Atmospheric responses to geoengineering in the stratosphere
Geoengineering by injection of aerosol into the stratosphere has been proposed as a way to counterbalance global warming driven by human emissions of greenhouse gases. Its effectiveness will vary regionally depending on the circulation response. In addition, the aerosol has a radiative effect on the temperature of the stratosphere, which affects the stratospheric circulation.
The first part of this thesis quantifies the stratospheric temperature response to a range of size distributions of sulphate, titania, limestone and soot aerosol. All aerosols produce tropical lower-stratospheric radiative heating, which increases the meridional temperature gradient. The magnitude of this temperature gradient change depends strongly on the aerosol type (with soot producing the greatest change and titania the least) and size.
The second part of this thesis uses a global climate model to investigate the dynamical response of the troposphere and the stratosphere to the radiative perturbations introduced by geoengineering aerosol. The increased lower-stratospheric meridional temperature gradient intensifies the stratospheric polar vortices. The variability in the strength of the Northern Hemisphere polar vortex is increased, implying a greater role of stratosphere-troposphere coupling in a geoengineered world.
The changes in the stratospheric polar vortices are associated with shifts in the tropospheric jets.
The tropical overturning circulation is weakened in the aerosol geoengineering simulations due to
upper-tropospheric radiative heating from the aerosol above. Thus, there is dynamical stratosphere-troposphere coupling of stratospheric aerosol geoengineering in midlatitudes, and radiative coupling in the Tropics. Since it has a smaller radiative impact on the stratosphere, titania aerosol has a smaller dynamical effect. A simulation with reduced solar irradiance shows that these dynamical effects are not captured when geoengineering is represented by solar dimming. These results demonstrate the important role played by the stratosphere in determining the climate response to stratospheric aerosol geoengineering.