Weakening the CO2 Greenhouse Effect via Stratospheric Aerosol Injection

Stratospheric aerosol injection (SAI) represents one of the primary potential options for intentionally modifying the climate to offset the warming from increasing greenhouse gases. The hypothesized strategy typically involves the injection of scattering aerosols in the lower stratosphere to increase the amount of sunlight reflected to space, thereby reducing the amount of sunlight absorbed by Earth. We demonstrate a new and potentially more efficient approach to SAI, using it to induce a weakening of the Earth’s greenhouse effect. We show that the injection of absorptive aerosols in the upper stratosphere (~ 10 hPa) increases the emission of top-of-atmosphere infrared radiation. Warming the emission level of CO ₂ weakens the greenhouse effect by altering the thermal structure of the upper stratosphere rather than the concentration of greenhouse gases. Climate model simulations indicate that the reduction in global temperatures induced through this process is an order of magnitude larger (per unit aerosol mass) than the injection of more traditional reflective aerosols. These results argue for further research into the possible impacts, particularly unintended deleterious side effects, of injecting absorptive aerosols in the upper stratosphere as a potential alternative strategy for solar radiation management.