Deliverable 2.1: Scoping notes on the state of solar radiation modification (SRM) research, field tests, and related activities

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Co-funded by the European Union. Views and opinions
expressed are however those of the author(s) only and do
not necessarily reflect those of the European Union or
CINEA. Neither the European Union nor the granting
authority can be held responsible for them.
This work was funded by UK Research and
Innovation (UKRI) under the UK government’s
Horizon Europe funding guarantee [No. 10123643 -
Climate Strategies; No. 10094614 - Trilateral Research
Limited; No. 10098060 - University College London].
Conditions for Responsible Research on SRM
D2.1 Scoping note on the state of Solar
Radiation Modification (SRM) research,
field tests, and related activities
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Information Sheet
Suggested reference: Redmond Roche, B.H. and Irvine, P.J. (2024) Deliverable 2.1: Scoping
notes on the state of solar radiation modification (SRM) research, field tests, and related
activities. Co-CREATE Project. Available on the Co-CREATE Website.
Project co-funded by the European Commission within the Horizon Europe Programme
[Grant Agreement No. GAP-101137642] and UKRI under the UK government’s Horizon
Europe funding guarantee [No. 10123643 - Climate Strategies; No. 10094614 - Trilateral
Research Limited; No. 10098060 - University College London].
Work Package
2 (Interdisciplinary scoping)
Deliverable
D2.1
Due Date
M10 (October 2024)
Submission Date
29 November 2024
The lead beneficiary for
this deliverable: UCL, Ben Redmond Roche, Pete Irvine (now at UChicago)
Contributors:
CNRS; PCR
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Deliverable 2.1. – Scoping note on the state of Solar Radiation
Modification (SRM) research, field tests, and related activities
Authors
Ben Redmond Roche (UCL), Pete Irvine (UChicago)
Reviewers
Anni Maattanen, Ivan Hernandez Galindo (CNRS), Matthias Honegger
(Perspectives Climate Group)
Date
29 November 2024
The Co-CREATE project
Experimental research on solar radiation modification (SRM) is controversial and
feared to distract from climate change mitigation or lead to dangerous SRM use. It is
not clear whether and under which conditions and governance arrangements
experimental SRM research may be desirable for reducing uncertainties and lowering
the probability of problematic outcomes including unilateral deployment elsewhere.
Co-CREATE seeks to help structure this decision problem through co-creative scoping,
analysis, and engagement for the development of principles and guidelines.
Deliverable description
Deliverable 2.1 includes a bundle of three scoping notes which offer a common basis
for the technical understanding of SRM and its possible forms of research. The task
on which this deliverable is based maps out SRM options and experimental SRM
research and testing currently being conducted, planned, or considered. In addition,
related activities that raise similar issues are also identified to help contextualize SRM.
An initial review of the SRM literature is the basis of the scoping notes on the potential
consequences of deploying each of the main SRM proposals. An international
workshop brought together SRM researchers and members of the Co-CREATE team
to examine the range of potential SRM field tests and the issues and opportunities
they present. The result of this is – in this document – a list of past, ongoing, and
planned SRM field tests. This includes analogous activities not necessarily considered
SRM as such but with relevant similarities, such as cloud seeding, and those
particularly relevant in the Arctic of Global South regions. The list is presented in a
common format that touches on physical aspects (geography and environmental
effects) and thus provides an overview of the science for further examination from
other perspectives in subsequent work packages
Keywords
Solar radiation modification (SRM); stratospheric aerosol injection (SAI); marine cloud
brightening (MCB); cirrus cloud thing (CCT); mixed-phase cloud thinning (MCT);
termination shock; and climate change; albedo; solar radiation; thermal radiation.
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Table of Contents
Information Sheet ................................................................................................................................................................. 2
List of figures, tables and glossary .................................................................................................................................... 5
Executive Summary .............................................................................................................................................................. 6
1. Introduction .........................................................................................................................................................
8
1.1 State of evidence on potential benefits to SRM: ........................................................................ 11
1.2 State of research on risks, uncertainties, and ethical considerations: ......................................... 12
2 Stratospheric Aerosol Injection (SAI) ...........................................................................................................
14
2.1 SAI – what is it and what might it do? ....................................................................................... 14
2.2 Mechanism and process of SAI ................................................................................................ 15
2.3 Positive environmental and climate effects of SAI ..................................................................... 15
2.4 Negative environmental and climate effects of SAI ................................................................... 16
2.5 Summary of the evidence on benefits and risks of SAI .............................................................. 17
2.6 Research and field testing on SAI ............................................................................................. 18
2.7 Conclusions on SAI .................................................................................................................. 18
Stratospheric Aerosol Injection (SAI): Key Takeaways ............................................................................................. 19
3. Marine Cloud Brightening (MCB) ............................................................................................................... 20
3.1 MCB – what is it and what might it do? ..................................................................................................... 20
3.2 Mechanism and process of MCB ............................................................................................................... 21
3.3 Positive environmental and climate effects of MCB ............................................................................... 21
3.4 Negative environmental and climate effects of MCB ............................................................................. 23
3.5 Summary of the benefits and risks of MCB .............................................................................................. 24
3.6 Research and field testing of MCB ............................................................................................................. 24
3.7 Conclusions on MCB ..................................................................................................................................... 24
Marine Cloud Brightening (MCB) Key Takeaways ..................................................................................................... 25
4. Cirrus Cloud Thinning (CCT) and Mixed-phase Cloud Thinning (MCT) ........................................ 26
4.1 CCT and MCT - what is it and what might it do? ..................................................................................... 26
4.2 Mechanism and process of CCT and MCT ............................................................................................... 27
4.3 Positive environmental and climate effects of CCT and MCT .............................................................. 27
4.4 Negative environmental and climate effects of CCT and MCT ............................................................ 28
4.5 Summary of evidence on expected benefits and risks of CCT and MCT ............................................ 29
4.6 Research and field testing of CCT and MCT ............................................................................................ 29
4.7 Conclusions on CCT and MCT .................................................................................................................... 29
Cirrus (CCT) and Mixed-Phase (MCT) Cloud Thinning ............................................................................................. 30
5. Other technologies with similar issues .....................................................................................................
31
6. Past and ongoing field experiments on SRM ...........................................................................................33
7. Conclusions and key takeaways ................................................................................................................ 36
8. References ..........................................................................................................................................................37
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List of figures, tables and glossary
List of Figures
Figure 1
Peak shaving
Page 9
Figure 2
SRM application
Page 9
Figure 3
The three SRM techniques
Page 10
Figure 4
Stratospheric aerosol injection (SAI)
Page 19
Figure 5
Marine cloud brightening (MCB)
Page 25
Figure 6
Cirrus- and mixed-phase cloud thinning (MCB and CCT)
Page 30
Figure 7
History of SRM field tests
Page 34
List of Tables
Table 1
SAI key takeaways
Page 19
Table 2
MCB key takeaways
Page 25
Table 3
CCT and MCT key takeaways
Page 30
Table 4
Past SRM field tests
Page 35
Glossary
Albedo The fraction of light that an object reflects: if all light is
reflected the value is 1, if no light is reflected it is 0. Page 14
CCT and
MCT Cirrus cloud thinning and mixed-phase cloud thinning Page 10
MCB
Marine cloud brightening
Page 10
SAI Stratospheric aerosol injection Page 10
Solar
radiation
Shortwave radiation emitted by the Sun, which includes
ultraviolet, visible light, and near-infrared wavelengths. Page 8
(SRM) Solar
radiation
modification
A collection of techniques that could help limit the
effects of climate change by reflecting more sunlight into
space.
Page 8
Stratosphere
The second lowest layer of the atmosphere, starting at
approximately 8-10 km at the poles and 16-18 km at the
equator.
Page 10
Thermal
radiation
Longwave radiation emitted from the Earth, primarily in
the far-infrared wavelength range. Page 8
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Executive Summary
Solar radiation modification (SRM) encompasses a range of hypothetical techniques
that if applied at scale would temporarily decrease some of the effects of climate
change. While the risks and uncertain climate risk-reducing benefits of SRM are being
researched as a potential complement to long-term emission reductions and carbon
removal strategies, its deployment is currently not considered an option. If considered
in the future, SRM could be applied in various scenarios, such as a temporary multi-
decadal ‘peak-shaving’ to limit global temperature rise until greenhouse gas
concentrations come down or to slow the rate of warming. In both cases, it could at
best serve as a complement to drastic mitigation and adaptation efforts.
The conversation on SRM research on feasibility and risks must be embedded in a
broader conversation on ethics and governance. Yet we need a shared understanding
of the object of conversation: what SRM techniques and what experiments are we
examining?
The three main SRM techniques that are examined in these scoping notes are:
•
Stratospheric aerosol injection (SAI):
Releasing sulphur dioxide or other material to
form particles in the stratosphere to reflect more solar radiation.
•
Marine cloud brightening (MCB):
Spraying sea salt into low-altitude marine clouds
to reflect more solar radiation.
•
Cirrus cloud thinning and mixed-phase cloud thinning (CCT and MCT):
Injecting
particles into cirrus or mixed-phase clouds to enlarge ice crystals allowing more
heat to escape from Earth into space.
SAI and MCB are regarded as potentially capable of achieving global cooling, whereas
CCT and MCT are considered in the context of localised or regional cooling. Why is
SRM being researched? If key uncertainties can be resolved, this would help make
informed decisions that consider both the potential benefits as well as the risks and
residual uncertainties.
The main potential benefits of SRM would be:
•
Rapid cooling potential:
SRM techniques can rapidly cool the planet and reduce
many climate impacts in the near term.
•
Cost-effectiveness:
SRM
techniques appear comparatively low-cost relative to the
projected or already observed damages from climate change.
•
Local environmental protection:
Some
SRM techniques could protect vulnerable
ecosystems, e.g., coral reefs and polar ice from heating.
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The main risks and uncertainties of SRM deployment are:
•
Unintended climate effects:
Regional rainfall patterns might be altered in
comparison to changes due to climate change without SRM, e.g., weakening
monsoon rainfall.
•
Side-effects:
SRM techniques could have significant environmental and health side-
effects, e.g., SAI could slow the ozone hole's recovery over the Antarctic.
•
Termination shock:
If SRM were exerting a large cooling, then abruptly and
permanently ending it would trigger rapid warming, with impacts which might
exceed even those of climate change without SRM.
Governance challenges of field experiments:
•
Urgency:
Field experiments have increased since 2020 and will likely increase in
future. The rapid worsening of climate impacts, including the possibility of crossing
tipping points, adds to the urgency of learning more about SRM potential and risks.
•
Unilateralism and international conflict:
SRM could theoretically be implemented by
an individual nation, which would inevitably disrupt international relations due to
significant transboundary effects in case of at-scale deployment. Some fear that
field experiments would make such a development more likely.
•
‘Moral hazard’:
There are concerns that the mere discussion of SRM might
undermine commitments for emissions reductions by moving some actors to delay
their investments in renewable energy. Some fear that the visibility of field tests in
media reporting would accentuate such a risk, but the opposite may also be true.
•
Slippery slope:
Some fear that even small-scale experiments would inevitably lead
to SRM deployment without sufficient public debate or governance structures in
place.
•
Equity and inclusion:
Given that field tests contribute to a broader conversation on
potentially using SRM, some argue for participatory research to ensure equity and
inclusion in knowledge production. Involving marginalised communities, who are
often the most vulnerable to climate change impacts is particularly challenging.
SRM research with a global scope faces a particular challenge given that there are
marginalised communities in the Global South as well as the North (such as
marginalised communities in the Arctic). Communities facing the brunt of climate
impacts or unintended consequences of SRM certainly need to be included in the
research ecosystem to avoid overlooking issues that matter to them.
Beyond optimal case scenarios of imagined SRM application, a wide range of less
ideal developments and geopolitical scenarios may also need to be examined to
assess their respective risks and uncertainties.
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1. Introduction
Solar Radiation Modification (SRM) encompasses a range of different techniques
designed to reflect a portion of sunlight back into space (solar radiation) or increase
the amount of radiation (thermal radiation) escaping into space. These approaches
are being researched as temporary measures to potentially mask, slow, or reduce
climate change while other measures, such as transitioning to clean energy and
implementing large-scale carbon removal methods (e.g., afforestation, direct air
capture, and carbon capture and storage), work to stabilise atmospheric greenhouse
gas concentrations.
The urgency of the climate crisis is the key rationale for SRM. In 2023, the average
global temperature had reached 1.45ºC above the 1850–1900 baseline, placing the
world on a trajectory that makes exceeding the 1.5ºC threshold almost inevitable [1].
Climate pledges have consistently remained inadequate even for 2ºC since the
adoption of the Paris Agreement [2, 3]. Consequently, the temperature trajectory
points to 2.7ºC of heating by 2100 [3]. As climate change accelerates and 25 of the
Earth’s 35 ‘vital signs’ reach critical levels, climate scientists are calling for
transformative actions to avoid severe consequences, including widespread societal
and environmental disruptions [4]. Moreover, the global annual damages from climate
change are projected to cost between $19 to 59 trillion dollars in 2049, with the
greatest losses affecting lower latitude regions, which have lower historical emissions
and present-day income [5].
Given these escalating risks, discussions surrounding SRM are expanding to
encompass both its potential to temporarily mitigate the effects of climate change
and the significant uncertainties and risks associated with its implementation. The
2024 State of the Climate report notes ongoing SRM research and argues for a
broadened research agenda [4]. There is growing interest in the potential for SRM
techniques to temporarily mitigate the effects of climate change. However, SRM
remains highly controversial. Its regional impacts on climate and ecosystems are
uncertain, with concerns over potential disruptions to precipitation patterns,
agriculture and biodiversity. Moreover, SRM poses significant governance and ethical
challenges, including questions about equitable decision-making, unintended
consequences, and global accountability. These uncertainties underscore the need for
a cautious, well-governed, and inclusive approach to any potential research
frameworks or potential SRM field experiments.
The context in which deployment is envisioned is crucial, as it shapes assumptions
about its political feasibility, ethical considerations, and broader social and
environmental implications. One strategy, known as peak-shaving, would see SRM
techniques deployed to temporarily reduce the peak of global temperature rise while
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long-term emissions reductions and carbon removal solutions are implemented
(Figure 1) [6, 7]. The core principle of peak-shaving is to alleviate the most immediate
effects of climate change without relying on SRM as a permanent solution.
Figure 1. The effects of emissions reductions and SRM, relative to business-as-
usual scenario. Note how SRM flattens the peak of climate change impacts [7, 8].
Another strategy would use SRM to slow the rate of warming, while the world
gradually transitions towards a warmer climate. This latter strategy would apply if
the global community proves unable or unwilling to take the substantial steps
required to actively lower atmospheric greenhouse gas concentrations. Achieving
such reductions would demand a massive, sustained international effort to fund
and implement large-scale carbon removal projects (i.e., removing more CO2 than is
then being emitted) over several decades.
Figure 2: The SRM application strategy in the absence of a return of greenhouse
gas concentrations: slowing the rate of warming [8, 9].
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The three main SRM techniques are stratospheric aerosol injection (SAI), marine
cloud brightening (MCB), and cirrus cloud thinning (CCT). Both SAI and MCB reflect
solar radiation back into space and are considered the most feasible SRM
techniques to have a substantial effect on climate [10]. CCT, and a similar process
called mixed-phase cloud thinning (MCT), aims to enhance the escape of thermal
radiation from the Earth’s atmosphere. While their potential impact on climate is
smaller compared to techniques like SAI and MCB, they may still play an important
role in climate mitigation. A summary of these techniques is given below.
•Stratospheric aerosol injection (SAI): Reflective particles such as sulphates are
introduced into the stratosphere to increase the reflection of shortwave radiation,
mimicking the natural cooling effects of large volcanic eruptions.
•Marine cloud brightening (MCB): Sea salt particles are sprayed into low-altitude
clouds in the troposphere, creating smaller and more numerous water droplets,
thereby enhancing the reflectivity to shortwave radiation.
•Cirrus cloud thinning (CCT): Ice-nucleating particles are injected into high-altitude
cirrus clouds, causing larger ice particles to form and precipitate out, thereby
thinning the clouds and allowing for more thermal radiation to escape into space.
Figure 3. A graphic of the three different SRM techniques examined in this scoping
note.
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The state of discussion on SRM research:
Some academics advocate for a more
systematic investigation into the field, which includes experiments [11]; others tend to
view research as development and are sweepingly opposed, including to international
assessments [12]; and a third group calls for a balanced approach that carefully
considers both the risks and opportunities [13]. Policymakers have also shown a
growing interest in SRM research governance, as evidenced by reports from the UK
House of Commons in 2009, the National Academies of Sciences, Engineering, and
Medicine, and the US White House in 2023 [14, 15, 16]. At the multilateral level, the
IPCC examined the literature in its seventh assessment report and UNEP produced a
high-level overview of key issues [17, 18].
1.1 State of evidence on potential benefits to SRM:
•
Rapid cooling potential:
SRM has consistently been shown to provide a rapid
cooling effect when deployed at scale, with the potential to stabilise global
temperatures within a few years [19]. Associated with such cooling are several
other climate effects, as the atmospheric heat content is a critical driver of
storms and other weather extremes. Evidence from computer simulations
suggests that moderate deployment of SAI could help to rapidly alleviate some of
the key impacts of climate change [20, 21, 22, 23].
•
Reduction of regional climate risks:
SRM could be deployed in specific regions to
mitigate local climate risks. For example, MCB could be used in tropical regions
experiencing marine heat waves to reduce sea surface temperatures. This
reduction could help mitigate the heat stress on coral reefs, which – alongside
ocean acidification – drives coral bleaching [24]. Similarly, CCT might be used
over the Arctic Ocean to increase the emission of longwave radiation, helping to
reduce sea ice melt [25]. However, regional environmental and weather risks may
limit the scalability of regional SRM techniques.
Francesco Ungaro / Unsplash
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•Continuity in masking the greenhouse effect: The burning of fossil fuels has not
only produced greenhouse gas emissions that have had a warming effect but
also released particles and tiny droplets (aerosols) that contribute to a cooling
effect. These aerosols are estimated to mask up to one-third of the full warming
impact of greenhouse gases [26]. As aerosol emissions decline due to stricter air
particulate regulations, the masking effect is decreasing, leading to an
acceleration of warming. SRM, particularly through SAI, could replicate the
cooling effects of these aerosols by dispersing them in the upper atmosphere. By
doing so, SRM could achieve a similar cooling effect while significantly reducing
the direct harm to human health and ecosystems, continuing to mask some of
the warming caused by greenhouse gas emissions.
•Cost-effective protection: The costs of SRM techniques are small compared to
the projected and already experienced global damages from climate change.
Some projections of climate destruction are between $19–59 trillion per year by
2049 [5]. In contrast, direct SAI deployment is estimated to cost 10,000 times less
($2.25–5.25 billion per year) [27]. However, such estimates do not account for
potential indirect arising from environmental, societal, or political challenges.
Direct cost comparisons are also complex, as investments in adaptation could
reduce climate damages while simultaneously delivering additional societal
benefits, such as improved infrastructure, public health, and ecosystem resilience.
1.2
State of research on risks, uncertainties, and ethical
considerations:
•Uncertain regional impacts: SRM could affect regional climates unpredictably,
potentially altering precipitation patterns or monsoons, which are critical for
agriculture and water resources in many parts of the World. For example, SAI
could shift the location of equatorial bands of clouds, such as the Intertropical
Convergence Zone (ITCZ), north or south [28]. These equatorial bands influence
seasonal monsoon wind patterns, which are vital for delivering rainfall to areas
such as South Asia and West Africa.
•Environmental and atmospheric side effects: SAI could increase tropospheric air
pollution, accelerate ozone depletion, and make the sky appear hazier [29].
Additionally, SRM techniques will have no effect on ocean acidification, which will
continue to worsen unless emissions are significantly reduced.
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•Termination shock: A sudden, sustained termination of SRM could lead to rapid
and extreme warming, as accumulated greenhouse gases would no longer be
counteracted by SRM’s cooling effect. This could also result in abrupt changes in
precipitation patterns, exacerbating the risk of climate-related challenges [30].
•Governance challenges: SRM raises complex governance challenges, particularly
concerning who would control, regulate, and maintain its deployment. The lack of
existing international frameworks for SRM creates a risk of unilateral use by a
single country or coalition, potentially leading to international tensions.
•‘Moral hazard’: Concerns about SRM include the risk that reliance on such
technologies might reduce the urgency for emissions reductions and carbon
removal if it is perceived as a quick, easy and relatively cheap fix [31]. Additionally,
small-scale field experiments raise the risk of a slippery slope, where incremental
steps towards broader SRM implementation occur without sufficient public
debate, transparent decision-making, or robust governance frameworks.
Consequently, there are uncertainties about the risks, benefits, and technical feasibility
of SRM, and the accuracy of the climate models used to predict their effects. Small-
scale field experiments will be an important component in reducing the uncertainty of
these models and assessing the potential impacts, allowing governance frameworks
to be established before any consideration of large-scale deployment. Ultimately, it
is important that SRM activities are transparent, include public engagement, and
where possible, be conducted multilaterally to ensure broad participation and
oversight [30].
Matt Palmer / Unsplash
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2. Stratospheric Aerosol Injection (SAI)
2.1
SAI – what is it and what might it do?
SAI involves injecting sulphur dioxide (SO2) into the stratosphere to reflect a portion of
the incoming shortwave solar radiation. SO2 is a gas that disperses easily in the
stratosphere and rapidly converts to sulphuric acid (H2SO4). The H2SO4 forms tiny
aerosol particles, either by nucleating to form tiny liquid droplets or by condensing
onto existing particles [32]. These H2SO4 aerosol particles create a whitish haze that
strongly scatters light, especially in visible light. The typical lifespan of the particles in
the stratosphere is over 1 year.
SAI aims to mimic the natural cooling effect observed after large volcanic eruptions,
by increasing Earth’s reflectivity (albedo). For example, the 1991 eruption of Mount
Pinatubo in the Philippines released 20 million tonnes of SO2 into the stratosphere,
forming a hazy layer of H2SO4 aerosol particles that covered the Earth in 22 days. This
layer reflected sunlight and decreased the average global temperature between 1991–
1993 by 0.6ºC [33, 34].
In addition to volcanic SO2 emissions, human activities – such as the burning of fossil
fuels – also release substantial amounts of greenhouse gasses. In 2005, 25 billion
tonnes of CO2 were emitted, along with 55 million tonnes of SO2 into the lower
atmosphere, causing significant air pollution. Approximately half of this SO2 was
converted to H2SO4 aerosol particles in the atmosphere, amongst other particles, with
the remainder deposited onto the surface of the Earth [35]. These aerosol particles
partially mask the warming effects of anthropogenic CO2 emissions, contributing to
an estimated cooling of ~0.5ºC [19]. However, In 2023, CO2 emissions reached a
record ~37.4 billion tonnes, while global SO2 levels have decreased by ~50% between
2005 and 2021 [36, 37]. Therefore, SAI also aims to counteract the warming effects
that could result from declining SO2 emissions, as fewer aerosols in the atmosphere
reduce their cooling contribution.
Kaushik Panchal / Unsplash
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2.2
Mechanism and process of SAI
1. Deployment:
Particle-forming SO2 gas would be injected into the stratosphere via
modified aircraft at altitudes of ~13 km in the polar regions or ~20 km in the
tropics. Alternative particles under consideration include calcium carbonate
(CaCO3) or aluminium oxides (Al2O3), which may offer reduced risks to the ozone
layer but are poorly understood compared to SO2.
2. Conversion to aerosols:
Once in the atmosphere, the SO2 particles rapidly react with
water vapour and other compounds, forming H2SO4 aerosols that are 0.1–1 µm in
diameter. These aerosols scatter shortwave solar radiation, increasing the Earth’s
reflectivity.
3. Particle lifespan and distribution:
The H2SO4 aerosols are dispersed globally by
stratospheric winds, forming longitudinal bands in ~22 days (east–west) and
gradually spreading latitudinally (north–south) over several months. The particles
typically remain suspended in the stratosphere for a year or two.
4. Monitoring and maintenance:
continuous injections of SO2 are required to maintain
a persistent aerosol layer as the aerosols eventually settle out of the stratosphere.
Ongoing monitoring using satellites and atmospheric sensors would be used to
detect any unintended environmental side effects, such as changes in precipitation
patterns or ozone depletion. Pre-established offramps (mitigation plans) would
allow for the suspension of operation if anomalies are detected. While the aerosols
would naturally dissipate over time, their typical lifespan of over a year means the
cooling effect would persist temporarily even after injections are halted.
2.3
Positive environmental and climate effects of SAI
•
Cooling effects:
SAI has high cooling efficacy, with one kilogram of SO2 injected
into the stratosphere capable of countering the warming caused by over 100
tonnes of CO2 [38]. For instance, injecting 8–16 million tonnes of SO2 into the
stratosphere per year could mask approximately 1ºC of warming, equivalent to a
~1% reduction in solar radiation [39]. Without mitigation, temperatures would
continue to rise, requiring increasing amounts of SO2 to maintain the cooling
effect. Although SAI has not yet been tested at scale, volcanic eruptions serve as
natural proxies, demonstrating that large releases of SO2 can produce significant
climatic impact [33, 34].
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•
Peak-shaving:
As temperatures rise, there is a risk of amplifying feedback loops –
such as permafrost thawing or Arctic sea ice loss – that could exacerbate warming
and trigger irreversible tipping points in Earth’s climate system [2]. Deploying SAI
could temporarily mitigate (i.e., peak-shave) the most immediate effects of climate
change, while carbon emissions reductions and carbon removal solutions increase
in their scale and capacity.
•
Reduction in extreme weather events:
By temporarily stabilising the atmospheric
temperatures, SAI may be able to reduce the frequency of extreme events.
Droughts may be reduced as the frequency of heatwaves and consecutive dry days
decrease [40]; flood risks in flood-prone areas such as Southeast Asia may
decrease [41]; and the frequency and intensity of tropical cyclones may decrease,
as evidenced by low-latitude volcanic eruptions [42].
•
Cost effectiveness:
The investment costs relative to the effects of climate change
are extremely low, with an estimated price of $2.25–5.25 billion per year to cut the
average projected increase in radiative forcing in half [27]. Releasing SO2 particles
at low latitudes would result in near-global aerosol coverage and decrease global
temperatures, which could feasibly be achieved with a fleet of a few hundred
modified aircraft. However, while a high-latitude injection program is considered
logistically feasible, its implementation is expected to require a decadal timescale
before deployment [43]. Logistical challenges are more pronounced in tropical
regions, where the higher tropopause necessitates additional aircraft modifications
to reach the required altitudes. Nevertheless, model studies and historical
analogues from volcanic eruptions indicate that once initiated, SAI could rapidly
reduce global temperatures within weeks to months.
2.4
Negative environmental and climate effects of SAI
•
Changes to precipitation patterns:
The effects that SAI can have on global and
regional precipitation patterns are complex and dependent on the location and
amount of injection [44]. Models predict a slight overall decrease in global
precipitation but indicate larger regional fluctuations. For example, SAI may reduce
monsoon rainfall in Asia and Africa, while increasing flood risks in typically arid
regions like Mexico, Australia, and the Southwestern U.S. [41, 45]. These
interactions introduce significant uncertainties regarding impacts on water
resources, ecosystems, and agriculture. Monsoon rains, crucial for agriculture and
biodiversity in affected regions, may be disrupted. While models provide initial
predictions, their results vary, emphasising the need for field tests to better
understand SAI’s full effects on the hydrological cycle [9].
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•
Environmental effects:
SAI affects stratospheric chemistry and dynamics, with one
of the most significant impacts being on stratospheric ozone. Aerosols provide
surfaces for chemical reactions that accelerate ozone depletion, particularly over
the polar regions [17]. The deepening of the Antarctic ozone hole and the delay in
ozone recovery are of particular concern. SAI with sulphates would also affect the
ozone layer, which protects the Earth from harmful ultraviolet rays, by delaying the
recovery of the ozone hole by up to a few decades [39]. Additionally, ocean
acidification will continue to worsen while SAI is undertaken if significant
reductions in emissions are not also undertaken.
•
Termination shock:
Termination shock is a rapid and substantial rise in global
temperatures following cessation of SRM deployment. It is not an inherent feature
of SAI deployment but is a potential risk if SAI is abruptly stopped after
implementation has begun. If SAI deployment is geographically distributed,
protected by redundant hardware, and managed multilaterally by several powerful
nations, the risk of termination shock can be minimised [46]. However, if SAI is used
to mask high levels of climate change without corresponding reductions in
greenhouse gas concentrations, the impacts of termination shock could be severe.
These impacts could include rapid warming, disrupted weather patterns,
ecosystem collapse, and food insecurity, which could outweigh the temporary
benefits of SAI deployment. Therefore, SAI implementation requires careful
planning and robust long-term governance frameworks to be established in order
to prevent abrupt termination.
2.5
Summary of the evidence on benefits and risks of SAI
SAI could have the potential to reduce some of the most immediate impacts of
climate change by rapidly cooling the planet. This rapid cooling might help reduce
extreme weather events, serving as a temporary buffer to the effects of climate
change while longer-term carbon mitigation strategies are scaled up. However, its
deployment may also unevenly affect regional climate change and create significant
unintended consequences. These risks include altered precipitation patterns, ozone
depletion, and the potential for termination shock, all of which could have severe
global consequences that may outweigh the temporary benefits of the technology.
Outside the realm of technical considerations, the complexities of (geo)politics and
the risks associated with imperfect use cases need additional consideration.
Therefore, the exploration of SAI as a potential climate intervention technique
underscores the necessity for a cautious, well-governed approach. Such an approach
should recognise and address its dual potential: to mitigate climate risks, such as
global temperature rise, while also inducing new risks to ecosystems, weather
patterns, and regional climates. This duality is further complicated by the significant
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uncertainties inherent in current scientific models, which may not fully capture the
complex, long-term interactions of SAI with Earth’s climate system, as well as by
political uncertainties, which are not comprehensively examined in this technical
scoping.
2.6
Research and field testing on SAI
The majority of knowledge about SAI comes from climate modelling and natural
analogues, particularly from the 1991 Mount Pinatubo eruption. Extensive modelling
research into the cooling efficacy of SAI has taken place, with useful and interactive
emulators now being developed allowing for the impact of different CO2 emissions
and SAI scenarios to be modelled (emulators are simplified models used to simulate
the impact of various SAI and CO2 emissions scenarios on global climate systems)
[47]. These emulators are also helpful in demonstrating the global impacts of SAI
deployment to policymakers and the public. However, physical field tests have been
limited, with small low-altitude sulphur releases in Russia aimed at testing the efficacy
of sulphur particles in reducing solar transmittance (the portion of solar radiation
passing through a medium), and small-scale performative releases by private
companies and individuals in the USA and the UK [48]. Larger projects – e.g. the SPICE
project in the UK, which focused on the delivery mechanisms for SO2, and SCoPEx in
the USA, which aimed to study aerosol behaviour in the stratosphere – were cancelled
due to governance issues and a lack of public engagement, respectively [48].
Moving forward, small-scale field tests may become relevant to advancing the
understanding of SAI’s impacts and verifying model predictions. Conducting these
tests under robust international governance could help ensure transparency and
minimise risks. Key areas of research areas include injection technology, aerosol
microphysics, plume physics, and the effects of SAI on physical processes in targeted
conditions.
2.7
Conclusions on SAI
From a theoretical and technical standpoint, the evidence suggests that SAI could
have the potential to serve as a temporary means to rapidly cool the planet. Practical
and broader considerations, which are not examined here, require much more
attention through a cautious and measured approach. Transparent, multilateral
governance will be essential to ensure that future research, including small-scale field
tests, addresses the many remaining uncertainties and risks. Fundamentally, SAI
cannot be regarded as a standalone solution but at best as a potential complement to
long-term carbon emissions reduction and removal policies, which remain the primary
means to addressing climate change alongside efforts for adaptation.
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What is SAI?
SAI involves injecting sulphur dioxide (SO ) into the stratosphere that forms
sulphuric acid (H SO ) aerosol particles.
The aerosol particles reflect a portion of the incoming solar radiation,
mimicking the cooling effects of large volcanic eruptions.
Key Benefits
Rapid and significant cooling of the climate, potentially helping to mitigate
climate change – confirmed by natural events such as large volcanic eruptions.
Potential reduction in extreme weather events e.g., regional heatwaves,
hurricanes, or droughts.
Very low-cost relative to the effects of climate change.
Allows long-term carbon reduction strategies to take effect.
Key Risks
Risk of adversely affecting global hydrological systems.
Decrease in atmospheric ozone.
Significant risk of termination shock if stopped abruptly.
Short-term solution that doesn’t address emissions.
Govenance
and Ethics
‘Moral hazard’ – reducing the urgency of emissions reduction.
Absence of a global governance framework.
Equity and justice concerns – ‘winners and losers’
‘Slippery slope’ – where broader SRM implementation occurs without
sufficient governance or robust debate.
Figure 4. SO particles released into the stratosphere turn into tiny H SO aerosols that remain
in the atmosphere for long periods and scatter incoming shortwave radiation.
19
Stratospheric Aerosol Injection (SAI): Key Takeaways
Table 1
2
2 4
2 2 4
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3. Marine Cloud Brightening (MCB)
3.1
MCB – what is it and what might it do?
While SAI focuses on injecting reflective aerosols into the stratosphere to reduce
global temperatures, MCB is a distinct SRM technique aimed at increasing the
reflectivity of low-lying marine clouds. Unlike SAI, which operates in the upper
atmosphere and has a broader, more uniform cooling effect, MCB targets specific
regions and leverages the natural properties of marine stratocumulus clouds to reflect
more sunlight back into space. These approaches could be complementary, with SAI
potentially addressing global warming on a large scale, while MCB could offer more
localised or adjustable cooling effects, particularly over oceanic regions.
MCB involves spraying fine particles of sea salt (NaCl) into the lower atmosphere to
form or enhance the brightness of marine stratocumulus clouds. NaCl is a non-toxic
and abundant resource, with approximately 3.3 billion tonnes naturally transported
from the ocean to the atmosphere in sea spray each year [49]. Small NaCl particles
act as cloud condensation nuclei (CCN), encouraging water vapour to condense. This
results in more, smaller droplets within the cloud, increasing its brightness and
enhancing the reflection of incoming solar radiation through a process known as the
Twomey Effect [50].
Currently, shipping inadvertently contributes to MCB by releasing sulphate aerosols in
exhaust fumes, which act as CCN and form ship tracks – a type of cloud associated
with shipping activity [51]. These ship tracks provide an estimated cooling of ~0.25ºC
by partially masking the warming effects of CO2 emissions [52]. However, 2020
regulations by the International Maritime Organization (IMO) require an 86% reduction
in sulphur content in shipping fuel, leading to reduced CCN formation from SO2
aerosols and a corresponding decrease in the cooling effect of ship tracks. Therefore,
MCB could help mask the warming effects that may arise from reduced ship tracks to
new shipping emissions regulations.
Ahmed Saalim Hussain / Unsplash
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3.2
Mechanism and process of MCB
1. Spraying:
Fine NaCl particles are released into the lower atmosphere from ship- or
land-based specialised sprayer devices. These particles are intended to brighten
marine clouds by increasing their reflectivity.
2. Cloud condensation nuclei (CCN):
The NaCl particles act as CCN, providing
surfaces onto which water vapour can condense to form cloud droplets. As more
droplets form, the same amount of water vapour is distributed across a larger
number of smaller droplets. This increases the cloud’s ability to reflect solar
radiation, effectively enhancing the Earth’s albedo.
3. Particle lifespan and distribution:
The suspended NaCl particles have a 2- to 3-day
lifespan in the atmosphere before they fall back to the ocean due to precipitation.
The effect of the additional CCN on clouds can last for up to a few weeks. MCB can
be deployed locally (over areas on the order of 10 kilometres) or regionally (over
100 kilometres or more), to influence solar radiation on a globally significant scale.
The warm marine boundary layer stratocumulus clouds, commonly found in the
southeast Atlantic Ocean and the eastern Pacific Ocean, are the primary target
areas for MCB [53].
4. Monitoring and maintenance:
Continuous spraying of NaCl particles is required to
maintain a consistent presence, as the particles will fairly rapidly settle out of the
atmosphere. Ongoing monitoring using satellite and remote sensing technologies
would be necessary to measure the changes in cloud albedo and detect any
unintended consequences, such as disruptions to precipitation or marine
ecosystems. Pre-established offramps would allow for the suspension of MCB
operations if anomalies or negative impacts are detected. The effects of halting
MCB are estimated to reverse within a time frame of 7-10 days.
3.3
Positive environmental and climate effects of MCB
•
Cooling effects:
By enhancing the albedo of marine stratocumulus clouds,
MCB can
cool the Earth’s surface by reflecting a larger portion of incoming solar radiation
back into space. While the total cooling capacity of MCB remains uncertain, one
study estimated that a fleet of 1500 vessels conducting MCB could mask
approximately 3ºC of warming [54]. Large-scale MCB deployment could therefore
have a significant effect on climate change, particularly in areas where marine
stratocumulus clouds are common. Modelling studies indicate that MCB
deployment in the eastern Pacific could decrease global temperatures similar to the
La Niña phenomenon and lead to increases in polar sea ice [55]. Additionally,
targeted local cooling can be deployed in ocean regions that are particularly
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sensitive to warming, such as polar regions and areas prone to experiencing marine
heat waves. Stabilising ocean surface temperatures in tropical storm development
areas may also help weaken storms before they develop into hurricanes, although
further research is needed to validate the efficacy of these efforts.
•
Protection of marine ecosystems:
Localised MCB deployment can lower ocean
surface temperatures in areas where climate change-induced marine heatwaves
threaten ecosystems. In 2024, coral reefs face significant risks from exceptionally
warm ocean temperatures, driving the fourth global-scale coral bleaching event
(previous events: 1998, 2010, 2014–17) [2]. To counter this, small-scale field tests
are currently being undertaken by researchers on the Great Barrier Reef to assess
the effectiveness of MCB in shading and protecting corals from bleaching [56]. The
results of these tests will be crucial in determining whether MCB can be used more
broadly to protect sensitive marine ecosystems.
•
Counteracting the reduction in tropospheric aerosols from shipping:
The
implementation of recent regulations on shipping SO2 emissions is expected to
lead to a decrease in inadvertent cooling effects caused by shipping aerosols. MCB
has been proposed as a potential approach to partially compensate for this
reduction by releasing non-toxic NaCl particles. This could potentially provide
continuity of the masking effect that is being undone with these regulatory
changes, but further research is needed to evaluate its feasibility, effectiveness, and
potential unintended consequences.
•
Cost effectiveness:
The estimated logistical costs for MCB are very low relative to
the potential impacts of climate change, with estimated logistical costs around
~$1–10 billion per year [10]. While the costs of scaling MCB are uncertain, it could
achieve significant cooling if implemented at a sufficient scale. However,
uncertainties persist regarding the technological readiness level of MCB, as key
cloud processes and the effects of varying particle sizes are not yet fully
understood [19, 57]. It has been suggested that 5-year research programs focusing
on modelling, technical development, and small-scale field experiments could
advance the operational feasibility of MCB [58]. Nevertheless, this timeline and its
outcomes remain subject to debate and further scrutiny.
Dillon Hunt / Unsplash
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3.4
Negative environmental and climate effects of MCB
•
Changes to precipitation patterns:
by influencing sea surface temperatures and
cloud properties, MCB carries the potential to pose serious risks to circulation
patterns and disrupt regional precipitation [53]. The cooling effects of MCB in some
areas can have teleconnections (i.e., significant relationship between weather
phenomena at distant locations on Earth) with negative repercussions in other
regions. For instance, modelling of MCB in the South Atlantic suggests that strong
localised cooling could lead to significant decreases in precipitation over the
Amazon [59]. Similarly, smoke from the 2019–20 Australian wildfires brightened
clouds in the southeastern Pacific, contributing to the abnormally persistent ‘Tiple-
Dip’ La Niña event of 2020–23, which may have masked rising global mean surface
temperatures [60]. Modelling also suggests that MCB in the northeast Pacific could
lead to decreased precipitation over the Sahel and western United States [61].
Consequently, the effects of MCB on precipitation patterns may be significant, but
substantial uncertainties between different models prevail.
•
Unequal temperature changes:
MCB may lead to teleconnections that result in
negative temperature repercussions in certain areas. For instance, MCB
deployment in the South Atlantic may cause temperature increases in addition to
drying in the Amazon [59]. At the same time, MCB in the North Pacific may
exacerbate heat stress in Northeast Asia, Europe, and Central America [61]. These
regional disparities underscore the need to better understand potential MCB risks
before considering widespread deployment. The uneven distribution of effects
(potentially resulting in regional ‘winners’ and ‘losers’) raises critical concerns about
equity and climate justice, as marginalised communities may be disproportionately
affected. Addressing these disparities within governance frameworks is essential
to ensure equitable outcomes and minimise potential injustices.
•
Impact on marine ecosystems:
There are concerns that MCB may reduce the
availability of light for phytoplankton to photosynthesise by increasing cloud cover
and lowering ocean temperatures. While this impact is unlikely to be significant
compared to the damage caused by climate change, it could still affect marine
ecosystems. Further research is necessary to understand the long-term effects on
marine biodiversity, particularly at the base of the marine food chain.
•
Termination shock:
MCB’s cooling effects are short-lived, as the NaCl particles fall
out of the atmosphere after a few days. Continuous deployment would be required
to maintain the cooling effect, and any sudden termination of MCB without
corresponding decreases in greenhouse gas concentrations could result in
termination shock. This sudden temperature rebound could lead to rapid warming,
potentially exacerbating current climate change and extreme weather events.
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3.5
Summary of the benefits and risks of MCB
By enhancing the brightness of marine stratocumulus clouds, MCB has the potential
to provide significant cooling. MCB may also protect vulnerable ecosystems, such as
coral reefs, from the threats of coral bleaching, and it may have far-reaching beneficial
teleconnections, such as slowing Arctic sea ice loss by spraying NaCl into the
atmosphere in the eastern Pacific. However, MCB and aerosol-cloud interactions in
general are fraught with uncertainties, including their potential to alter precipitation
patterns and increase local temperatures, potentially creating regional ‘winners’ and
‘losers’ with important equity implications. Of particular concern are the threats to the
Amazon from MCB deployment in the southern Atlantic. To consider MCB as a viable
option for limiting the effects of climate change, small-scale field testing may become
an important means to address the significant uncertainties between models.
3.6
Research and field testing of MCB
The majority of current knowledge about MCB comes from climate modelling, but
several field tests have been undertaken to examine its efficacy. The earliest
experiment, E-PEACE (July–August 2011), off the coast of California, USA, explored
how SO2 and NaCl particles affect cloud albedo [62]. Since 2020, field experiments
have been taking place in the Great Barrier Reef, off the coast of Queensland,
Australia, testing the potential of NaCl particles to increase cloud albedo and protect
coral from warming-induced bleaching [24, 56]. In April 2024, an experiment off the
California coast aimed to test equipment designed to create and measure aerosol
plumes over 20 weeks but was cut short due to concerns over transparency raised by
the local Council [63]. Future small-scale studies should prioritise critical research
areas, such as the behaviour of aerosol particles in varying atmospheric conditions, to
refine existing climate models.
3.7
Conclusions on MCB
Current MCB modelling studies reveal substantial variability of its potential impacts,
and reducing these uncertainties would be essential for considering MCB viability or
risks. Small-scale local deployments, such as those in the Great Barrier Reef, may
offer valuable protection for vulnerable ecosystems, providing important insights into
the feasibility of MCB. Nevertheless, large-scale regional deployments aimed at
cooling the planet would entail significant risks and should only be considered after
exhausting all other research approaches, including small-scale field tests. Such
deployments might only be deemed permissible if conducted within a robust,
multilateral, and transparent governance framework that ensures public engagement
and fosters international cooperation throughout the process.
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What is MCB?
MCB involves spraying fine sea salt particles (NaCl) into marine clouds to
increase their reflectivity, enhancing the amount of sunlight reflected back
into space.
Key Benefits
Localised cooling effects in heat or drought-prone areas, depending on the
area of deployment.
May cool sea surface temperatures to protect coral reefs.
Low cost relative to the effects of climate change.
Reversible within a few days to weeks.
Can alternate temperature overshoot while long-term carbon reduction
strategies take effect.
Key Risks
Potential disruption to global hydrological cycles e.g., teleconnections linking
regional cooling in certain areas with regional drought in other areas.
Limited global cooling effect compared to SAI.
Difficulty controlling scale and geographical extent of cooling.
Risk of termination shock if stopped abruptly.
Short-term solution that doesn’t address the root cause.
Govenance
and Ethics
Moral hazard – reducing urgency of emissions reductions.
Absence of a global governance framework.
Equity and justice concerns – ‘winners and losers’.
Slippery slope’ – where broader SRM implementation occurs without
sufficient governance or robust debate.
25
Figure 5. NaCl particles sprayed into the troposphere form a larger number of smaller
cloud droplets, increasing cloud brightness and scattering shortwave radiation.
Marine Cloud Brightening (MCB): Key Takeaways
Table 2
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Simon Fitall / Unsplash
4. Cirrus Cloud Thinning (CCT) and Mixed-phase Cloud
Thinning (MCT)
4.1
CCT and MCT - what is it and what might it do?
While SAI and MCB focus on reflecting more sunlight back into space to reduce global
temperatures, CCT and MCT take a different approach by targeting Earth’s thermal
radiation balance. These techniques aim to increase the amount of thermal radiation
escaping into space, rather than primarily addressing incoming solar radiation. These
complementary strategies could work in tandem with SAI and MCB, as they target a
different aspect of the climate system.
CCT involves modifying high-altitude cirrus clouds in the upper troposphere, which
are composed of small ice crystals that form at temperatures below -38ºC and
typically trap heat by absorbing more thermal radiation than they reflect solar
radiation [64]. By injecting non-toxic particles that encourage the formation of larger
ice crystals, CCT reduces the optical thickness of cirrus clouds, allowing more thermal
radiation to escape into space. This process also causes larger ice crystals to
precipitate out as snow, further diminishing the warming effect of these clouds.
MCT works similarly to CCT, but targets clouds that contain both supercooled liquid
water droplets and ice crystals. These mixed-phase conditions typically occur at lower
altitudes than cirrus clouds and temperature ranges between 0ºC and -38ºC [65]. MCT
promotes ice crystal formation in these clouds, thinning them and reducing their
capacity to trap thermal radiation. Together, CCT and MCT address distinct types of
clouds, providing additional pathways for cooling the planet by mitigating heat
retention in the atmosphere.
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4.2
Mechanism and process of CCT and MCT
1. Injection: For CCT, particles are injected into the upper troposphere using balloons,
drones, or aircraft. Once injected, these particles act as catalysts for the formation
of larger ice crystals. For MCT, if the temperature of the ground is near -5ºC, it may
be possible to inject particles at low altitudes as they may form ice crystals at
temperatures below -10ºC.
2. Ice crystal formation and cloud thinning: the injection of ice nucleating particles
promotes the formation of fewer but larger ice crystals, which precipitate out of the
cloud as snow. This process reduces the amount of water vapour and ice in the
cloud, causing more quickly, reducing the amount of water vapour and ice in the
cloud, causing it to become physically and optically thinner. As thinner clouds trap
less thermal radiation, this leads to a cooling effect. However, because the particles
precipitate out, regular injections are necessary to maintain the desired impact on a
broader scale [65].
4.3
Positive environmental and climate effects of CCT and MCT
•Cooling effect: CCT and MCT reduce the ability of cirrus and mixed-phase clouds to
trap thermal radiation, allowing more heat to escape into space, resulting in a
cooling effect. While the global cooling potential is debated, modelling studies
estimate that CCT and MCT could cool global mean temperatures by
approximately 1ºC or more [66]. The traditional SRM techniques such as SAI and
MCB are most effective when the sun is high in the sky as they reflect solar
radiation. In contrast, CCT and MCT are most effective when the sun is low in the
sky or during the nighttime when there is little to no incoming solar radiation and
these clouds only trap thermal radiation [67]. CCT and MCT are therefore
particularly useful in the polar regions during winter, where cirrus clouds trap
longwave radiation, contributing to a warming effect. Consequently, modelling
studies indicate there is a significant cooling potential in the Arctic, with a twofold
increase in escaping thermal radiation occurring in response to CCT and MCT [68].
Additionally, CCT and MCT have high potential in mountainous regions, where
cirrus clouds or supercooled stratus clouds are frequently found [69].
•Cryosphere benefits: CCT and MCT have substantial potential to cool Arctic and
mountainous regions during the winter, potentially helping to slow the decrease in
polar sea ice and retreat of mountain glaciers [68].
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•Cost-effectiveness: The estimated investment costs relative to the effects of
climate change are low, indicating that CCT and MCT may be cost-effective
techniques. However, operating in the polar regions poses challenges, and the rapid
precipitation of the particles means they will need constant replacement to sustain
a cooling effect. While these techniques could complement mitigation efforts by
reducing thermal radiation trapping and cooling targeted regions, their overall cost-
effectiveness and scalability are likely secondary to SAI and MCB.
4.4
Negative environmental and climate effects of CCT and MCT
•Changes to precipitation patterns: Since the purpose of CCT and MCT is to reduce
the warming effect of cirrus and mixed-phase clouds, they could impact regional
weather patterns, particularly precipitation patterns. Although there are significant
uncertainties, modelling indicates that CCT and MCT could cause minor
enhancements to the global hydrological cycle, resulting in small increases in
global precipitation. However, these interventions could also lead to large regional
and seasonal shifts in rainfall, including potential impacts on the monsoon systems
[66].
•Unequal temperature changes: CCT and MCT may create regional disparities in
their climatic effects. For example, while these techniques could result in significant
cooling effects in the polar regions, they might cause drying or warming in other
areas. Some modelling studies have even suggested that if ‘overseeding’ occurs
(i.e., where too many particles are injected), cirrus clouds could increase in physical
and optical thickness, inadvertently causing a warming effect instead of cooling
[70]. However, uncertainties remain regarding the exact conditions under which
overseeding might occur and its broader climatic implications. Determining the
correct injection thresholds is crucial to prevent such negative consequences,
particularly in regions already experiencing rapid warming.
•Termination shock: The cooling effects of CCT and MCT are short-lived, as the
injected particles precipitate out of the atmosphere. To sustain the cooling effect,
continuous deployment of the injected materials is required. If CCT or MCT
deployment were to suddenly stop, it could lead to a rapid rebound in temperatures,
exacerbated by the presence of high levels of greenhouse gases in the atmosphere.
Brian Cook / Unsplash
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4.5
Summary of evidence on expected benefits and risks
of CCT and MCT
By decreasing the physical and optical thickness of cirrus clouds and mixed-phase
clouds, CCT and MCT have the potential to induce cooling in polar and mountainous
regions. This could substantially slow the decline of sea ice and glaciers in these
areas. However, the cooling efficacy of CCT and MCT varies significantly between
different models, making it uncertain whether these techniques can achieve the
desired cooling or if they could even cause inadvertent warming (through the
‘overseeding’ of clouds). Additionally, there are concerns about the negative impacts
on regional precipitation patterns, though models indicate a small overall effect on the
global hydrological cycle. Resolving these model uncertainties, particularly around
‘overseeding’, would eventually require small-scale field testing in polar and
mountainous regions. Given the large uncertainties, CCT and MCT cannot currently be
considered viable options for counteracting the effects of climate change at scale.
4.6
Research and field testing of CCT and MCT
The effects of CCT and MCT have primarily been studied through modelling [e.g., 67,
68, 70, 71]. However, the CLOUDLABS project in Eriswell, Switzerland has been
conducting small-scale MCT field experiments since 2021. These experiments use
stable wintertime stratus clouds as a natural laboratory to study ice crystal formation
from injected particles, contributing to the validation of existing models [69]. Future
studies could focus on understanding the plume physics of various particles
introduced under a range of atmospheric and cloud conditions, including polar
regions, to better inform potential applications and assess associated risks.
4.7
Conclusions on CCT and MCT
There are important avenues to explore the uncertain potentials of CCT and MCT to
temporarily and regionally cool temperatures, particularly in polar and high-altitude
regions, but uncertainties regarding their efficacy and potential impacts remain very
large. Small-scale experiments could help reduce model uncertainties substantially.
Large-scale tests raise concerns of inter-regional knock-on effects and thus may only
be considered at a later stage – under robust, multilateral, and transparent
government frameworks. As with all SRM techniques, ensuring public engagement
and international cooperation already at the research stage can be essential.
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What is CCT
and MCT?
CCT and MCT aim to reduce the thickness of cirrus and mixed-phase clouds,
respectively, which trap heat by absorbing thermal radiation.
Key Benefits
Localised cooling, particularly in polar or high-altitude regions.
May decrease polar sea ice loss and glacier retreat.
Low cost relative to the effects of climate change.
Reversible within a few days.
Provides time for long-term carbon reduction strategies to take effect, providing
they are implemented.
Key Risks
High uncertainty regarding feasibility and cooling efficacy.
Limited scope and scale to address global climate change. Much lower global
cooling capacity than SAI and MCB.
Potential disruption to global hydrological cycles.
Unequal regional temperature changes.
Risk of termination shock if stopped abruptly.
Short-term solution that doesn’t address the root cause.
Govenance
and Ethics
Moral hazard – reducing urgency of emissions reductions.
Absence of a global governance framework.
Equity and justice concerns – ‘winners and losers’.
Slippery slope’ – where broader SRM implementation occurs without sufficient
governance or robust debate.
Cirrus (CCT) and Mixed-Phase (MCT)
Cloud Thinning: Key Takeaways
30
Figure 6. Particles released into the troposphere/stratosphere increase ice crystal size,
causing precipitation and decreasing absorption of outgoing long-wave radiation.
Table 3
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5. Other technologies with similar issues
In addition to the three main SRM techniques covered in this scoping note, other
techniques aim to reflect incoming solar radiation. However, these alternative
techniques are mostly theoretical, with low technology readiness levels and limited
scope, often focusing on specific regions, such as the Arctic, with only modest cooling
potential [71, 72]. Several of the most relevant technologies are described below.
Space-based reflectors:
This would involve placing giant mirrors or shading materials
between the Earth and the Sun to block or deflect a fraction of incoming solar
radiation [73]. Space-based reflectors could potentially cool the planet on a scale
comparable to SAI and without the associated risks to the hydrological cycle or ozone
layer. However, this technology is not broadly considered viable owing to its low
technology readiness level, long development timeline, and prohibitive costs, which
are estimated at between $1–20 trillion [73].
Sea ice modification:
This would involve increasing the albedo of sea ice by adding
reflective materials, such as silica microspheres, or increasing ice thickness by
pumping water onto the ice surface. However, both approaches may hold significant
limitations and risks. Albedo modification through silica microspheres would require a
dramatic increase of their global production volumes, and the potential environmental
risks of these materials remain unclear. There are also fears that the microspheres
would inadvertently accelerate sea ice loss [74]. Ice thickening may only temporarily
delay the disappearance of Arctic summer ice. The effectiveness for global cooling is
very low, with costs estimated at $50 billion per year to thicken merely 10% of the
Arctic by one meter [75].
Vadym Shashkov / Unsplash
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Surface albedo enhancement:
This strategy increases the reflectivity of urban areas,
croplands, or deserts by using reflective building materials, cultivating brighter crops,
or applying reflective coverings on the ground. While urban and cropland strategies
would be costly and offer only minor cooling effects, desert-based strategies could
have a larger climatic impact but may also lead to significant changes in global
circulation and the hydrological cycle [76]. There are also significant concerns about
secondary effects, particularly those arising from the large-scale use of artificial
materials, which could have unforeseen environmental and ecological consequences.
Therefore, whilst some urban planning practices, such as the brightening of urban
spaces, have a high technology readiness level, they lack scalability and are expensive
relative to other SRM techniques such as SAI, MCB, CCT and MCT. However, as
adaptation measures, they hold significant potential to counter the urban heat island
effect and thus save lives and raise building efficiency standards.
Contrail avoidance:
This involve altering flight paths or altitudes to reduce the
formation of contrails, which are created by aircraft exhaust fumes. Like to cirrus
clouds, contrails consist of tiny ice crystals that absorb outgoing thermal radiation
and account for 57% of the warming potential of aviation emissions [77]. Optimising
flight paths may reduce contrail formation by up to 59%, while improved engine
designs might reduce them by 90% [78]. Modelling indicates contrail avoidance poses
very low operational risks and could be achieved with limited additional costs,
estimated at around 0.19% [79]. Consequently, contrail avoidance may offer a
successful strategy to significantly decrease the ~4% of warming attributed to global
aviation [80].
Toa Heftiba / Unsplash
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6. Past and ongoing field experiments on SRM
Since 2008, SRM field experiments have expanded from initial feasibility trials to more
specialised and regionally focused projects. Early SAI studies, such as the Russian
SO2 injection experiments (2008–2010) and the UK-based SPICE project (2011),
primarily investigated the feasibility of particle injection methods. The cancellation of
SPICE before deployment underscored governance and public engagement
challenges, which have since shaped subsequent SRM trials.
In 2011, California’s E-PEACE experiment explored MCB’s effects on marine
stratocumulus clouds, laying foundational knowledge for future MCB projects by
illustrating how particles impact cloud albedo. Since 2020, field experiments have
grown more numerous and complex, exemplified by Australia’s Great Barrier Reef
MCB project, which aims to protect coral reefs from bleaching through cloud
brightening. In 2021, the CLOUDLABS project in Switzerland was the first to explore
how MCT affects stratus clouds by spraying particles into persistent winter clouds
over the Swiss Alps.
In recent years, several initiatives have undertaken small-scale experiments involving
the release of sulphur into the atmosphere via weather balloons:
•
SATAN:
A UK-based independent researcher launched high-altitude weather
balloons that released approximately 400 grams of SO2 into the stratosphere,
marking a potential first in SRM field experiments, albeit based on a very limited
scientific basis.
•
Make Sunsets:
A California-based startup releasing multiple balloons containing
SO2 into the stratosphere, marking the first private SRM company to sell ‘cooling
credits’, claiming that each gram of SO2 released offsets the warming effect of one
ton of CO2 for one year. These activities have sparked significant controversy due
to the very limited scientific basis and lack of regulatory oversight.
•
Stardust Initiative:
A US-Israeli company that may have conducted initial hardware
test flights and is planning further application-oriented tests related to atmospheric
interventions. Specific details about their experiments and objectives are currently
limited.
Engin Akyurt / Unsplash
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The increasing number of SRM field tests underscores a growing interest in reducing
uncertainties in computer models and advancing hardware development. However,
the scientific merit of many experiments has been disputed, making clarity on the
purpose and objectives of field tests crucial for assessing their overall desirability. As
field tests and hardware development progress, the technical focus remains on
addressing environmental risk, overcoming technical challenges, and resolving key
model uncertainties.
At the same time, the involvement of private actors introduces broader concerns
regarding desirability, political risks, and ethical implications. Developing robust
governance frameworks and regulations must navigate this complex landscape,
ensuring that field tests are transparent, accountable, and publicly funded, where
appropriate, to resolve uncertainties while serving the public interest.
There is also a pressing need to ensure that private actors operate transparently and
responsibly, with any technological advancements aligned with the public good. Such
efforts could contribute to a more informed and balanced discussion on the potential
benefits and risks of SRM, fostering greater understanding and guiding decisions
about its future development and deployment.
Figure 7. The history of different SRM field tests
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Table 4. SRM field experiments table
Title
Date
Type
Location
Russian SAI Experiments
[81]
2008–2010
SAI
Saratov Oblast, Russia
•Low altitude injection of SO2 particles from a car and helicopter over several different
tests
•Assessing aerosol behaviour and cooling effect.
SPICE
[81] 2008–2010 SAI England, UK
•Equipment testing of the pipe injecting water particles into the atmosphere at 1 km.
•
Cancelled
before starting – concerns regarding governance structures and IP rights.
E-PEACE [62] 2011 MCB California, USA
•SO2 and NaCl released from a ship and aircraft over a two-month campaign.
•Assessing how aerosol perturbations affect marine stratocumulus clouds.
SCoPEx [48, 72]
2021
SAI
Kiruna, Sweden
•Equipment testing of the gondola/balloon system used to inject CaCO3 or SO2 particles.
•
Cancelled
before starting – objections from the Saami Council about a lack of prior
engagement, potential socio-environmental impacts, and moral hazard.
SATAN [84] 2021 SAI England, UK
•<1 kg of SO2 released into the stratosphere by balloon on two separate occasions.
•Performative proof of concept experiments to prove SAI as controllable and low-cost.
Make Sunsets [48, 85]
2022-
SAI
California, USA
•<1 kg of SO2 released into the stratosphere by balloons on over 100 separate occasions.
•Performative for-profit experiments monetised via non-verified cooling credits.
CLOUDLABS [69]
2021-
MCT
Eriswell, Switzerland
•<1 kg AgI particles released from UAVs into mixed-phase stratus clouds during winter.
•Assessing ice crystal formation in stable winter stratus clouds to verify MCT models.
Great Barrier Reef MCB [24, 56]
2020-
MCB
Queensland, Australia
•NaCl particles released from a ship repeatedly during Austral Summer.
•Assessing the cloud response NaCl particles and MCBs effectiveness against bleaching.
University of Washington MCB [63]
2024
MCB
California, USA
•NaCl particles released from a ship to assess MCB plume development.
•Project duration was planned for 20 weeks.
•
Stopped
early – objections from Alameda Council regarding project transparency.
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7. Conclusions and key takeaways
Solar radiation modification (SRM) is a temporary measure to slow down or reduce
the impacts of climate change while longer-term strategies – primarily emissions
reduction and carbon removal – are developed and scaled up. SRM might eventually –
at best – be considered a complementary intervention that could help address the
risks of climate change alongside urgent global decarbonisation and adaptation.
The three most significant SRM techniques are stratospheric aerosol injection (SAI),
marine cloud brightening (MCB), and cirrus cloud thinning and mixed-phase cloud
thinning (CCT and MCT). SAI and MCB operate by increasing the reflection of
shortwave radiation back into space, and CCT and MCT operate by increasing the
amount of longwave radiation that escapes into space. These techniques have
potential as relatively low-cost methods for rapid cooling, with their costs to
implement likely being a fraction of the projected damages of unchecked climate
change, estimated at $38 trillion per year in 2049 [5]. SAI and MCB are regarded as the
most impactful SRM techniques for achieving global cooling, whereas CCT and MCT
are considered as having more localised effects.
However, SRM poses significant risks and uncertainties, including:
•
Regional climate effects:
SRM could introduce uneven impacts on regional climate
systems, particularly influencing temperature patterns and disrupting the
hydrological cycle, with potential consequences for monsoon systems.
•
Ozone depletion:
SAI which releases sulphate particles into the stratosphere, could
decrease ozone and slow the recovery of the ozone hole over the Antarctic.
•
Termination shock:
A sudden and permanent cessation of SRM could trigger intense
warming, with potential impacts that might surpass those of climate change itself.
Consequently, given the increase in field testing since 2020 and the likelihood of
further growth, robust governance frameworks are needed for research, including field
tests. We do not address issues related to potential deployment, but this would
require even more robust global governance. The next steps in SRM research may
include controlled, small-scale field experiments conducted under strict oversight.
These experiments could help address technical challenges, validate model
uncertainties, and enhance understanding of SRM’s potential impacts, but must be
approached cautiously. Field tests should balance potential benefits against risks of
unintended consequences and ethical concerns. Rigorous governance and
transparency will be essential to ensure such experiments are conducted only when
necessary and within frameworks prioritising environmental safety, equity, and public
trust.
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Funded by the European Union. Views and opinions
expressed are however those of the author(s) only and do
not necessarily reflect those of the European Union or
CINEA. Neither the European Union nor the granting
authority can be held responsible for them.
This work was funded by UK Research and
Innovation (UK RI) under the UK government’s
Horizon Europe funding guarantee [No. 10123643 -
Climate Strategies; No. 10094614 - Trilateral Research
Limited; No. 10098060 - University College London]
PENDING EC APPROVAL