Crédits de refroidissement : le financement du déploiement des MRS pourrait-il compenser les émissions de CO₂ ?

Helium-filled stratospheric balloons are regularly used by scientists and hobbyists to carry scientific instruments, cameras, and other objects into the upper atmosphere. In 2022, a US startup – Make Sunsets – began repurposing these balloons for micro-scale deployments of stratospheric aerosol injection (SAI).

Her device is rudimentary: she manually fills these balloons with a cocktail of helium and sulfur dioxide (SO₂) ¹  but her ambitions are great.

Launched with the backing of a few investors, it sells consumers unverified « cooling credits. » By releasing about a kilogram of SO₂ into the stratosphere at a time, which should form many tiny, reflective particles, it claims to offset the warming effects of a much larger amount of CO₂. ²

Could these cooling credits actually offset the effects of CO₂ and other greenhouse gases? And would trading them help encourage cost-effective climate action, like carbon credits?

Carbon credits

CO2 is a long-lived greenhouse gas, ³ which disperses roughly uniformly around the planet 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 impact on the environment.

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

In response to these disparities in the costs of reducing carbon emissions, carbon markets were set up. Governments, organizations, and others were allowed to trade “carbon credits”—instead of reducing their own emissions, they would pay an entity that removes or reduces emissions. In essence, a carbon credit represents a ton of CO₂ or an equivalent amount of other greenhouse gases.

Does it really work?

One of the main challenges with carbon credits is verifying that they work as intended and that the promised emissions reductions actually occur.

Each of the many types of carbon credits 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 sustainable, albeit costly, solution.

The least reliable carbon credits include those based on hypothetical baselines that are impossible to verify and susceptible to manipulation. For example, Chinese chemical companies were paid under the EU’s carbon trading scheme to avoid emitting HFCs , extremely potent greenhouse gases. However, these payments provided incentives for companies to produce excess HFCs so they could promise to reduce them further.

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

CO₂ is very sustainable, while cooling from SRM is not.

Regarding MRS 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 go far beyond simply verifying the cooling potential of MRS.

For example, CO₂ is the largest contributor to global warming, but it is not the most powerful greenhouse gas . On the contrary, 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 than CO₂ at trapping heat, with a global warming potential about 30 times greater than that of CO₂ . This means that, on average over 100 years, one tonne of methane would warm the atmosphere 30 times more than one tonne of CO₂ .

IAS/SAI and other MRS approaches, such as marine cloud brightening (MCB), would require relatively small amounts of material to produce a significant cooling effect. However, the cooling effect of IAS/SAI deployment would persist for only 1–2 years ⁸ and the effects of MCB for only a few days. ⁹

This short lifetime implies a long-term commitment to large-scale MRS once it has begun, as rapid global warming would ensue if the deployment were suddenly and permanently halted (a so-called termination shock). ¹⁰

The environmental effects of CO₂ and MRS 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 MRS are very different from those of CO₂ . If companies bought MRS cooling credits instead of reducing their CO₂ emissions, the planet might not be warmer (assuming they were effective), but it would certainly experience greater environmental changes.

CO2 and other greenhouse gases affect the climate in a similar way by trapping some heat as they leave the Earth’s atmosphere, warming the planet and producing other climate effects. MRS would instead reduce the amount of sunlight absorbed at the surface, counteracting the warming effects of CO₂ but producing different climate effects overall.

Changes in precipitation patterns could have a significant impact on climate outcomes. ¹¹ Compared with an equivalent reduction in CO₂ warming, the IAS/SAI would result in larger reductions in global precipitation, with a different pattern of change. ¹² The MCB could have even larger effects on global precipitation and lead to larger changes in regional patterns. ¹³

Both CO₂ and SRM have collateral effects that go beyond their impact on climate. For example, CO₂ acidifies the oceans – a collateral effect that SRM has not been able to address. SRM, on the other hand, is thought to affect the ozone layer , acid rain and the appearance of the sky. ¹⁴

Cooling credits represent a solution to a problem that MRS does not have

Decarbonizing the economy will require significant investment , particularly in certain sectors. Carbon credits aim to address this problem by incentivizing cost-effective carbon reduction actions.

High costs are one of the major barriers to decarbonization, but this problem is not shared by MRS, particularly MRS/SAI . The challenges facing MRS ideas are more fundamental than costs – they include complex risk trade-offs, significant governance issues, and profound ethical questions.

Widespread adoption of cooling credits could accelerate and expand the micro-scale deployment of IAS or other MRS approaches, thereby producing minimal global cooling effects. However, this could undermine efforts to address the accumulation of greenhouse gas emissions that is driving the climate change problem.

The adoption of cooling credits could also lead to opposition that could undermine efforts to establish international collaboration on MRS research and governance. Make Sunsets initially launched balloons in Mexico without authorization or oversight. In response, the Mexican government announced that it would ban MRS testing and deployment.

In December 2024, the European Commission’s Chief Scientific Advisors published recommendations on MRS, which include a ban on the sale of MRS cooling credits.

While MRS may play a role in future climate policy, it would be a poor substitute for emissions reductions. Thus, proposals for cooling credits that incentivize such substitution and encourage early micro-scale deployments lack both academic and policy support.