How could stratospheric aerosol injection affect regional rainfall and monsoon patterns
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Executive summary
Stratospheric aerosol injection (SAI) — spraying reflective particles into the stratosphere — can cool the planet but will not do so uniformly, and those uneven cooling and circulation effects are likely to alter regional rainfall and monsoon systems in complex, sometimes harmful ways [1] [2]. Model experiments show consistent tendencies toward reduced tropical and monsoon precipitation in many scenarios, but outcomes depend strongly on injection latitude, particle strategy, and the exact metric used, leaving large policy and ethical trade‑offs [3] [4].
1. How SAI changes the physics of rain: cooling, land–sea contrast and the ITCZ
SAI reduces incoming solar radiation and tends to cool land more than ocean, weakening the thermal contrast that drives summer monsoons and thereby reducing monsoon precipitation in many model runs [1] [5]. The cooling is also typically stronger in the Northern Hemisphere after volcanism‑like forcing, which can shift the intertropical convergence zone (ITCZ) and relocate the main rainfall belts north or south of their historical positions, with commensurate regional wetting or drying [1] [2].
2. Latitude and strategy matter — where you spray will change who gets rain
Climate model ensembles demonstrate that the latitude of injections markedly alters the hydrological response: injections at northern tropical latitudes (e.g., 15–30°N) can cut Indian summer monsoon rainfall by substantial percentages in single‑point experiments, while other injection strategies mitigate or exacerbate different regions’ drought risk [3] [6]. Multi‑latitude versus equatorial strategies produce contrasting stratospheric heating, jet changes, and Walker/Hadley circulation shifts, so “how” SAI is done is as decisive as “whether” it is done [7] [4].
3. Regional patterns observed in model experiments: Asia, Africa, Americas
Large multi‑model projects like GLENS and other SAI ensembles repeatedly show suppressed tropical precipitation especially over Indian, West African, and parts of the Americas and southern Africa, reversing some greenhouse‑gas induced wetting in places but producing drier conditions in others — for example, SAI can intensify drought in tropical Africa in some scenarios [6] [8]. The East Asian summer and winter monsoons also respond differently by season and model: summer precipitation generally decreases while winter circulation patterns can strengthen, illustrating seasonally variable effects [5] [9].
4. Mechanisms beyond simple cooling: stratospheric heating, ozone, and circulation feedbacks
Injected aerosols heat the lower stratosphere and can moisten it, which alters stratosphere–troposphere coupling, ozone chemistry, and planetary wave patterns; those changes modulate jet streams, Rossby wave trains, and large‑scale circulation like the Hadley and Walker cells, producing non‑intuitive regional rainfall shifts beyond direct radiative cooling [4] [9]. These dynamical and chemical side‑effects mean rainfall responses are not a simple proportional “less sunlight → less rain” relationship [4].
5. Uncertainties, risks and social consequences — termination shock and governance
Models converge on the likelihood of uneven regional impacts but disagree on magnitude and sign for many regions, leaving high uncertainty for agriculture, water security, and health; abrupt cessation of SAI would risk rapid “termination shock” with fast climate change that could be worse for monsoons and dependent societies [10] [11]. The technical variability in outcomes creates a geopolitical dilemma: different countries would be harmed or helped by given strategies, raising governance, equity and hidden‑agenda risks if decisions are made without broad, transparent international consent [10] [11].
6. Bottom line: a potential patch that redistributes risk, not a regional fix
SAI can reduce global mean temperature and mitigate some greenhouse‑gas driven extremes, but it is not a regionally precise tool — in many model experiments it weakens monsoon systems or shifts rainfall patterns in ways that could harm billions dependent on seasonal rains, with outcomes that hinge on injection choices and model assumptions [2] [6] [8]. Given the scientific uncertainties and profound societal stakes documented in the literature, any discussion of deployment must center equity, robust international governance, and further targeted research on regional hydrological responses [10] [4].