Frequently Asked Questions
What is Solar Radiation Modification?
Solar Radiation Modification (SRM) encompasses a suite of technologies designed to cool the planet by enhancing the Earth’s ability to reflect sunlight. The most developed of these is stratospheric aerosol injection, which involves releasing aerosols into the stratosphere to reflect a small fraction of sunlight back into space. The technique mirrors the natural phenomenon that occurs after volcanic eruptions, when aerosols are thrust into the stratosphere, resulting in measurable cooling.
While there is a growing body of literature and advanced modelling on the topic, which has been developing for many years, there is very little evidence or research on the risks and impacts of SRM.
Why is the United Nations engaging in this topic, and why are we concerned?
Earth's climate and environmental systems are shaped by a complex interplay of natural and human-induced forces. SRM, as a potential means to rapidly cool the planet, could trigger unforeseen changes that may jeopardize both human health and natural ecosystems. The risks of SRM are largely unknown, and its operational deployment could lead to a cascade of unintended effects. Given the limited scientific understanding and uncertainty surrounding these technologies, particularly on the environmental and social impacts, the United Nations and UNEP remain concerned about the viability and potential risks of SRM and strongly cautions against such measures as an alternative to emissions abatement.
While SRM technologies remain largely speculative (theoretical) and full-scale deployment is not anticipated in the immediate future, research is advancing, technologies and schemes are being discussed, and outdoor experiments are being pursued. As these technologies gain momentum as a potential “last resort” or emergency option, UNEP stresses the need for caution. As part of its mandate to keep the environment under review, UNEP must carefully examine and monitor technologies that may impact or affect aspects of the environment.
What prior international initiatives have been undertaken in SRM?
Various bodies have also examined the issue of solar radiation modification technologies. In January 2022, the Montreal Protocol’s Scientific Assessment Panel (SAP) first assessed the impact of Stratospheric Aerosol Injection (SAI) on stratospheric ozone. It warned of unintended consequences, including disruptions to stratospheric temperatures, circulation, and ozone production. A year later, the UN Human Rights Council's Advisory Committee flagged risks to human rights from climate-altering interventions like SAI, citing significant uncertainties.
The Intergovernmental Panel on Climate Change (IPCC) touched on SAI in its Sixth Assessment Report (AR6), acknowledging critical knowledge gaps but excluding it from the climate solutions mix. Despite calls for more attention, particularly regarding its effects on the water cycle, SAI will not be a focus in the upcoming AR7. Some Member States have called for a clearer distinction between SAI and carbon sequestration in the broader climate intervention debate.
In July 2024, UNEP’s Navigating New Horizons report identified SRM deployment as a potential weak "signal of change," underscoring the vast gaps in understanding its environmental and socio-economic impacts.
In December 2024, the European Commission’s Scientific Advice Mechanism Group released a report offering recommendations on SRM research governance, addressing both its risks and opportunities.
The World Climate Research Programme (WCRP), co-sponsored by the World Meteorological Organization (WMO), the International Science Council (ISC), and the Intergovernmental Oceanographic Commission (IOC) of UNESCO, has initiated a Lighthouse Activity focusing on Climate Intervention Research. This initiative aims to advance scientific understanding of the physical, biogeochemical, and societal dimensions of two climate intervention (CI) approaches, Solar Radiation Modification (SRM) and Carbon Dioxide Removal (CDR). The primary objective is to explore future scenarios involving climate interventions, providing an unbiased assessment of associated Earth system risks, opportunities, uncertainties, and knowledge gaps. A perspective paper on CI research was published in December 2024, followed by an assessment of SRM research gaps in early 2025. WCRP continues to collaborate with the IPCC, providing critical insights for upcoming assessments.
Are there any existing international treaties or UN resolutions that are relevant to SRM?
Parties to the Convention on Biological Diversity (CBD) issued non-binding decisions regarding geoengineering specifically, Decision X/33 (2010) which calls for Parties and other Governments to ensure that ‘no climate-related geoengineering activities that may affect biodiversity take place’ in the absence of an adequate scientific basis, impact assessment and a global mechanism for governance. Further, Decision XIII/14 (2014) noted that “more transdisciplinary and sharing of knowledge among appropriate institutions is needed in order to better understand the impacts of climate-related geoengineering on biodiversity and ecosystem functions and services, socio-economic, cultural and ethical issues and regulatory options”.
At the thirty fifth Meeting of the Parties to the Montreal Protocol on Substances that Deplete the Ozone Layer considered and took decisions XXXV/3 and XXXV/4 (2023) focused on assessing the potential effects of SRM on the Ozone layer, and on one SRM technologies in particular, namely stratospheric aerosol injection (SAI) – noting the limited scientific information available about the risks posed by this technology to the ozone layer. It also called for more scientific understanding about the potential impacts of this technology on the ozone layer.
In February 2024, at the sixth session of the UN Environment Assembly (UNEA-6), Member States were unable to agree to a resolution that would launch an independent scientific review of SRM, with many countries reaffirming their positions on the complex and contentious nature of SRM research.
The International Law Commission has also considered SRM in the context of discussing Intentional large-scale modification of the atmosphere and highlights the need to ensure prudence and caution while conducting activities aimed at intentional large-scale modification of the atmosphere, subject to any applicable rules of international law, including those relating to environmental impact assessment.
