Drought repeatedly triggered periods of violence, famine and conflict during the Dark Ages, with researchers able to pinpoint when periods of severe drought led to plague, mass death and social unrest.
Modern societies may also struggle to survive given changes on the way due to global warming, the researchers warn.
Drought had occurred in 1014 when the Battle of Clontarf took place, with the climatic conditions of the time confirmed by studying tree rings.
Other severe droughts occurred at times that can be matched up with crop failures and plague as described in the Irish annals from 1575.
The Annals of the Four Masters from 1252 describe “great heat and drought” during which “people crossed the principal rivers of Ireland with dry feet”.
Tree-ring growth
Years when major droughts occurred can be determined by studying annual oak tree-ring growth.
Trees produce a growth ring for every year they are alive. Years when there is plenty of rain produce a wider ring than do years where rain is reduced.
Periods of severe drought would show as a sequence of very narrow rings.
"This combination of evidence from human archives like medieval chronicles and written archives like oak trees can help to advance our understanding of how climate played a role in human history," said Dr Francis Ludlow of Trinity College Dublin.
A research fellow at Trinity's school of histories and humanities, Dr Ludlow was part of an international team that used tree ring studies and medieval manuscripts to map the impact of drought on past societies.
The team was led by Prof Edward Cook of Columbia University and involved dendrochronologists and scientists from a number of disciplines.
The worst periods tended to coincide with droughts, and these years can be catalogued and matched up with medieval documents.
Societal stresses
Major societal stresses such as famine, plague and mass mortality were the result, say the researchers. The droughts played a key role shaping modern European societies, they add.
They collected tree ring evidence from across Europe to build a reconstruction of spring and summer weather right across the Dark Ages.
A recurring link was seen between droughts and the historical recording of violence and conflict in the Annals, the researchers said.
Dr Ludlow presented their findings this morning at the European Geosciences Union General Assembly under way in Vienna.
The study shows how climate can affect human history, and modern societies can also be vulnerable to climatic shocks, Dr Ludlow said.
Understanding this interplay is important given our ongoing interference with the global climate system, he said.
