Climate, conflict, and social stability: what does the evidence say?

The "sister paper" to our recent Science article on climate and conflict has come out in Climatic Change. This new article is a traditional review article that walks readers through individual studies in the literature and discusses some of the debates in less technical terms than the Science article. The sample of studies included is slightly different since the submission timeline for the two studies was different.

Climate, conflict, and social stability: what does the evidence say?
Solomon Hsiang and Marshall Burke

Abstract: Are violent conflict and socio-political stability associated with changes in climatological variables? We examine 50 rigorous quantitative studies on this question and find consistent support for a causal association between climatological changes and various conflict outcomes, at spatial scales ranging from individual buildings to the entire globe and at temporal scales ranging from an anomalous hour to an anomalous millennium. Multiple mechanisms that could explain this association have been proposed and are sometimes supported by findings, but the literature is currently unable to decisively exclude any proposed pathway. Several mechanisms likely contribute to the outcomes that we observe.

How stark is the reversal in global temperature trend?

Last week I put a link to the recent Marcott et al. Science paper reconstructing Holocene temperatures in the weekend links. I've since taught it to both my graduate econometrics class (as a motivating example for statistical inference) and to an undergraduate research methods class (as part of a larger lecture on environment and development economics), and after mulling it over for a while think that the core message of the paper is actually fairly subtle.

The most amazing graph I've seen all month: magnetization of lake sediments and the collapse of Chinese dynasties

This is from the innocuously titled paper Influence of the intertropical convergence zone on the East Asian monsoon published in Nature (2007) by Yancheva et al.

Quote from abstract: "Here we present high-resolution records of the magnetic properties and the titanium content of the sediments of Lake Huguang Maar in coastal southeast China over the past 16,000 years, which we use as proxies for the strength of the winter monsoon winds. We find evidence for stronger winter monsoon winds before the Bølling–Allerød warming, during the Younger Dryas episode and during the middle and late Holocene, when cave stalagmites suggest weaker summer monsoons. We conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone. Similar migrations of the intertropical convergence zone have been observed in Central America for the period AD 700 to 900, suggesting global climatic changes at that time. From the coincidence in timing, we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the Tang dynasty in China and the Classic Maya in Central America."

Click to enlarge

From the last two paragraphs of the paper's discussion:

The role of climate and environmental change in the success or failure of societies is a matter of intense debate. It would be simplistic to imagine that all episodes of societal change are driven by climatic events, especially in an advanced and complex society such as dynastic China. Nevertheless, we note that, on the basis of our new Huguang Maar data, major changes in Chinese dynasties occurred when the winter monsoon was strong (Fig. 3). The anti-correlation between winter and summer monsoon strength indicated by com- parison of the Huguang Maar data with the cave records would suggest that dynastic transitions tended to occur when the summer monsoon was weak and rainfall was reduced. Dynastic changes in China often involved popular uprisings during phases of crop failure and famine, consistent with a linkage to reduced rainfall. The Tang dynasty has been described as a high point in Chinese civilization, a golden age of literature and art. The power of the dynasty began to ebb in the eighth century, starting with a defeat by the Arab army in AD 751. Rebellions further weakened the Tang empire, and it fully collapsed in AD 907. 

It is intriguing that the rise and collapse of the Classic Maya coincided with the golden age and decline of the Tang dynasty in China. Comparison of the Ti records from Lake Huguang Maar and the Cariaco basin reveals similarities, including both a general shift towards drier climate at about AD 750 and a series of three multi-year rainfall minima within that generally dry period (Fig. 3), the last of which coincides with the final stage of Maya collapse as well as the end of the Tang dynasty. Given these results, it seems possible that major circum-Pacific shifts in ITCZ position catalysed simultaneous events in civilizations on opposite sides of the Pacific Ocean.

Related: Human migration and the green Sahara, three other paleoclimate papers on social collapse

h/t Peter

Insights from past millennia into climatic impacts on human health and survival

David Biello (@dbiello) points us to a great lit review by Anthony McMichael in PNAS on climate impacts throughout human history:

Climate change poses threats to human health, safety, and survival via weather extremes and climatic impacts on food yields, fresh water, infectious diseases, conflict, and displacement. Paradoxically, these risks to health are neither widely nor fully recognized. Historical experiences of diverse societies experiencing climatic changes, spanning multicentury to single-year duration, provide insights into population health vulnerability—even though most climatic changes were considerably less than those anticipated this century and beyond. Historical experience indicates the following. (i) Long-term climate changes have often destabilized civilizations, typically via food shortages, consequent hunger, disease, and unrest. (ii) Medium-term climatic adversity has frequently caused similar health, social, and sometimes political consequences. (iii) Infectious disease epidemics have often occurred in association with briefer episodes of temperature shifts, food shortages, impoverishment, and social disruption. (iv) Societies have often learnt to cope (despite hardship for some groups) with recurring shorter-term (decadal to multiyear) regional climatic cycles (e.g., El Niño Southern Oscillation)—except when extreme phases occur. (v) The drought–famine–starvation nexus has been the main, recurring, serious threat to health. Warming this century is not only likely to greatly exceed the Holocene's natural multidecadal temperature fluctuations but to occur faster. Along with greater climatic variability, models project an increased geographic range and severity of droughts. Modern societies, although larger, better resourced, and more interconnected than past societies, are less flexible, more infrastructure-dependent, densely populated, and hence are vulnerable. Adverse historical climate-related health experiences underscore the case for abating human-induced climate change.

Not to be that guy, but there's a huge number of potential paper ideas lurking in this one.

Volcanoes and the Little Ice Age

The Little Ice Age is the period in the middle of the last millennium when global temperatures seem to have been particularly low. It's famous for, among other things, the bias towards winter scenes in European landscape paintings during that period, but there is still much disagreement over why it occurred. In today's issue of Geophysical Research Letters a large team of researchers from UC Boulder's NCAR and the University of Iceland present evidence that it was caused by volcanoes:

Abrupt onset of the Little Ice Age triggered by volcanism and sustained by sea-ice/ocean feedbacks
Northern Hemisphere summer temperatures over the past 8000 years have been paced by the slow decrease in summer insolation resulting from the precession of the equinoxes. However, the causes of superposed century-scale cold summer anomalies, of which the Little Ice Age (LIA) is the most extreme, remain debated, largely because the natural forcings are either weak or, in the case of volcanism, short lived. Here we present precisely dated records of ice-cap growth from Arctic Canada and Iceland showing that LIA summer cold and ice growth began abruptly between 1275 and 1300 AD, followed by a substantial intensification 1430–1455 AD. Intervals of sudden ice growth coincide with two of the most volcanically perturbed half centuries of the past millennium. A transient climate model simulation shows that explosive volcanism produces abrupt summer cooling at these times, and that cold summers can be maintained by sea-ice/ocean feedbacks long after volcanic aerosols are removed. Our results suggest that the onset of the LIA can be linked to an unusual 50-year-long episode with four large sulfur-rich explosive eruptions, each with global sulfate loading >60 Tg. The persistence of cold summers is best explained by consequent sea-ice/ocean feedbacks during a hemispheric summer insolation minimum; large changes in solar irradiance are not required.

The aerosol forcing argument is interesting as for a long time the Little Ice Age was hypothesized to be entirely the result of the above-mentioned "hemispheric summer insolation minimum," also known as the Maunder Minimum. Some recent work has argued that the decrease in solar activity wouldn't have been large enough to trigger substantial cooling, and this paper to a certain degree is an answer to that. That said, as with all paleoclimate work, this is essentially an attempt to engage in "forensic" climatology using proxy measures, and getting to answers with certainty is as difficult as it is in, say, archaeology.

I should also note, since several readers have asked recently about geoengineering, that the cooling detailed here is mechanistically the same as using stratospheric sulfate aerosols to arrest anthropogenic climate change. The potential risks of doing so should be self-evident.

Orbital forcing, the green Sahara, human migration and the rise of civilization in the Nile Valley

In a recent talk, Peter B. deMenocal was showing results from this 2006 paper in Science. If you're interested in this stuff, I also recommend looking at Peter's work.  [More paleo-climate/social papers here.]

Climate-Controlled Holocene Occupation in the Sahara: Motor of Africa’s Evolution
Rudolph Kuper and Stefan Kropelin

Abstract: Radiocarbon data from 150 archaeological excavations in the now hyper-arid Eastern Sahara of Egypt, Sudan, Libya, and Chad reveal close links between climatic variations and prehistoric occupation during the past 12,000 years. Synoptic multiple-indicator views for major time slices demonstrate the transition from initial settlement after the sudden onset of humid conditions at 8500 B.C.E. to the exodus resulting from gradual desiccation since 5300 B.C.E. Southward shifting of the desert margin helped trigger the emergence of pharaonic civilization along the Nile, influenced the spread of pastoralism throughout the continent, and affects sub-Saharan Africa to the present day.

In some of my work with colleagues (notably Jesse), we've been finding evidence that it's very costly for populations to adapt to their climate, even in the long run (see here, here, here, and here).  This implies that populations may will endure large welfare losses to the climate without employing adaptation (eg. migrating away).  What I find interesting about the figure above is that even after an abrupt drying event, it takes populations ~1000 years to completely abandon a location.  Presumably, over the course of that millenium, the abruptly dried climate exacted a substantial welfare toll on the population.  I think this general idea has real implications for how we think about human responses to climatic changes. In general, it's assumed that adaptation is relatively cheap so that we adjust quickly to reduce welfare losses to the climate.  However, if the costs of adaptation are high, the adjustments will be slow and the welfare losses will be large. (A more detailed discussion is in this paper.)

Did the global climate cause dark ages and golden ages?

Yesterday in PNAS:

The causality analysis of climate change and large-scale human crisis

David D. Zhang, Harry F. Lee, Cong Wang, Baosheng Li, Qing Pei, Jane Zhang, and Yulun An

Abstract: Recent studies have shown strong temporal correlations between past climate changes and societal crises. However, the specific causal mechanisms underlying this relation have not been addressed. We explored quantitative responses of 14 fine-grained agro-ecological, socioeconomic, and demographic variables to climate fluctuations from A.D. 1500–1800 in Europe. 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. We identified a set of causal linkages between climate change and human crisis. Using temperature data and climate-driven economic variables, we simulated the alternation of defined “golden” and “dark” ages in Europe and the Northern Hemisphere during the past millennium. 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.

Comment I gave to a journalist:

Zhang et al. have done an impressive job of collecting, organizing and making sense of many pieces of data.  If links from the global climate to food supply to conflict were underlying historical periods of unrest, this will certainly change how we understand historical developments in Europe.  The work by Zhang et al. is particularly interesting because they have tried to examine the detailed pathways that link the global climate to social upheaval.  Their finding that agricultural productivity, agricultural wages and food prices all might have contributed to conflict agrees with a number of leading hypotheses in the field, and they are certainly consistent with observations made in other studies.


It is difficult to know exactly how much these findings can tell us about the modern world, since many things have changed since 1800.  However, it is worth noting that per capita incomes in Europe at 1800 were similar to those we observe in many modern low income countries, such as Bangladesh, Haiti, Nepal and much of Sub-Saharan Africa.  So if poverty is a key factor in the link between the global climate and conflict, the global population in the 21st century is still not wealthy enough that we can consider ourselves "out of the woods."

[Related here, here, here, here, here, here, here and here]

Paleoclimate papers on climactic changes and civilization collapse

My recent work led me to these three interesting paleo papers. All of these groups seem to come from chemistry-oriented paleoclimate backgrounds, so their methods are something like the inverse of regression discontinuity (RD) research design that we're taught in econometrics class.  The RD approach would take some known exogenous break in an independent variable (here climate), and then looking for responses in dependent variables (here civilization stability).  But these groups all do the reverse. Since, they know when civilizations collapsed (more or less) from archeological records, they look in tree cores or marine sediments for evidence of sharp changes in the climate that match the timing of collapse. (I'd be curious to hear what other statisticians/econometricians think about this; particularly if there is a "right" way to do hypothesis-testing with this inverted approach.)

Climate change and the collapse of the Akkadian empire: Evidence from the deep sea

(Geology, 2000)

H. M. Cullen, P. B. deMenocal, S. Hemming, G. Hemming, F. H. Brown, T. Guilderson and F. Sirocko

The Akkadian empire ruled Mesopotamia from the headwaters of the Tigris-Euphrates Rivers to the Persian Gulf during the late third millennium B.C. Archeological evidence has shown that this highly developed civilization collapsed abruptly near 4170 ± 150 calendar yr B.P., perhaps related to a shift to more arid conditions. Detailed paleoclimate records to test this assertion from Mesopotamia are rare, but changes in regional aridity are preserved in adjacent ocean basins. We document Holocene changes in regional aridity using mineralogic and geochemical analyses of a marine sediment core from the Gulf of Oman, which is directly downwind of Mesopotamian dust source areas and archeological sites. Our results document a very abrupt increase in eolian dust and Mesopotamian aridity, accelerator mass spectrometer radiocarbon dated to 4025 ± 125 calendar yr B.P., which persisted for 300 yr. Radiogenic (Nd and Sr) isotope analyses confirm that the observed increase in mineral dust was derived from Mesopotamian source areas. Geochemical correlation of volcanic ash shards between the archeological site and marine sediment record establishes a direct temporal link between Mesopotamian aridification and social collapse, implicating a sudden shift to more arid conditions as a key factor contributing to the collapse of the Akkadian empire.

Climate and the Collapse of Maya Civilization
(Science 2003)

Gerald H. Haug, Detlef Günther, Larry C. Peterson, Daniel M. Sigman, Konrad A. Hughen and Beat Aeschlimann

In the anoxic Cariaco Basin of the southern Caribbean, the bulk titanium content of undisturbed sediment reflects variations in riverine input and the hydrological cycle over northern tropical South America. A seasonally resolved record of titanium shows that the collapse of Maya civilization in the Terminal Classic Period occurred during an extended regional dry period, punctuated by more intense multiyear droughts centered at approximately 810, 860, and 910 A.D. These new data suggest that a century-scale decline in rainfall put a general strain on resources in the region, which was then exacerbated by abrupt drought events, contributing to the social stresses that led to the Maya demise.

Climate as a contributing factor in the demise of Angkor, Cambodia
(PNAS 2009)

Brendan M. Buckleya, Kevin J. Anchukaitis, Daniel Penny, Roland Fletcher, Edward R. Cook, Masaki Sano, Le Canh Nam, Aroonrut Wichienkeeo, Ton That Minh, and Truong Mai Hong

The “hydraulic city” of Angkor, the capitol of the Khmer Empire in Cambodia, experienced decades-long drought interspersed with intense monsoons in the fourteenth and fifteenth centuries that, in combination with other factors, contributed to its eventual demise. The climatic evidence comes from a seven-and-a-half century robust hydroclimate reconstruction from tropical southern Vietnamese tree rings. The Angkor droughts were of a duration and severity that would have impacted the sprawling city’s water supply and agricultural productivity, while high-magnitude monsoon years damaged its water control infrastructure. Hydroclimate variability for this region is strongly and inversely correlated with tropical Pacific sea surface temperature, indicating that a warm Pacific and El Niño events induce drought at interannual and interdecadal time scales, and that low-frequency variations of tropical Pacific climate can exert significant influence over Southeast Asian climate and society.