The Harvard Solar Geoengineering Research Program

The Harvard Solar Geoengineering Research Program (SGRP) produces knowledge to help society decide whether—and if so, how—solar geoengineering should play a role in global efforts to manage the growing risks from climate change.

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About the Harvard Solar Geoengineering Research Program

The Harvard Solar Geoengineering Research Program (SGRP) aims to reduce uncertainties surrounding solar geoengineering; generate critical science, technology, and policy insights; and help inform the public debate surrounding this controversial idea. Recognizing that solar geoengineering could not be a replacement for reducing emissions or adapting to climate impacts, SGRP draws on Harvard’s research capabilities and global convening power to provide the knowledge necessary in considering solar geoengineering as a supplement to broader mitigation and adaptation efforts. SGRP supports a broad array of natural science, social science, and humanities research, both at Harvard and in collaboration with other academic, civil society, and government organizations and institutions.

Harvard has been engaged on the topic of solar geoengineering for decades:

  • In 1965, Professor Roger Revelle was responsible for drafting the climate change section of the President’s Science Advisory Committee’s report to President Lyndon B. Johnson, Restoring the Quality of Our Environment, which discussed the role that solar geoengineering could play in offsetting the warming associated with climate change.
  • In 1980, Professor Tom Schelling chaired a National Academy of Sciences committee whose report, Changing Climate, addressed the potential for solar geoengineering to counter global warming.
  • In 1992, a National Academies report co-authored by several Harvard affiliates explored a vast portfolio of strategies for reducing climate change risks, including various solar geoengineering technologies.
  • In 2009, former Harvard Professor David Keith served as a member of the UK Royal Society Working Group that produced the landmark report Geoengineering the Climate: Science, Governance, and Uncertainty.

The Harvard Solar Geoengineering Research Program (SGRP) was launched in April 2017. Since its establishment, SGRP has supported research resulting in the publication of dozens of peer-reviewed articles on the science and governance of solar geoengineering, as well as multiple non-technical publications (see ‘publications’ tab). SGRP hosts workshops, seminars, forums, and speakers covering solar geoengineering from a wide variety of natural science, social science, and humanities perspectives. SGRP provided funding and support to the Stratospheric Controlled Perturbation Experiment (SCoPEx), which ended in March 2024.


Geoengineering refers to a set of emerging technologies that could manipulate the environment and partially offset some of the impacts of climate change. Solar geoengineering in particular could not be a replacement for reducing emissions (mitigation) or coping with a changing climate (adaptation); yet, it could supplement these efforts.

Geoengineering is conventionally split into two broad categories: The first is carbon geoengineering, often also called carbon dioxide removal (CDR). The other is solar geoengineering, often also called solar radiation managmenet (SRM), albedo modification, or sunlight reflection.

There are large differences:

Carbon geoengineering seeks to remove carbon dioxide from the atmosphere, which would address the root cause of climate change — the accumulation of carbon dioxide in the atmosphere. In the chain from emissions to concentrations to temperatures to impacts, it breaks the link from emissions to concentrations.

Solar geoengineering seeks to reflect a small fraction of sunlight back into space or increase the amount of solar radiation that escapes back into space to cool the planet. In contrast to carbon geoengineering, solar geoengineering does not address the root cause of climate change. It instead aims to break the link from concentrations to temperatures, thereby reducing some climate damages.

Solar Geoengineeringam

There are several proposed solar geoengineering technologies. These include marine cloud brightening, cirrus cloud thinning, space-based techniques, and stratospheric aerosol scattering, amongst others.

Marine cloud brightening would attempt to brighten marine clouds to reflect more sunlight back into space.

Cirrus cloud thinning would attempt to reduce the thin, high-altitude cirrus clouds to emit more long-wave radiation from the earth to space.

Space-based technologies would attempt to reflect a small fraction of sunlight away from the earth by positioning sun shields in space.

Lastly, stratospheric aerosol scattering would introduce tiny reflective particles, such as sulfate aerosols or perhaps calcium carbonate, into the upper atmosphere, where they could scatter a small fraction of sunlight back into space.

More information can be found on the Technology Factsheet: Solar Geoengineering from the Harvard Belfer Center and Center for Research on Computation and Society (CRCS).

Solar geoengineering benefits and risksam

Climate models have consistently shown that solar geoengineering, when used in moderation and combined with emissions cuts, has the potential to reduce climate changes around the globe. For example, it could reduce climate impacts such as extreme temperatures, changes in water availability, and intensity of tropical storms.

However, any benefits come with novel risks and significant uncertainty. For example, while the latest science might show some benefits globally, local impacts could vary more widely. There are a lot of other scientific uncertainties that are not yet well understood, not least the enormous governance challenges.

Also, solar geoengineering (largely) does not address ocean acidification. Every year, the ocean absorbs about one-quarter of the carbon dioxide we emit into the atmosphere, changing the chemistry of the oceans and harming marine ecosystems. Given that solar geoengineering would not remove carbon dioxide from the atmosphere directly, but rather reflect sunlight back to space, it could do little to address this serious problem except via carbon cycle feedbacks, the process through which additional carbon is emitted into the atmosphere upon additional warming.

That said, solar geoengineering could reduce rising temperatures, offsetting many impacts on the oceans. For example, by reducing sea surface temperatures, it could reduce the risk of coral bleaching events and help to maintain conditions favorable for coral reefs (as the damage to coral reefs is largely caused by rising sea surface temperatures, followed by intensifying ocean acidification). Solar geoengineering could also reduce poleward shifts in species ranges, which has been posing serious risks to tropical fisheries. And it could lessen the amount of sea-ice loss, which could reduce the impacts on high-latitude ecosystems and climate, and help to limit changes in ocean circulation and glacier melt.

In any case, solar geoengineering could not be a substitute for cutting carbon dioxide pollution. It could only be a potential supplement.

Solar geoengineering research

Research could reduce uncertainty about the technology’s potential benefits and risks, but, for decades, research in solar geoengineering has been limited. This has been in part because of a fear that it could lesson efforts to cut emissions. There have also been concerns pertaining to its ethics, governance, and potential impacts to the climate system. Recently the U.S. National Academy of Sciences and major environmental groups such as the Environmental Defense Fund and the Natural Resources Defense Council have begun to support careful research. The U.S. also published the Climate Science Special Report, which discussed geoengineering and called for further research. The report was a key part of the Fourth National Climate Assessment, which the U.S. Global Change Research Program (USGCRP) oversaw.



Clark, Britta. “How to Argue about Solar Geoengineering.” Journal of Applied Philosophy 40, no. 3 (2023): 505-520. Publisher’s Version

Harding, Anthony R., Mariia Belaia, and David W. Keith. “The value of information about solar geoengineering and the two-sided cost of bias.” Climate Policy 23, no. 3 (2023): 355-365. Publisher’s Version

Horton, Joshua B., Kerryn Brent, Zhen Dai, Tyler Felgenhauer, Oliver Geden, Jan McDonald, Jeffrey McGee, Felix Schenuit, and Jianhua Xu. “Solar geoengineering research programs on national agendas: a comparative analysis of Germany, China, Australia, and the United States.” Climatic Change 176 (2023). Publisher’s Version


Horton, Joshua. Carbon Removal Scoping Study, 2022. carbon_removal_scoping_study.pdf

Horton, Joshua. Solar Geoengineering Scoping Study, 2022. solar_geoengineering_scoping_study_v2.pdf

Rabitz, Florian, Marian Feist, Matthias Honegger, Joshua Horton, Sikina Jinnah, and Jesse Reynolds. “A preliminary framework for understanding the governance of novel environmental technologies: Ambiguity, indeterminateness and drift.” Earth System Governance 12 (2022). Publisher’s Version


Belaia, Mariia, Juan B. Moreno-Cruz, and David W. Keith. “Optimal Climate Policy in 3D: Mitigation, Carbon Removal, and Solar Geoengineering .” Climate Change Economics 12, no. 3 (2021). Publisher’s Version

Horton, Joshua B. “Solar Geoengineering at a Standstill?” Global Policy Opinion, 2021. Publisher’s Version

Dove, Zachary, Joshua Horton, and Katharine Ricke. “The middle powers roar: Exploring a minilateral solar geoengineering deployment scenario.” Futures 132 (2021). Publisher’s Version

Aldy, Joseph E, Tyler Felgenhauer, William A Pizer, Massimo Tavoni, Mariia Belaia, Mark E Borsuk, Arunabha Ghosh, et al. “Social science research to inform solar geoengineering: What are the benefits and drawbacks, and for whom?” Science 374, no. 6569 (2021): 815-818. Publisher’s Version

Felgenhauer, Tyler, Joshua Horton, and David Keith. “Solar geoengineering research on the U.S. policy agenda: when might its time come?” Environmental Politics (2021): 1–21. Publisher’s Version

Fan, Yuanchao, Jerry Tjiputra, Helene Muri, Danica Lombardozzi, Chang-Eui Park, Shengjun Wu, and David Keith. “Solar geoengineering can alleviate climate change pressures on crop yields.” Nature Food 2, no. 5 (2021): 373-381. Publisher’s Version

Irvine, Peter, Elizabeth Burns, Ken Caldeira, Frank Keutsch, Dustin Tingley, and David Keith. “Expert judgments on solar geoengineering research priorities and challenges.” EarthArXiv (2021). Publisher’s Version

Dai, Zhen, Elizabeth Burns, Peter Irvine, Dustin Tingley, Jianhua Xu, and David Keith. “Elicitation of US and Chinese expert judgments show consistent views on solar geoengineering.” Humanities and Social Sciences Communications 8, no. 1 (2021). Publisher’s Version

Horton, Joshua, and David Keith. “Can Solar Geoengineering Be Used as a Weapon?” Council on Foreign Relations, 2021. Publisher’s Version

Golja, C. M., L. W. Chew, J. A. Dykema, and D. W. Keith. “Aerosol Dynamics in the Near Field of the SCoPEx Stratospheric Balloon Experiment.” Journal of Geophysical Research (2021). Publisher’s Version

Seeley, Jacob T., Nicholas J. Lutsko, and David W. Keith. “Designing a radiative antidote to CO2.” Geophysical Research Letters (2021). Publisher’s Version


Harding, Anthony R., Katharine Ricke, Daniel Heyen, Douglas G. MacMartin, and Juan Moreno-Cruz. “Climate econometric models indicate solar geoengineering would reduce inter-country income inequality.” Nature Communications 11 (2020).

Keith, David. “The world needs to explore solar geoengineering as a tool to fight climate change.” Boston Globe, 2020. Publisher’s Version

Reynolds, Jesse, and Joshua Horton. “An earth system governance perspective on solar geoengineering.” Earth System Governance 3 (2020). Publisher’s Version

Dai, Zhen, Debra K. Weisenstein, Frank N. Keutsch, and David W. Keith. “Experimental reaction rates constrain estimates of ozone response to calcium carbonate geoengineering.” Communications Earth & Environment 1, no. 63 (2020). Publisher’s Version

Reynolds, Jesse, and Joshua Horton. “An earth system governance perspective on solar geoengineering.” Earth System Governance 3 (2020). Publisher’s Version

Reynolds, Jesse L., and Joshua B. Horton. “An earth system governance perspective on solar geoengineering.” Earth System Governance 3 (2020).

Horton, Joshua B., and Barbara Koromenos. “Steering and Influence in Transnational Climate Governance: Nonstate Engagement in Solar Geoengineering Research.” Global Environmental Politics 20, no. 3 (2020): 93-111. Publisher’s Version

Lutsko, Nicholas J., Jacob T. Seeley, and David W. Keith. “Estimating Impacts and Trade‐offs in Solar Geoengineering Scenarios With a Moist Energy Balance Model.” Geophysical Research Letters 47, no. 9 (2020). Publisher’s Version

Horton, Joshua B., Penehuro Lefale, and David Keith. “Parametric Insurance for Solar Geoengineering: Insights from the Pacific Catastrophe Risk Assessment and Financing Initiative.” Global Policy, no. Special Issue (2020). Publisher’s Version

Keith, David, and Peter Irvine. “Halving warming with stratospheric aerosol geoengineering moderates policy-relevant climate hazards.” Environmental Research Letters 15, no. 4 (2020). Publisher’s Version


MacMartin, Douglas, Peter Irvine, Ben Kravitz, and Joshua Horton. “Technical characteristics of a solar geoengineering deployment and implications for governance.” Climate Policy 19, no. 10 (2019): 1325-1339. Publisher’s Version

Dagon, Katherine, and Daniel Schrag. “Quantifying the effects of solar geoengineering on vegetation.” Climatic Change 152, no. 1-2 (2019): 235–251. Publisher’s Version

Burns, Lizzie, David Keith, Peter Irvine, and Joshua Horton. “Belfer Technology Factsheet Series: Solar Geoengineering” (2019).

Keith, David, and Joshua Horton. “Multilateral parametric climate risk insurance: a tool to facilitate agreement about deployment of solar geoengineering?” Climate Policy (2019). Publisher’s Version

Vattioni, Sandro, Debra Weisenstein, David Keith, Aryeh Feinberg, Thomas Peter, and Andrea Stenke. “Exploring accumulation-mode H2SO4 versus SO2 stratospheric sulfate geoengineering in a sectional aerosol–chemistry–climate model.” Atmospheric Chemistry and Physics 19 (2019). Publisher’s Version

Heyen, Daniel, Joshua Horton, and Juan Moreno-Cruz. “Strategic implications of counter-geoengineering: Clash or cooperation?” Journal of Environmental Economics and Management 95 (2019): 153-177. Publisher’s Version

Svoboda, Toby, Peter Irvine, Daniel Callies, and Masahiro Sugiyama. “The potential for climate engineering with stratospheric sulfate aerosol injections to reduce climate injustice.” Journal of Global Ethics (2019). Publisher’s Version

Irvine, Peter, Kerry Emanuel, Jie He, Larry Horowitz, Gabriel Vecchi, and David Keith. “Halving warming with idealized solar geoengineering moderates key climate hazards.” Nature Climate Change (2019). Publisher’s Version


Horton, Joshua B. “Parametric Insurance as an Alternative to Liability for Compensating Climate Harms.” Carbon & Climate Law Review 12, no. 4 (2018): 285-296. Publisher’s Version

Smith, Wake, and Gernot Wagner. “Stratospheric aerosol injection tactics and costs in the first 15 years of deployment.” Environmental Research Letters 13 (2018). Publisher’s Version

Irvine, Peter J., David W. Keith, and John Moore. “Brief communication: Understanding solar geoengineering’s potential to limit sea level rise requires attention from cryosphere experts.” The Cryosphere 12 (2018): 2501-2513. Publisher’s Version

Horton, Joshua B., Jesse L. Reynolds, Holly Jean Buck, Daniel Callies, Stefan Schäfer, David W. Keith, and Steve Rayner. “Solar Geoengineering and Democracy.” Global Environmental Politics (2018): 5-24. Publisher’s Version

Parker, Andy, and Peter Irvine. “The Risk of Termination Shock From Solar Geoengineering.” Earth’s Future 6 (2018): 456-467. Publisher’s Version

Eastham, Sebastian D., Debra K. Weisenstein, David W. Keith, and Steven R. H. Barrett. “Quantifying the impact of sulfate geoengineering on mortality from air quality and UV-B exposure.” Atmospheric Environment (2018). Publisher’s Version

Mahajan, Aseem, Dustin Tingley, and Gernot Wagner. “Fast, cheap, and imperfect? U.S. public opinion about solar geoengineering.” Environmental Politics (2018). Publisher’s Version

Parker, Andy, Joshua Horton, and David Keith. “Stopping Solar Geoengineering Through Technical Means: A Preliminary Assessment of Counter-Geoengineering.” Earth’s Future (2018). Publisher’s Version

Wagner, Gernot. “Chemtrails Aren’t the Geoengineering Debate We Should Be Having (Because They Aren’t Real).” Earther, 2018. Publisher’s Version

Wagner, Gernot, and Martin Weitzman. “A Big-Sky Plan to Cool the Planet.” The Wall Street Journal, 2018. Publisher’s Version

Smith, Jordan P., John Dykema, and David Keith. “Production of Sulfates Onboard an Aircraft: Implications for the Cost and Feasibility of Stratospheric Solar Geoengineering.” Earth and Space Science (2018). Publisher’s Version

MacMartin, Douglas G., Katharine L. Ricke, and David W. Keith. “Solar geoengineering as part of an overall strategy for meeting the 1.5°C Paris target.” Philosophical Transactions of the Royal Society 376, no. 2119 (2018).

Dai, Zhen, Debra Weisenstein, and David Keith. “Tailoring Meridional and Seasonal Radiative Forcing by Sulfate Aerosol Solar Geoengineering.” Geophysical Research Letters 45 (2018).


Keith, David, and Ted Parson. “Solar geoengineering: Science fiction – or saviour?” The Globe and Mail, 2017. Publisher’s Version

Keith, David W., and Gernot Wagner. “Fear of solar geoengineering is healthy – but don’t distort our research.” The Guardian, 2017. Publisher’s Version

Keith, David. “Toward a Responsible Solar Geoengineering Research Program.” Issues in Science and Technology 33, no. 3 (2017). Publisher’s Version

Sugiyama, Masahiro, Shinichiro Asayama, Atsushi Ishii, Takanobu Kosugi, John C. Moore, Jolene Lin, Penehuro F. Lefale, et al. “The Asia-Pacific’s role in the emerging solar geoengineering debate.” Climatic Change (2017).

Burns, Elizabeth, David Keith, Edward Parson, and Gernot Wagner, ed. Report on the Forum on U.S. Solar Geoengineering Research. Washington, D.C. 2017.

Keith, David W., Gernot Wagner, and Claire L. Zabel. “Solar geoengineering reduces atmospheric carbon burden.” Nature Climate Change 7 (2017): 617–619. Publisher’s Version

Tingley, Dustin, and Gernot Wagner. “Solar geoengineering and the chemtrails conspiracy on social media.” Palgrave Communications 3, no. 12 (2017). Publisher’s Version


Harvard’s Solar Geoengineering Research Program
Harvard University Center for the Environment
26 Oxford Street
Cambridge, MA 02138

Joshua Horton
Program Manager

Harvard’s Solar Geoengineering Research Program is funded by the following foundations and individuals. All donations are philanthropic gifts.

J. Baker Foundation
The Blue Marble Fund
OW Caspersen Foundation
The Crows Nest Foundation
The William and Flora Hewlett Foundation
Constance C. and Linwood A. Lacy Jr. Foundation
The Open Philanthropy Project
Pritzker Innovation Fund
Reflective Earth
Ronin Private Investments LLC
The Alfred P. Sloan Foundation
The Tansy Foundation
Teza Technologies LLC
VoLo Foundation
The Weatherhead Center for International Affairs

Laura and John Arnold
G. Leonard Baker, Jr.
Alan Eustace
Rob Fergus
Howard Fischer
Ross Garon
Bill Gates
Jonathan Golderg
The Imperial Family
Drew Myers
John Rapaport
Chris and Crystal Sacca
Michael Smith
Andrew Stark
Bill Trenchard

Fundraising Policies

In addition to Harvard’s standard funding policies, SGRP follows two further policies:

  1. We do not accept anonymous donations.
  2. We do not accept donations from corporations, foundations, or individuals if the majority of their current profits or wealth come from the fossil fuel industry unless they can clearly demonstrate that they do not have a conflict of interest and present a strong track record of supporting efforts to address climate change.

We are concerned that fossil fuel companies or other interests will seek to exploit solar geoengineering as a pretext for delaying reductions in greenhouse gas emissions. We do not want donors who are (or could reasonably be construed as being) motivated to support solar geoengineering research to protect fossil fuel industries. For purposes of excluding such donors, we consider a rough weighting system as a guide. We rate the donor’s ties to fossil fuels on a 1 to 5 scale, where 1 has no connection with fossil fuels and 5 has nearly all of their current wealth and social connections tied to coal. Then, we rate the donor’s commitment to climate from 1 for a donor who has long devoted a majority of their time and resources to climate action to 5 for a donor who has no visible interest in climate. We then take the product of the two ratings, rejecting donors with a multiplicative combined rating that is larger than 10.

We would like to elaborate on this last point. We take issues of conflict of interest very seriously. And we take the “moral hazard” concern very seriously—the idea that research or even discussion on solar geoengineering could reduce incentives to mitigate. The world must reduce greenhouse emissions to zero, and remove carbon dioxide from the atmosphere, to address the root cause of climate change.  Solar geoengineering does and will not change this fact.

We offer a few examples of our funding decisions:

  • We would not accept funding from Exxon both because the company would benefit from prolonging the use of fossil fuels and because it has clearly undermined efforts to meaningfully address climate change. In other words, we would rate Exxon with a 5 x 5 = 25.
  • We would accept funding from Tom Steyer or The Rockefeller foundation because they no longer would benefit from a delay in fossil fuel use even though their wealth was generated from investments in the fossil fuel industry (N.B. neither have donated to SGRP, this is illustrative.) Here, we would rate Rockefeller as 3 x 2 = 6.

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