Wars, Social Upheavals, Caused by Climate Changes, New Analysis Claims


The Hanging by Jacques Callot
Les Grandes Misères de la guerre (The Great Miseries of War) by Jacques Callot, 1632.

Was the “30 Years’ War” in Europe (1618–1648) caused by a shift in climate? Were history’s “golden ages” and “dark ages” ultimately due to climate changes?

A new analysis offers compelling, though debated, evidence of a causal relationship between historical climate fluctuations and “large-scale human crisis” events, such as famines, epidemics, social unrest, migrations, and wars .

University of Hong Kong geographer David Zhang wanted to investigate the impacts of historical cold and hot periods on human civilization by simulating  “the alternation of defined ‘golden’ and ‘dark’ ages in Europe and the Northern Hemisphere during the past millennium.

Zhang and colleagues selected 14 socioeconomic, ecological, and demographic variables as inputs for this analysis, including the price of gold, tree ring width, temperatures from pre-industrial Europe (1500 – 1800), and even individual height (a proxy indicator of famine via malnutrition).

famine years
Famine Years (Zhang et al)

The team then ran these variable through what’s known as a Granger causality analysis which assesses time series data and allows investigators to establish whether an actual, causal relationship exists between any two points of data, and not just a statistical correlation.

In the final step of the analysis, the researchers divided their studies time period into smaller periods of between 40 and 150 years to more precisely determine the causal link between temperature differences during these times and human crisis events (such as war).

Quoting from the paper abstract:

“Results show that cooling from A.D. 1560–1660 caused successive agro-ecological, socioeconomic, and demographic catastrophes, leading to the General Crisis of the Seventeenth Century. “

Of all the variables used in the analysis, climate was the strongest, causally linked variable to famine, economic downturns and other catastrophic events.

Magnitude of Social Disturbances
Magnitude of Social Disturbances (Zhang et al)

Again, quoting from the abstract:

“Our findings indicate that climate change was the ultimate cause, and climate-driven economic downturn was the direct cause, of large-scale human crises in preindustrial Europe and the Northern Hemisphere. “

The most direct route from a climate shift to human crisis is through agriculture. For example, the 100 year long cold period (which followed the  start of the ‘Little Ice Age’ in early, 16th Century Europe*) led to wide-spread crop failures, persisting for many years, which led to famines. Disease epidemics are worsened by famines, which tend to leave people too malnourished (thus with weakened immune systems) to successfully fight off the contagion.

(Author note: epidemics can also result from other causes such as trade, which brought the flea infested rodents ashore during the ‘Back Death’ plague of 14th Century Europe. This was also a period of general warming, occurring between cooling eras.)

Number of Wars
Number of Wars (Zhang et al)

Further, in more modern societies (and even in many pre-industrial ones), a widespread crop failure (due to drought or flooding), forces investors to move their money to “safer” havens, like gold. This drives the price of gold higher and, historically, has led to high inflation (like the classic ‘cartload of money to buy a loaf of bread’ effect), which can quickly lead to social unrest and upheaval. And entrenched governments, wishing to avoid both internal change and depletion of wealth, will often find some external conflict to redirect the anger of its people. leading to wider, regional conflicts/wars [for details on how the war data was determined, see end note].

2000 year temperature comparison
The reconstructed depth of the Little Ice Age varies between different studies (anomalies shown are from the 1950–80 reference period).

All that said, the study results are being met with some skepticism and doubt. There is some debate between historians and the data analysis camp. Typically, criticisms surround either the choice of variables (too limited or selective; see: the experimenter effect), or the shortness or broadness of the time series data (the length of time over which data was sampled).

Some critics question whether the research is relevant to today’s modern societies whose industrial and technological advances (and wealth mechanisms) make said societies less sensitive to climate changes (such as the rising temperatures we are seeing today), although this ‘lack of sensitivity’ theory has never been tested. And, the researchers do admit that the evidence supporting the theory that war will result from the current rise in temperatures is not extensive (but see the author comment, below).

Some historians criticize that analysis for not including other factors like religious tensions, international trade, and colonization (such as in the New World, where disease was “imported” by settlers).

Other, scientific criticisms have asserted that the results have been made too precise by attaching exact dates and numerical values to these conflicts. Others criticize the study for not including other variables like rainfall, or for “smoothing” the data into neat, 40-year segments, which can impact the statistical significance of the putative causal links found in the analysis.

Defenestration of Prague
Contemporary woodcut depicting the Second Defenestration of Prague (1618), which marked the beginning of the Bohemian Revolt, which began the first part of the Thirty Years' War.

However, the team was able to indirectly validate their results; through mapping temperatures onto historical periods, the researchers were able to determine a “crisis threshold” at which, for example, food prices rise so much (as a function of climate change) that conflict and crisis followed. This threshold, in turn, allowed them to “predict” when a crisis event should have occurred during a previous historical period. Their backward predictions were correct, that is, historical periods that met the critical threshold experienced more of these events.

Thus, beyond war and conflict, this analysis shows that climate change has the potential to impact the whole of society.

On a more hopeful note, the researchers point out that so-called “golden ages” can emerge from these “dark ages”, as for example, temperatures began to warm around 1650 and a new king (Charles II) was crowned. This event marked the beginning of The Enlightenment era in Europe.

* Climate researchers differ on the exact dates of this ‘Little Ice Age’, but generally concur that it transpired between 1550 and 1850, with slight warming periods occurring between three colder ones which lasted roughly 1oo years each.

End Note: (quoting from the research paper)

“We obtained  the  total number of wars  from  the most  inclusive global war  series  so  far,  the Conflict  Catalogue,  which  was  compiled  by  Prof.  Peter  Brecke from  the  Sam  Nunn School of International Affairs, Georgia Institute of Technology. The catalogue documents a total  of 582  wars  fought  in  Europe  in  1500–1800, which  includes  all  recorded  violent conflicts  that meet Richardson’s magnitude 1.5 or higher criterion  (32 or more deaths).”

Due to a lack of precise fatality data for all conflicts, the researchers then

“…composed a war  fatality  index  (in  annual  units)  to  estimate  annual  war fatalities.  It was  done  by  the  following  steps:  Firstly,  the  fatality  of  each war  (with  known fatality  record) was divided by  its duration. Secondly, we summed up the annual fatality of all of the wars on a yearly basis. Finally, the resultant figure was divided by 10,000 to give the annual war fatality index. “

Author Comment:

Global temperatures are a contributing factor to the global heat budget; heat is what drives precipitation. If, during a ‘Little Ice Age’, temperatures dropped (thus the heat budget is reduced), then this would mean less rain fall (plus possibly a shorter growing season). This would indeed impact agriculture and social stability (consider the impact of the food shortage in Egypt this past Spring and the subsequent ousting of the Mubarak regime).

In the case of New World colonization (and subsequent migrations), and the importation of disease, this period coincided with the cold shift in Europe (and its consequent famines and conflicts). Climate change in Europe may have prompted the search for a ‘New World’. So, indirectly, climate change could indeed have been factor in the epidemics of influenza and smallpox that devastated native populations here in North America.

Further, there is persuasive evidence that the continued, tribal and ethnic conflicts in Sudan are the result of prolonged water shortages and drought conditions. The African continent is being (arguably) hardest hit by climate impacts, largely due to it’s pervasive poverty.

I would also note that, as of this date,  here in the US, we have had a season of widespread crop failures and droughts. Meanwhile, the price of gold has risen to historical highs, popular uprisings continue in the Middle East, and the ‘occupy Wall street” movement is growing.

There seems to be growing evidence lending support to the validity of Zhang’s findings for today’s world

The paper, ‘The causality analysis of climate change and large-scale human crisis’ (Zhang et al) was published in Proceedings of the National Academy of Sciences, Sept. 6, 2011. The research team members were: David D.Zhang, Harry F. Lee, Cong Wang, Baosheng Li, QingPei, Jane Zhang, and Yulun An.

Some source material for this article came from the Science Magazine on-line article ‘Got War? – Blame the Weather, by Sara Reardon

Top charts: (Famine, Social Disturbances, Wars) The Authors, PNAS supplemental material

Bottom Chart: (200o year temperature comparison) Robert A. Rohde

For further related reading, check out my earlier articles:

Climate Change, Conflict & Ill Health Linked, Pose ‘Grave Threat’

As Arctic Ice Shrinks, Pacific Species Invade Atlantic, Causing Ecological ‘Tumult’


Human Race Faces Three Paths Forward – TED Talk {VIDEO}


1 thought on “Wars, Social Upheavals, Caused by Climate Changes, New Analysis Claims”

  1. Terrific article, especially for us wonks. I will be highlighting this on our Facebook Page and Twitter Feed. Also worth reading: Tropic of Chaos, Climate Change and the New Geography of Violence by Christian Parenti.

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