Meridional Temperature Gradients

Meridional temperature gradients here refer to the zonal mean pattern of temperature change relative to the baseline (target) climate state that was at the same global mean temperature as the SAI case (but lower CO2). This includes both inter-hemispheric and equator-to-pole gradients as well as more regional variations; we choose to focus the metric here solely on the inter-hemispheric balance, measured as the projection of zonal mean temperature onto the sin of latitude (T1 in past literature). The ability to achieve target states is important for minimizing regional climate changes, including those mediated by circulation changes and ITCZ position, and T1 in particular is loosely correlated with ITCZ. While our assumed scenario specifies equal injection rate in each hemisphere, that ratio could readily be controlled for with a feedback algorithm as described in Kravitz et al. (2017) to maintain not just global mean temperature but also some (single) additional measure of inter-hemispheric temperature gradient, however by adjusting that ratio it could also affect the ability to simultaneously reduce risks at higher latitudes in one or the other hemisphere (e.g., AMOC is helped by injection in the NH but not SH (Bednarz et al 2025), while risks from Antarctic melt are helped by injection in the SH and hurt by injection in the NH (Goddard et al 2023)). Note that the extent to which hemispherically symmetric injection maintains inter-hemispheric temperature balance is mostly not about SAI specific processes but rather uncertainties in how the climate system responds to CO2 increases (see Fasullo and Richter, 2023), as well as changes in tropospheric aerosol forcing.