Response to Overshoot Commission Report 

The privately funded “Global Commission on Governing Risks from Climate Overshoot” released on 14 September 2023 a report that proposes a range of measures to reduce risks from climate “overshoot,” which the commission defines as exceeding the goal of limiting average global warming to 1.5oC. The report attempts to be comprehensive by acknowledging the need to phase out fossil fuels, reduce greenhouse gas emissions, and adapt to climate risks. Yet it also promotes novel technologies to remove carbon dioxide from the atmosphere and more research into technologies that would reflect sunlight in an attempt to cool the Earth (referred to in the report as Solar Radiation Modification, or SRM). This commentary focuses on those aspects of the message that deal with SRM, also known as “solar geoengineering.”

The report includes five recommendations specific to solar geoengineering


First, the commission recommends an immediate moratorium on the deployment of solar geoengineering. A moratorium is indeed much needed. Given the growing debates in some quarters about solar geoengineering being a possible “Plan B” in climate policy, governments need to set a clear signal that solar geoengineering is not an option. As one commissioner wrote in a separate blog, “SRM could potentially have major unintended consequences, such as changing precipitation patterns and on biodiversity, on a global scale. It also would not stop major problems like ocean acidification. That is why we are calling for countries to adopt a moratorium on any type of SRM intervention with a risk of significant transboundary harm, consistent with the moratorium called for by the Convention on Biological Diversity.” 

However, while the call for a moratorium on solar geoengineering is an important intervention, the overshoot commission does not go far enough. The moratorium that the overshoot commission proposes would apply only to “deployment of solar radiation modification and large-scale outdoor experiments,” and it would focus only on interventions with a risk of significant transboundary harm. 

This contrasts with the much wider and more inclusive global ban called for by 450+ academics from around the world: an International Non-Use Agreement on Solar Geoengineering, which recognizes that already small-scale outdoor experimentation incurs a risk of developing deployable technology. The specification by the overshoot commission that only risks of “transboundary harm” would trigger the moratorium they propose is problematic as well, and also from a social justice perspective. Instead, it might suggest that an experiment could be considered acceptable even if it put millions of people at risk, as long as those people were all in one jurisdiction.

Expanded research governance

Second, the commission calls for expanded governance of solar geoengineering research, but does not offer any detail on the politically important questions of what kinds of research are to be governed, and how. The document fails to recognize that even small-scale field experiments pose technological lock-in risks and that existing governance mechanisms are ill-equipped to regulate this kind of research. The overshoot commission signals their vision when they write that “outdoor experiments [would] expand in scale,” implying a dangerous inevitability that experiments will expand across time towards possible technology development within some jurisdictions. The recommendation that “a group of independent scientific experts should write guidelines and best practices…” is vacuous, as it says nothing about how such a group would be convened and leaves the “best practices” a matter of voluntary compliance. Conversely, the persistent power asymmetries in global governance, much discussed in the literature, are not mentioned by the commission, even though such global asymmetries will shape the implementation of any possible global research and deployment program on solar geoengineering.

Some parts of the second recommendation are laudable but would be difficult to enforce. SRM research is extensively funded by private foundations, which in many cases are not required to disclose their donors. For example, the Open Philanthropy Foundation is largely funded by a Facebook founder but is not obligated to disclose other donors. The Degrees Initiative states voluntarily that it is funded by Open Philanthropy, but the UK Charities Commission, under which Degrees is constituted, does not require donors to be disclosed. The commission also states here that “SRM research should not be led by for-profit firms and should not be funded by sources with an interest in maintaining greenhouse gas emissions, such as fossil fuel interests.” This is in theory a reasonable requirement, but would be difficult to enforce in real-world circumstances, especially given a general lack of transparency and numerous opportunities for hidden cross-subsidies. 

Expanded solar geoengineering research

The third recommendation is a general call for strengthening solar geoengineering research. Here the commission discusses research on solar geoengineering without offering much detail of the dangers associated with this set of hypothetical technologies, concluding that further research will mitigate the dangers—rather than compound them. This reinforces a misleading claim that more research will reduce the risks. More research, however, cannot resolve the social and political risks of any government or small group of countries unilaterally deploying solar geoengineering. More research will not reduce the risk of delaying urgently needed transformative policies, which might be an outcome of further legitimizing and popularizing an imaginary future where solar geoengineering halts climate chaos. Conducting more research on this speculative future technological “solution” will not prevent powerful organizations or countries from deploying this technology unilaterally without global consent or oversight. New research also cannot prevent the global impacts of solar geoengineering from being inequitably distributed. New research will not help to alleviate the fundamental challenges of governing, in a fair and equitable manner, the potential future deployment of a speculative technology that carries such complex risks and unequal global impacts. These concerns remain unanswered by parties proposing to expand solar geoengineering research. The old and battered response that inclusive governance is important should, by now, be seen as too simplistic to the deeper, complex questions above.

Outdoor field experiments, in particular, involve the release of substances into the atmosphere and are associated with many complex risks. Small-scale experiments shed little light on what happens with a release into a stratosphere that has been saturated with aerosol-forming substances from decades of artificial Pinatubo-sized injections, or into the nature of long-term response of the global climate system to hammering the Earth’s radiation budget in such a fashion. In fact, it is far from clear that any outdoor experimentation short of a full-scale deployment could provide the needed sort of data, and even then, it could take decades to collect enough data to sort out what we had done to the climate. Also, small-scale outdoor experiments are already politically dangerous, in that they inevitably risk development of technology that some third party might decide to deploy—and the decision about whether to deploy will not be in the hands of organizations like the overshoot commission.

Overall, the overshoot commission offers little discussion of the physical science aspects of solar geoengineering. While the risk of termination shock is mentioned (that is, the catastrophic warming once unforeseen circumstances force an abrupt halt to solar geoengineering), the commission fails to highlight the millennial-long lifetime of carbon dioxide in the atmosphere. This long lifetime means that avoiding termination shock requires relentless (and perhaps ever-increasing) deployment of solar geoengineering over centuries to millennia, posing an unprecedented intergenerational commitment to collaborative global governance. The commission states that any solar geoengineering deployment would need to be preceded by a “decade-long research program and possibly a multi-decade phased testing period.” But why the commission believes a decade would be a sufficient timescale is not explained. No details are provided about what analysis methods and observational platforms would be needed for a phased testing period. The document also claims that “most research currently envisioned can be adequately regulated at the national level using existing regulatory frameworks.” For the most part, existing regulatory frameworks were not written with the physical and geopolitical risks of solar geoengineering experimentation in mind. When Harvard’s ScoPex project tried to experiment with solar geoengineering in northern Sweden, it was public protest and Indigenous resistance, not Swedish environmental regulations, that stopped the experiment. The inadequacy of existing regulatory frameworks has also been demonstrated with the venture-capital-funded startup Make Sunsets which began selling deployment of solar geoengineering through “cooling credits” in late 2022. So far, no regulatory framework has stopped them, and it seems clear that there is no regulatory mechanism to prevent some billionaires from investing in this company to scale up this kind of deployment. 

International and independent scientific review and broader consultation

The fourth recommendation is for international, independent scientific review, and it is reasonable so far as it goes. However, many such assessments to date are far from “independent.” Notably, many authors of the recent UNEP independent expert review (not mandated by governments) on solar geoengineering were scholars known to be long-standing advocates of solar geoengineering research, and the report did not equally include experts critical of solar geoengineering. 

The fifth and final recommendation calls for “broad consultations and dialogue,” and is largely vacuous because of the lack of a stated mechanism for translating such broad consultations into action, and for determining who should be included in such broad consultations. Harvard’s attempt to carry out the SCoPEx experiment in northern Sweden failed because their notion of “broad consultations” did not extend to consulting either the local Indigenous peoples or the various Swedish environmental movements. Broad consultations and dialogue are not a replacement for global governance and global control for some of the most risky and dangerous technology developments that humankind has ever contemplated—the intentional modification of the entire planetary system by means of “solar geoengineering”. What is needed is not a global research program but a global agreement on restricting the development of solar geoengineering technologies

Effective global governance of solar geoengineering is highly implausible at present, something the document seems to ignore. The report cites nuclear arms control treaties as an example of successful international diplomacy governing a dangerous technology, but recent developments show that even in the face of the obvious dangers of nuclear warfare, it has proved impossible to maintain a stable nuclear arms control regime over even a half-century, once the technology was developed. The complexity of the global control regime that would be needed to control solar geoengineering, and the multi-centennial time frame over which it would have to be adhered to, poses a likely insurmountable barrier to governance of solar geoengineering deployment. 

History shows that nationalistic interests often prevent international governance, and it would likely be the powerful and privileged nations that would control and optimize the deployment of solar geoengineering for their own interests – at the cost of small and less powerful nations. Seen this way, any deployment of solar geoengineering could become a form of atmospheric colonialism providing yet one more mechanism for rich and powerful people, organizations, and nations to control and exploit those who are disempowered and vulnerable. When it comes to questions of global governance, it is important to also consider the power and influence of corporate elites and technology billionaires who have been strongly influencing the “governance” of solar geoengineering research with their philanthropic support for advancing the technology. Notably, the overshoot commission’s report includes no self-reflection on the origins and funding of the commission itself. 

In the end, it is essential to note that the climate overshoot commission is, despite its official-sounding name, a privately funded initiative not mandated by, nor accountable to, any international organization or government. The “commission” has no relationship with the Intergovernmental Panel on Climate Change (IPCC) and no formal links to any UN-based global governance system. Moreover, Professors David Keith (then Harvard) and Edward Parson (UCLA) were up until very recently listed as senior advisors to the secretariat. Keith and Parson both endorse and strive for expanded SRM research; for example, Parson has stated that the risks of SRM research are overblown and that “It would be a big step forward if we have a research program.” Likewise, the commission’s secretariat is staffed in part with advocates of SRM research. The commissioners themselves are mostly prominent, respected (retired) political leaders, therefore not directly involved in scientific research. While the commissioners are surely motivated by the desire to engage with what should be done to address the global climate crisis, interpretation of the report should nonetheless be informed by the fact that SRM advocates seem to have played an important role in convening this group. Some further background on the commission and its history can be found in this article.

There is no doubt that individual members of the commission are working towards, and believe in, a more just and livable future; at least one commissioner has expressed views on solar geoengineering that seem far more critical than those embodied in the commission report. However, the overshoot commission document is pointing in the wrong direction. What is needed is mitigation and massive programs to phase out coal, oil and gas. Arguably, the work of an overshoot commission could be to propose and draft political proposals and public policy to define clear pathways to enact such programs. This is the only way forward. The pipedream of solar geoengineering, too loudly suggested by the overshoot commission, is a dangerous distraction.

This blog post is based on discussions within the initiative for an International Non-Use Agreement on Solar Geoengineering and has been written with contributions from Raymond Pierrehumbert, Jennie Stephens, Frank Biermann, and Carol Bardi. All specific views expressed here do not necessarily represent the views of all of the over 450 signatories of the open letter calling for this non-use agreement.

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