1 What inspires us to examine the
climateclimate-war relationship?
GEOG 3211
Climate Change and Social
Responses
1.1 Iraq War in 2003
Topic 4:
Climate change & social stability
1.2 Field trip to Northern China in 2002
2 What have been done by other scholars?
2.1 Climate change & social responses
• “The Pulse of Asia” (Huntington 1907) is the 1st
study examining the association between climate
change & human history
• With the recent advancement in paleo–climate
reconstruction, there are many studies about the
impact of climate change upon human societies in
history
1
2.2 Dynastic change & warfare studies
Study 1: Civilian wars & important historic events in
Chinese history, 221BC–
221BC–AD1929
Study 2: Temperature, aridity threshold & the latitudinal
variations of the southern boundaries of the state power
for nomadic tribes in the past 4,000yrs
3 How do we conduct our research?
3.1 Theoretical framework
Solid arrow: Direct association
Dotted arrow: Feedback effect
Study 3: Comparison of the reconstructed seasea-surface
salinity for the last 3,000yrs with periods of droughts,
floods, peasant uprisings, construction phases of the
Great Wall & changes in the ruling dynasties
2
Maslow’
Maslow’s Hierarchy of Needs
• Food shortage manifested itself in 2 causal
pathways:
• Direct cause,
cause resource-oriented wars erupted
as most of the world pop still struggled to satisfy
the lower levels of Maslow’s Hierarchy of Needs
• Indirect cause,
cause constrained food resources &
economic difficulties stemming from that
intensified different social contradictions, which
increased the likelihood of war outbreaks
• We delimit our scope of research to historic
agrarian societies instead of modern industrial
societies
• We focus on the impact of longlong-term climate
change instead of short-term climate change
• We base on all known cases instead of narrow
historic examples
3.2 Data source
• Fine-grained paleo-climate reconstructions &
historic socio-economic datasets such as
agricultural production, economic, warfare &
population data
3.3 Methodology
• Time series analysis
• We examine the climate-war relationship at the
macro scale instead of micro scale
3
4 What are our findings?
• 4.1.1. Temperature change & war frequency
4.1 China
• Reduction of thermal energy input in cold periods
reduced land productivity in agrarian society 
weakened state power & brought about famine 
wars became more likely, especially rebellions &
northern tribe invasions
Northern China:
continental humid,
semi-humid, semiarid & arid
temperate climate
• Agricultural yields in China in 1840–1890 (cold
phase)  10–25% compared with 1730–1770
(warm phase), because of shorter growing season
& more frost days & cold spells
Central China:
monsoonal climate
Southern China:
subtropical &
tropical climate
10–
10–25% reduction, so what?
Time series of temperature
change & social responses
in China, 1000–
1000–1911
Food per capita index,
1730–
1730–1910. It is calculated
by (harvest index X
cultivated land area) /
population size
“Warfare was an adaptive
ecological choice under
limitation of resources &
increase of population”
population”
(a) NH temp anomaly
(b) No. of wars & rebellions
(c) No. of wars in Northern
China,
China, Central China &
Southern China
Taiping
Rebellion
(1851)
End of
Qing
(1911)
(d) Dynastic changes &
population size
Gray strips represent cold
phases
4
Pearson’
Pearson’s r between temperature anomalies & war
records (annual scale), 1000–
1000–1911
Pearson’
Pearson’s r between temperature anomalies & war
records (decadal scale), 1000–
1000–1911
Pearson’
Pearson’s r between temperature anomalies & the
highest war frequencies (phase scale), 1000–
1000–1911
• 4.1.2 Geographical Pattern
• Wars in Northern China & Central China
significantly correlated to temperature changes,
because their land are ecologically sensitive to
climate change
Region
Northern
China
Points to note
The main subsistence was grazing,
grazing which was sensitive
to cooling. Besides, pastoral animal resources couldn’
couldn’t
be stored for a long time
In Southern Song, Yuan & Qing dynasties, the whole
China or part of Central China was ruled by northern tribes,
During the time, livelihood resources were massively
transported from the south to the north
Central
China
Controlled by the monsoons,
monsoons cold periods could have
imposed both coldness & dryness
Population density remained the highest among the 3
regions in the past millennium
Southern
China
Humid subtropical & tropical climate, with a wide range of
domesticated species. Alternative crops could be
adopted in the cold period
5
• 4.1.3 Temperature change & population
Pearson’
Pearson’s r between annual temperature anomalies &
annual war records of Northern China during the period
without northern tribe occupation
• 5 demographic collapses, each with population
losses ~30–80 million, occurred during the last
millennium
• All of them happened in cold phases & coincided
with high war frequencies & dynastic changes
• Such collapses were caused by war, famine &
epidemics. But, they were mainly caused by war
At the annual scale correlation analysis, if we disregard the
northern tribe occupation years, the correlation between
temperature & war in Northern China was more significant
than that in other areas
• 4.1.4 Climatic cycles & dynastic cycles
• On the contrary, all warm phases had fast
population growth
Relationship between climatic phases, wars & dynastic
changes
• The starts & ends of dynastic cycles in China
were associated with cold periods during the last
millennium
• Climate change was closely associated with war
frequency because the shortage of livelihood
resources in cold periods could trigger wars. The
outbreak of wars would further weaken state
power, eventually leading to dynastic collapse
6
4.2 Global
• 4.2.1 Temperature change & the cyclic
patterns of wars
• At the continental scale, the no. of wars in the
NH, Europe, Asia & the arid areas of the NH
followed the cycles of the NH temp variations
• At the global scale, different worldwide war
databases with different violence thresholds also
followed the cycles of the NH temp variations
Map of different geographical regions. North America (gold), South
South
America (blue–
(blue–gray), Western Europe (gray), Eastern Europe (lime),
North Africa (wheat), West & Central Africa (lavender), East & South
South
Africa (rose), Asia (light yellow), Australia (white), arid & semiarid
areas of NH (shaded areas)
PaleoPaleo-temperature change,
war & population growth rate,
1400–
1400–1900
(A) NH temp anomaly
(B) Wars in the NH, Asia, Europe &
the arid areas in the NH
(C) Wars worldwide as recorded by
Wright, Luard & Brecke
• War fatality peaked in the cold 17th & the early
19th centuries, which was coincided with 2 of the
greatest population declines since 1400
• Worldwide war ratio in a cold climate was
almost doubled that of a mild climate. Similar
phenomenon was also observed for the NH,
Asia, arid areas of the NH & Europe
• >80% countries & regions around the world had
higher war ratios in a cold climate
(D) Population growth rate in
Europe, Asia & the NH & the NH
fatality index
Cold phases are shaded as gray
stripes. The bright green curves
correspond to the right y axis
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Correlations between
climate change & war
frequency, 1500–
1500–1900
These figures show that the
match between cooling & war
outbreak is not accidental
* <5% is by chance
*** <0.1% is by chance
Regions
World (>50,000 troops)
–0.26***
0.26***
World (principal wars)
–0.32***
0.32***
World (armed conflicts)
–0.36***
0.36***
Northern Hemisphere (NH)
–0.46***
0.46***
Southern Hemisphere
–0.48***
0.48***
Asia
–0.24***
0.24***
Arid areas in NH
–0.64***
0.64***
Europe
–0.63***
0.63***
Western Europe
–0.56***
0.56***
Eastern Europe
–0.63***
0.63***
North America
–0.08
South America
–0.12*
0.12*
West & Central Africa
–0.49***
0.49***
East & South Africa
–0.55***
0.55***
China
–0.26***
0.26***
Regions
Correlations between
climate change &
population growth, 1500–
1500–
1900
These figures show that the
match between cooling &
population decline is not
accidental.
*** <0.1% is by chance
Correlations
Correlations
World
0.52***
0.52***
NH
0.53***
0.53***
Asia
0.59***
0.59***
Europe
0.36***
0.36***
North America
0.33***
0.33***
China
0.57***
0.57***
• 4.2.2 Temperature change & population
growth
• Cooling induced population collapses around
the world
• In a mild climate, population growth rates in
various major geographic regions increased; in
a cold climate, they shrank together
Climate &
population cycles
in the NH in 1000–
1000–
1900 are also
closely matched
Phase I
Phase II
Phase III
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• Phase I (1000–
(1000–1400):
1400) World pop stagnated. Temp
& pop growth rate dropped simultaneously. Every
sudden temp drop induced a ‘‘demographic shock’’
• Phase II (1400–
(1400–1700):
1700) The 1st half of this period
coincided with a warm phase & the beginning of the
“Early Modern Times.’’ NH pop growth rate rose
rapidly. But, when the coldest epoch of the LIA took
place (17th century), pop declined followed
• Phase III (1700–
(1700–1900):
1900) A dramatic increase in
human pop occurred because of the Industrial &
Agricultural revolutions. But, a significant cooling in
the mid–19th century still caused a worldwide
demographic shock
Cooling engendered
disastrous impact on Europe
& China synchronously
When agri prod went down,
wheat prices went up. When
prices reached a certain level,
more wars erupted. Pop growth
rates were influenced by both
war freq & food supply per
capita (reflected in cereal prices)
& dramatically dropped to –ve
values when agri prod was at its
bottom, cereal prices & war freq
reached their peaks. Such a
socio-ecological disaster
reduced pop sizes
• 4.3.3 Comparison between Europe & China
• We compared Europe & China (1500–1800) to
further verify the impact of climate change on
human societies
• They shared 60% of the total world population
during the time
• We found that cooling induced disastrous
impact on Europe & China synchronously
Correlations between climate change & its impact on
Europe & China,
China, 1500–
1500–1800
These figures confirm that climate change engendered the
same socio–economic impact on the 2 regions
synchronously *** <0.1% is by chance
Causal linkage
Europe
China
0.70***
0.70***
Temperature change  Agricultural production
0.84***
0.84***
Agricultural production  Food supply per capita
–0.41***
0.41*** –0.13***
0.13***
Food supply per capita  Population growth
–0.32***
0.32*** –0.64***
0.64***
Food supply per capita  War frequency
0.64***
0.64***
War frequency  Population growth
–0.31***
0.31*** –0.66***
0.66***
Population size  Food supply per capita
0.25***
0.25***
0.34***
0.34***
Population growth  Agricultural production
0.49***
0.49***
0.41***
0.41***
0.46***
0.46***
9
• Europe & China were politically, economically &
geographically detached during the time. But,
their socio-economic fluctuations were the same
in terms of their macro-trends, turning points &
oscillation magnitude
• Those fluctuations were in a successive order &
corresponded to the temperature change
• To conclude, historical war-peace, population &
economic cycles were induced by climate
change!
5 What are the theoretical implications of
our research?
5.1 Other pathways for human adaptation
• Migration?
• Economic change?
• Human innovation?
• Trade & peaceful resource redistribution?
5.2 Cycle theories
• There are many theories explaining the cyclical
pattern of history & the dynamics behind the
cycles
e.g., population size  
state strength  
warfare   population
collapses
• But, they can’t explain parallel occurrence of
these crises in widely separated regions that
were in different stages of civilization, culture &
resource endowment, nor predict the timing of
such crises
Solid curve: pop size
Broken curve: state
strength
Dotted curve: war intensity
10
• 5.4 War theories
5.3 Demographic theories
• We argue that the level of livelihood resources
will shrink in some periods due to climate
change. This is the root cause of human
miseries (e.g., wars, famines & epidemics). This
point is overlooked by Malthus, Darwin & many
other demographers
• The speed of global warming is beyond our
imagination. At the moment, scientists can’t
accurately predict the chain ecological effects
induced by climate change
Northern Hemisphere Temperature Anomaly
0.00
(oC)
-0.15
-0.30
-0.45
400
600
800
1000
1200
Year AD
1400
1600
• Our research results have explained WHEN,
WHEN
WHERE & WHY most of wars occurred. This is
also the thing not achieved by other scholars
• “Recent global warming” & “past cooling” are
both extreme climate. The last 10yrs is the
hottest period over the last 2,000yrs
6 What are the social implications of
our research?
200
• Many scholars have investigated the causes of
war. Some of them can explain certain wars &
some of them may even explain sizable classes
of wars, but none of them can explain the
temporal & spatial patterns of warfare
1800
2000
• If global warming continues, the associated
shortages of livelihood resources such as fresh
water, arable land & food may trigger more
armed conflicts (e.g., Darfur in Africa) or even
general crises in the world
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