Is Turning Down the Sun a Good Proxy for Stratospheric Sulfate Geoengineering?

Deliberately blocking out a small portion of the incoming solar radiation would cool the climate. One such approach would be injecting SO2 into the stratosphere, which would produce sulfate aerosols that would remain in the atmosphere for 1-3 years, reflecting part of the incoming shortwave radiation. The cooling produced by the aerosols can offset the warming produced by increased greenhouse gas (GHG) concentrations, but it would also affect the climate differently, leading to residual differences compared to a climate not affected by either. Many climate model simulations of geoengineering have used a uniform reduction of the incoming solar radiation as a proxy for stratospheric aerosols, both because many models are not designed to adequately capture relevant stratospheric aerosol processes, and because a solar reduction has often been assumed to capture the most important differences between how stratospheric aerosols and GHG would affect the climate. Here we show that dimming the sun does not produce the same surface climate effects as simulating aerosols in the stratosphere. By more closely matching the spatial pattern of solar reduction to that of the aerosols, some improvements in this idealized representation are possible, with further improvements if the stratospheric heating produced by the aerosols is included. This is relevant both for our understanding of the physical mechanisms driving the changes observed in stratospheric-sulfate geoengineering simulations, and in terms of the relevance of impact assessments that use a uniform solar dimming. Plain Language Summary Injecting SO2 in the stratosphere has been proposed as a method to temporarily cool the planet by partially reflecting the incoming solar radiation. To assess the eventual side-effects of this method, some climate model simulations have simply reduced the solar constant in the model rather than simulating the actual aerosols that would be produced. We show here what the limits of emulating stratospheric sulfate injection this way are, and what are the physical causes behind the differences from simulations where stratospheric aerosols are simulated.

Automated summary

Injecting SO2 into the stratosphere to produce sulfate aerosols for reflecting solar radiation is proposed as a method to cool the climate, but simulating aerosols directly in models shows different effects from simply reducing incoming solar radiation.