How Does the Latitude of Stratospheric Aerosol Injection Affect the Climate in UKESM1?

Abstract. Stratospheric Aerosol Injection (SAI) refers to a climate intervention method by which aerosols are intentionally added to the lower stratosphere to enhance sunlight reflection and offset some of the adverse effects of global warming. The climate outcomes of SAI depend on the location, amount, and timing of injection, as well as the material used. Here, we isolate the role of the latitude of SO₂ injection by comparing different scenarios which have the same global-mean temperature target, altitude of injection, and hemispherically symmetric injection rates. These are: injection at the equator (EQ), and injection at 15° N and S (15N+15S), at 30° N and S (30N+30S), and at 60° N and S (60N+60S). We show that injection at the equator leads to many undesirable side effects, such as a residual Arctic warming, significant reduction in tropical precipitation, reductions in high-latitude ozone, tropical lower stratospheric heating, and strengthening of the stratospheric jets in both hemispheres. Additionally, we find that the most efficient injection locations are the subtropics (15 and 30° N and S), although the 60N+60S strategy only requires around 30 % more SO₂ injection for the same amount of cooling; the latter also leads to much less stratospheric warming but only marginally increases high-latitude surface cooling. Finally, while all the SAI strategies come with trade-offs, we demonstrate that the 30N+30S strategy has, on balance, the least negative side effects and is easier to implement than a multi-latitude controller algorithm; thus it is a good candidate strategy for an inter-model comparison.