What goes up must come down: impacts of deposition in a sulfate geoengineering scenario

Abstract

The problem of reducing the impacts of rising anthropogenic greenhouse gas on warming temperatures has led to the proposal of using stratospheric aerosols to reflect some of the incoming solar radiation back to space. The deliberate injection of sulfur into the stratosphere to form stratospheric sulfate aerosols, emulating volcanoes, will result in sulfate deposition to the surface. We consider here an extreme sulfate geoengineering scenario necessary to maintain temperatures at 2020 levels while greenhouse gas emissions continue to grow unabated. We show that the amount of stratospheric sulfate needed could be globally balanced by the predicted decrease in tropospheric anthropogenic SO2 emissions, but the spatial distribution would move from industrialized regions to pristine areas. We show how these changes would affect ecosystems differently depending on present day observations of soil pH, which we use to infer the potential for acid-induced aluminum toxicity across the planet.

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

“…This can be observed in Figures and , where we show the precipitation changes in two of the seasons (DJF and JJA). As an example, over India the magnitude of precipitation changes in JJA is larger in the 3 × 3 SS simulations than in other seasons, compared to SD and SDH: in this case, differences in cooling over the Tibetan plateau, driven by the seasonal variation of the AOD, would affect the monsoonal circulation, combined with energetic changes in the column produced by the stratospheric heating (Simpson et al., 2018; Visioni, MacMartin, Kravitz, Richter, et al., 2020).…”

“…For the 3 × 3 cases, this effect is less pronounced, since the injection locations are chosen so as to have a similar profile to the one actually achieved by the solar dimming (MacMartin et al., 2017). At very high latitudes in both hemispheres, however, some differences are present mostly due to the polar transport barriers (Visioni, MacMartin, Kravitz, Lee, et al., 2020) that reduce the high‐latitude AOD. It is likely that a more uniform AOD distribution using more latitudes of injection (see for instance Dai et al., 2018) could produce results more closely resembling those from 1 × 1 SD: however, some differences would still remain due to the considerable variation across different months of the AOD (Figure 2) compared to the constant dimming produced by the SD cases: as shown by Visioni, MacMartin, Kravitz, Richter, et al.…”

“…Even without considering geoengineering, there are uncertainties in the projected local changes under climate change, although with improvements in climate models, these uncertainties are decreasing (Christensen et al., 2007; Matte et al., 2019). For solar geoengineering, our assessment of local changes does however depend on more factors than for climate change: aside from the uncertainty in specific physical processes (Kravitz & MacMartin, 2020), these factors include (i) the desired level of cooling (P. Irvine et al., 2019; MacMartin et al., 2019; Tilmes et al., 2020); (ii) the specific technique simulated (i.e., the method chosen to reduce surface temperatures, Gasparini et al., 2020; Niemeier et al., 2013), and (iii) within the same technique, the specific strategy deployed (Kravitz et al., 2019; Visioni, MacMartin, Kravitz, Richter, et al., 2020). There is thus a compound of different kinds of uncertainties (those listed, and those we do not know we do not know about) that result in challenges in clearly determining—and communicating—what effects geoengineering would have locally.…”

“…In our simulations, combining solar dimming with stratospheric heating helps further reduce the differences with the 3 × 3 SS strategy The aerosols scatter part of the incoming sunlight, modifying the ratio of direct to diffuse radiation, possibly modifying the projected changes on vegetation and evapotranspiration. Stratospheric aerosols affect stratospheric chemistry (principally ozone), and also ultimately result in the deposition of sulfate at the surface that might have environmental effects (albeit those have been projected to be small, see Kravitz et al., 2009; Visioni, Slessarev, et al., 2020)…”

“…These changes, however, would very likely be smaller in magnitude than those produced by climate change itself (Irvine & Keith, 2020; MacMartin et al., 2019). Another important difference is to be found in the stratosphere, where the sulfate aerosols would absorb some near‐infrared radiation and heat the air locally, resulting in changes to stratospheric dynamics (Aquila et al., 2014; Niemeier & Schmidt, 2017; Niemeier et al., 2020; Richter et al., 2017; Visioni, MacMartin, Kravitz, Lee, et al., 2020), chemistry (Tilmes, Richter, Mills, et al., 2018; Visioni, Pitari, Aquila, Tilmes, et al., 2017), and upper tropospheric clouds (Kuebbeler et al., 2012; Visioni, Pitari, Di Genova, et al., 2018). Furthermore, the stratospheric heating may also affect the surface climate due, for instance, to shifts in the atmospheric circulation (Simpson et al., 2019).…”