Should We Cool the Earth?

Amy, Oliver, Colleen, Alistair and Mark mid-chat, in front of the geoengineering acronym guide

October 2026

Featuring: Oliver Morton, senior editor at The Economist and author of The Planet Remade; Dr Colleen Golja, atmospheric scientist at Imperial College London and co-director of CLARO; Dr Alistair Duffey, senior scientist at Reflective and visiting researcher at UCL; and Professor Mark Symes, programme director of ARIA’s £56.8m Exploring Climate Cooling programme.

As global temperatures continue to rise, solar geoengineering is moving from a largely theoretical debate towards serious scientific research. At October’s Earth Set, we explored what it might involve, what we know about the risks, and who would ever get to decide whether to use it.

Having inadvertently heated the planet, could we deliberately cool it by reflecting a small proportion of incoming solar energy back into space?

Could we actually cool the planet?

Oliver described geoengineering as a means of “decoupling climate outcomes from cumulative emissions”. Normally, the more greenhouse gases accumulate in the atmosphere, the warmer the planet becomes. Carbon dioxide removal changes that relationship by taking CO₂ back out; solar geoengineering would instead reduce some of the energy coming in.

Colleen framed the underlying problem through Earth’s Energy Imbalance (EEI). The Earth normally receives energy from the Sun and radiates energy back into space. Greenhouse gases reduce the amount of heat escaping, creating an imbalance; the planet then warms as it moves towards a new equilibrium. Because CO₂ accumulates and persists in the atmosphere, that imbalance reflects not simply what we are emitting today, but the cumulative effect of past emissions.

Solar geoengineering essentially intervenes on the other side of that equation: rather than removing the greenhouse gases trapping outgoing heat, it reduces the solar energy coming in. That is why it can lower temperatures without addressing the underlying accumulation of CO₂. The most studied technique is Stratospheric Aerosol Injection (SAI), introducing reflective particles high into the atmosphere. Large volcanic eruptions already demonstrate the underlying mechanism: sulphur dioxide injected into the stratosphere forms sulphate aerosols which reflect sunlight and temporarily cool the Earth. As Colleen put it, the effect is rather like putting “a pair of sunglasses” on the planet. Climate models agree that SAI could lower average surface temperatures, although it could also affect regional rainfall and climate patterns. Crucially, it would not remove the CO₂ already in the atmosphere: oceans would continue to acidify and the underlying greenhouse effect would remain.

Reflective has chosen to concentrate on SAI, principally using sulphur, because it believes it is the most plausible near-term option for global-scale cooling. ARIA is deliberately taking a broader approach. Its Exploring Climate Cooling programme is funding 22 teams to investigate several potential routes, alongside the modelling, monitoring and governance needed to understand their consequences. These include marine cloud brightening, using fine sea-salt spray to make low marine clouds more reflective; attempts to thicken Arctic sea ice by pumping seawater onto its surface during winter; and work on making existing ship tracks brighter and longer-lived. The programme is also investigating different materials that might be used for SAI, rather than assuming sulphur is the only option.

Why research it now?

Solar geoengineering has been scientifically plausible for decades, but often regarded as a subject best left alone. That is changing as temperatures rise and confidence that the world can remain below 1.5°C through emissions reductions alone has diminished. For Alistair, the case is one of “responsible urgency”. Policymakers in twenty or thirty years may want to know, for example, what large-scale SAI would do to the Indian monsoon. The argument for research now is that those answers should exist before they are urgently needed.

This is also why the distinction between research and deployment matters. None of the panel was arguing for deployment now; all argued, from different perspectives, that the consequences need to be much better understood.

The risk of an escape hatch

One of the strongest arguments against solar geoengineering is mitigation deterrence: if governments believe they have a relatively cheap way of suppressing rising temperatures, does that reduce the pressure to eliminate emissions? Oliver thought some mitigation deterrence would probably be inevitable if SRM were ever deployed. Permanent carbon removal remains extremely expensive; increasing the amount of solar cooling could be much cheaper. The danger is a world that continues to emit while masking an increasing proportion of the resulting warming.

That creates another problem: termination shock. If a world reliant on SRM stopped reflecting sunlight while atmospheric CO₂ remained high, temperatures could rise very rapidly. The greater the amount of warming being suppressed, the greater the risk. So SRM cannot simply substitute for decarbonisation. Large-scale, long-term use without reaching net zero could create an obligation for future generations to continue the intervention while dealing with the accumulated CO₂.

Who gets to decide?

Governance may be harder than the science.

Different interventions would have different regional effects. SAI undertaken in one hemisphere, for example, could shift tropical rain belts; marine cloud brightening intended to produce a local effect could have consequences elsewhere through interconnected weather systems. Colleen described ARIA-funded diplomatic simulations in which participants were given hypothetical climate crises involving SAI. The issue rapidly became “securitised” rather than remaining primarily a climate problem, countries treated it as a question of national security and geopolitical power. In all three simulations, no agreement was reached.

Oliver was sceptical that there would ever be a single global “we” making a deployment decision. In practice, a sufficiently powerful state might eventually decide that intervention was in its interests and that other countries were unlikely to prevent it, although any serious deployment would almost certainly involve extensive diplomacy.

The audience pushed this further: what if a private actor decided to intervene? What are China and the US doing? And could anyone meaningfully restrict a technology that might be relatively inexpensive compared with conventional climate action?

Public science or private technology?

That question also shaped the discussion about commercial involvement.

Mark drew a parallel with AI, where much of the leading technical knowledge is concentrated within a small number of private companies, leaving governments dependent on those companies for expertise. His argument was that solar geoengineering should develop differently: governments, scientists and regulators need a substantial body of open, shared knowledge before private actors acquire the capability to act. Researching geoengineering and governing geoengineering are therefore not necessarily opposing objectives; the former may be necessary for the latter.

There are some precedents for international governance. The Montreal Protocol, repeatedly cited during the discussion, has successfully coordinated global action on ozone-depleting substances and continues to assess the implications of solar geoengineering for the ozone layer. But the geopolitical challenge of deliberately altering global temperature would be of a different order.

When might intervention become justified?

Towards the end, the panel was asked what would need to happen before they could personally support intervention.

There was no common threshold. Colleen described reaching a point where the climate itself had become sufficiently unfamiliar that intervention no longer seemed the stranger option. Alistair wanted evidence that emissions were falling and the world was genuinely moving towards net zero, while also arguing that the views of countries most vulnerable to climate change should carry particular weight. Oliver offered perhaps the neatest formulation: the question changes when the world without geoengineering becomes stranger than the world with it.

No one on the panel was proposing that we cool the Earth now. But there was broad agreement on something more immediate: decisions about whether to intervene in the climate may arise sooner than we expect, and making them without sufficient science, monitoring or public capacity would be considerably more dangerous than doing the research now.

And yes, there were a lot of acronyms.