Abstract
Abstract. As part of the Geoengineering Model Intercomparison Project a numerical experiment known as G6sulfur has been designed in which temperatures under a high-forcing future scenario (SSP5-8.5) are reduced to those under a medium-forcing scenario (SSP2-4.5) using the proposed geoengineering technique of stratospheric aerosol intervention (SAI). G6sulfur involves introducing sulfuric acid aerosol into the tropical stratosphere where it reflects incoming sunlight back to space, thus cooling the planet. Here, we compare the results from six Earth-system models that have performed the G6sulfur experiment and examine how SAI affects two important modes of natural variability, the northern wintertime North Atlantic Oscillation (NAO) and the Quasi-Biennial Oscillation (QBO). Although all models show that SAI is successful in reducing global mean temperature as designed, they are also consistent in showing that it forces an increasingly positive phase of the NAO as the injection rate increases over the course of the 21st century, exacerbating precipitation reductions over parts of southern Europe compared with SSP5-8.5. In contrast to the robust result for the NAO, there is less consistency for the impact on the QBO, but the results nevertheless indicate a risk that equatorial SAI could cause the QBO to stall and become locked in a phase with permanent westerly winds in the lower stratosphere.
Highlights
As well as absorbing outgoing terrestrial radiation, stratospheric aerosols absorb sunlight in the near infra-red region of the solar spectrum and increase the mean photon path and absorption of solar radiation in ozone-absorbing bands. This absorption leads to heating in the sunlit parts of the stratosphere at lower latitudes, thereby strengthening the temperature gradient and the polar vortex and inducing a positive phase of the North Atlantic Oscillation (NAO), increasing precipitation in northern Europe while decreasing it in southern Europe (Shindell et al, 2004; Scaife et al, 2008; Marshall et al, 2009)
The results presented above support the conclusions of Jones et al (2021) regarding the impact on the NAO in G6sulfur using a larger number of models
A robust agreement is found across the models that stratospheric aerosol intervention (SAI) in G6sulfur tends to induce a positive phase of the wintertime NAO leading to warmer and wetter winters over northern Eurasia with cooler and drier winters over southern Europe than in ssp245
Summary
Global warming has accelerated swiftly over the last decade, with the last 7 years being warmer than any preceding years in the climatological record (e.g. https://climate.nasa.gov/ vital-signs/global-temperature/, last access: 17 June 2021). A further concern is that warming levels could be reached whereby key elements of the climate system such as the Amazon rainforest or the West Antarctic ice sheet could change dramatically in response to only a little additional warming (Lenton et al, 2019; Wunderling et al, 2021) These concerns have led to calls for research into less-conventional mitigation strategies (e.g. Royal Society, 2009; MacMartin et al, 2018; NAS, 2021). As well as absorbing outgoing terrestrial radiation, stratospheric aerosols absorb sunlight in the near infra-red region of the solar spectrum and increase the mean photon path and absorption of solar radiation in ozone-absorbing bands In northern wintertime, this absorption leads to heating in the sunlit parts of the stratosphere at lower latitudes, thereby strengthening the temperature gradient and the polar vortex and inducing a positive phase of the NAO, increasing precipitation in northern Europe while decreasing it in southern Europe (Shindell et al, 2004; Scaife et al, 2008; Marshall et al, 2009).
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