Dendrochronology and the discovery of the Late Antique Little Ice Age

I first came across dendrochronology when I was studied prehistory and archaeology in my undergraduate degree. Since then, the field has exploded.

Wikipedia describes dendrochronology in these terms:

Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed in order to analyze atmospheric conditions during different periods in history. Dendrochronology is useful for determining the timing of events and rates of change in the environment (most prominently climate) and also in works of art and architecture, such as old panel paintings on wood, buildings, etc. It is also used in radiocarbon dating to calibrate radiocarbon ages. 

New growth in trees occurs in a layer of cells near the bark. A tree's growth rate changes in a predictable pattern throughout the year in response to seasonal climate changes, resulting in visible growth rings. Each ring marks a complete cycle of seasons, or one year, in the tree's life As of 2013, the oldest tree-ring measurements in the Northern Hemisphere are a floating sequence extending from about 12,580 to 13,900 years.

A very user friendly description by Matthew Mason can be found in Environment Science: Dendrochronology: What Tree Rings Tell Us About Past and Present.

As indicated, dendrochronology can be used in dating including the calibration of radio carbon dates. However, variations in the tree rings can also provide evidence for climatic variation. This February 2018 Cambridge University promo, Silent witnesses: how an ice age was written in the trees, describes a little of the work being done here focused especially on the discovery of LALIA, the Late Antique Little Ice Age which dated from AD 536 to around AD 660.

The promo references a 2016 letter published in Nature Geoscience, Cooling and societal change during the Late Antique Little Ice Age from 536 to around 660 AD.. The abstract follows. Details of the footnotes can be found in the original. :

Climatic changes during the first half of the Common Era have been suggested to play a role in societal reorganizations in Europe 1,2 and Asia 3,4. In particular, the sixth century coincides with rising and falling civilizations 1,2,3,4,5,6, pandemics 7,8, human migration and political turmoil 8,9,10,11,12,13. Our understanding of the magnitude and spatial extent as well as the possible causes and concurrences of climate change during this period is, however, still limited. Here we use tree-ring chronologies from the Russian Altai and European Alps to reconstruct summer temperatures over the past two millennia. We find an unprecedented, long-lasting and spatially synchronized cooling following a cluster of large volcanic eruptions in 536, 540 and 547 AD (ref. 14), which was probably sustained by ocean and sea-ice feedbacks 15,16, as well as a solar minimum 17. We thus identify the interval from 536 to about 660 AD as the Late Antique Little Ice Age. Spanning most of the Northern Hemisphere, we suggest that this cold phase be considered as an additional environmental factor contributing to the establishment of the Justinian plague 7,8, transformation of the eastern Roman Empire and collapse of the Sasanian Empire 1,2,5, movements out of the Asian steppe and Arabian Peninsula 8,11,12, spread of Slavic-speaking peoples 9,10 and political upheavals in China 13.

Ulf Büntgen, Vladimir S. Myglan, Fredrik Charpentier Ljungqvist, Michael McCormick, Nicola Di Cosmo, Michael Sigl, Johann Jungclaus, Sebastian Wagner, Paul J. Krusic, Jan Esper, Jed O. Kaplan, Michiel A. C. de Vaan, Jürg Luterbacher, Lukas Wacker, Willy Tegel & Alexander V. Kirdyanov "Cooling and societal change during the Late Antique Little Ice Age from 536 to around 660 AD".. Nature Geoscience volume 9, pages 231–236 (2016) doi:10.1038/ngeo2652

Discussion

One of my weaknesses when I first started studying history at school and then as an undergraduate is that I underestimated the importance of variations across space and time in environmental conditions. I became much more conscious of these issues when I started Aboriginal history. Even the, I had unconscious blind spots.  As a simple and much later example, until I visited the Greek Islands in 2010 I really had no idea of the importance of water, food production and transport to life and politics on particular islands. Perhaps more precisely, I knew in an abstract sense but hadn't made the concrete connection.

As we gain more knowledge of the past, our views shift. The knowledge that the still new colony at Port Jackson was hit by an El Nino event that counters a previously prevailing view that its food problems were primarily due to lack of knowledge of soil and farming techniques in a new land.

The discovery from dendrochronology of the Late Antique Little Ice Age, although I'm not sure that I buy all the hype attache, is actually exciting. Here we have a case of difficulties that were identified in the documentary record but that, like the Sydney example, lacked a context to interpret them in a coherent way.