Identifying climate variables that interchange with volcanic eruptions as cooling forces during the Common Era&rsquo;s ice ages

Seip, Knut Lehre; Grøn, Øyvind; Wang, Hui

doi:https://doi.org/10.5194/egusphere-2024-1874

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https://doi.org/10.5194/egusphere-2024-1874

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20 Aug 2024

| 20 Aug 2024

Status: this preprint is open for discussion and under review for Climate of the Past (CP).

Identifying climate variables that interchange with volcanic eruptions as cooling forces during the Common Era’s ice ages

Knut Lehre Seip, Øyvind Grøn, and Hui Wang

Abstract. Volcanism is known to be an instigating factor for the Late Antique Little Ice Age (LALIA, 536–660) and the Little Ice Age (LIA, 1250–1850), but little is known about when the effect of volcanism ends, and which other mechanisms prolong a cold period that includes the ice-ages’ cold periods, but also continued periods with persistent cooling. Here we show, with a high-resolution lead-lag method, where the stratospheric aerosol optical depth (SAOD) generated by volcanic emissions ceases to precede the Northern Hemisphere summer temperature (NHST). We find that five climate mechanisms cool the Northern Hemisphere (percentage time in parentheses): SAOD (51 %), total solar irradiance (TSI, 2 %), the North Atlantic oscillation (NAO, 11 %), the interdecadal Pacific oscillation (IPO, 28 %) and CO₂ (16 %). The last four variables overlap, and altogether the five climate variables cover 89 % of the cold period that includes LALIA and LIA. In contrast, we find an increase in atmospheric CO₂ over a brief period just after large volcanic eruptions. During the cold period, the five variables lead NHST, are in a cooling mode, and have sufficient strength to cool the Northern Hemisphere.

Received: 21 Jun 2024 – Discussion started: 20 Aug 2024

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Knut Lehre Seip, Øyvind Grøn, and Hui Wang

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RC1:
'Comment on egusphere-2024-1874', Anonymous Referee #1, 29 Aug 2024 reply

This is the third time I have reviewed this paper, earlier versions of which were submitted to Bulletin of Volcanology and Science of the Total Environment. In both cases I recommended rejection, and my recommendation is the same here. Resubmitting papers without fixing them wastes editors’ and reviewers’ time. This may not even be only the third time, as I may have not been sent other versions to review.
The fundamental problem is that a purely statistical approach is not correct for this problem. Such a statistical approach ignores the physics of the system and is confusing. Correlation is not causation. The forcing from volcanic eruptions is very short, but the response has a much larger time scale, and is non-linear. In the case of the Little Ice Age, a resetting of the state of the Arctic climate prolonged the response for decades and centuries. This is already well known. The authors claim “to our knowledge, few studies examine the timings when the effect of volcanism ceases, and other mechanisms continue the cooling effect.” But to my knowledge there are many papers that address this. For example, look at papers referenced in this submission such as Miller et al. (2012), the title of which is “Abrupt onset of the Little Ice Age triggered by volcanism and sustained by sea-ice/ocean feedbacks.” By ignoring the physical system and its response, it is not possible to understand how climate changes. By treating SAOD, TSI, NAO, IPO, and CO₂ as normalized time series, and not with units and actual radiative forcing, the results are meaningless, particularly as NAO and IPO are internal variability and not forcing. The conclusion that CO₂ changes are as important as volcanic eruptions is ridiculous, if you account for the magnitude of their forcings, except for the period since 1850 when CO₂ and other greenhouse gases dominate. The claim on lines 529-530 that “Volcanic eruptions instigated directly or indirectly increases in atmospheric CO₂ concentrations for 136 ±42 years” is completely wrong, and based on statistical artifacts of the analysis that have no basis in physics. CO₂ emissions from volcanic eruptions for the past 2000 years have been very small. They have been important for massive flood basalt eruptions, but the contribution from eruptions during the period studied is trivial.
The authors mix up external forcing and the internal climate system. Oceanic and atmospheric variations (such as the NAO) are internal responses to external forcings, such as volcanic eruptions and solar variations.
Some of the references are messed up with parts of them on different lines, such as Miller et al. (2012).

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Citation: https://doi.org/10.5194/egusphere-2024-1874-RC1
- AC1: 'Response to EGUSPHERE-2024-1874', Knut Seip, 02 Sep 2024 reply
  
  R … Resubmitting papers without fixing them wastes editors’ and reviewers’ time.
  Response. I find the introduction to the review personal, and to me this contrast with the reviewer choosing to be anonymous. Second, we have “fixed” the paper, partly based on the present reviewer’s recommendations. However, there are issues the reviewer pointed to that we do not agree with. We appreciate the opportunity to respond to the reviewer’s – sometimes harsh (the term ridiculous was used!) –comments. And for the record, there have been other reviewers that have been positive to the study.
  R The fundamental problem is that a purely statistical approach is not correct for this problem. Such a statistical approach ignores the physics of the system and is confusing.
  Response. The paper is basically statistical, but we invoke time series, e.g., the North Atlantic Oscillation (NAO) and other ocean variability series that have been discussed extensively in physical terms. The ocean variability series has been shown to impact global temperature. Wu et al. (2019) is cited in the paper line 537, but a new article in Nature corroborate this result, Samset et al. (2024, p.3):” What was special about 2023 as rather that multiple ocean basins had warm anomalies at the same time”. Apart from this, to address issues around a phenomenon from a statistical perspective, is in our opinion correct and may give valuable information on the theme studied. We will add the reference to Samset et al. (2024).
  R. Correlation is not causation.
  Response. We are not sure if the referee assumes that we believe correlation is causation. We do not. We outline our method in Section 3 line 145 and below, we list three additional techniques, in addition to ordinary linear regression (OLR), that may strengthen a causal interpretation. Furthermore, we discuss causality interferences when there are known and unknown potential causes, line 235.
  R. In the case of the Little Ice Age, a resetting of the state of the Arctic climate prolonged the response for decades and centuries. This is already well known.
  Response. We are not sure what the reviewer means by “resetting”. There is a large literature on ocean teleconnections across the globe, and changes in the Arctic climate is one element. Other variabilities have also been given prominent roles, like El Niño and the Pacific decadal oscillation.
  R The authors claim, “to our knowledge, few studies examine the timings when the effect of volcanism ceases, and other mechanisms continue the cooling effect.” But to my knowledge there are many papers that address this. …” Miller et al. (2012) is given as an example.
  Response. The study by Miller et al. (2012) identify when summer cold begins (1275), but not when it ends. There are several studies that suggest mechanisms that would continue the cooling effect, but they are generic. The Miller et al. (2012, p. 5) study says: “Climate modeling reveals one such possible feedback mechanism.” Modeling studies have their advantage in being mechanistically driven, but also their drawbacks. Examples on this were given on Line 60. In contrast, the time series we use are observed series, although often based on proxies.
  R By ignoring the physical system and its response, it is not possible to understand how climate changes.
  Response. “To understand” is a complex and interesting expression. For example, you understand a phenomenon if you can refer to something well known (freezing water increase in volume, IPO gets warmer) and accepted a causal link between the familiar and what you want to understand (the water bottle breaks, GTA increases). The water bottle example is from Campbell (1952).
  R. By treating SAOD, TSI, NAO, IPO, and CO₂as normalized time series, and not with units and actual radiative forcing, the results are meaningless.
  Response. We treat the series in a normalized form to identify correlations and lead-lag relations. We also distinguish components of the series. To convert the series to a common unit is a challenge, but we refer to studies that suggest that e.g. NAO and IPO would have the strength to affect the global temperature, line 535. Presently we could have added a study by Samset et al. (2024) who focus on ocean basins having positive anomalies at the same time and on upwelling of previously stored deeper water with high temperatures. Text will be changed, a reference to Samset et al. (2024) will be added.
  R particularly as NAO and IPO are internal variability and not forcing.
  Response. NAO (indirectly) and IPO show characteristics of surface water temperatures in the Atlantic and the Pacific respectively, but they are also part of a teleconnection system and therefore exert force on other part of the global surface temperature system. (NAO is measured as sea surface pressure difference, but it is associated with sea surface temperatures in the literature). If GTA is restricted to the surface system, the oceanic deep waters will be a force outside this system. However, the referee is right, presently all the ocean waters are treated as one system. Text will be changed.
  R. The conclusion that CO₂changes are as important as volcanic eruptions is ridiculous, if your account for the magnitude of their forcings, except for the period since 1850 when CO₂and other greenhouse gases dominate.
  Response.The role of CO₂ in the atmosphere before, say 1850, has been revised in recent literature. It is probably not as small as previously believed. For example, Hansen et al. (2023) states that “We infer from paleoclimate data that aerosol cooling offset GHG warming for several millennia as civilization developed” The authors suggest that the effects of aerosols and GHG were comparable. However, their suggestions are discussed further in subsequent literature.
  R. The claim on lines 529-530 that “Volcanic eruptions instigated directly or indirectly increases in atmospheric CO₂concentrations for 136 ±42 years” is completely wrong and based on statistical artifacts of the analysis that have no basis in physics.
  Response. We give the source for the time series for CO₂ around line 140. To our knowledge, it is developed without references to volcanic emissions. Figure 3 shows that following three (pre-1850) volcanic eruptions, atmospheric CO₂ concentrations increase giving the numbers 136± 42 years. The CO₂ curve in the Figure is a slightly LOESS (0.1,2) smoothed version of the raw curve (Line 228, to avoid possible high frequency noise) and the droplines shows volcanic eruptions. KLS remade the calculations the result is correct, and the calculations are available from him.) We do not know what artefacts the referee refers to.
  R CO₂emissions from volcanic eruptions for the past 2000 years have been very small. They have been important for massive flood basalt eruptions, but the contribution from eruptions during the period studied is trivial.
  Response. We quote Werner et al. (2019) and Buono et al. (2023). We should not have restricted the Werner reference to one figure. Did we misunderstand the information in the Werner article? Text will be changed if the sources do not support our claim.
  R The authors mix up external forcing and the internal climate system. Oceanic and atmospheric variations (such as the NAO) are internal responses to external forcings, such as volcanic eruptions and solar variations.
  Response.. The GTA is a global surface phenomenon with measurements at the surface. If “the system” is the surface system, then the deep ocean waters are external to this system. This was the case for many early publications. It is also rational; the surface waters have been monitored for many years ≈ 1850, whereas deep waters have only recently, ≈ 1990, been monitored and understood, e.g., (Cheng et al., 2024). We believe the referee’s second statement is a conjecture. The causes of variations in NAO are not known, although several mechanisms have been proposed. (Meehl et al., 2013; Wu et al., 2019).
  R. Some of the references are messed up with parts of them on different lines, such as Miller et al. (2012).
  Response. We will take care of this.
  Literature
  Buono, G., Caliro, S., Paonita, A., Pappalardo, L., & Chiodini, G. (2023). Discriminating carbon dioxide sources during volcanic unrest: The case of Campi Flegrei caldera (Italy). Geology, 51(4), 397-401. https://doi.org/10.1130/G50624.1
  Campbell, N. R. (1952). What is science? Dover Publications.
  Cheng, L. J., Abraham, J., Trenberth, K. E., Boyer, T., Mann, M. E., Zhu, J., Wang, F., Yu, F. J., Locarnini, R., Fasullo, J., Zheng, F., Li, Y. L., Zhang, B., Wan, L. Y., Chen, X. R., Wang, D. K., Feng, L. C., Song, X. Z., Liu, Y. L., . . . Lu, Y. Y. (2024). New Record Ocean Temperatures and Related Climate Indicators in 2023. Advances in Atmospheric Sciences, 41(6), 1068-1082. https://doi.org/10.1007/s00376-024-3378-5
  Hansen, J. E., Sato, M., Simons, L., Nazarenko, L. S., von Schuckmann, K., Loeb, N. G., Osman, M. B., Pushker Kharecha, Qinjian Jin, George Tselioudis, Andrew Lacis, Reto Ruedy, 9, Gary Russell, Junji Cao, & Li11, J. (2023). Global warming in the pipeline. Oxford Open Climate Change, 3.
  Meehl, G. A., Hu, A. X., Arblaster, J. M., Fasullo, J., & Trenberth, K. E. (2013). Externally Forced and Internally Generated Decadal Climate Variability Associated with the Interdecadal Pacific Oscillation. Journal of Climate, 26(18), 7298-7310. https://doi.org/10.1175/Jcli-D-12-00548.1
  Miller, G. H., Geirsdóttir, A., Zhong, Y. F., Larsen, D. J., Otto-Bliesner, B. L., Holland, M. M., Bailey, D. A., Refsnider, K. A., Lehman, S. J., Southon, J. R., Anderson, C., Björnsson, H., & Thordarson, T. (2012). Abrupt onset of the Little Ice Age triggered by volcanism and sustained by sea-ice/ocean feedbacks. Geophysical Research Letters, 39. https://doi.org/Artn L02708 10.1029/2011gl050168
  Samset, B. H., Lund, M. T., Fuglestvedt, J. S., & Wilcox, L. J. (2024). 2023 temperatures reflect steady global warming and internal sea surface temperature variability. Communications Earth & Environment, 5(1). https://doi.org/ARTN 460 10.1038/s43247-024-01637-8
  Werner, C., Fischer, T., Aiuppa, A., Edmonds, M., Cardellini, C., Carn, S., & Allard, P. (2019). Carbon Dioxide Emissions from Subaerial Volcanic Regions: Two Decades in Review. In B. Orcutt, I. Daniel, & R. Dasgupta (Eds.), Deep Carbon: Past to Present (pp. 188-236). Cambridge University Press.
  Wu, T. W., Hu, A. X., Gao, F., Zhang, J., & Meehl, G. A. (2019). New insights into natural variability and anthropogenic forcing of global/regional climate evolution. Npj Climate and Atmospheric Science, 2. https://doi.org/UNSP 18 10.1038/s41612-019-0075-7
  
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  Citation: https://doi.org/10.5194/egusphere-2024-1874-AC1
- AC2: 'Reply on RC1', Knut Seip, 14 Oct 2024 reply
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1874/egusphere-2024-1874-AC2-supplement.pdf
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  Citation: https://doi.org/10.5194/egusphere-2024-1874-AC2
RC2:
'Comment on egusphere-2024-1874', Anonymous Referee #2, 10 Oct 2024 reply

The authors have made a commendable effort in this manuscript, presenting original approaches to determining the onset and duration of cooling episodes caused by large volcanic eruptions during the Common Era. The study is interesting and has potential offering contributions to our understanding of volcanic impacts on climate.
However, the clarity of the manuscript should be improved. At times, it is difficult to follow due to long and complex sentences. For instance, the explanation of the LL hypothesis—"The LL hypothesis comes with a caveat; if two variables impact the temperature, the second variable may distort the temperature series so that the first series appears to lag the temperature series even if it is leading"—is quite dense and could benefit from simplification.
The first line of the discussion is actually helpful, but should be moved up to the introduction.
Overall, I recommend revising sections with similarly complicated phrasing to improve readability. A clearer structure would enhance the impact of this important work and make it more accessible to the wider audience.
Despite the effort and interesting approaches presented in the manuscript, my primary concern lies with the tree-ring width (TRW) reconstruction, which forms the foundation of the study. The authors apply a loess filter to the data (what happens if you use the raw data?), but they fail to mention that the reconstruction is based on tree-ring widths, which are known to exhibit biological memory effects (see Esper et al., 2015, among many other references). This omission is critical, as memory effects in TRW can extend the cooling signal by up to 10 years, whereas the actual volcanic forcing and cooling feedbacks may be much shorter in duration.
Additionally, the issue of volcanic dating raises further concerns. While the dataset of Sigl et al. (2015) is referenced, the authors do not consider the potential impact of the double-event effect (see Tejedor et al., 2021). This is a complex matter that could potentially undermine the validity of the results.
Therefore the paper could benefit from using Northern Hemisphere reconstructions based on maximum latewood density (MXD), such as those by Schneider et al. (2017) or Büntgen et al. (2024), as MXD reconstructions are known to mitigate the memory effects associated with TRW. This shift in proxy data would likely result in more accurate reconstructions of volcanic cooling episodes.
In Figure 3, if you include the titles (which i dont know if its allowed), you should also point out which are your NHST raw temperature and which are the PCAs..

Esper et al., 2015 (Dendrochronologia). https://www.sciencedirect.com/science/article/abs/pii/S112578651500048X
Tejedor et al., 2021 (P&P). https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020PA004128
Schneider et al., 2017 (ERL).https://iopscience.iop.org/article/10.1088/1748-9326/aa7a1b
Büntgen et al. 2024 (GPC). https://www.sciencedirect.com/science/article/pii/S0921818123003107?via%3Dihub

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Citation: https://doi.org/10.5194/egusphere-2024-1874-RC2
- AC3: 'Reply on RC2', Knut Seip, 18 Oct 2024 reply
  
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  Citation: https://doi.org/10.5194/egusphere-2024-1874-AC3

Knut Lehre Seip, Øyvind Grøn, and Hui Wang

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Short summary

The Little Ice age in the Northern Hemisphere is suggested to be instigated by volcanic eruptions, but cooling of the earth that is due to volcanism last for a few years whereas the little ice age lasted for 600 years. So, even if there are several volcanic eruptions, what continue the cool temperature during the ice age? Here, we use a new technique that let us know when volcanic cooling cease, and which mechanisms lead the temperature and are strong enough to cool the Northern Hemisphere.


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