SAI is a proposed solar geoengineering technique aimed at artificially cooling the Earth’s surface to counter global warming.1
- Mechanism: It involves deliberately injecting reflective aerosols, such as sulfur dioxide (SO₂), into the stratosphere.2 These aerosols then reflect a portion of incoming solar radiation back into space, mimicking the natural cooling effect seen after large volcanic eruptions.
- Cooling Process:
- Chemical Conversion: Injected SO₂ reacts with water vapor in the stratosphere to form sulfuric acid aerosols (H₂SO₄).3 These aerosols are tiny suspended particles that can linger for months to years.
- Radiative Forcing Reduction: These aerosols increase Earth’s albedo (the measure of solar energy reflected by Earth), leading to less solar radiation reaching the Earth’s surface and thus a cooling effect.4
- Stratosphere: This is the stable atmospheric layer located approximately 10–50 km above the troposphere.5 It contains the ozone layer and can hold aerosols for extended periods.
- Geoengineering: This term refers to large-scale, deliberate interventions in the climate system, broadly categorized as Carbon Dioxide Removal (CDR) and Solar Radiation Management (SRM), with SAI being an SRM technique.6
Significance:
- Climate Cooling Potential: Studies suggest that injecting 12 million tonnes of SO₂ annually at 13 km altitude could cool the Earth by about 0.6°C, while 21 million tonnes would be needed for a 1°C cooling.
- Aerosol Behavior: Aerosols injected at higher altitudes remain in the atmosphere longer (months to years), enhancing their cooling effect, particularly near the equator where the stratosphere is higher.
Challenges:
- Environmental Risks: SAI could disrupt monsoon patterns and hydrological cycles, delay ozone layer recovery (leading to increased UV radiation and acid rain), and potentially mask the urgency for reducing greenhouse gas emissions.7
- Unequal Climate Effects: The cooling effect might be more pronounced in polar regions than in the tropics, raising concerns about fairness and potential regional imbalances.8
- Global Governance Challenges: Since aerosol injection affects all nations, it presents significant geopolitical risks due to a lack of global regulatory frameworks.9
- Scientific Uncertainties: Continuous injection is required for sustained cooling.10 A sudden cessation of injections could lead to rapid warming, known as “termination shock.”11