The 852/3 CE Mount Churchill eruption: examining the potential climatic and societal impacts and the timing of the Medieval Climate Anomaly in the North Atlantic region
Authors: Mackay Helen , Plunkett Gill , Jensen Britta J. L. , Aubry Thomas J. , Corona Christophe , Kim Woon Mi , Toohey Matthew , Sigl Michael , Stoffel Markus , Anchukaitis Kevin J. , Raible Christoph , Bolton Matthew S. M. , Manning Joseph G. , Newfield Timothy P. , Di Cosmo Nicola , Ludlow Francis , Kostick Conor , Yang Zhen , Coyle McClung Lisa , Amesbury Matthew , Monteath Alistair , Hughes Paul D. M. , Langdon Pete G. , Charman Dan , Booth Robert , Davies Kimberley L. , Blundell Antony , Swindles Graeme T. 
Published: June 2022

Abstract. The 852/3 CE eruption of Mount Churchill, Alaska, was one of the largest
first-millennium volcanic events, with a magnitude of 6.7 (VEI 6) and a
tephra volume of 39.4–61.9 km3 (95 % confidence). The spatial extent of the ash fallout from this event is considerable and the cryptotephra (White River Ash east; WRAe) extends as far as Finland and Poland. Proximal ecosystem and societal disturbances have been linked with this eruption; however, wider eruption impacts on climate and society are unknown. Greenland ice core records show that the eruption occurred in winter 852/3 ± 1 CE and that the eruption is associated with a relatively moderate sulfate aerosol loading but large abundances of volcanic ash and chlorine. Here we assess the potential broader impact of this eruption using palaeoenvironmental reconstructions, historical records and climate model simulations. We also use the fortuitous timing of the 852/3 CE Churchill eruption and its extensively widespread tephra deposition of the White River Ash (east) (WRAe) to examine the climatic expression of the warm Medieval Climate Anomaly period (MCA; ca. 950–1250 CE) from precisely linked peatlands in the North Atlantic region. The reconstructed climate forcing potential of the 852/3 CE Churchill eruption
is moderate compared with the eruption magnitude, but tree-ring-inferred
temperatures report a significant atmospheric cooling of 0.8 ∘C
in summer 853 CE. Modelled climate scenarios also show a cooling in 853 CE, although the average magnitude of cooling is smaller (0.3 ∘C). The simulated spatial patterns of cooling are generally similar to those generated using the tree-ring-inferred temperature reconstructions. Tree-ring-inferred cooling begins prior to the date of the eruption suggesting that natural internal climate variability may have increased the climate system's susceptibility to further cooling. The magnitude of the reconstructed cooling could also suggest that the climate forcing potential of this eruption may be underestimated, thereby highlighting the need for greater insight into, and consideration of, the role of halogens and volcanic ash when estimating eruption climate forcing potential. Precise comparisons of palaeoenvironmental records from peatlands across
North America and Europe, facilitated by the presence of the WRAe isochron,
reveal no consistent MCA signal. These findings contribute to the growing
body of evidence that characterises the MCA hydroclimate as
time-transgressive and heterogeneous rather than a well-defined climatic
period. The presence of the WRAe isochron also demonstrates that no
long-term (multidecadal) climatic or societal impacts from the 852/3 CE
Churchill eruption were identified beyond areas proximal to the eruption.
Historical evidence in Europe for subsistence crises demonstrate a degree of temporal correspondence on interannual timescales, but similar events were reported outside of the eruption period and were common in the 9th
century. The 852/3 CE Churchill eruption exemplifies the difficulties of
identifying and confirming volcanic impacts for a single eruption, even when
the eruption has a small age uncertainty.