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

Abstract

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.

Identifying the sources of uncertainty in climate model simulations of solar radiation modification with the G6sulfur and G6solar Geoengineering Model Intercomparison Project (GeoMIP) simulations

“…For G6sulfur, there is a general agreement in the inability of sulfate geoengineering to completely cool down the northern high latitudes, partly due to the focus of the geoengineering strategy on reducing global mean temperatures (Kravitz et al, 2019) but also probably due to the presence of stratospheric heating (Jiang et al, 2019), as evident by the absence of high-latitude warming with the same magnitude in the G6solar simulations. The residual warming also present in the G6solar simulations can be partly explained by the differences in the radiative forcing from the CO 2 and solar reduction (Ban-Weiss and Caldeira, 2010;Henry and Merlis, 2020;Visioni et al, 2021). Differences in the surface response between models would thus depend on how different models physically reproduce some of the processes mentioned but also on the differences in the stratospheric response reported in the previous section.…”

“…When geoengineering the climate, reducing incoming solar radiation (either by simulating stratospheric aerosols or by reducing the solar constant in models) to obtain the same global surface temperature as a scenario with lower GHGs does not assure that regional temperatures follow the same pattern. This has been reported in climate model simulations of various complexity, from 1-D models (Henry and Merlis, 2020) to Earth system model simulations (i.e., Ban-Weiss and Caldeira, 2010;Niemeier et al, 2013;Jones et al, 2018;Visioni et al, 2021). These differences may be reduced if, together with reducing global temperatures, the geoengineering strategy also aims to reduce differences in higher-order temperature gradients (Kravitz et al, 2016;Tilmes et al, 2018a), but they cannot be completely canceled due to various factors.…”

“…These differences may be reduced if, together with reducing global temperatures, the geoengineering strategy also aims to reduce differences in higher-order temperature gradients (Kravitz et al, 2016;Tilmes et al, 2018a), but they cannot be completely canceled due to various factors. The main factor would be a fundamental difference in the radiative perturbation from CO 2 (that warm throughout the atmospheric column) and from the reduction in solar constant (that cool from the bottom-up)(Ban-Weiss and Caldeira, 2010; Henry and Merlis, 2020); then, seasonal and latitudinal differences (Govindasamy et al, 2003;Ban-Weiss and Caldeira, 2010;Visioni et al, 2020b) and surface climate effects (such as precipitation changes) of the stratospheric heating produced by the aerosols (Simpson et al, 2019;Visioni et al, 2021;Jones et al, 2021). Other factors may also be an inability to restore the same state for the ocean circulation.…”

“…4) given a similar injection and those produced by different response of the surface climate. While comparing results with those from a similar, more uniform experimental design such as G6solar might help, the lack of the potential response produced by the aerosols (Banerjee et al, 2021;Visioni et al, 2021) may suggest the use of a prescribed aerosol distribution for various models (Tilmes et al, 2015) as an intermediate approach. This can also be seen in the comparison between the two versions of MPI (that differ only in their horizontal resolution, which is twice as high in the HR version).…”

“…G6solar, on the other hand, decreases total incoming solar irradiance. While the latter does not aim to reproduce the effects of an actual sulfate aerosol intervention, comparisons of its results with simulations of stratospheric aerosols in the same model may help understand the contributions to inter-model differences in the response to aerosols (Niemeier et al, 2013;Visioni et al, 2021). Both reductions of incoming solar radiation at the surface (directly, by turning down the Sun, or indirectly, by having the aerosols reflect the solar radiation) are adjusted at least every decade to ensure that the target temperature is being met.…”