1AR – Mitigation Fails

Lee et al. 21 – *Sibley School for Mechanical and Aerospace Engineering, Cornell University, Douglas G. MacMartin, Senior Research Fellow Sibley School for Mechanical and Aerospace Engineering, Cornell University, Ph.D. in Aeronautics and Astronautics from MIT, Daniele Visioni, Research Associate at the Sibley School for Mechanical and Aerospace Engineering, Cornell University, Ben Kravitz, Professor in the Department of Earth and Atmospheric Science, Indiana University, Ph.D., Atmospheric Sciences, Rutgers University [Walker Raymond, “High-Latitude Stratospheric Aerosol Geoengineering Can Be More Effective if Injection Is Limited to Spring,” Geophysical Research Letters, Volume48, Issue9 16 May 2021, e2021GL092696, , DKP]

Mitigation of future CO2 emissions alone may not be sufficient to prevent future climate impacts due to uncertainty in both the rate of future mitigation and in the climate response (Rogelj et al., 2016), and stratospheric aerosol injection (SAI) has been suggested as a possible temporary supplement to mitigation and carbon dioxide removal. There have been a number of simulations of “global” SAI strategies with the aim of maintaining a desired global mean temperature or other climate goals: The G3 and G4 experiments of the Geoengineering Model Intercomparison Project (GeoMIP) injected SO2 above the equator to offset increases in radiative forcing and global mean temperature (Kravitz et al., 2011), and the Geoengineering Large Ensemble (GLENS) study injected SO2 at 30°N, 15°N, 15°S, and 30°S independently to try and stabilize global mean temperature alongside the interhemispheric and equator-to-pole temperature gradients (Kravitz et al., 2017; Tilmes, Richter, Kravitz, et al., 2018). Several studies have evaluated the Arctic impacts of these “global” approaches, finding that low- or mid-latitude injections of SO2 could reduce global-warming-induced losses of sea ice and permafrost (Chen et al., 2020; Jiang et al., 2019; Lee et al., 2020; Moore et al., 2019). However, high-latitude SAI intended specifically to preserve the Arctic has been hypothesized to provide greater Arctic cooling per unit of injection than low-latitude SAI with smaller effects at low latitudes; for example, high-latitude injections have been shown to have smaller impacts on tropical precipitation than equatorial injections (Sun et al., 2020). Simulations of localized solar reduction northwards of 50°, 60°, and/or 70°N (or similar) have been shown to slow or reverse the loss of sea ice to varying extents (Caldeira & Wood, 2008; Kravitz et al., 2016; MacCracken et al., 2013; Tilmes et al., 2014). However, solar reduction is at best a limited proxy for SAI because it fails to account for stratospheric heating, changes to stratospheric ozone concentrations, changes to the stratospheric circulation, and perhaps most important of all, the actual spatial and seasonal distributions of AOD, which are driven by stratospheric transport (Visioni et al., 2021). There have been a few studies of Arctic-focused SAI, its effects on sea ice, and potential extra-Arctic impacts: Robock et al. (2008) injected 3 Mt/yr of SO2 at 68°N, which restored September Sea Ice (SSI) comparably to 5 Mt per year injected at the equator; Jackson et al. (2015) successfully managed the restoration and stabilization of SSI through injections at 78.55°N; and Sun et al. (2020) showed that aerosols present at high-latitudes injections have smaller impacts on tropical precipitation than equatorial injections.

There is substantial evidence that high-latitude SAI could, to some degree, counteract Arctic warming and consequent impacts on the Arctic more efficiently than low-latitude injection. However, to date, most simulated Arctic SAI strategies have been ad hoc; Jackson et al. (2015) and Lee et al. (2020) are the only strategies which have actively managed injection rates to maintain a desired SSI concentration, and Jackson et al. (2015) is the only strategy which has done so by injecting at a high latitude. Little effort has been undertaken to design an Arctic-focused SAI strategy: what combination of latitude, altitude, quantity, or timing of the injection will best preserve Arctic climate? Of these, time of year of injection is likely critical. Visioni, MacMartin, Kravitz, Richter, et al. (2020) found that injection in different seasons (at lower latitudes) has different regional climate outcomes. Moreover, the difference between injecting SO2 year round, as has been the case for most simulations of SAI to date, and injecting seasonally, is likely even more critical for higher-latitude-injection Arctic-focused geoengineering due to both shorter aerosol lifetime and higher seasonality of insolation. Insolation north of 60°N is at least 10 times greater in summer than in winter (Peixoto & Oort, 1992); at low latitudes, insolation is comparable year-round, but at high latitudes, there is little to no sunlight in winter, meaning aerosols injected to reflect sunlight are largely useless. Furthermore, SO2 injections are oxidized by OH, which forms in the presence of sunlight; since there is little to no sunlight in winter, SO2 injected during the winter will oxidize into aerosols much more slowly, if at all. Finally, while sulfate aerosols present in winter oxidize more slowly and reflect less sunlight, interactions with other components of the atmosphere would still take place, including the trapping of longwave radiation, stratospheric ozone depletion, and sulfur deposition. Therefore, there is little purpose to injecting in winter, and aerosols should be concentrated in the summer for maximum effect. SO2 injected in the spring will oxidize in time to be present as aerosols throughout the summer period of peak insolation (our results show an oxidation lifetime of approximately one month), producing greater effects on the climate than if the same quantity of SO2 were distributed year-round. This hypothesis is supported by the results of Dai et al. (2018), who compared June and December injections of SO2 and H2SO4 at 66.3°N; summer SO2 injections produced 2–3 times greater maximum changes in radiative forcing than winter SO2 injections, and summer H2SO4 injections (which do not need to oxidize and begin reflecting sunlight immediately) produced 3–5 times greater maximum changes in radiative forcing than winter H2SO4 injections. While Jackson et al. (2015) injected SO2 seasonally, their primary focus was to determine whether sea ice was controllable rather than on the climate response, and they evaluated neither the differences relative to annually constant injection nor the dependence on injection latitude and timing.