Geoengineering
The emergency brake no one wants to pull
Description
On 15 June 1991, Mount Pinatubo in the Philippines erupted in one of the largest volcanic events of the twentieth century. The eruption injected approximately 20 million tons of sulfur dioxide into the stratosphere, where it formed a layer of sulfate aerosols that spread around the planet over the following months. These aerosols reflected sunlight back into space. Average global surface temperatures dropped by about 0.5 degrees Celsius over the next two years. By 1993, temperatures had returned to their previous trajectory. Pinatubo was a natural experiment, instructive for atmospheric scientists, who got an unscheduled demonstration of how stratospheric aerosols could cool the planet.
In 2006, the Dutch atmospheric chemist Paul Crutzen, who had won the Nobel Prize in 1995 for his work on ozone depletion, published a paper in Climatic Change suggesting that the Pinatubo mechanism could be deliberately replicated as a last-resort response to climate change. Crutzen was not advocating for the technique. He was arguing that the topic could no longer be left undiscussed. His paper broke a taboo. Geoengineering — the deliberate large-scale modification of Earth's climate system — had been considered too dangerous to talk about openly. After Crutzen's paper, the conversation became permissible.
Almost two decades later, the conversation has not produced anything resembling consensus. Solar geoengineering remains highly controversial within the climate science community. Some serious researchers think it should be developed as an option. Others think researching it makes deployment more likely and that deployment would be catastrophic. A small number of private actors have already begun experiments, including a startup called Make Sunsets that began launching balloons of sulfur dioxide into the stratosphere in 2022, generating an international diplomatic response.
The question we're asking: what is solar geoengineering, what does the evidence suggest about its effects, and what is the governance vacuum it now sits in?
What we'll see: Pinatubo, Crutzen's paper, the Make Sunsets controversy, and what the absence of agreed-upon rules means.
Table of contents
01Pinatubo as evidence
The Pinatubo eruption was the most thoroughly studied volcanic event in history. Atmospheric chemists tracked the sulfate aerosol plume from its formation through its dispersal over two years. The data confirmed what physical models had predicted: small particles in the stratosphere reflect a fraction of incoming solar radiation, lowering surface temperatures. The cooling was not uniform. Precipitation patterns shifted. The Asian and African monsoons weakened slightly. None of these effects were catastrophic, but they served as a reminder that altering the planetary energy budget produces complex secondary effects.
The eruption also damaged the ozone layer. Sulfate aerosols provide surfaces on which heterogeneous reactions destroy ozone, and ozone loss in the years following Pinatubo was measurably greater than expected from CFC chemistry alone. The damage was small relative to the existing ozone hole, but it was real. Anyone considering deliberate stratospheric aerosol injection has to factor in this side effect.
02Crutzen's paper
Crutzen's 2006 paper argued that the climate science community could no longer treat geoengineering as an off-limits topic. The dilemma he identified was that emissions reductions were happening too slowly, that the political consensus required for adequate reductions did not exist, and that the climate community had a responsibility to think carefully about what options would be available if worst-case scenarios materialized. Crutzen explicitly did not advocate for deployment. He advocated for research.
The paper's reception was mixed. Some researchers thought Crutzen had performed a service by making the discussion legitimate. Others thought he had opened a door that would be difficult to close that the existence of a published paper proposing the technique would make deployment more likely, distract from emissions reductions, and create constituencies that might push for premature deployment. The debate over whether even researching geoengineering is wise has continued ever since.
03The Make Sunsets affair
In late 2022, a small American startup called Make Sunsets launched two weather balloons in Mexico carrying small quantities of sulfur dioxide gas, released at high altitude. The company's business model was to sell cooling credits to corporations interested in offsetting their carbon footprint. The quantities were tiny and would have had no detectable climatic effect. The founders, Luke Iseman and Andrew Song, knew this. The point was to demonstrate that small-scale stratospheric aerosol injection was technically and commercially feasible, and to establish themselves as first movers.
The reaction was sharper than the founders had anticipated. The Mexican government banned solar geoengineering experiments on its territory in January 2023, citing concerns about transboundary effects and the absence of regulatory framework. The American government had no specific authority to prevent the launches but expressed disapproval. Major scientific organizations working on geoengineering distanced themselves publicly from Make Sunsets. The startup pivoted toward balloon launches in the United States and continued operating at small scale.
04The governance vacuum
The international institutions that exist for climate-related issues the UNFCCC, the Convention on Biological Diversity, the Montreal Protocol were not designed to address solar geoengineering. The CBD adopted a non-binding moratorium in 2010, but it has no enforcement mechanism and excludes small-scale research. The UNFCCC has not addressed geoengineering substantively. The Montreal Protocol would arguably have jurisdiction over stratospheric aerosol injection because of the ozone implications, but this jurisdiction has not been formally exercised. Individual countries have generally not legislated, with Mexico's 2023 ban being a partial exception.
Several proposals for governance have been developed by academic and policy groups. The Asilomar conference in 2010, modelled on the famous biotechnology conference of the 1970s, produced voluntary principles for conducting research. The Carnegie Climate Governance Initiative has been working with governments to develop frameworks. The 2023 open letter calling for a non-use agreement, signed by hundreds of researchers, articulated the position that solar geoengineering should not be deployed. None of these proposals has produced binding international agreements.
05Conclusion
Solar geoengineering occupies an unusual position in climate policy. It is a technique that almost certainly works at the basic physical level. It is a technique that almost no one wants to deploy, and that almost no one wants to rule out. It is a technique with very low direct costs and very high indirect risks, both ecological and political. It is a technique whose discussion has split the climate community and whose deployment would split the international system. The debate over what to do about it has been substantively serious and institutionally unproductive in roughly equal measure.