Alternative Particles Could Reduce the Side Effects of SAI

Sunlight Reflection Methods (SRM) are approaches that aim to cool the climate artificially by increasing the amount of sunlight the Earth reflects back to space. One of the most studied methods is Stratospheric Aerosol Injection (SAI). This involves releasing tiny particles called aerosols into the upper atmosphere to reflect some incoming light.

Natural processes, as well as industrial pollution, release a large number of aerosols into the atmosphere that reflect sunlight and cool the climate. However, most of these are released into the troposphere – the lower 10–12 km of our atmosphere – where aerosols last only 5–10 days because they are washed out by rain. By contrast, in the dry stratosphere – located above the troposphere – there is no rain, so aerosols persist for 1–2 years before they slowly settle down, thus contributing more effectively to the cooling of the planet.

Research on SAI has primarily focused on the release of sulphur dioxide (SO₂), which reacts to form sulphuric acid aerosols – tiny sulphur-containing liquid particles. The idea was first suggested by Mikhail Budyko in 1974¹ and was inspired by major explosive volcanic eruptions, which can emit large amounts of SO₂ into the stratosphere. A well-known example is the 1991 Mt. Pinatubo eruption in the Philippines, which injected about 15 million tonnes of SO₂ into the stratosphere. This cooled the global climate by about 0.5°C in the 1–2 years following the eruption.²

Volcanic eruptions provide a useful natural analogue for SAI with sulphuric acid aerosols, allowing scientists to test their theories and models of the effects of stratospheric sulphuric acid aerosols on the climate against observations.³ For example, sulphuric acid aerosols are known to affect the stratospheric ozone layer, which protects life on Earth from harmful ultraviolet radiation. For this reason, the physical and chemical properties of stratospheric sulphuric acid aerosols have been studied for many years.⁴

This existing scientific knowledge made it relatively easy to begin evaluating the risks and benefits of SAI via SO₂ injection. However, it also soon became clear that SO₂-based SAI could have unwanted side effects such as depleting the stratospheric ozone layer⁵ or warming the stratosphere, which would impact global wind and precipitation patterns,⁶ possibly exacerbating the effects of climate change in some regions of our planet.⁷

Recent studies have shown that SAI using solid particles, such as aluminium oxide, calcite, or diamond particles, could reflect light with much smaller negative effects than liquid sulphuric acid aerosols.^7–10 It has been demonstrated that the surfaces of certain solid particles are likely much less reactive than sulphuric acid aerosols,⁸ which could minimise their impact on the stratospheric ozone layer. In addition, certain solid particles would result in substantially less warming of the stratosphere,^7,9,10 which in turn would reduce the impact on global winds and precipitation patterns.⁷ Furthermore, some of these solid particles would reflect more sunlight per unit mass, reducing the amount of material required to cool the planet.¹⁰

However, these benefits are subject to considerable uncertainty. Unlike sulphuric acid aerosols, relatively little is known about the chemical and physical properties of solid particles in the stratosphere. Solid particles do not occur naturally in the stratosphere, making it difficult to predict how they would interact with each other, with radiation, or with stratospheric ozone. As a result, there have been few scientific publications on SAI with solid particles to date – the vast majority of publications on SAI have focused on SO₂ injections.¹¹

The main uncertainties associated with solid particles identified in these few studies relate to how the solid particles would age in the stratosphere, i.e., how their properties would change as they interact with stratospheric gases and aerosols.^8,9 Would this change their chemical and physical properties, such as their reflectivity? And how would these property changes affect stratospheric ozone and warming? One study highlights the uncertainty of potential interactions between the solid particles and clouds in the troposphere,¹² which could reduce the overall cooling effect of solid particles.

In addition to these uncertainties, there are also technological challenges related to dispersing solid particles into the stratospheric air. Compared to solid particles, the release of compressed gaseous SO₂ from aircraft would be relatively simple. After release, e.g. through a simple valve, SO₂ mixes with air and forms liquid sulphuric acid aerosols after reacting over the course of about 30 days.

For solid particles, the key challenge would be to take size selected particles and release them in such a way that they are individually separated. This may provide technical challenges as particles would need to start off separated and could clump back together shortly after being released. This may limit the rate at which particles could be released from aircraft, which could impact the costs of deployment since more flights and aircraft would be required.

To reduce these uncertainties, we need coordinated experimental laboratory studies of the chemical and physical properties of solid particles under stratospheric conditions, i.e., very low temperatures and humidity, combined with small-scale field experiments and modelling. The recent advances in research on SAI using solid particles suggest a way to reduce these uncertainties through collaborative inter-institutional efforts.

This is urgently needed, because the current focus on SO₂ injection risks biasing any assessment of SAI towards sulphuric acid aerosols. If the uncertainties surrounding SAI with solid particles can be resolved and they actually perform much better than SO₂, it may be necessary to redo assessments of the climate risks and benefits of SAI.

We are a long way from being able to do this as there are currently only a few climate models that can simulate the interactions of solid particles in the stratosphere,¹³ making a timely assessment difficult. It is therefore important to develop these modelling capabilities now, while also utilising experimental work to narrow down the physical and chemical properties of potential solid particle candidates.

Not doing so could mean the world becomes locked into a second-best option. If the decision is made that deploying SAI would be preferable, then the technology that is easier to implement will likely have a head start. Similar to how internal combustion engines dominate the automotive industry and thus prevail over the development of climate-friendly electric cars, technological simplicity and lower uncertainty of SAI with SO₂ could prevail over SAI with solid particles, even if the latter had significantly fewer side effects.

Injection of SO₂ is currently better understood than any of the alternatives, but if the uncertainties can be resolved, solid particles could offer a way to greatly reduce the side effects of SAI. Given this considerable potential, it is high time these alternatives got the research attention they need.

The views expressed by Perspective writers are their own and are not necessarily endorsed by SRM360. The goal of our Perspectives is to present ideas from diverse viewpoints, further supporting informed discussion of sunlight reflection methods.