What Is Solar Radiation Modification?

By Abdul Ahad NazakatReviewed by Sarah KellyJun 19 2026

What SRM Is
Primary Methods Under Investigation
The Termination Shock Problem
Governance: The Unresolved Core Problem
The UK Research Context
Outlook
References and Further Readings

Global average temperatures have been recorded at 1.5 °C above pre-industrial levels for consecutive years. The World Meteorological Organization recorded 2024 as the warmest year in documented history, while the Met Office ranked 2025 the third warmest on record.¹

Image Credit: buradaki/Shutterstock.com

With current emissions-reduction commitments widely acknowledged as insufficient to meet Paris Agreement targets, attention has increasingly turned to a class of proposed interventions known as Solar Radiation Modification (SRM).

SRM refers to approaches that aim to cool the Earth by altering how much solar radiation the planet absorbs. The principle is straightforward: reflect more sunlight away from Earth's surface, and surface temperatures fall. However, the practical reality is considerably more complex.

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What Is Solar Radiation Modification?

The United Nations Environment Program's 2023 independent expert review, One Atmosphere, established that SRM is not a substitute for emissions reduction.²

The panel found that large-scale deployment remains premature given current levels of scientific understanding and that critical gaps persist around environmental impacts, social consequences, and questions of equity and consent. This is particularly true for communities in the low- and middle-income countries most exposed to climate risk.

This framing matters because SRM proposals are sometimes discussed in ways that conflate research with deployment readiness. The two are not the same.

Currently, SRM primarily exists in modeling, simulation, and limited observational science domains, and no intervention has been deployed at scale.

Primary Methods Under Investigation

Past research attention has concentrated on a small number of candidate approaches. Two of these are the most scientifically developed:

• Stratospheric aerosol injection (SAI)
• Marine cloud brightening (MCB)

Stratospheric Aerosol Injection

Stratospheric aerosol injection involves releasing reflective particles, typically sulfate compounds, though calcium carbonate and other materials have been proposed, into the upper atmosphere, where they scatter a fraction of incoming solar radiation back into space.

The approach draws on a natural analog: large volcanic eruptions, such as the 1991 eruption of Mount Pinatubo, produced measurable short-term global cooling through similar aerosol loading.

NOAA's Earth Radiation Budget Program, which has been conducting modeling and stratospheric observation work since 2020 under Congressional direction, notes that SAI and MCB are the two strategies that have attracted the most sustained research interest based on a combination of projected feasibility and estimated cost.³

Marine Cloud Brightening

A 2025 study published in Geophysical Research Letters introduced an additional complication: SAI would not simply reduce direct solar radiation. The scattering of sunlight by stratospheric aerosols significantly increases the fraction of diffuse radiation reaching the lower atmosphere, from approximately 20% under clear conditions to over 59% with an aerosol layer. This enhances cloud reflectivity by roughly 10%.⁴

Following the publication of this work, lead author Gristey described this as an inadvertent "bonus" marine cloud brightening effect that could substantially amplify the overall cooling output of SAI, while simultaneously complicating the predictability of its regional impacts.⁵

Marine cloud brightening targets low-lying marine clouds rather than the stratosphere. Sea salt particles are seeded into clouds to increase the number of small droplets, raising cloud albedo. Natural analogs include “ship tracks”, which are the visible brightening of clouds along shipping lanes due to aerosol emissions.

MCB is considered more localized and more reversible than SAI, though modeling studies show it can produce La Niña-like sea surface temperature responses, with cooling concentrated in lower latitudes.⁶

Other candidate methods, including cirrus cloud thinning, surface albedo modification, and speculative proposals such as space mirrors, remain at early conceptual stages.

The Termination Shock Problem

One of the most significant constraints on any SRM deployment scenario is termination shock. If SRM was deployed at sufficient scale to offset accumulated warming and then abruptly halted due to political instability, conflict, or loss of institutional capacity, global temperatures could rise rapidly, potentially faster than they would have without the intervention.

Parker and Irvine's analysis of this risk concludes that some relatively simple backup deployment policies could protect against most plausible scenarios, but this presupposes sustained international coordination that does not currently exist.⁷

The termination shock problem is inseparable from the question of governance. An intervention that requires continuous maintenance, or else triggers accelerated warming upon cessation, cannot be managed by any single nation.

Governance: The Unresolved Core Problem

The governance landscape for SRM has been called both "ungoverned" and, more accurately, "governed unequally."⁸ A chronological review of SRM governance from 2006 to 2024 finds that while a range of initiatives and principles frameworks have emerged, including the Oxford Principles (2013), the Tollgate Principles (2018), and the American Geophysical Union's Ethical Framework Principles (2024), governance activity remains concentrated in the Global North, and binding international frameworks do not exist.⁸

This gap is recognized across the research community. Existing multilateral institutions face structural limitations. The UN Security Council's veto architecture, for instance, would create significant power asymmetries in any SRM governance body modeled on current UN mechanisms. Yet no purpose-built alternative institution exists.

The governance deficit takes on particular salience in light of equity concerns. Regional climate effects of SRM interventions are not expected to be evenly distributed. SAI deployment, for example, could alter monsoon patterns and precipitation regimes in ways that affect agricultural systems in South Asia, sub-Saharan Africa, and other climate-vulnerable regions. These areas have contributed least to the accumulated emissions that SRM would nominally address.

UNEP's One Atmosphere review flags the inadequacy of existing research on these differential impacts as a critical knowledge gap, and specifically notes the near-absence of literature drawing on indigenous knowledge systems.²

The UK Research Context

The UK has moved to formalize its investment in SRM research. The Natural Environment Research Council (NERC) has committed £10.5 million to four projects running from 2025 to 2030, all of which use computer modeling rather than field experimentation.

In parallel, the Advanced Research and Invention Agency (ARIA) has launched a £56.8 million program, “Exploring Climate Cooling,” that funds a broader range of methodologies, including limited, governed outdoor experiments.¹

Alongside this research investment, NERC commissioned a public dialogue, conducted by Hopkins Van Mil in late 2025, with 52 participants broadly representative of the UK population. Participants generally supported modeling research while remaining cautious about field experimentation and opposed to deployment without robust international governance.

Six principles emerged from the process:

1. Do no harm
2. Do not distract from emissions reduction
3. Global collaboration and equity
4. Public engagement
5. Prioritization of the public good
6. Transparency and accountability.¹

These align closely with existing specialist frameworks, including the Oxford Principles and the AGU's 2024 Ethical Framework.

Outlook

SRM remains a research-stage proposition, not a deployment-ready technology. There are still key scientific uncertainties, including regional precipitation effects, interactions with stratospheric ozone, impacts on biodiversity and agricultural systems, and the long-term consequences of sustained aerosol loading, all of which are insufficiently characterized.

UNEP's assessment that the evidence base to support informed deployment decisions is "simply not there" reflects the current consensus among cautious observers.²

At the same time, the deteriorating trajectory of global emissions, combined with the possibility that some warming thresholds may prove effectively irreversible, has sustained interest in SRM as a potential emergency measure: not a solution, but a possible means of buying time. The scientific community remains divided, with open letters both defending and challenging the case for continued investment in research.

What is uncontested is that the question of governance must precede any serious conversation about deployment. Research that advances faster than the institutional capacity to manage its findings creates its own category of risk.

References and Further Readings

1. Hopkins Van Mil. (2026). Solar Radiation Modification: A Public Dialogue. UKRI-NERC/Sciencewise. https://www.ukri.org/wp-content/uploads/2026/05/NERC_280526-PublicDialogueonSRM-Report.pdf.
2. UNEP. (2023). One Atmosphere: An Independent Expert Review on Solar Radiation Modification Research and Deployment. UN Environment Programme. https://wedocs.unep.org/items/c46f8f63-0a21-4995-a4ef-29a6f83a3ccd.
3. NOAA Climate.gov. (2024). Solar Radiation Modification: NOAA State of the Science Factsheet. Climate.gov. https://www.climate.gov/news-features/understanding-climate/solar-radiation-modification-noaa-state-science-factsheet.
4. Gristey, J.J., and Feingold, G. (2025). Stratospheric Aerosol Injection Would Change Cloud Brightness. Geophysical Research Letters. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024GL113914.
5. NOAA Chemical Sciences Laboratory. (2025). Injecting light-reflecting particles into the stratosphere could also make marine clouds brighter. NOAA CSL News. https://csl.noaa.gov/news/2025/426_0324.html.
6. Chen, C.-C. and Richter, J.H. (2024). Rethinking the Susceptibility-Based Strategy for Marine Cloud Brightening Climate Intervention: Experiment With CESM2 and Its Implications. Geophysical Research Letters. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024GL108860.
7. Parker, A. and Irvine, P. (2018). The Risk of Termination Shock From Solar Geoengineering. Earth’s Future. 6. pp.456–467. https://salatainstitute.harvard.edu/wp-content/uploads/2024/06/parker_et_al-2018-earths_future.pdf.
8. Reynolds, J.L. (2024). A chronological review of SRM governance history, 2006-2024. ResearchGate. https://www.researchgate.net/publication/353308580_Solar_geoengineering_Scenarios_of_future_governance_challenges.

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