The 30^th United Nations Climate Conference (COP30) concluded on November 21 with limited progress. Reported national climate targets fell far short of Paris Agreement goals, at present suggesting a 2.5°C temperature increase.  As it stands, global efforts to combat climate change appear to be failing, and we are moving towards an uncertain future.

But what if the situation got so bad that someone decided to try and hack the global climate?

A recent survey of 120 leading climate scientists revealed that two-thirds of respondents anticipate atmospheric “tweaking” interventions by 2100, potentially initiated by a “rogue actor” like a private company, billionaire, or nation. Additionally, just over 20% agreed that such measures should be considered if global temperatures became certain to exceed 2°C.

The types of interventions addressed in this survey are referred to as “Solar Radiation Modification” (SRM), which aim to reduce the amount of solar energy absorbed by Earth by deflecting sunlight or allowing more infrared radiation to escape into space. SRM does not address the root causes of climate change, such as rising greenhouse gas levels, but rather seeks to mitigate the warming effect, at a local, regional or global level. As such, SRM could potentially act as a “band-aid” solution to limit severe global warming impacts until greenhouse gas concentrations are lowered to sustainable levels by other means.

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Although SRM approaches have been suggested for decades, most techniques remain theoretical or in early trial stages and are not yet part of official policy frameworks. However, interest in SRM has grown in recent years as significant progress in reducing global emissions has proved elusive, and challenges in deploying Carbon Dioxide Removal (CDR) timely and at scale have become more apparent to policy makers.

Potential SRM technologies have been discussed in reports by various international and scientific organizations and governmental agencies, including IPCC, UNEP, UNESCO, US National Academy of Sciences and the European Commission, and have been studied at several universities. Nevertheless, SRM remains controversial, with strong voices from the scientific community both for and against its research and potential future deployment.

What technologies are considered as SRM?

There are five broad categories of generally discussed SRM options, with large variations in scope, cost, technological maturity and potential risks. Presented roughly in order of increasing geographical scope, these include:

Surface brightening (SB): The aim of these techniques is to increase the surface brightness (albedo) of the land or sea, to increase their reflectivity. Methods include whitening urban areas (e.g. painting walls and roofs), growing more reflective crops, brightening oceans with foams or microbubbles, and covering deserts or unproductive areas with reflective or radiative materials. While some of these measures can be implemented easily at a local scale, studies indicate they are unlikely to counteract warming effectively on a large scale.

Cloud brightening: Cloud brightening aims to increase reflectivity and extent of shallow clouds, especially in regions with persistent low-level cloud cover. This would be achieved by ‘seeding’ the clouds with microscopic, nucleating particles to influence cloud droplet generation. Marine areas are ideal, in part due to their low background albedo. Readily available sea salt is often proposed as seeding material. While models suggest that substantial cooling at a global scale is possible, the approach could also be deployed for limited periods and regions, relevant for the protection of specific vulnerable ecosystems (e.g. coral reefs), or arctic sea ice. Nevertheless, the scientific foundation is still highly uncertain and there are significant technological challenges to address.

Cirrus cloud thinning: Cirrus clouds can contribute to warming by trapping more heat than they reflect. This method aims to reduce their heat-trapping effect by seeding them with specific microscopic particles. While there is conflicting evidence on its global cooling potential, it might also be applied to specific regions and periods. The scientific and technological understanding of this method is still in its infancy.

Stratospheric aerosol injection (SAI): Widely considered the leading SRM approach, SAI is generally agreed to be capable of achieving global cooling in principle. It involves injecting aerosols, like sulfur dioxide, into the stratosphere to mimic the cooling effect of large volcanic eruptions. Injection could be done with high-altitude aircraft. SAI has a longer lasting climate effect than cloud manipulation, spanning years. However, it requires sustained, costly efforts, with detailed planning and monitoring, and its effects can currently only be assessed with climate models.

Space mirrors: Theoretical studies suggest that we could use space-based mirrors to shield the Earth from incoming solar radiation, either in the form of large sunshades or thousands of smaller objects, positioned at low Earth orbit, geosynchronous orbit, or (gravitationally quasi-stable) Lagrange points. These shields would demand continuous maintenance and vast material resources. This approach is seen as an extremely costly, long-term option for global cooling and relies on technology that is currently far from available.

Damned if you do – but, damned if you don’t?

There are significant challenges and objections to applying SRM in climate mitigation. Predicting the beneficial and adverse impacts of large-scale SRM interventions on the environment and climate is difficult, and all proposed methods likely have wide-ranging, poorly understood side effects, which may include changes in rainfall patterns, negative consequences for the ecosystem, and decreased food security.

Large-scale outdoor experiments are often infeasible due to high risk, ethical concerns, and legal or public perception issues. Additionally, proper international governance and rules for SRM applications are lacking, and unilateral use could well lead to international conflict. Moreover, SRM is narrowly focused on reducing warming, and does not address other adverse effects of higher atmospheric CO₂ concentrations, such as ocean acidification or vegetation fertilization.

If SRM is used to control global temperatures, any interruption of this strategy could trigger a “termination shock”, where temperatures rapidly rise to levels dictated by high GHG concentrations, leading to dramatic consequences. Opponents also cite the “moral hazard”, where the potential benefits of SRM may weaken international resolve to cut emissions, the only long-term solution to the climate crisis. Finally, there is concern that costly development and deployment of SRM could divert resources from more effective emission-reduction efforts.

However, in certain scenarios the ethical argument could be flipped, as indicated by almost 50% of the survey respondents who were open to SRM measures if global temperatures reach critical levels.  For instance, should we abstain from using regional SRM measures in cases where this may plausibly save vulnerable ecosystems facing imminent loss before a permanent climate solution is devised?  It is possible that the danger of unmitigated climate change at some point might outgrow the danger of applying far-reaching climate fixes. 

A key challenge in SRM discussions is the lack of an accepted scientific framework to balance the risks of SRM with the substantial risks of poorly mitigated climate change on the other. Until scientific evidence can convincingly show that the benefits of SRM outweigh its risks, this problem will remain unresolved.