Article

Cooling Credits: Could Paying for SRM Deployment Offset CO2 Emissions?

“Cooling credits” are being offered as a way for companies and individuals to offset their carbon dioxide (CO2) emissions by investing in the deployment of sunlight reflection methods (SRM), also known as solar geoengineering. But could SRM really offset the effects of CO2 emissions?

Key takeaways

  • The physical effects of SRM are not equivalent to cutting CO2 emissions due to its much shorter lifetime and different environmental impacts.
  • There are currently no standards for verifying claims regarding “cooling credits” from SRM deployment, and policymakers have received recommendations to prohibit their sale.
  • Carbon credits aim to incentivise cost-effective emissions cuts, addressing one of the biggest barriers to action. However, the financial costs of SRM are one of the least significant concerns about these technologies.

High-altitude balloons filled with helium are regularly used by scientists and hobbyists to carry scientific instruments, cameras, and more into the upper atmosphere. In 2022, a startup from the United States – Make Sunsets – began repurposing these balloons for micro-scale deployments of stratospheric aerosol injection (SAI).

Their set-up is crude – they manually fill these balloons with a cocktail of helium and sulphur dioxide (SO2)1 – but their ambitions are grand.

A person fills a balloon using a hose and canister while another person looks on.

The founders of Make Sunsets filling a ballon before a launch (Photo: Make Sunsets).

Launching with the backing of some investors, they sell unverified “cooling credits” to consumers. By releasing of about a kilogram of SO2 in the stratosphere at a time, which should form many tiny, reflective particles, they claim to be offsetting the warming effects of a much larger amount of CO2.2

Could such cooling credits really offset the effects of CO2 and other greenhouse gases? And would trading them help incentivise cost-effective climate action, in a similar way to carbon credits?

Carbon credits

CO2 is a long-lived greenhouse gas,3 which disperses roughly evenly across the globe within a few years regardless of where it is emitted. This means that one tonne of CO2 emitted anywhere is equal to any other in terms of its environmental impact.

However, the costs of avoiding one tonne of CO2 emissions are far from consistent. Some sources of CO2 are very costly or even impossible to eliminate, such as emissions from aviation.4 In other cases, small investments, such as in the energy efficiency of buildings, can lead to substantial emissions cuts.5

In response to these disparities in the costs of cutting carbon emissions, carbon markets were set up. Governments, organisations, and others were enabled to trade “carbon credits” – instead of cutting their own emissions, they would pay an entity that removes or reduces emissions. In essence, one carbon credit accounts for one tonne of CO2 or the equivalent amount of other greenhouse gases.

Is this really working?

A key challenge for carbon credits is verifying that they are working as intended and the promised emissions reductions are actually happening.

Each of the many types of carbon credit poses its own verification challenges. Some carbon credits offer to capture CO2 from the atmosphere and store it in geological formations. Such an approach, in principle, offers a verifiable and durable, albeit expensive, solution.

Less robust carbon credits include those based on hypothetical reference scenarios, which are impossible to verify and susceptible to manipulation. For example, Chinese chemical companies were paid through the EU carbon trading scheme to avoid emitting HFCs – incredibly powerful greenhouse gases. However, the payments incentivised the companies to overproduce HFCs so they could promise to cut more.

Other credits are based on promises to plant or protect forest plots for decades. In many cases, these projects have been ineffective or significantly less beneficial than claimed.6 Many have also harmed Indigenous peoples and local communities, for example, by forcibly removing them from their land.

CO2 is very long-lived, and the cooling from SRM is not

When it comes to SRM cooling credits, there are no official standards for calculating the equivalence between the amount of reflected sunlight and the amount of CO2 offset.7 However, the problems with this idea run much deeper than verifying SRM’s cooling potential.

For example, CO2 makes the largest contribution to global warming, but it is not the most powerful greenhouse gas. Instead, it is abundant and long-lived, persisting in the atmosphere for centuries.3

Methane, on the other hand, only has a lifetime of about 12 years in the atmosphere. However, it is more effective at trapping heat than CO2, with a global warming potential roughly 30 times greater than CO2. This means, averaged over 100 years, one tonne of methane would warm the atmosphere 30 times more than one tonne of CO2.

SAI and other SRM approaches, such as marine cloud brightening (MCB), would require relatively small amounts of material to produce a large cooling effect. However, the cooling effect of SAI deployment would persist for only 1–2 years8 and the effects of MCB for only a few days.9

This short lifetime implies a long-term commitment to large-scale SRM once it has begun, as a rapid global warming would follow if deployment were suddenly and permanently stopped (known as a termination shock).10

The environmental effects of CO2 and SRM are very different

While a tonne of CO2 from aviation has the same environmental impact as any other tonne of CO2, the environmental impacts of SRM are very different to those of CO2. If companies bought SRM cooling credits instead of cutting their CO2 emissions, the planet may not be any warmer (presuming they were effective), but it would certainly see greater environmental change.

CO2 and other greenhouse gases affect the climate in similar ways by trapping some heat as it leaves the Earth’s atmosphere, both warming the planet and producing other climate effects. SRM would instead reduce the amount of sunlight absorbed at the surface, counteracting the warming effects of CO2 but producing different climate effects overall.

Changes to rainfall patterns would be an important difference in climate effects.11 Compared to an equivalent reduction in CO2 warming, SAI would cause larger reductions in global rainfall with a different pattern of change.12 MCB could have even larger effects on global rainfall, and lead to greater shifts in regional patterns.13

Both CO2 and SRM have side effects beyond their climate impacts. For example, CO2 acidifies the ocean – a side effect that SRM could not address. SAI, on the other hand, would have impacts on the ozone layer, acid rain, and the appearance of the sky.14

Cooling credits are a solution to a problem that SRM does not have

Decarbonising the economy will require significant investment, especially in some sectors. Carbon credits aim to address this problem by incentivising cost-effective carbon-cutting actions.

High costs are one of the most significant barriers to decarbonisation, but this is not a problem that SRM, particularly SAI, shares. The challenges that SRM ideas face are more fundamental than cost – they include complex risk-risk trade-offs, substantial governance issues, and deep ethical questions.

The widespread adoption of cooling credits could accelerate and expand micro-scale deployment of SAI or other SRM approaches, producing a tiny global cooling effect. However, this could risk undermining efforts to address the build-up of greenhouse gas emissions – the root of the climate change problem.

Adopting cooling credits might also lead to pushback that could undermine efforts to build international collaboration around SRM research and governance. Make Sunsets initially launched balloons in Mexico without permission or oversight. In response, the Mexican government announced they would ban SRM experimentation and deployment.

In December 2024, the chief scientific advisors to the European Commission issued recommendations on SRM that included a call to prohibit the sale of SRM cooling credits.

While SRM may play a role in future climate policy, it would be a poor substitute for emissions cuts. As such, proposals for cooling credits that incentivise this substitution and push early micro-scale deployments lack both academic7 and policy support.

Open questions

  • Could policymakers support the development of standards for cooling credits, or could they instead ban their sale?
  • Do cooling credits undermine emissions cuts? Do consumers who buy cooling credits consider them equivalent to cutting emissions?
  • What impact will commercial interests in SRM have on public trust and international collaboration?

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Endnotes

  1. SO2 is hazardous to human health, especially if inhaled in high concentrations. During one of Make Sunsets’ launches, they exposed an NPR journalist to SO2.
  2. As of January 2025, Make Sunsets claim to have released 108 kg of SO2 into the stratosphere.
  3. Archer D, Eby M, Brovkin V, et al. (2009). Atmospheric lifetime of fossil fuel carbon dioxide. Annual review of earth and planetary sciences. 37(1):117-34. https://doi.org/10.1146/annurev.earth.031208.100206
  4. Bergero C, Gosnell G, Gielen D, et al. (2023). Pathways to net-zero emissions from aviation. Nature Sustainability. 6(4):404-14. https://doi.org/10.1038/s41893-022-01046-9
  5. Slabe-Erker R, Dominko M, Bayar A, et al. (2022). Energy efficiency in residential and non-residential buildings: Short-term macroeconomic implications. Building and environment. 222:109364 https://doi.org/10.1016/j.buildenv.2022.109364
  6. West TA, Wunder S, Sills EO, et al. (2023). Action needed to make carbon offsets from forest conservation work for climate change mitigation. Science. 381(6660):873-7. https://doi.org/10.1126/science.ade3535
  7. Diamond MS, Wanser K, Boucher O. (2023). “Cooling credits” are not a viable climate solution. Climatic Change, 176(7). https://doi.org/10.1007/s10584-023-03561-w
  8. Laakso A, Niemeier U, Visioni D, et al. (2022). Dependency of the impacts of geoengineering on the stratospheric sulfur injection strategy–Part 1: Intercomparison of modal and sectional aerosol modules. Atmospheric Chemistry and Physics. 22(1):93-118. https://doi.org/10.5194/acp-22-93-2022
  9. Feingold G, Ghate VP, Russell LM, et al. (2024). Physical science research needed to evaluate the viability and risks of marine cloud brightening. Sci. Adv (Vol. 10). https://doi.org/10.1126/sciadv.adi8594
  10. Parker A, Irvine PJ. (2018). The risk of termination shock from solar geoengineering. Earth’s Future. 6(3):456-67. https://doi.org/10.1002/2017EF000735
  11. Tracy SM, Moch JM, Eastham SD, et al. (2022). Stratospheric aerosol injection may impact global systems and human health outcomes. Elementa. University of California Press. https://doi.org/10.1525/elementa.2022.00047
  12. MacMartin DG, Ricke KL, Keith DW. (2018). Solar geoengineering as part of an overall strategy for meeting the 1.5 C Paris target. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 376(2119):20160454. https://doi.org/10.1098/rsta.2016.0454
  13. Haywood JM, Jones A, Jones AC, et al. (2023). Climate intervention using marine cloud brightening (MCB) compared with stratospheric aerosol injection (SAI) in the UKESM1 climate model. Atmospheric Chemistry and Physics, 23(24), 15305–15324. https://doi.org/10.5194/acp-23-15305-2023
  14. Lemon A, Keith DW, Albers SC. (2024). Under a not so white sky: visual impacts of stratospheric aerosol injection. Environmental Research Letters. https://doi.org/10.1088/1748-9326/ada2ae

Citation

Pete Irvine, Kimberly Samuels-Crow, Caitlin Soch (2025) – "Cooling Credits: Could Paying for SRM Deployment Offset CO2 Emissions?" [Article]. Published online at SRM360.org. Retrieved from: 'http://srm360.org/article/cooling-credits-could-srm-offset-co2/' [Online Resource]

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