Reduction of rainfall or change to its seasonal patterns in the tropics is often cited as a key risk of SAI, with impacts on the Indian/South Asian Summer Monsoon a particular focus. As in the extratropical case, such changes could arise from spatial and temporal forcing patterns which differ from GHGs, and lower-stratospheric heating. Strong asymmetry in aerosol forcing between the hemispheres would drive large changes in low-latitude precipitation, with ITCZ movement towards the warmer hemisphere (e.g. Haywood et al., 2013), but here, we restrict the uncertainty to our scenario of hemispherically balanced injection. Even under this scenario, substantial changes have been found - for example, Simpson et al. (2019) report substantial drying in many high-precipitation tropical regions forced by tropical lower-stratospheric heating, under injection strategies that attempted to balance interhemispheric temperature gradients (GLENS). Variation in ITCZ position is not in general tightly coupled to the overall inter-hemispheric temperature difference, and even balanced injection scenarios can produce significant shifts (Henry et al., 2024).
Metric
A detectable reduction in Indian monsoon strength on a 50-year timescale at 0.5°C cooling, relative to a reference world with the same global mean temperature but neither higher CO2 nor SAI.
Uncertainty
While various studies have addressed aspects of this, we have low confidence in our assessment of the degree of uncertainty under hemispherically balanced injection at moderate magnitude (i.e. 0.5C cooling).
Decision relevance
Detectable changes to the Indian Monsoon would be a highly significant impact, given its importance for rain-fed agriculture and water reserves in the region. Note that some analysis is required to calculate how large a change would be detectable on this timescale.