Summary
The study tests how the altitude of stratospheric sulphate aerosols (22 km, 18 km, 16 km) would affect global and tropical precipitation under solar geoengineering. Although higher aerosol layers cool the planet more effectively, precipitation shows little sensitivity to altitude. This is because stronger cooling at higher altitudes increases slow‑response precipitation suppression, while fast atmospheric adjustments become less suppressive, nearly cancelling each other out. As a result, total precipitation reduction changes only marginally across altitudes despite differing cooling strengths.
Abstract
Within SRM research, there is an evolving discussion about the differing impacts of SRM based on the altitude at which the aerosols would be deployed in the stratosphere. The altitude would significantly influence the cooling effectiveness of the particles and their impact on other climate variables, such as precipitation. Previous studies have shown that SRM would have a stronger cooling effect when sulphate aerosols are at higher altitudes.
The Degrees modelling team in India has been investigating the sensitivity of the global hydrological cycle to different scenarios for SRM aerosols. In their new study, the team, led by scientist Govindasamy Bala, explored how different altitudes of sulphate aerosol layers (22 km, 18 km, and 16 km) would affect average precipitation on a global scale and in tropical regions. Using a global climate model, they found that while the cooling efficacy of SRM increases with higher altitude deployment, precipitation is less sensitive to the height at which the aerosols are injected. This is because knock-on fast climatic adjustments to precipitation patterns almost entirely offset the increased cooling effect of aerosols at higher altitudes.
Aerosols have a dual impact on the climate. By reflecting more sunlight into space and thus reducing the amount of radiation that reaches the Earth, they have a cooling effect on average temperatures. This cooling, in turn, influences precipitation—a slow response. Simultaneously, because sulphate aerosol particles also absorb some radiation, these aerosols have a warming effect locally in the atmospheric layer where they are injected, creating knock-on responses in both temperature and precipitation changes—a fast climate response. The team finds that as the altitude of the aerosol layer changes, these two opposing forces balance each other out when it comes to precipitation.
The team finds that higher deployment altitudes would cause a larger precipitation decrease through slow climatic responses but a smaller decrease through fast adjustment. In contrast, lower altitude deployment would cause a smaller decrease through the slow precipitation responses but a larger decrease from the fast response. This trade-off means that precipitation is overall less sensitive to the altitude of SRM aerosols than average temperature.
The paper’s findings suggest that although SRM could effectively cool the planet, its impact on precipitation is complex and influenced by a combination of fast and slow atmospheric responses to aerosols and the altitude at which they are injected. It highlights the need for a multi-model assessment to better understand these dynamics and the broader climatic impacts of SRM on precipitation.
The research also underscores the importance of considering both the altitude of aerosol layers and the atmospheric energy budget when evaluating the potential of SRM to mitigate the impacts of climate change. First author Dr K.H. Usha and co-authors recommend further research to explore the hydrological implications of different SRM deployment altitudes. This work will be crucial for better understanding the impact of SRM on locally relevant hydrological cycles like the Indian monsoon.
