The UnProxy: Archives corroborate tree ring and ice core records

by Eric Ellman

Over the past decade, YCEI climate historian and geographer Francis Ludlow has surveyed over a million words of Gaelic and Latin from the chronicles of medieval Ireland and correlated them with modern ice core and tree ring records. The story that has emerged – of abrupt climate change prompting societal mayhem – adds a human dimension to the quantitative analyses of climate change that have predominated until now. 

Scientists have long known that the ice-age cycles involved huge climatic changes.  What few realized until the 1970s is that the Holocene (which is the most recent and current interglacial) was not stable. And while it didn’t see changes of the magnitude of ice-age cycles, the changes it did see were big enough to likely impact human civilization.  The realization that climate could change abruptly within the scale of a human lifespan was a surprise for many who study the topic; for people living through those changes, the reality was far more severe.

Bloody battles, slave and cattle raids, burning of crops and settlements, and the killings of secular and ecclesiastical elites feature prominently in Ludlow’s review of 1200 years of Irish chroniclers’ accounting of yearly events.  When mapped against tree ring and ice core records, a recurring link emerges between periods of climatic stress and extreme weather, and increased reports of violence and conflict (see Figure 1). The pathways connecting climate to violence are undoubtedly complex, with cultural and political factors playing a large role and mediating any influence of weather and climate. But the Irish chronicles make abundantly clear how conflict and violence can be triggered by the consequences of extreme weather, with the Annals of Connacht reporting in 1465 CE howExceeding great frost and snow and stormy weather [occurred] this year, so that no herb grew in the ground and no leaf budded on a tree until the feast of St. Brendan [16th May], but a man, if he were the stronger, would forcibly carry away the food from the priest in church…”. As Ludlow remarks, “it is time to take climatic pressures seriously as a recurring factor in human history.”

The Irish chroniclers’ meticulous chronology of severe frosts, droughts, dried rivers, and discolored sunsets has also helped to corroborate and then correct a 7-to-8 year discrepancy between climatic extremes identified by dendrochronology and ice core records. Ludlow’s collaboration with Michael Sigl and other colleagues, published in Nature in 2015, presents a new chronology that reconciles these two critical tools for understanding Earth’s climate past, correcting the dating of many known volcanic events, and identifying many new eruptions through the past 2,500 years.

On Monday, November 16, Ludlow presents his work as part of a Yale Climate and Energy Institute (YCEI) speakers series that highlights the work of YCEI postdoctoral fellows and colleagues.  The next day, J.G. Manning presents a follow-up talk on how collaboration with Ludlow is re-writing the history of Ancient Egypt. The new developments, says Manning, the William K. and Marilyn M. Simpson Professor of History & Classics, are the greatest disruption in the study of ancient history since the decipherment of ancient Egyptian and cuneiform writing systems in the 19th century.

Statements about the importance of climate can ruffle feathers among historians.  The community remembers how some 19th and early 20th Century geographers utilized aspects of the physical environment to explain perceived characteristics of people living in regions such as the tropics, with their work contributing to the justification of colonialist domination of cultures there.  Even today, discussion of climate in history opens one to accusations of “environmental determinism,” (an objection that historians have lodged against Jared Diamond’s Guns, Germs and Steel and Collapse.) Acemoglu and Robinson’s Why Nations Fail argues that climate and environment are unimportant in explaining economic success.  Get the institutions right, and all else follows. The problem with that view, says Manning, is that other things get in the way. One of these, in a society based on irrigation agriculture such as Egypt, is climatic shock. 

Manning always suspected that shocks lay behind the problems that the Ptolemaic kings faced in the 3rd century BCE. “We always knew that the Nile deeply effects Egyptian civilization in every way.  But in terms of social dynamics, he says, “it wasn’t so easy to see.”

When Manning met Ludlow through the YCEI and Whitney Humanities-funded Climate History Initiative, he learned how sulfate levels in ice cores recorded some of the largest volcanic eruptions in human history.  “To my astonishment,” Manning says, “many seemed to align with Egypt’s years of greatest hardship.” This observation complemented historical references to failures of Nile flooding that Manning had collected in a shoebox over his career. Subsequent conversations with atmospheric scientists Bill Boos and Trude Storelvmo revealed that IPCC climate models suggest a linkage between high-latitude eruptions and Nile flow.

The new chronology of volcanism, says Manning, “opens our eyes to a past we’ve been pretty blind to.”  Combined with written archives from the Greco-Roman period, he says, fresh understanding of climate’s history helps to explain food crises, social unrest, political bargaining, and major wars through a new lens.

In anticipation of research proposed by a consortium of historians and climate scientists from Yale, Manning predicts “we are on the cusp of major revisions to ancient history.”


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