New findings on the global cooling of 1627 BC

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Aniakchak caldera on the Alaska Peninsula with a diameter of around 10 kilometre
Aniakchak caldera on the Alaska Peninsula with a diameter of around 10 kilometres. Together with the Aleutian Island chain, this region is one of the most active volcanic regions in the world. Wikicommons / M.Williams_Alaska_National_Park_Service

An interdisciplinary study, in which the University of Bern played a major role, sheds new light on two extreme volcanic events and a subsequent global cooling in antiquity. With the help of a highly precise analysis of volcanic ash and sulphur in ice cores from Greenland and Antarctica, it became clear that the global cooling around 1627 BC was not attributable to the Thera volcano in Santorini, as previously assumed, but to a volcano in distant Alaska.

The smallest droplets of sulphuric acid, which reach heights of up to 40 kilometres in the stratosphere as a result of explosive volcanic eruptions, can lead to sudden climate shocks with far-reaching consequences. This was already the case in ancient times, when probably the most famous volcanic eruption in history occurred around 1600 BC, the eruption of Thera on the Aegean island of Santorini, an important ancient trading centre in the Mediterranean.

Far less well known is a second volcanic eruption around the same time: the eruption of Aniakchak II, a remote volcano in the Aleutian Range in what is now Alaska. With over 100 megatonnes of sulphur dioxide emitted, most of it into the stratosphere, and the proven global spread of aerosols as far as Antarctica, this was the strongest eruption affecting the climate in the past 4,000 years. This is shown by a study with the participation of the University of Bern, which was published in the Proceedings of the National Academy of Sciences-(PNAS) Nexus. In the study, the eruption years and the climate impact of all significant volcanic eruptions between 1700 and 1500 BC were reconstructed.

The true trigger of global cooling
’Until now, global cooling in 1627 BC has been attributed to the eruption of Thera. This has proven to be wrong. We were able to show that the colossal eruption of Aniakchak was responsible,’ says Michael Sigl, co-author of the study. He is an assistant professor of climate and environmental physics at the University of Bern and heads the Paleovolcanism and Climate Impacts research group at the Oeschger Centre for Climate Research.

The volcano’s eruption left a blast crater 10 kilometres in diameter in the Arctic landscape. And it was so large that the researchers have even detected the sulphuric acid in Antarctica, over 16,000 kilometres away - and in addition, with the help of tree rings from the long-lived pine species Pinus longaeva, the only living witnesses of that time, they can date the eruption to the exact year 1628 BC. This finding settles a scientific controversy about the causative agent of the volcanic winter (a cold phase triggered by a volcanic eruption) of 1627 B.C. The eruption of Aniakchak II, according to the study, is the first recorded in the mid-latitudes of the northern hemisphere that distributed sulphur globally and was thus responsible for that very volcanic winter.

Geochemical detective work
The first author of the study is Charlotte Pearson, a geoarchaeologist and dendroclimatologist at the University of Arizona who is researching the Bronze Age Thera eruption. Together with Bernese researcher Michael Sigl, she has assembled an international team of researchers from the USA, the UK, Italy and Switzerland, who specialise primarily in geochemical methods for determining the composition of cryptotephra (ash particles in the micrometre range that are invisible to the naked eye) and sulphuric acid.

The aim of the project, co-funded by the ERC Horizon programme, was to link geochemical fingerprints of volcanic eruptions preserved in the perpetual ice of Greenland and Antarctica to the footprints these eruptions have left in the global climate. Studying past volcanic eruptions is important for assessing the likelihood and consequences of such extreme events in the future. In addition, such volcanic events provide precisely dated anchor points to better synchronise disparate climate reconstructions and thus characterise natural climate variability in the context of global warming.

This study was co-funded by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (THERA project 820047).