Injection near the stratopause mitigates the stratospheric side effects of sulfur-based climate intervention

Abstract. Stratospheric aerosol injection (SAI) using sulfur has been proposed to cool the planet by reflecting sunlight back to space. A commonly proposed SAI, with sulfur dioxide injection rate of 10 Tg yr−1 at 25 km, accumulates aerosols in the tropical lower stratosphere, causing a 6 K warming of the tropical lower stratosphere that impact the entry value of stratospheric water vapor and jet positions. This approach could also delay October Antarctic total column ozone (TCO) recovery to 1980s values by 25–55 years. We propose a novel SAI approach of injecting sulfur at 50 km (SAI50) that substantially reduces these negative impacts. In SAI50, the mean meridional overturning circulation near the stratopause rapidly transports gaseous SO2 to mid-high latitudes, preventing sulfate aerosol accumulation in the tropical lower stratosphere. This approach reduces tropical stratospheric warming to 3 K and shortens the Antarctic ozone recovery delay to 5 years. Furthermore, SAI50 demonstrates greater cooling efficiency, enhancing global and polar surface cooling by 22 % and 40 % respectively. Consequently, SAI50 preserves 20 % more Arctic September sea ice compared to lower-altitude SAI. These findings suggest that SAI50 could offer a more effective and less disruptive approach to climate intervention.