Assessing the consequences of including aerosol absorption in potential stratospheric aerosol injection climate intervention strategies

Theoretical stratospheric aerosol intervention (SAI) strategies model the deliberate injection of aerosols or their precursors into the stratosphere, thereby reflecting incident sunlight back to space and counter-balancing a fraction of the warming due to increased concentrations of greenhouse gases. This cooling mechanism is known to be relatively robust through analogues from explosive volcanic eruptions which have been documented to cool the climate of the Earth. However, a practical difficulty of SAI strategies is how to deliver the injection high enough to ensure dispersal of the aerosol within the stratosphere on a global scale. Recently, it has been suggested that including a small amount of absorbing material in a dedicated 10 d intensive deployment might enable aerosols or precursor gases to be injected at significantly lower, more technologically feasible altitudes. The material then absorbs sunlight, causing a localised heating and "lofting" of the particles and enabling them to penetrate into the stratosphere. Such self-lofting has recently been observed following the intensive wildfires in 2019-2020 in south-eastern Australia, where the resulting absorbing aerosol penetrated into the stratosphere and was monitored by satellite instrumentation for many months subsequent to emission. This study uses the fully coupled UKESM1 climate model simulations performed for the Geoengineering Model Intercomparison Project (GeoMIP) and new simulations where the aerosol optical properties have been adjusted to include a moderate degree of absorption. The results indicate that partially absorbing aerosols (i) reduce the cooling efficiency per unit mass of aerosol injected, (ii) increase deficits in global precipitation, (iii) delay the recovery of the stratospheric ozone hole, (iv) disrupt the Quasi-Biennial Oscillation when global-mean temperatures are reduced by as little as 0.1 K, and (v) enhance the positive phase of the wintertime North Atlantic Oscillation which is associated with floods in northern Europe and droughts in southern Europe. While these results are dependent upon the exact details of the injection strategies and our simulations use 10 times the ratio of black carbon to sulfate that is considered in the recent intensive deployment studies, they demonstrate some of the potential pitfalls of injecting an absorbing aerosol into the stratosphere to combat the global warming problem.