We need to start talking about Solar Radiation Modification

The UK Government is about to fund climate cooling research projects, adding to the growing International interest. What’s involved, why is it needed, what are the risks and how can it be governed?

Fifteen years ago the Royal Society published a report on Geoengineering the Climate¹. The UK Government is now about to fund a series of climate cooling research projects, adding to the growing interest from Universities, philanthropists and venture capitalists around the world. This article looks at what’s involved, why it is needed, how it could be deployed, what the risks might be and how it could be governed?²

The Earth is warming because the incoming solar radiation is not being balanced by the amount of re-radiated energy from the warming surface and the amount of reflected radiation from clouds and ice. This Earth Energy Imbalance has been steadily growing as we have been emitting greenhouse gases into the atmosphere. The trapped heat from our carbon dioxide, methane and nitrous oxide emissions act in two main ways, firstly they directly block long wave radiation from the Earth’s surfaces, which warms the climate, but secondly the warming feedbacks reduce the reflectiveness of the surface to incoming shortwave solar radiation through the melting of ice and the reduction of cloud cover. Both the direct warming and this feedback warming contribute to the overall energy imbalance and accelerating warming we are experiencing today.³

Since there is a dual aspect to the energy imbalance there are twin approaches to dealing with the problem. The most important and the one pursued through the Paris Agreement and the work of the UNFCCC and IPCC is reducing the concentration of greenhouse gases in the atmosphere. Principally this is to be done through reduction of emissions all the way to net-zero, followed by deployment of negative emissions technologies on a massive scale that will remove carbon from the atmosphere for long term storage. All the safe(ish) IPCC scenarios employ huge amounts of (as yet unavailable) carbon capture in an attempt to limit temperature rises. There is no doubt that this is the only solution to climate change and must be the priority, however huge the challenge.

There is however, another complimentary approach being discussed. That is to deploy methods to increase the reflectiveness or albedo of the Earth. By reflecting more of the sun’s radiation back into space, the energy imbalance can be lowered, so reducing the rate or even pausing the warming. According to the Royal Society report, if the reflectivity of the Earth was increased by 1% that would offset about 1ºC of surface warming.

This approach has many names; Solar Radiation Modification, Solar Radiation Management, Sunlight Reflection Methods (all helpful sharing the acronym SRM) and Solar Geoengineering.

Note I said these methods can reduce or pause the warming, this is because only stopping the buildup of greenhouse gasses can permanently stop the warming. SRM is a temporary measure since when it is ceased, for whatever reason, the climate will swiftly revert to the temperature level it would have been if the SRM had not been deployed at all. More of this ‘termination shock’ phenomena later. In this way SRM treats some, but not all, of the symptoms of climate change, but it is not a cure.

Types of Solar Radiation Modification

There are five main methods under discussion which reflect incoming sunlight at different elevations from all the way out in space to on the ground:

• Space based. Putting mirrors in space, normally at a position where the mirrors remain in-between the Sun and the Earth. The amount of reflection achieved depends on the total area of the mirrors, but you’d need a lot.

• Cirrus cloud altering. High level or cirrus clouds reflect heat back to the surface. Many are caused by airplane contrails which we’ve all noticed can turn a perfect blue sky morning into a hazy hot afternoon. Methods are proposed to reduce cirrus clouds by adding particles to thin the clouds, but there is a lot of uncertainty.

• Marine cloud brightening. Ship tracks are observed when ocean going shipping routes become cloudy due to high levels of pollution in their exhaust gases. Principally sulphur dioxide, these pollution particles make for more small water droplets in the clouds making them brighter and more reflective to sunlight. This is reducing now due to shipping regulations designed to prevent this pollution which is harmful to health. The principle, however, could be emulated by spraying fine saltwater spray into the air from ships, enhancing the clouds that would then form. There are however unresolved technical challenges as well as uncertainty about the impact of cloud changes on local and regional weather.

• Surface brightening. Measures that increase the albedo of the Earth’s surface locally such as planting slightly more reflective crops, building with brighter materials, or brightening and thickening sea ice. These ideas might have some potential to alleviate local impacts, but none could be scaled up to have a substantial global cooling effect.

• Stratospheric Aerosol Injection (SAI). Large volcanos such as the 1991 Mount Pinatubo eruption shot 20 million tonnes of sulphate particles into the stratosphere in 12 hours and the earth cooled by a measurable amount for about 18 months. SAI would involve specially built aeroplanes spraying millions of tonnes of sulphate aerosols into the air at very high altitudes to form a global aerosol cloud which would reflect incoming sunlight.

If you are new to this subject, the last one, SAI, might seem like the most outlandish but it is actually the clear front runner. A huge amount of knowledge has been built up about the science and chemistry, both from volcanos and tropospheric pollution reduction efforts. It is also technically doable assuming planes and engines can be developed for the job. For this reason SAI will be considered as the intended method for the rest of this article.

The case for SRM and when could it be used?

Reaching net-zero emissions will stop further warming after a decade or two once the feedbacks settle out. It will not return the planet to pre-industrial conditions, or even to the relatively calm and cool conditions we experience today. It will be slightly worse than when net-zero is achieved, so with all likelihood global temperatures will be upwards of +3ºC, even if we do everything countries have currently committed to. Massive amounts of carbon dioxide removal (CDR) will need to follow for centuries to return the planet to +1.5ºC or lower.

Only SRM has the potential to reduce temperatures on a decadal timescale. An important point to note however is that it will not return the climate to the old normal since the forcing of the climate will be different. SRM works from the top-down while the climate is being changed from the bottom up. It will also not reduce the impact of ocean acidification as it does nothing to reduce CO₂ levels in the atmosphere.

SRM will also only work as part of a planned net-zero and negative emissions strategy. The diagram below from SRM360.org explains why.

With no emissions cuts, the temperature keeps rising following the red line with all the risks associated. Deploying SRM to keep the temperature at say +2ºC would see it needing to be increased in line with the accumulated emissions forever. At some stage though it would inevitably stop. The sulphate particles would rain out over the course of a year or two and then expose the Earth to the full forcing of the accumulated greenhouse gases. Temperatures would rocket up as a result, causing absolute chaos in terms of extreme weather as the climate struggled to find an equilibrium. Studies show that the impacts of such a ‘termination shock’ would be worse than if the SRM had not been carried out in the first place. This would be partly due to the lack of adaptation measures that would have otherwise been built up under gradual warming.

The purple line indicates the climate temperature if net-zero is achieved. Again, there is no point deploying SRM here as it would again be required to be deployed forever, with the same termination shock when it was stopped.

The blue line shows reaching net-zero and then deploying carbon dioxide removal (CDR) to bring the forcing to a target level. Under this scenario the amount of SRM would start high but then reduce down to zero as the CDR brought down atmospheric concentrations. There would be no termination shock as long as the CDR was effective. This is the only safe(ish) SRM scenario.

There is an issue here as well though. The blue line implies starting SRM quickly and with rapid strengthening up to point where net-zero is achieved, whereas a more sensible approach would see a slow ramp-up of SRM that would not be noticed straightaway, but could be scientifically analysed for efficacy as well as for side effects. Ramping up slowly, it would be decades before the levels had sufficient impact on rising temperatures. Projecting into the future, we would be very lucky to be able to even keep temperatures at +2ºC given the current accelerating warming.

Only under a scenario with rapid decarbonisation and scaling carbon removal does it makes sense to deploy SRM for the few centuries until CO₂ was reduced to the desired level. This is known as peak shaving⁴. It has the advantage of being deployed for a finite period, it can help to reduce the risk of triggering tipping elements and it would actually reduce the time required to reach an equilibrium temperature since less heat would accumulate in the oceans while it was being deployed.

This would be the rational approach, but there is a risk that as tipping points get closer and with, let’s be honest, a near failure of efforts to reduce emissions, SRM is one of only a few things that could rescue climate politics. Simulations show that SAI would indeed have the effect of reversing AMOC weakening, if one was detected in time.⁵ The graph below, from Bednarz et al, shows the recovery of AMOC strength and density when SAI is deployed at different latitudes. The black line is the projected decline under the moderate SSP2-4.5 scenario with no interventions.

Up until recently SRM has belonged to the list of topics which were completely off the table in terms of climate politics. Because economic de-growth and lifestyle curtailments remain off the table, speculative large scale technologies become the answer with all scenarios and plans including huge amounts of carbon capture and storage, using yet to be developed technologies.

Geoengineering in the form of SRM is likely to be the next card that is played in order to provide the illusion that business as usual can persist. SAI will be the first of these as its simple, cheap and naturally inspired.

What are the risks?

The risks of SAI fall into two types, physical risks and socio-political risks.

Currently humanity dumps 100 million tonnes of sulphur aerosols into the lower atmosphere each year through the burning of fossil fuels. This pollution is known to kill between 5 and 8 million people a year. Efforts to reduce this pollution are ongoing and is in part leading to the acceleration of climate change, since the cooling this pollution was providing is being reduced. The areas where the reductions are happening the fastest are also the regions warming the quickest such as Europe and the northern oceans.

To achieve the same level of cooling in the stratosphere we would only need 2-5% of the current sulphate emissions. The reason for this is that in the stratosphere the particles last much longer. Up to two years compared with a few weeks in the lower atmosphere. The coverage would also be global rather than regional, diluting the pollution effect on highly populated areas. In this way, stratospheric injection would cause about 1,000 times less harm in terms of human health than current fossil fuel pollution.

Although based on experience from volcanic eruptions, those last for days, not years decades or centuries. The physics and chemistry of long duration injection is much more in doubt and could lead to unknowable and unforeseen climate effects.

Since the mechanism of SRM is not the same as reducing greenhouse gases, there are important implications for the climate system. One of the main issues is that it does not alleviate growing ocean acidification which is a significant threat to the marine ecosystem and marine carbon sink capability.

Weather changes and the position of global rain patterns could also be affected, moving tropical rain bands from areas currently receiving rain to areas currently not. This would have profound implications for agriculture potentially affecting billions that live in monsoon areas as well as leading to catastrophic flooding in currently arid areas. Which areas are affected and by how much depends on the stratospheric distribution. Since the aerosols migrate to the poles, to keep a balance, injection would be required in both hemispheres, but research is still needed to examine the ‘best’ latitudes and altitudes against the ‘worst’ side effects.

The effect of sulphate aerosols on the ozone layer is also a significant concern. Sulphates degrade ozone to the extent that deployment could put back the recovery achieved since the Montreal Protocol by 35 years.

It is however, often suggested that the physical risks of doing SRM are lower than the risks of not doing SRM and letting climate change run its course.

The most cited socio-political risk is known as the ‘moral hazard’. The concern is that if SRM was deployed it would take the urgency out of decarbonisation. The temptation would be to keep burning fossil fuels and reduce carbon dioxide removal efforts since the situation would be portrayed as under control. As discussed above, this would lead to the level of SRM having to be continually increased and be locked in forever under the threat of an ever growing termination shock.

The next risk category are the geo-political risks. Put simply, if SAI is started and then whether as a result of it, or through coincidental natural variability, a severe drought, flood or other major catastrophe affects a region, the ones carrying out the intervention will get the blame. The ramifications could range from demanding compensation, demanding immediate cessation, civil unrest, regional destabilisation or even military action to prevent the practice or in retaliation. The Arab Spring was first initiated by crop failures, so this is not a far fetched scenario.

Even agreement on the location and scale of deployment could be a political issue. What works best for India may be different from what suits China. How would the populations of South America and Africa be informed, consulted and considered?

The final socio-political risk is how the general and voting public would react. Earlier planned trials in the US have been cancelled and the media response to the recent UK funding is mixed to say the least⁶. Some climate scientists are heavily opposed, likening it to treating cancer with aspirin, while others also use a cancer analogy claiming banning SRM would be like banning chemotherapy for fear of hair loss. More informed debate is required to allow the public to understand and support any decisions.

How would SRM be regulated internationally?

This leads to probably the most important unanswered question surrounding SRM. How would the world come together to agree a regulatory framework, proper plan and implementation strategy to control deployment?

The first problem to overcome is that the level of knowledge within the population as well as policy makers is really low at the moment. This is partly because it has been off the table within IPCC discussions, but also the moral hazard fear has prevented people discussing it seriously in case it took momentum out of the required climate actions of carbon mitigation and climate adaptation. To address this, physical scientists, social scientists, policy makers and elected officials need to start coming together to discuss SRM. The media also needs to be properly informed.

Young people also need to be engaged and involved. Future generations will be affected whether it is implemented or not, either through having to adapt to a 3ºC plus world or through having to continue SRM deployment for centuries.

Today there are no international laws or conventions that legally prohibit a country deploying SRM. The Convention on Biological Diversity in 2010 made a non-binding decision on geoengineering research. The London Protocol has language about geoengineering in the oceans. There are pieces of the Montreal Protocol that talk about protecting the ozone layer. The latest meeting in 2022 included a report on the impacts of SAI on the ozone layer. The UNFCCC however has no language at all on SRM.

The first step would be to agree some governance for in-silico research and small scale outdoor experiments. The momentum is building anyway, so it is important to get this under an umbrella before it gets too far ahead. This is seen as a relatively easy first step compared to the much more difficult and geo-politically challenging governance required for deployment. The upcoming UK trials, for example, are associated with governance measures to limit the scale, the chemicals used and ensure environmental impact studies are carried out.⁷ Ultimately the decision on whether an outdoor experiment takes place or not is made by Aria’s chief executive, who is ultimately responsible to the UK Parliament. This will be the first government backed set of governance methods and recommendations and should be a key output of the research programme.⁸

The complexity of the required governance for full deployment is mind boggling. The Montreal Protocol simply had to ban the use of a class of chemicals and has been successful. The whole UNFCCC journey including the Paris Agreement simply had to reduce greenhouse gas emissions. Measured against progress in this mission it has been a failure. Getting the international community to agree a plan with break points on the back of triggers involving successful decarbonisation progress would have seemed extremely difficult even before the current inhabitant of the White House moved in. The Paris Agreement took 23 years to be agreed and then was only achieved because the country contributions were voluntary. A comprehensive SRM Protocol would appear to be orders of magnitude harder.

Without guard rails however, there is nothing currently stopping unilateral deployment. If a country sees a clear and present danger to its citizens through mass casualty heat wave events, mega-droughts or other risks, nothing exists today to stop them deploying SAI. Alternatively a country determined to support its fossil fuel industry at all costs, may deploy to lesson the demands for decarbonisation in a short term move to buy time. The geo-political shockwaves this would create could be extremely de-stabilising. The only body that could issue a legal moratorium to prevent this would be the UN Security Council. Even getting this through would be a long process with potential veto from at least one of the permanent members.

Another potential way forward may be a consortium of countries (or large companies) working within a Manhattan Project type model. Current geo-politics may make that difficult for a while, but its a possibility. This might help govern research and lower scale trial deployments, but every country would need some say in a full deployment decision. This equity is important, the global south are most at risk of climate impacts but are the least informed and consulted about preventative measures. The regional impact questions are not known and this needs addressing ahead of a global decision to proceed.

There is a narrow window for regulation as some small Start-ups are already doing experiments, essentially without any oversight.

Conclusions

We need more discussion of this topic, we need more people involved from all aspects of the technical science, the social science, the young, religious groups as well as policy makers and the general public. These discussions need to be inclusive and equitable especially for the global south and regions most effected by both the ravages of climate change and susceptible to potential SRM side effects.

We need to do the research to prepare for its use as part of an emergent decarbonisation strategy and for use in an emergency to prevent mass mortality or the triggering of a tipping element. This will of course include modelling work but should also include outdoor experiments.

We need to do policy research to try and figure out a way of agreeing and regulating the required research as well as possible future deployment. Transparency, monitoring, data sharing are key starting points.

The bottom line is that this is the least we can do for future generations to give them a tool which may help their inheritance of our arrogance and ineptitude thus far. Any belief that we can deliver them a world on course to stay within the Paris Agreement limits under current policy trends is incredibly naive. They deserve as much help as we can give them.

If the whole idea of SRM seems repugnant and unthinkable, the only alternative is that we should accelerate decarbonisation efforts exponentially, invest hugely in climate adaptation and resilience capacity, accept the costs, lifestyle impacts and steer society towards a sustainable zero-carbon future. We can’t have it both ways.

1. Royal Society Report on Geoengineering the Climate, 2009,

https://royalsociety.org/-/media/policy/publications/2009/8693.pdf

2. Unless otherwise stated with a citation link, the material for this article was sourced through recorded lectures and interviews posted by Climate Chat, The Great Simplification and the Salata Institute at Harvard University. These include presentations by David Keith, Cynthia Scharf, Daniele Visioni, Shuchi Talati, Ben Kravitz, Doug MacMartin and Jim Stock.

3.

4. Peak Shaving with Solar Radiation Modification Would Shorten Global Temperature Overshoot. Linus Boselius et al, 2025,

https://academic.oup.com/oocc/advance-article/doi/10.1093/oxfclm/kgaf013/8107970

5. Stratospheric Aerosol Injection could prevent future Atlantic Meridional Overturning Circulation decline, but injection location is key. Ewa M. Bednarz et al. 2024,

https://doi.org/10.22541/essoar.173482135.52320055/v1

6. The Guardian - The UK’s gamble on solar geoengineering is like using aspirin for cancer, Raymond Pierrehumbert and Michael Mann, March 2025

https://www.theguardian.com/commentisfree/2025/mar/12/solar-geoengineering-uk

7. Research Professional News - Geoengineering trials defended as ‘missing part’ of climate science, April 2025

https://www.researchprofessionalnews.com/rr-news-uk-innovation-2025-4-geoengineering-trials-defended-as-missing-part-of-climate-science/

8. UK Advanced Research and Invention Agency - Future Proofing our Climate and Weather

https://www.aria.org.uk/opportunity-spaces/future-proofing-our-climate-and-weather