would be affected differently, raising the question of who controls the system.
Moral hazard. If geoengineering looks like a fix, the incentive to cut emissions weakens.
Cost. Space mirrors and direct air capture are currently far too expensive to deploy at scale.
WORKED EXAMPLE
Distinguish between solar radiation management and carbon dioxide removal. [2]
Mark 1 — SRMSRM reflects incoming sunlight away from Earth to lower surface temperature, for example by injecting reflective aerosols into the stratosphere.Mark 2 — CDRCDR removes carbon dioxide already in the atmosphere and stores it, for example by direct air capture or BECCS.2 / 2One reduces energy in; the other reduces the gas that holds energy. Each with an example.
WORKED EXAMPLE
Evaluate the use of geoengineering as a response to climate change. [6]
Point 1 — define and frameGeoengineering deliberately alters climate systems at large scale, treating symptoms rather than the underlying cause of rising greenhouse gases.Point 2 — argument for speedSRM could cool the planet far faster than emissions cuts, which act only over decades, so it may reduce short-term risk.Point 3 — argument for insuranceIt offers a backup if mitigation fails, and CDR methods such as BECCS may be necessary to reach net zero in sectors that cannot decarbonise.Point 4 — scientific objectionEffects are highly uncertain: aerosols could shift rainfall patterns and cause regional drought, and SRM does nothing about ocean acidification.Point 5 — political objectionBecause countries would be affected unequally, control of any global system would be contested, and its apparent availability could reduce pressure to cut emissions.Point 6 — judgementOn balance, CDR has a defensible role within a mitigation strategy, while SRM is better viewed as an emergency measure of last resort rather than an alternative to reducing emissions.6 / 6A six-mark evaluate needs an actual judgement at the end. Sitting on the fence costs the final mark.
💡 Exam tip
Sort every technique into SRM or CDR before writing. Half the marks in this topic come from that classification being right.
Use the phrase “treats the symptoms, not the causes”. It is the syllabus framing and it earns credit.
Learn three SRM and three CDR examples. Six names is enough for any question here.
For evaluation, use termination shock, uneven regional effects and moral hazard. All three are discriminating points.
Remember SRM does nothing about ocean acidification, since the carbon dioxide is still there.
End evaluate answers with a clear judgement, and justify it in the same sentence.
⚠️ Common mix-up
Calling all geoengineering the same thing. SRM and CDR differ in mechanism, cost, speed and risk.
Treating SRM as a cure. It masks warming while the cause continues to build.
Confusing CCS with BECCS. CCS captures fossil emissions; BECCS captures biomass emissions and can therefore go negative.
Saying space mirrors are in use. They exist only as proposals and experiments.
Assuming ocean fertilisation is harmless because algae are natural. Deliberately altering ocean nutrient levels can disrupt whole food webs.
Forgetting the governance question. Deciding who is allowed to change the global climate is as hard as the technology.
Up next: Why Climate Intervention Is Hard — the belief, money, leadership and inequality barriers that stop good strategies being carried out, and the economic logic underneath all of them.
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