Stratospheric Aerosol Activity Implementation Plan 2025-2030

The importance of satellite observations with good spatial and temporal coverage can’t be stressed enough. Such observations are needed for climate models to simulate and predict regional climate impacts on seasonal to sub-seasonal time scales following episodic events like volcanic eruptions and wildfires, and they will be crucial to detect and monitor any deliberate SAI endeavors by governments or private stakeholders. Long-term measurements are critical to understanding a changing climate, but this area remains challenging as instruments are retired, updated and hopefully replaced. Ensuring continuity from an ever-changing collection of advancing instruments that span different techniques, sampling characteristics and quantities remains a primary challenge of the community that will be addressed by the Stratospheric Aerosol Activity with high priority. While discrepancies between instruments (e.g. reconciling solar occultation and limb sounding measurements under enhanced conditions) remains an ongoing source of consternation when merging records, the variety of current and planned missions, both in situ and remote, provide a unique opportunity for measurement synergy and aerosol understanding that was not possible in the past. The ability to improve detection, attribution and understanding of aerosol changes will rely both on the continuation of measurements and the community to maximize the science return from these records. Details on specific challenges and necessary tasks to be addressed in the context of stratospheric aerosol observations over the coming years are given in a white paper by an ISSI team on Perspectives of Stratospheric Aerosol Observations that was initiated by the Stratospheric Aerosol Activity.

Global warming alters the thermal structure and circulation of the upper atmosphere, which in turn alters the stratospheric aerosol lifecycle and the magnitude of aerosol radiative forcing (Aubry et. al., 2022). Using climate model simulations, Aubry et. al., 2021 showed that a higher future tropopause means moderate-magnitude tropical eruptions inject proportionally less sulfur into the stratosphere, reducing stratospheric aerosol optical depth by about 75% in a high-end warming scenario. Plume-rise height from less-frequent large-magnitude eruptions on the contrary increases under global warming, which together with an acceleration of the Brewer–Dobson circulation has been suggested to produce smaller-sized sulfate particles and thus a larger global-mean radiative forcing compared to present-day. Overall, our understanding of the role of climate change in altering volcanic aerosol–climate interactions is still limited and requires further research also in the context of SAI.

With more types of upper tropospheric and stratospheric aerosols being detected and measured remotely and in-situ, there is a need to consider their various roles in the climate system. Currently, most climate models consider only sulfate aerosol in the stratosphere. More evidence now shows the potential importance of meteoric dust, aerosols from satellite reentry and rocket launches, organics, ammonium and nitrate aerosols to be included in aerosol simulations in the climate models. Those non-sulfate particles are important to understand the stratospheric aerosol composition, microphysical properties, and their radiative and chemical impacts.

Natural processes play an important role in the global sulfur cycle. Different sulfur compounds are emitted or taken up by vegetation, soils, the oceans and wildfires. Climate change induces significant changes to and in many ecosystems and may thus alter natural sulfur fluxes substantially. A quantitative understanding of these processes and fluxes is necessary to predict such changes and possible climate feedbacks.

Calls to consider climate engineering (CI) as a means to mitigate climate change are growing louder, and SAI is nowadays openly discussed as one CI method. Much research has already been conducted over the past two decades, looking at potential benefits in terms of global radiative forcing but also at unwanted regional climate impacts as well as other risks such as chemical ozone destruction. As the community with comprehensive expertise on stratospheric aerosol, we naturally engage in this research striving to fill relevant knowledge gaps. We support ongoing and anticipated reviews and evaluations to provide a sound scientific foundation for scenario simulations in climate models and a comprehensive risk assessment of SAI. This will be done in close collaboration with the WCRP Lighthouse Activity on Climate Intervention Research (see below under Projects and Activities).