Given the severity of climate change, scientists and scientific bodies have recommended research on potential approaches to increasing the reflection of sunlight (or release of long wave radiation) from the atmosphere, referred to as “solar radiation modification” (SRM), to slow climate warming and reduce climate impacts. In particular, this research is important for understanding their potential for responding to climate change rapidly, in order to reduce the dangers to people and ecosystems of the climate warming that is projected to occur over the next few decades while society reduces greenhouse gas emissions and concentrations in the atmosphere.

The following open letter is from more than 90 physical and biological scientists studying climate and climate impacts about the role of physical sciences research, including the central role it plays in effective governance. The letter affirms the importance of proceeding with responsible research to objectively evaluate the potential for SRM to reduce climate risks and impacts, to understand and minimize the risks of SRM approaches, and to identify the information required for governance. While not addressed in this letter, any decisions to actively use SRM would also need to be preceded by work to address the complex legal, ethical, and political aspects of making such a decision.

The letter is being shared openly to support consideration by, and dialogue among, stakeholders around the world.

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27 February 2023

Letter of support for research on atmospheric aerosols and their potential to increase the reflection of sunlight from the atmosphere to address climate risk

Climate change is causing devastating impacts on communities and ecosystems around the world, posing grave threats to public health, economic security, and global stability. Natural systems are approaching thresholds for catastrophic changes with the potential to accelerate climate change and impacts beyond humans’ ability to adapt. 

While reducing emissions is crucial, no level of reduction undertaken now can reverse the warming effect of past and present greenhouse gas emissions. The Earth is projected to continue to warm for several decades in all of the climate change scenarios considered by the UN’s IPCC.¹ Even with aggressive action to reduce GHG emissions it is increasingly unlikely that climate warming will remain below 1.5-2°C in the near term. This is because reversing current warming trends will require a significant reduction in the atmospheric concentrations of greenhouse gases, which significantly lag behind reductions in emissions due to their long atmospheric lifetime. Notably, even 1.5-2°C of warming brings with it significant risks to human, ecological, and global economic systems and may be sufficient to cross the threshold of earth system “tipping points.”²

In contrast to greenhouse gases, another category of emissions from human activities, particulate (aerosol) emissions, can act to cool climate. Aerosols cool climate by scattering sunlight and, when they mix into clouds can increase cloud reflectivity and lifetime.  Through these mechanisms, aerosols from human activities are currently estimated to be offsetting about a third of greenhouse gas climate warming. This aerosol effect is also the most uncertain human driver of climate change, adding considerable uncertainty to the amount of climate warming attributable to greenhouse gases. 

While greenhouse gas emissions continue to increase, aerosol emissions have been declining in much of the world and are projected to globally decline in the coming decades due to increasingly rigorous particulate air pollution regulations motivated by their negative health and environmental impacts. Because the lifetime of aerosols in the atmosphere is less than a week, reductions in aerosol emissions rapidly reduce this source of climate cooling. As such, reductions in aerosol emissions in the coming few decades will rapidly “unmask” a significant but very uncertain amount of climate warming. 

Based on analyses of a broad range of feasible greenhouse gas and aerosol emissions scenarios, recent scientific assessments indicate that holding near-term climate warming to below 1.5°C is unlikely without significant carbon dioxide removal from the atmosphere.³ There are substantial environmental, technical, and cost challenges in using carbon dioxide removal (CDR) at the scale needed to significantly reduce global warming. While using CDR to remain below 1.5°C may be physically possible, these challenges and the slow response of the climate system make it unlikely that CDR could be implemented rapidly enough or at sufficient scale to entirely avoid dangerous levels of climate warming in the near term.

Multiple scientific assessments^4,5,6 have concluded that, as a complement to greenhouse gas emissions reductions and CDR, the most rapid way to potentially counter some near-term climate warming is through an important class of climate intervention techniques that slightly change the energy entering and leaving the planet. The first two of these approaches are based on observations of how aerosols in the atmosphere already affect climate. They include:

• increasing the amount of light-scattering aerosols in the stratosphere (stratospheric aerosol injection, SAI),
• adding sea salt aerosols to low marine clouds to increase their reflectivity (marine cloud brightening, MCB), and 
• seeding cirrus clouds with aerosols to reduce the amount of infrared radiation retained by Earth (cirrus cloud thinning, CCT). 

The first two techniques would slightly decrease the amount of sunlight reaching Earth’s surface and so they are all often called solar radiation modification (SRM) methods (although CCT does not significantly modify sunlight reaching the surface). Each of these techniques may have the potential to rapidly reduce warming and counter many greenhouse gas warming impacts. However, the compensation would be imperfect and none of them could reverse the increase in ocean acidity driven by increasing CO₂ concentrations. Each SRM technique would also generate other distinct features and affect climate risks differently. These differences are potentially useful, since currently it is unclear whether any of these approaches can be determined to be safe and effective enough for use, and outcomes might be optimized if multiple techniques were used in combination.

Significant uncertainties remain around how any of these SRM interventions would affect climate risk under different scenarios of greenhouse gas and background aerosol concentrations. Yet as the impacts of climate change grow and become more tangible, there will be increasing pressure to reduce climate warming using one or more SRM approaches.

The current level of knowledge about SRM interventions is not sufficient to detect, attribute or project their consequences for climate risks.

Given the above findings, we believe that scientific research should be conducted to support the assessment of:

• the effectiveness of different SRM interventions to reduce climate warming;
• how different SRM interventions would affect climate change and climate impacts under different greenhouse gas scenarios; and
• the capabilities for detecting and attributing the impacts of various SRM interventions.

This research should be designed to provide understanding of SRM techniques and help develop relevant scenarios to enable informed decision-making in developing climate policy – specifically, to support decisions surrounding whether and how climate policy should include SRM in addition to mitigation, adaptation and carbon draw-down in response to climate change.

While we fully support research into SRM approaches, this does not mean we support the use of SRM. Uncertainties in how SRM implementation would play out in the climate system are presently too large to support implementation. Decisions about whether and how SRM might eventually be implemented must be preceded by both:

• a comprehensive, international assessment of how SRM interventions would affect climate risks regionally and globally, modeled after the independent scientific assessment process that supports the world’s most successful environmental treaties, like the Montreal Protocol, and
• cooperative international decision-making on the use of SRM based on the latest science. 

Indeed, we support a rigorous, rapid scientific assessment of the feasibility and impacts of SRM approaches specifically because such knowledge is a critical component of making effective and ethical decisions about SRM implementation. 

The state of scientific knowledge about SRM is also currently insufficient for it to be included as part of a climate credit system or other commercial offering, as some have started to propose. Even for stratospheric aerosol injection (the most well-understood SRM approach), the amount of cooling achieved by the injection of a given mass of material and how SAI will affect the climate system are still highly uncertain. Even with improved understanding of these effects, since SRM does not address the cause of climate change, nor all of the effects of increased greenhouse gas concentrations, it likely will never be an appropriate candidate for an open market system of credits and independent actors.

Since decisions on whether or not to implement SRM are likely to be considered in the next one to two decades, a robust international scientific assessment of SRM approaches is needed as rapidly as possible. Notably, much of the fundamental research needed to assess SRM interventions would also address existing knowledge gaps about our atmosphere and climate, such as how particulate pollution is already affecting clouds and how much this effect has been countering the warming from GHG increases – which continues to be the largest source of uncertainty in how humans are presently affecting climate.

A necessary foundation of any robust assessment is a body of peer-reviewed research that addresses the range of key knowledge gaps and uncertainties. A research program in support of such an assessment would therefore encompass the range of research activities needed to address critical information gaps and uncertainties about SRM, as long as these activities produce negligible effects on the environment and Earth system. The scope of research activities needed includes computer model simulations, observations, analytical studies, and small-scale field experiments. Such experiments are needed to help understand and test models’ ability to simulate how aerosols evolve in the atmosphere and how they affect clouds in order to more accurately project SRM effects at larger scales.

An optimal research program to inform decisions about SRM must be conducted transparently, with open access to data, results, and, wherever possible, the models used to assess SRM interventions and their impacts. To the greatest extent feasible, scientists globally must be involved in defining metrics of relevance for assessing climate risk both with and without SRM, and in the modeling and analysis of these risks, as well as with SRM research more broadly. Research must be undertaken independently, so that research findings and assessments are protected from political influence, business interests, and public pressure.  Where possible, governments, philanthropists and the scientific community must seek ways to expand scientific capacity for Global South researchers to both engage in and direct research on SRM.

Signed,

NOTE: Affiliations are given for identification purposes only and do not imply endorsement by the signer’s institute

• Sarah J. Doherty PhD, Atmospheric Sciences, University of Washington, Seattle (USA)
• Philip J. Rasch PhD, Atmospheric Sciences, University of Washington, Retired, Seattle (USA)
• Robert Wood PhD, Atmospheric Sciences, University of Washington, Seattle (USA)
• Jim Haywood, PhD, University of Exeter (UK)
• Piers M Forster PhD, Atmospheric Sciences, University of Leeds (UK)
• James E. Hansen, PhD, Columbia University Earth Institute, New York, NY (USA)
• Govindasamy Bala PhD, Atmospheric and Oceanic Science, Indian Institute of Science, Bengaluru (India)
• Alan Robock PhD, Dept. of Environmental Sciences, Rutgers University, NJ (USA)
• Hansi Singh PhD, School of Earth and Ocean Sciences, University of Victoria, BC (Canada)
• Olivier Boucher PhD, Institute Pierre-Simon Laplace, Sorbonne Université / CNRS, Paris (France)
• Paolo Artaxo PhD, Instituto de Fisica, Universidade de Sao Paulo (Brazil)
• David L. Mitchell PhD, Desert Research Institute, Reno, Nevada (USA)
• Seong Soo Yum PhD, Atmospheric Sciences, Yonsei University (South Korea)
• Michael S. Diamond PhD, Earth, Ocean, & Atmospheric Science, Florida State University (USA)
• Anna Possner PhD, Atmospheric Sciences, Geothe University, Frankfurt, Hesse (Germany)
• Philip Stier PhD, Department of Physics, University of Oxford (UK)
• Stephen G. Warren PhD, Atmospheric Sciences, University of Washington, Seattle (USA)
• Prof. Heri Kuswanto, Center for Disaster Management and Climate Change, Institut Teknologi Sepuluh Nopember (ITS) (Indonesia)
• David Keith PhD, Environmental Science & Engineering and Kennedy School, Harvard University, Cambridge MA (USA)
• Trude Storelvmo PhD, Department of Geoscience, University of Oslo (Norway)
• Timothy S. Bates, PhD, CICOES, University of Washington, Seattle, WA (USA)
• Haruki Hirasawa PhD, School of Earth and Ocean Sciences, University of Victoria, BC (Canada)
• Fabian Hoffmann, Dr., Meteorological Institute, Ludwig Maximilian University Munich (Germany)
• John T. Fasullo PhD, Astrophysical, Planetary and Atmospheric Sciences, University of Colorado, Boulder (USA)
• Douglas MacMartin, Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY (USA)
• Amadou Coulibaly PhD, Institut Polytechnique Rural de Formation et de Recherche Appliquée [IPR/IFRA] de Katibougou, Bamako (Mali)
• Becky Alexander PhD, Atmospheric Sciences, University of Washington, Seattle (USA)
• Daniele Visioni, PhD Atmospheric Chemistry and Physics, Cornell University, Ithaca NY (USA)
• Pornampai Narenpitak PhD, National Electronics and Computer Technology Center [NECTEC], National Science and Technology Development Agency (NSTDA), Pathum Thani (Thailand)
• Ben Kravitz PhD, Atmospheric Sciences, Indiana University, Bloomington (USA)
• Franklin J. Opijah PhD, Earth and Climate Sciences, University of Nairobi (Kenya)
• Tianle Yuan, PhD, GESTAR-II, University of Maryland, Baltimore County and Climate and Radiation Lab, NASA Goddard Space Flight Center (USA)
• Abu Syed PhD, Climate Change Adaptation and Risk Assessment Expert, C4RE, Dhaka (Bangladesh)
• Ehsan Erfani PhD, Desert Research Institute, Reno, NV (USA)
• Ryan Eastman PhD, Atmospheric Sciences, University of Washington, Seattle (USA)
• Duncan Watson-Parris PhD, Scripps Institute of Oceanography, University of California San Diego (USA)
• Mou Leong Tan PhD, Hydroclimatic Modelling, Universiti Sains Malaysia (Malaysia) 
• Lili Xia PhD,  Dept. of Environmental Sciences, Rutgers University, NJ (USA)
• Lucas McMichael PhD, Atmospheric Sciences, University of Washington, Seattle (USA)
• Matthew Henry PhD, Atmospheric and Oceanic Sciences, University of Exeter (UK)
• Abdoulaye Ballo PhD, West African Science Service Center on Climate Change and Adapted Land Use (WASCAL) Competence Centre (CoC), Ouagadougou (Burkina Faso)
• Valentina Aquila, PhD, American University (USA)
• Sebastian D. Eastham PhD, Center for Global Change Science, Massachusetts Institute of Technology, Cambridge, MA (USA)
• Gabriel Chiodo PhD, Atmospheric Sciences, Swiss Federal Institute of Technology, Zurich (Switzerland)
• Armin Sorooshian PhD, Chemical and Environmental Engineering, University of Arizona, Tucson, AZ (USA)
• Blaž Gasparini, PhD, University of Vienna, Vienna (Austria)
• Kelsey E. Roberts PhD, Marine Science, Louisiana State University, Baton Rouge, LA (USA) 
• Joel Thornton PhD, Atmospheric Sciences, University of Washington (USA)
• Timofei Sukhodolov PhD, Chemistry-Climate Modelling, Physikalisch-Meteorologisches Observatorium Davos and World Radiation Center, Davos (Switzerland)
• Khalil Karami, PhD, Leipzig University, Leipzig (Germany)
• Paul B. Goddard PhD, Earth and Atmospheric Sciences, Indiana University, Bloomington, IN (USA)
• Alfonso Fernández PhD, Department of Geography, Universidad de Concepción, Concepción (Chile)
• Cheng-En Yang, PhD, Civil and Environmental Engineering, University of Tennessee, Knoxville, TN (USA)
• Frank N. Keutsch PhD, Department of Chemistry and Chemical Biology, Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA (USA)
• Hosea O. Patrick PhD, Geography, Geomatics, and Environment, University of Toronto (Canada)
• Valerio Lembo PhD, CNR-ISAC, Rome (Italy)
• Kyoungock Choi PhD, Atmospheric Sciences, University of Washington, Seattle (USA)
• Forrest M. Hoffman PhD, Climate Change Science Institute, Oak Ridge National Lab, Tennessee (USA)
• Robyn Schofield PhD, School of Geography, Earth and Atmospheric Sciences, University of Melbourne (Australia)
• Jyoti Singh PhD, Dept. of Environmental Sciences, Rutgers University, NJ (USA)
• Claudia Wieners PhD, Institute for Marine and Atmospheric research, Utrecht (IMAU), Utrecht University, Utrecht (Netherlands)
• Arshad Arjunan Nair PhD, University at Albany, State University of New York (USA)
• Chris Lennard PhD, University of Cape Town (South Africa)
• Paris Rivera PhD, Universidad Mariano Gálvez de Guatemala (Guatemala)
• Alan Gadian PhD, National Centre for Atmospheric Sciences, University of Leeds (UK)
• Bassem Sabra PhD, Notre Dame University, Louaize (Lebanon)
• Sir David King PhD, Centre for Climate Repair at Cambridge University (UK)
• Isabelle Steinke PhD, Delft University of Technology (Netherlands)
• Michael Schulz PhD, Norwegian Meteorological Institute, Oslo (Norway)
• Herman Russchenberg PhD, Delft University of Technology (Netherlands)
• Nicholas Lutsko PhD, Scripps Institution of Oceanography/UCSD, San Diego, California (USA)
• Kate Ricke PhD, Scripps Institution of Oceanography and School of Global Policy & Strategy, University of California San Diego, San Diego, CA (USA)
• Richard Gammon PhD, University of Washington Professor Emeritus, Chemistry, Oceanography, Atmospheric Sciences, Seattle (USA)

• Wener Ochoa PhD, University of San Carlos of Guatemala (Guatemala)
• Albin J. Gasiewski PhD, University of Colorado at Boulder, Center for Environmental Technology, Dept of ECEE, Boulder, CO (USA)
• Fangqun Yu PhD, Atmospheric Sciences Research Center, State University of New York at Albany (USA)
• Gerrit de Leeuw PhD, Professor Emeritus (Netherlands)
• Don Wuebbles PhD, University of Illinois (USA)
• Russell Seitz PhD, Fellow of the Department of Physics, Emeritus, Harvard University, Cambridge, MA (USA)
• Jorge Ivan Cifuentes Castillo, University of San Carlos of Guatemala, Water and Circular Economy Researcher (Guatemala)
• Yaping Zhou PhD, University of Maryland Baltimore County & NASA Goddard Space Flight Center (USA)
• Shaun Fitzgerald, Centre for Climate Repair at Cambridge University (UK)
• Alice Wells, PhD Candidate, Environmental Intelligence, University of Exeter (UK)
• Mahjabeen Rahman, PhD Candidate, Rutgers University, NJ (USA)
• Ilaria Quaglia, PhD student, Università degli Studi dell’Aquila (Italy)
• Travis Aerenson, PhD Candidate, Atmospheric Sciences, University of Washington, Seattle (USA)
• Hongwei Sun, PhD Candidate, Harvard University, Cambridge, MA (USA)
• Adrian Hindes, PhD Candidate, Fenner School of Environment and Society, Australian National University, Canberra, ACT (Australia)
• Celeste Tong, PhD student, Atmospheric Sciences, University of Washington, Seattle (USA)
• Yan Zhang, PhD Candidate, Mechanical Engineering, Cornell University, Ithaca, NY (USA)
• Burgess Langshaw Power, PhD Candidate, Balsillie School of International Affairs – University of Waterloo, Waterloo, ON (Canada)
• Nina Grant, PhD student, Atmospheric Science, Rutgers University, New Brunswick, NJ (USA)
• Jessica S. Wan, PhD student, Climate Sciences, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA (USA)
• Iris de Vries, PhD student, Climate Physics, ETH Zurich (Switzerland)
• John Virgin, PhD Student, University of Waterloo, Department of Geography of Environmental Management, Waterloo, Ontario (Canada)
• Alistair Duffey, PhD student, Centre for Polar Observation and Modelling, Earth Sciences, University College London (UK)
• Marissa Saenger, PhD student, Scripps Institution of Oceanography, UC San Diego (USA)
• Cindy Wang, PhD Student, Atmospheric & Oceanic Sciences, Princeton University, Princeton, NJ (USA)

¹ IPCC, 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2391 pp., https://doi.org/10.1017/9781009157896.

² Armstrong McKay, D. I., A. Staal, J. F. Abrams, R. Winkelmann, B. Sakschewsi, S. Loriani, I. Fetzer, S. E. Cornell, J. Rockström and T. Lenton, 2022: Exceeding 1.5°C global warming could trigger multiple climate tipping points, Science, 377, 6611, https://doi.org/10.1116/science.abn7950.

³ Kriegler, E., G. Luderer, N. Bauer, L. Baumstark, S. Fujimori, A. Popp, J. Rogelj, J. Strefler and D. P. van Vuuren, 2018: Pathways limiting warming to 1.5°C: a tale of turning around in no time?, Phil. Trans. Royal Soc. – A., 376:20160457, https://doi.org/10.1098/rsta.2016.0457.

⁴ Shepherd, J et al., 2009: Geoengineering the climate: science, governance and uncertainty. London, UK: The Royal Society. https://royalsociety.org/topics-policy/publications/2009/geonengineering-climate/

⁵ National Academies of Sciences, Engineering, and Medicine, 2015: Climate Intervention: Reflecting Sunlight to Cool Earth. Consensus Study Report. NASEM. https://doi.org/10.17226/18988.

⁶ National Academies of Sciences, Engineering, and Medicine, 2021: Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governance. Consensus Study Report. NASEM. https://doi.org/10.17226/25762.

This letter was written and organized by members of the physical and biological science community.