Solar geoengineering can reduce global warming without causing unacceptable environmental, political, or governance risks

What's this about?

People disagree about whether reflecting some sunlight could cool Earth without causing harms we cannot accept.

This plan could act fast, but we are not sure it would stay safe or fair.

What supporters say

• Good rules, open reports, world talks, and public input could lower some risks during tests and use.
• Reflecting sunlight could lower the highest heat soon, which might lessen some heat dangers.
• It might cut some climate harms at a fairly low direct cost, especially if Earth warms more than expected.

What critics say

• Cooling the whole world could bring harmful heat, rain, or light changes to some places.
• No clear world leader or group can fairly control this plan or punish harmful use.
• If people stopped the plan after years, warming could jump fast and hurt nature and people.
• We still do not know enough about its effects on air, rain, crops, oceans, and living things.

How to read this

The number of points on each side does not show who is right; check how strong the proof is.

The bottom line

Solar geoengineering might cool Earth fast and lower some dangers, but its safety remains unproven.

The evidence leans against using it now; it should not replace cuts in heat-trapping gases.

Solar geoengineering could cool the planet relatively quickly by reflecting some sunlight back into space. But whether it could do so without unacceptable environmental, political and governance risks remains unproven.

The case for

The strongest argument is speed. Studies assessed by the National Academies and the IPCC indicate that solar-radiation-modification methods could lower global average temperatures or offset some warming on relatively short timescales. That could reduce certain heat-related dangers if emissions cuts fall short or climate damage becomes more severe.1 However, this is a possible supplement to cutting greenhouse-gas emissions, not a replacement for it.

There is also a weaker economic argument. If the climate proves especially sensitive to greenhouse gases, retaining the option to use solar geoengineering could have value. That finding supports further research, but it does not show that real-world deployment would be affordable, safe or effective.

Governance could reduce some risks, particularly during research and any staged decision-making. The National Academies has recommended transparency, disclosure, international coordination, public engagement and oversight. Such safeguards could make experiments more accountable and reduce the chance of poorly monitored or unilateral action.3 But there is no proof that countries would accept, enforce or maintain these protections during deployment.

The case against

The strongest challenge is that global cooling would not produce uniformly safe local conditions. Models show that the location of any atmospheric injection and the chosen strategy would affect regional temperature and rainfall patterns. Agriculture could also be helped in some places and harmed in others, because the intervention would change temperature, precipitation and the amount and distribution of sunlight. A favorable global average, therefore, would not be enough to establish safety.4 (see Figure 1) (see Figure 2)

Important environmental effects remain uncertain. Atmospheric chemistry, ecosystems, agriculture, radiation and human health have not been fully studied at deployment scale. Existing evidence is largely based on models and is incomplete. The National Academies and the IPCC have identified possible effects on ozone and the wider climate system, but the absence of demonstrated catastrophe is not evidence that those risks are settled.

Deployment could also create long-term dependence. If geoengineering masked substantial warming and then stopped while greenhouse-gas concentrations remained high, temperatures could rise rapidly in a “termination shock.” Avoiding that outcome would require reliable funding, technology, monitoring and political cooperation for many decades.6

The political problem is equally serious. Solar geoengineering would affect a shared atmosphere, while distributing benefits, harms, responsibility and consent unevenly between countries. Unresolved questions include who could authorize deployment, how it would be monitored, who would be liable for damage, whether affected populations would receive compensation, and how poorer or more vulnerable countries would be represented.7 Existing international arrangements do not clearly answer these questions. Normalizing the technology could also weaken political support for emissions cuts or deepen existing power imbalances.

Research and deployment must be treated differently. The National Academies supports cautious and transparent research to reduce uncertainty, not deployment itself. Even research, critics warn, could influence political incentives and shift power between countries. Solar geoengineering also would not reverse all the effects of carbon dioxide: ocean acidification and other harms would remain, leaving mitigation and adaptation essential.

The bottom line

The evidence strongly supports the narrower conclusion that solar geoengineering could cool the planet and reduce some forms of warming relatively quickly. It also provides meaningful, though less certain, support for carefully governed research.

But the broader claim—that it could do this without unacceptable environmental, political or governance risks—is not established. The strongest evidence against the claim concerns uneven regional effects; other concerns, including ecological uncertainty, termination shock and unresolved international authority, are also significant. The overall balance is therefore cautious and unresolved rather than affirmative.

The central gap is not evidence that the technology could cool the planet. It is the lack of deployment-scale evidence and the absence of durable, legitimate institutions able to manage unequal harms, liability, compensation and long-term dependence. Whether those risks are “unacceptable” also involves a political and moral judgment that science alone cannot settle.