Stratospheric ozone response to sulfate geoengineering: Results from the Geoengineering Model Intercomparison Project (GeoMIP)

Abstract

Geoengineering with stratospheric sulfate aerosols has been proposed as a means of temporarily cooling the planet, alleviating some of the side effects of anthropogenic CO 2 emissions. However, one of the known side effects of stratospheric injections of sulfate aerosols under present-day conditions is a general decrease in ozone concentrations. Here we present the results from two general circulation models and two coupled chemistry-climate models within the experiments G3 and G4 of the Geoengineering Model Intercomparison Project. On average, the models simulate in G4 an increase in sulfate aerosol surface area density similar to conditions a year after the Mount Pinatubo eruption and a decrease in globally averaged ozone by 1.1À2.1 DU (Dobson unit, 1 DU = 0.001 atm cm) during the central decade of the experiment (2040-2049). Enhanced heterogeneous chemistry on sulfate aerosols leads to an ozone increase in low and middle latitudes, whereas enhanced heterogeneous reactions in polar regions and increased tropical upwelling lead to a reduction of stratospheric ozone. The increase in UV-B radiation at the surface due to ozone depletion is offset by the screening due to the aerosols in the tropics and midlatitudes, while in polar regions the UV-B radiation is increased by 5% on average, with 12% peak increases during springtime. The contribution of ozone changes to the tropopause radiative forcing during 2040-2049 is found to be less than À0.1 W m À2. After 2050, because of decreasing ClO x concentrations, the suppression of the NO x cycle becomes more important than destruction of ozone by ClO x , causing an increase in total stratospheric ozone.

Quantification of sulfur deposition changes under sulfate geoengineering conditions

“…The explicitly calculated effective radius (ULAQ-CCM) or indirectly derived 15 using the Grainger et al (1995) method (GEOS-Chem) are both consistent with the SAGE-II derived estimates approximately one year after the Pinatubo eruption, with comparable integrated stratospheric sulfate mass (Pitari et al (2014); Visioni et al (2017b)). The breakdown of global SO x deposition fluxes, among SO 2 , SO 4 dry and wet deposition terms, is summarized in Table 4 for the two models, and a comparison is made with multimodel data presented in Lamarque et al (2013).…”

“…Important model updates regarding horizontal and vertical resolution (now T21 with 126 log pressure levels), species cross sections and Schumann-Runge bands treatment, and upgrades of the radiative transfer code were described and tested in Pitari et al (2014). This radiative module, crucial for a good prediction of the sulfate aerosol interaction 10 with shortwave solar and longwave planetary radiation has been tested for tropospheric aerosols in SPARC-AEROCOM (Randles et al (2013)) and also for stratospheric aerosols after major volcanic eruptions (Pitari et al (2016b)).…”

Effects of Arctic geoengineering on precipitation in the tropical monsoon regions

“…Early modeling research in geoengineering showed that solar geoengineering schemes could markedly diminish regional and seasonal climate change caused by increase in CO 2 concentration (Govindasamy and Caldeira 2000;Govindasamy et al 2003). In the recent past, several modeling studies have investigated solar geoengineering using stratospheric sulfate aerosols (Robock et al 2008;Modak and Bala 2014;Tilmes et al 2009;Pitari et al 2014;Niemeier and Timmreck 2015). Other geoengineering methods such as marine cloud brightening and ocean albedo modification are also discussed in detail in several reviews (Bala 2009;Lenton and Vaughn 2009;Caldeira et al 2013;NRC Report 2015).…”

Stratospheric solar geoengineering without ozone loss

“…Sulfuric acid, H 2 SO 4 , can accelerate ozone loss by forming aqueous aerosol that catalyzes reactions such as HCl + ClONO 2 → Cl 2 + HNO 3 , shifting halogens from reservoir species to reactive compounds and altering the NO x budget via hydrolysis of N 2 O 5 to HNO 3 . Previous studies found that injection of sufficient SO 2 or particulate sulfate to produce −2 W·m −2 of radiative forcing-a useful benchmark for SRM-reduced average column ozone by 1 to 13% (2,(6)(7)(8).…”