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Timing of Holocene volcanic eruptions and their radiative aerosol forcing

Periodic Reporting for period 4 - THERA (Timing of Holocene volcanic eruptions and their radiative aerosol forcing)

Okres sprawozdawczy: 2023-09-01 do 2024-08-31

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. 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 overarching goal of THERA is to generate the longest and most comprehensive reconstruction of the climate impact potential from past volcanic eruptions to understand the role of volcanism on climate. The unique ability to extract from these ice cores a complete record of all major volcanic eruptions for over 12,000 years, including such magnitudes scarcely recorded in recent times, offers an ideal opportunity to analyze the likelihood of these global-scale natural hazards.

Conclusions of THERA:

1) The climate system is more sensitive to volcanic eruptions than previously thought. This comprises the magnitude of climate responses, its spatial and temporal extent and its scope. Next to global surface cooling, volcanic eruptions disrupted atmospheric circulation and the global distribution of rainfall, promoting extreme weather, monsoon failures and increased streamflow variability. Some of these climatic changes may even have influenced societal trajectories.
2) Many eruptions during the past 12,000 years released more sulfur than the colossal eruption of Tambora in 1815, suggesting eruptions of this size occur more than twice as often globally as previously thought. The chance to experience the consequences of such an eruption until the end of the 21st century is as high as 1 in 6, or orders of magnitude higher than a comparable extraterrestrial impacts.

An increased sensitivity of the climate system to volcanic eruptions which are also more likely to occur means that the hazard risks for the global population are currently strongly underestimated. At the same time, awareness of these risks remains low and preparedness to address these global catastrophic risks following future large eruptions are still in their infancy. This is also reflected in the latest IPCC’s Assessment Report, the go-to document of the state of climate change science for many policy makers, scientists and communicators, in which the consequences of future volcanic eruptions on the climate, environment and society play a negligible role.
Using the geochemical fingerprint of cryptotephra we pinpointed major eruptions from Iceland, New Zealand, Asia and North America as well as tropcal eruptions from Mesoamerica. Global climate effects were studied using model simulations and climate proxies following the 43 BCE Okmok eruption (Alaska). Exceptional strong sulfur burden in the Northern Hemisphere resulted in extreme weather in 43 BCE affecting the Nile River flow with economic impacts during late Roman Republic. 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, prominently covered by leading media outlets (e.g. NYT, CNN, FOX News, FAZ).

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 (9500 BCE-1900 CE). This new long-term reconstruction of past VSSI variability confirms evidence from regional volcanic eruption chronologies by showing that the early Holocene 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.

Cryptotephra and heavy metals in ice cores from Greenland showed that volcanic activity in Iceland can persist for up to decades-to-centuries, providing context to the possible durations and associated risks of the recent volcanic activity in Iceland. Such a magnitude, duration and frequency of volcanic ash, sulfur, metal, and halogen emissions from Iceland is unprecedented within the Common Era. While this is the youngest long-lasting eruptive episode from Iceland, similar periods were more frequent and extensive during the early Holocene. This type of activity is comparable to the anthropogenic release of sulfur peaking in the 20th century and it will require additional efforts to understand its role in the climate system.

Results:

1) We reconstructed sulfur injection, atmospheric aerosol properties and their climate forcing potential for close to 1000 eruptions over the past 12,000 years and analyzed long-term trends of volcanic activity and changes in climate forcing.

2) For key eruptions we used the geochemical composition of ash to identify source volcanos and discriminated the sulfate contributions transported via the stratosphere and troposphere to improve our estimates of climate forcing.

3) Tropospheric sulfur emitted from long-lasting eruptive episodes over decades to centuries emerged as potentially important new climate forcing agent.

4) Proxy records and output from fully coupled Earth System Climate Model simulations show diverse effects of volcanic eruptions on global climate parameters.

We disseminated the project results in the science community through hosting three topical international workshops at the University of Bern. We engaged with the media and the general public through press releases, interviews on television, radio and in newspapers, social media and other outreach activities.
HolVol v.1.0 incorporates new-generation ice-core aerosol records with sub-annual temporal resolution and has sub-decadal dating accuracy and precision. With HolVol we provide for the first time precise ages and atmospheric sulfur injections (i.e. global climate impact potential) for close to 1000 eruptions including all volcanic eruptions similar in strength to Pinatubo 1991, which cooled the globe by 0.5 °C. 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.

A key part of our approach was the holistic combination of various geochemical methods characterizing different volcanic fallout parameters (i.e. volcanic ash and volcanic acids) because it is their exact stratigraphic position within the polar ice sheets that hold clues on the volcanic sources and the eruption’s climatic impact. 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.
Fig05_Okmok caldera (Alaska) from space
Fig08_"Icleandic Active Period" from 750 to 940 CE
Fig04_Geochemical analyses of cryptotephra at University of Bern
Fig14_Workshop_VICS_Bern2023
Fig12_Size-frequency-relation based on HolVol1.0 with Pinatubo 1991 and Tambora 1815 highlighted
Fig09_Winter sea-ice anomaly in Constantinople in 763/764 CE
Fig06_Extreme cooling in CESM simulations following the 43 BCE Okmom II eruption
Fig02_Ice-core sampling — Reno, Denver, Bremerhaven & Copenhagen
Fig01_The National Science Foundation Ice Core Facility (NSF-ICF) — Repository Denver, USA
Fig11_Holocene volcanic sulfur injections (HolVol1.0)
Fig13_Volcanism, Climate and Societies during the 939-40 Eldgja eruption
Fig10_Bristlecone pine frost rings (California) in 1627 BCE after the Aniakchak II eruption (Alaska)
Fig03_Method flow chart for tephra analyses at University of Bern
Fig15_Volcanic forcing and temperatures in the Northern Hemisphere in the past 8,000 years
Fig07_Volcanic weather anomalies in context with ancient human history