the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Aug 2026
Volcanic eruptions alter sky colours in a geoengineered world
Abstract. This study examines sky colour changes in a geoengineered world following a volcanic eruption, combining CESM2-(WACCM6) climate model simulations, SCIATRAN radiative transfer calculations, and colour modelling. Bridging a gap in existing research, this is the first study to analyse how sky colours respond when a volcanic eruption occurs in the context of an ongoing stratospheric aerosol injection (SAI) deployment. The SAI experiments consist of simulations with feedback-controlled sulphur dioxide injections amounting to 7.5 Tg per year at the time of the volcanic eruption, with eruption magnitudes of 10 and 50 Tg sulphur dioxide. The results show several optical phenomena, including Bishop's Rings near the solar horizon, anomalous greenish twilight colouration, and pronounced purple light. The latter is consistent with prior findings linking vivid purple colours to elevated aerosol loading. Despite variations in SAI scenario design, with Northern Hemisphere suspension in one scenario and complete suspension in the other, no significant differences in sky colour were found between the scenarios, months, and latitude investigated following the volcanic eruption with a magnitude of 10 Tg sulphur dioxide.
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- CC1: 'Comment on egusphere-2026-3347', Alan Robock, 11 Aug 2026 reply
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RC1: 'Comment on egusphere-2026-3347', Anonymous Referee #2, 30 Oct 2026
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This paper aims to simulate the color of the twilight sky when volcanic eruptions occur during ongoing geoengineering by stratospheric aerosol injection. Climate model runs with both SAI and volcanic eruptions are used to create aerosol inputs to a radiative transfer model to simulate radiance spectra that are then transformed to sky colors. The paper is interesting but there are gaps in the study design and the radiative transfer calculations.
I have several concerns regarding the radiative transfer calculations.
- I think it is really critical that the SCIATRAN convergence in approximate spherical mode is verified for the cases studied here. Referencing the previous study is not sufficient as the scenarios are different and this geometry is known to be quite sensitive to the radiative transfer assumptions. In the cases studied here, the stratosphere is optical depth is significant and the geometry goes deep into twilight where multiple scattering and strong aerosol forward scattering dominates the analyzed signals. The authors should at least check against a fully spherical model, either SCIATRAN’s full spherical mode or a Monte Carlo model.
- The description of how the aerosol is included in SCIATRAN is incomplete. Please clarify the assumed refractive index (and/or weight fraction) and exactly how the geometric width of the distribution is set.
- The use of an ozone climatology might be influencing the results. Can the ozone from WACCM be used in SCIATRAN? Ozone depletion from SAI/volcanic aerosol might cause changes to the observed spectrum through Chappuis band absorption and influence the perceived green and purple colors.
In terms of the overall study, I think that to make the claims robust, the authors need to add two control cases in a self consistent way: (1) no geoengineering or volcanic eruption, and (2) with a volcanic eruption but no geoengineering. This is really needed to show that the impacts on sky color are really from the combination of the eruption and the geoengineering.
I am also concerned that the study chooses only months 1 and 5 and a single latitude for the analysis. The residence time of the aerosol (both geoengineering and volcanic) is much longer than 5 months so I think the study needs to look at months something like 12-24 and at higher latitudes to really see any difference in the scenarios.  And is a monthly mean for the first month after the eruption really meaningful due to the ongoing transport and dispersion of the plume? It might be that the limited choice of analysis points is driving the conclusion that there are no significant differences between the scenarios, months, and latitude. (Only one latitude is studied!)
Finally I don’t quite appreciate the function of the Reinhard tone mapping operator here. Fig 4 shows that most of the Y values are << 1, so this means that Y’ is essentially equal to Y and the operator isn’t doing anything.Â
Citation: https://doi.org/10.5194/egusphere-2026-3347-RC1
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With respect to the 1883 Krakatau eruption, I want to make two small comments:
1. The correct spelling is Krakatau, and not Krakatoa. Even though the British spelled it wrong, it should be spelled the way the Indonesians spell the name of their own volcano.
2. I recommend you consider this paper, which is relevant to your paper. One idea, which I support, is that the sky in The Scream was based on what Munch saw after the Krakatau eruption, and the paper goes into detail about volcanic sunsets.
Prata, Fred, Alan Robock, and Richard Hamblyn, 2018: The sky in Edvard Munch’s The Scream. Bull. Amer. Meteorol. Soc., 99, 1377-1390, doi:10.1175/BAMS-D-17-0144.1.