To guide future work, an international team of scientists led by the U.S. National Science Foundation’s National Center for Atmospheric Research (NSF NCAR) has published a paper with specific recommendations for evaluating proposals to inject sulfur dioxide, known as stratospheric aerosol intervention (SAI). The paper also suggests criteria for halting those scenarios that are not feasible due to scientific, technical or societal challenges.

“The goal is to work toward an assessment that can be used to identify the most feasible and legitimate scenarios, based both on how much they reduce natural and societal risks, and on undesirable side effects,” said NSF NCAR scientist Simone Tilmes, the lead author. “If society were ever to consider implementing SAI, it is imperative that we provide policymakers and the public with the best possible scientific insight.”

The paper, based on work funded by NSF and NOAA, was published in Oxford Open Climate Change.

Imitation of volcanic eruptions

Once injected into the stratosphere, sulfur dioxide would form sunlight-reflecting sulfate aerosols. Previous studies, based on computer models and observations of large volcanic eruptions, have shown that these aerosols would have a cooling effect similar to that of a large volcanic eruption.

The injections could cool the Earth for decades or even centuries, buying time until the amount of heat-trapping greenhouse gases in the atmosphere falls again.

However, previous research has also shown that SAI poses potential risks, such as changes in the stratospheric ozone layer and changes in global precipitation patterns.

Because such injections cannot perfectly offset the impact of greenhouse gas emissions, Tilmes and her co-authors write that informed policy decisions require a comprehensive understanding of the benefits and risks of SAI. They emphasize the need for a research and governance structure, with fair representation from both the Global South and the North, to oversee SAI research and technology developments.

“Research into different methods of solar geoengineering has been ongoing for decades, but there has been no formal assessment that collects all the information in one place that is useful for policymakers and the public,” said NOAA scientist Karen Rosenlof, a co-author of the new paper. “It is time for such an assessment to take place, one that includes the criteria outlined in this paper, and that is repeated regularly.”

The article proposes eight research criteria to assess SAI developments. The criteria are:

• Technical and economic limitations
• Cooling potential
• Ability to achieve climate goals
• Infrastructure for monitoring, detection and attribution
• Large-scale and regional climate response
• Impact on human and natural systems
• Social risks
• Risk mitigation through governance

The article recommends that assessment reports on SAI developments be issued every few years with globally representative participation. The criteria may also apply to other proposals for solar radiation modification, such as marine cloud brightening.

“The aim of these criteria is to promote optimal approaches from a climate perspective, carefully weighing benefits and risks and taking into account the perspectives of underrepresented groups and the global South,” Tilmes said.