the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Sep 2024
Modelled surface climate response to Icelandic effusive volcanic eruptions: Sensitivity to season and size
Abstract. Effusive, long-lasting volcanic eruptions impact climate through emission of gases and subsequent production of aerosols. Previous studies, both modelling and observational, have made efforts in quantifying these impacts and untangle them from natural variability. However, due to the scarcity of large and well observed effusive volcanic eruptions, our understanding remains patchy. Here we use an Earth system model to systematically investigate the climate response to high-latitude, effusive volcanic eruptions, similar to the 2014–15 Holuhraun eruption in Iceland, as a function of eruption season and eruptive size. The results show that the climate response is regional and strongly modulated by different seasons, with mid-latitude cooling during summer and Arctic warming during winter. Furthermore, as eruptions become larger in terms of sulfur dioxide emissions, the climate response becomes increasingly insensitive to variations in the emission strength, levelling out for eruptions between 20 and 30 times the size of the 2014–15 Holuhraun eruption. Volcanic eruptions are generally considered to lead to surface cooling, but our results indicate that this is an oversimplification, especially in the Arctic where we find warming to be the dominating response during fall and winter.
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RC1: 'Comment on egusphere-2024-2651', Anonymous Referee #1, 04 Oct 2024
Zoëla et al. are interested in the climate impacts of effusive volcanic emissions of sulfur via their interactions with radiation and with clouds. They ask how the effect depends on magnitude of sulfur emission rate and on seasonal timing of the emission. The study makes use of an atmospheric general circulation model. The study in general is well written and of interest to the readership of Atmos. Chem. Phys.
I have two main comments and a number of specific ones.
1) Model-dependency of the results. This is a model-only study. The qualitative results are not very surprising, and so it is the quantitative results that matter. Most of the relevant results (cloud response, related radiation response especially in the terrestrial spectrum, and subsequently, temperature and sea ice extent responses) hinge on the simulated response of cloud liquid and ice water path and cloud horizontal cover to the emitted sulfur. The model the authors use (CAM6) shows a strongly positive response of cloud liquid water path and cloud cover to increasing aerosols. In the study of Malavelle et al. (Nature 2017, cited), a number of general circulation models were evaluated for their response to Holuhraun aerosol. The predecessor to the model this study uses, CAM5, was one of these models. CAM5 showed, erroneously, a strong positive LWP response. The data, and some other models, showed no net response of LWP. This result has been confirmed since then from other model studies (e.g. Haghighatnasab et al. ACP 2022). So the authors should at least discuss how peculiarities of the model they use, especially biases, affect the outcome.
2) Detection attempts. One of the simulations the authors perform (September) is starting at the time the effusion from Holuhraun in 2014 indeed started. It would be useful if the authors in particular discussed this simulation and discussed in particular to which extent the outcomes they simulate might be attributable in reality.
Minor remarks:
l14 – this statement depends on what is considered “high”. There are also many small emission sources.
l19 – this is an old-fashioned term (actually both), typically one nowadays would talk about “radiative forcing due to aerosol-cloud interactions”. Also it is the drop number enhancement that leads to a higher albedo (if smaller droplets are mentioned, one needs to say something about the cloud liquid water path)
l23 – this is an outdated term, nowadays one would describe it as “adjustments to aerosol-cloud interactions”. The cloud lifetime hypothesis is only one of many.
l32 – previously, the authors have described effusive emissions in contrast to eruptions, why is now effusion just a variant of explosion?
l59 – more details on the aerosol scheme are required. What types are considered, are they internally mixed, are conversion processes simulated?
l64 – the CMIP6 historical forcing stops in 2014, what is the assumption for 2015?
l76 – the authors earlier explained that the largest-ever eruption had 21 m³ lava, Holuhraun 1 m³. So a factor of 50 seems not plausible. It may be interesting, of course, nevertheless
l115 – CCN?
l146 – what is the energetic argument for the precipitation reduction? is a significant cooling of the atmosphere seen?
l155 – this can be tested, since it is model-world only. Does the model precipitate more efficiently via ice phases?
l185 – why only “dominate”? during polar night, aerosol-radiation interactions are zero
l192 – is this not potentially testable? even one-fifth of this signal could be detectable. Was this the case in reality?
l205 – same here, isn’t this signal large enough to be detectable in reality?
l274 / Table 1 – the physical units for the a and b coefficients are missing
Throughout: References, where multiple publications are cited for one fact, in general should be ordered chronologically.
Citation: https://doi.org/10.5194/egusphere-2024-2651-RC1 -
RC2: 'Comment on egusphere-2024-2651', Anonymous Referee #2, 07 Oct 2024
This study investigates the aerosol-climate interactions following effusive volcanic eruptions that emit large amounts SO2. The study builds on previous analyses of aerosol-cloud interactions following the 2014-15 Holuhraun eruption, but adds a novel direction by simulating the climate impact if a similar eruption happened in different seasons or of increased magnitude of emissions. The results show how the climate response varies with the seasons of the eruption start – particularly there is opposing effects on temperature between summer and winter, and how the response plateaus for higher magnitude eruptions.
Overall, the study presents a nice piece of an analysis. The results are clearly presented and the article is well laid out. This study is an interesting addition to the literature on aerosol-climate interactions following volcanic eruptions. I recommend minor revision prior to publication.
Comments:
- Coupled simulations can evolve differently due to the variability of the climate. This study seems to use only use one initial condition ensemble member. How do you know that differences in cloud cover between the eruption vs control simulations could be just due to differences in climate variability or if the results might be different if another initial condition ensemble member is used? I’m not sure how much impact the variability in coupled simulations would have in the short timescale of this study but I think a bit of discussion/acknowledgment of this is needed since most other studies on this topic have used nudged atmosphere-only simulations.
- Most climate models don’t represent the entrainment processes that would lead to a decrease in LWP for an increase in aerosol. There is no discussion on if CESM2(CAM6) represents that process, or how the results could differ it did.
Minor comments:
P1 L2: Untangling rather than untangle makes more sense here.
P3 L75: Is x50 emissions a plausible scenario for an effusive eruption?
Figure 2 caption: I’m not clear what the ensemble mean refers to here if there is 1x simulation per start month and scaling as described in the methods.
P11 L188: I think ‘we model’ should be replaced with the ‘model shows’ or similar throughout as this is a result of the experiment rather than modelling a climate response explicitly.
Figure 6: By eye it looks that there is less difference in the cloud properties and temperature response of eruptions starting due to starting in different months, and more that the impacts follow a seasonal cycle regardless of the start month of the eruption.
P15 L258: Wang et al. 2024 Hidden Large Aerosol-Driven Cloud Cover Effect Over High-Latitude Ocean discusses an Nd threshold for cloud fraction changes – might be of relevance here?
Citation: https://doi.org/10.5194/egusphere-2024-2651-RC2
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