Geoengineering beyond SO2

Stratospheric aerosol injection (SAI) for climate modification has been modeled primarily assuming that SO₂ is the preferred aerosol precursor for this purpose. The natural analog with volcanoes and the available observational data on atmospheric sulfate made SO₂ a logical choice for the majority of early and ongoing geoengineering studies. However, SO₂ injection has a number of limitations in this application, including a less-than-linear radiative forcing response to injection magnitude, undesirable stratospheric heating, and projected depletion of ozone. We present studies with the AER 2-D model of stratospheric aerosol injection with alternative materials, including H₂SO₄, alumina particles, and calcite particles. These substances overcome some of the drawbacks of SO₂ injection by improving the response linearity (H₂SO₄), minimizing longwave stratospheric heating (alumina and calcite), and potentially restoring the ozone layer (calcite). However, significant uncertainties in chemical and microphysical behavior, and hazards such as cirrus clouds impacts, remain. Lab experiments at Harvard University are ongoing to narrow some of these uncertainties.