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Timing of Holocene volcanic eruptions and their radiative aerosol forcing

Periodic Reporting for period 2 - THERA (Timing of Holocene volcanic eruptions and their radiative aerosol forcing)

Reporting period: 2020-09-01 to 2022-02-28

Volcanic eruptions produce regional to global scale impacts on the climate system. There is growing evidence of a strong vulnerability of past economies and societies to volcanic impacts on climate drawn from numerous historical examples of “years without a summer”, famines and pandemics coinciding with volcanic extreme events. Despite the known impact of volcanic eruptions on climate and societies in the past, volcanic eruptions are currently not taken into account for climate projections of the 21st century “due to the episodic nature of volcanic eruptions” (IPCC AR6). In order to mitigate and adapt to the climate effects of future large volcanic eruptions we need to better quantify the risk of these eruptions including 1) the probability of their occurrence and 2) their expected climatic impact.

The observational record of the timing of volcanic eruptions, their location, magnitude of sulphate aerosol injection and its atmospheric life-cycle, however, is geographically biased and often incomplete, with gaps in our record of past volcanic activity increasing dramatically before the Modern (pre-1800) era. This shortage in observational data strongly limits our understanding of the sensitivity of the Earth system to volcanism and the vulnerability of social and economic systems to the climate impact of past and future eruptions. However, traces from past volcanic eruptions are encapsulated in polar ice-sheets and can be retrieved, dated and analyzed using ice-cores drilled in Greenland and Antarctica.

The primary goal of this project is to precisely estimate the probability (i.e. return intervals) of volcanic eruptions able to impact global climate, humanity and economy: What is the likelihood of an eruption as large as the eruption of Tambora in 1815 to occur somewhere on the globe within the next 30 years? To answer this research question, my team extracted comprehensive data on the timing, magnitudes and source locations of volcanic eruptions occurring since the last Glacial (i.e. over the past 13,200 years), using a bi-polar array of ice-cores and by employing novel, precisely dated, high-time resolution aerosol measurements from these ice cores.
With volcanic synchronization, we aligned the sulphate records ice cores in Antarctica and Greenland thus creating a bipolar chronological framework. We conducted high-resolution sulphur isotope analysis (33S, 34S) of historic eruptions from Alaska and Indonesia demonstrating the ability to trace the highly diagnostic 33S evolution following tropical eruptions in two ice cores from opposing hemispheres.

Using the geochemical fingerprint of cryptotephra we pinpointed prehistoric eruptions from Iceland, as well as the 431 CE Tierra Blanca Joven eruption of Ilopango (El Salavador) which is only the second time that tephra from a tropical eruption has been detected in polar ice cores. Climate effects in the Northern Hemisphere were studied using model simulations and climate proxies following the 43 BCE Okmok II eruption (Alaska) identified through cryptotephra. Exceptional strong burden sulphate in the middle and higher latitudes of the Northern Hemisphere resulted in extreme weather anomalies in 43 BCE affecting the Nile River flow with economic impacts during late Roman Republic and Ptolemaic Kingdom. Since the eruption date and climatic effects coincided with the civil war unfolding following the murder of Julius Caesar in 44 BCE, this study has sparked global interest far beyond the geosciences and was prominently covered by leading media outlets (e.g. NYT, CNN, FOX News, FAZ).

Finally, based on a set of continuous sulphate records from a suite of ice cores from Greenland and Antarctica, the HolVol v.1.0 volcanic eruption catalogue was developed and includes estimates of the magnitudes and approximate source latitudes of major volcanic stratospheric sulphur injection (VSSI) events for the Holocene (from 9500 BCE to 1900 CE). This new long-term reconstruction of past VSSI variability confirms evidence from regional volcanic eruption chronologies (e.g. from Iceland) by showing that the early Holocene (9500-7000 BCE) experienced a higher number of volcanic eruptions and cumulative VSSI compared to the past 2,500 years. This increase coincided with the rapid retreat of ice sheets during deglaciation, providing an analogy for the future if volcanic activity increased in regions under projected glacier melting in the 21st century.
HolVol v.1.0 incorporates new-generation ice-core aerosol records with sub-annual temporal resolution and has sub-decadal dating accuracy and precision. By accurately aligning and stacking the ice-core records on the WD2014 chronology from Antarctica we resolve long-standing inconsistencies in the dating of ancient volcanic eruptions that arise from biased (i.e. dated too old) ice-core chronologies over the Holocene for Greenland. Accurately characterizing volcanic forcing in climate models is crucial for the robust attribution of the drivers of surface temperature trends. Our new volcanic forcing dataset can readily be implemented for transient climate simulations throughout the Holocene to detect and attribute natural climate drivers on interannual-to-centennial scales.
Based on targeted analyses of the soluble and insoluble volcanic aerosols in polar ice cores we could precisely constrain key eruptions source parameters of major past eruptions. These analyses were possible, because co-registered high-resolution measurements of coarse insoluble particles and sulphur provide a clear-cut indicator for the presence of very low concentrations of tephra glass shards (cryptotephra) in polar ice. This work revealed the importance of considering the VSSI rather than magnitude (M) or volcanic explosivity index (VEI) when assessing the climate impact of past eruptions and the future hazard risk.
Our targeted, near-surgical, approach to volcanic horizon identification and analysis based on unambiguous diagnostic features for tephra layers in ice, sets new standards for future ice core projects. Our methodology is tailored to projects where only very small cross-sections/volumes of very valuable ice are available, such as Beyond EPICA Oldest Ice. (2006)

We will continue to reconstruct the most comprehensive and accurate reconstruction of Holocene volcanism and analyze its relationship with the global climate system throughout the Holocene. We will gain new insights in the frequency and climate forcing of volcanic eruptions for example during the deglaciation in the early Holocene or during the transition between Late Antique Little Ice Age (536-660 CE) and the Medieval Warm Period (950-1100 CE) in the North Atlantic region.

Extratropical volcanic eruptions are commonly thought to be less effective at driving large-scale surface cooling than tropical eruptions, and only the latter are commonly thought to be able to distribute sulphate globally. We will test both assumptions using a network of ice cores from the polar regions of Antarctica and Greenland covering the past 13,000 years and climate aerosol modeling. We will use this information to estimate the climate impact potential due to negative radiative forcing caused by Earth’s largest volcanic eruptions since the last Glacial and the return intervals of such eruptions.
Okmok caldera (Alaska) from space
Geochemical analyses of cryptotephra at University of Bern
Extreme cooling in CESM simulations following the 43 BCE Okmom II eruption
Ice core Sampling
The National Science Foundation Ice Core Facility (NSF-ICF) — Repository Denver, USA
Method flow chart for tephra analyses at University of Bern
Volcanic weather anomalies in context with ancient human history