“When north-south relations were east-west:

 urban and empire synchrony (500 BCE-1500 CE)”

Christopher Chase-Dunn, Richard Niemeyer, Alexis Alvarez,

Hiroko Inoue, Kirk Lawrence and Anders Carlson

Institute for Research on World-Systems

University of California-Riverside

(v. 3-19-06)

Frederic Teggart’s Rome and China

 

Earlier studies have demonstrated a curious interregional synchrony in the growth and decline of large cities and empires. From about 500 BCE until about 1500 CE cities and empires in East Asia and the West Asian/Mediterranean region were growing and declining in the same periods, whereas intervening South Asia did not conform to these patterns. This study reports our efforts to examine the possible sources of this interregional synchrony by examining the patterns of climate change. We think that an emerging multicore Afroeurasian world-system may be the culprit, but other possibilities need also to be considered. Central Asia was a peripheral region relative to the citified core regions of West, South and East Asia. It is in this sense that North/South relations were East/West in Iron Age Eurasia.

 

To be presented at the 2006 conference of the International Studies Association, San Diego, March 23, 3:45-5:30 PANEL: The Historical Long-Term. 5006 words

This paper is available at http://irows.ucr.edu/papers/irows16/irows16.htm

 

            The growth and decline patterns of the world’s largest cities and empires and their changing locations over the past three millennia provide an important window on world history. Earlier research has repeatedly demonstrated a fascinating synchrony in the growth/decline phases of largest cities and empires in East Asia and the West Asian/Mediterranean region (e.g. Chase-Dunn and Willard 1993; Chase-Dunn, Manning and Hall 2000; Chase-Dunn and Manning 2002). 

Using data on the population sizes of largest cities and the territorial sizes of largest empires it has been discovered and repeatedly confirmed that medium-term growth/decline phases in East Asia and the West Asian/Mediterranean regions experienced synchronous cycles between 500 BCE  and 1500 CE.[1] So for two thousand years, when the largest city was growing in East Asia, the largest city in West Asia was also growing, and they shrank also simultaneously. With a completely different data set based on the territorial sizes of the largest states and empires, the same phenomenon is found. But this synchrony is not found in the South Asian subcontinent (India) where states were growing and declining, but not in the same time periods as in East Asia or West Asia.

 

Earlier Findings

            The population and areal sizes of human settlements have increased since the emergence of sedentism around 12,000 years ago, and so have the sizes of the largest polities. But these general long-term trends have been complicated by sequential middle-term declines in the sizes of the largest cities and empires in all regions where urban and polity sizes have been studied quantitatively. The population size estimates of both modern and ancient cities are subject to large errors, and existing compilations (Chandler 1987; Modelski 2003) badly need to be improved using better methods of estimation (e.g. Pasciuti and Chase-Dunn 2002). The same can be said for existing compilations of estimates of the territorial sizes of the world’s largest empires (Taagepera 1978a, 1978b, 1979, 1997). When these upgraded estimates become available, the East/West synchrony findings discussed here will need to be reexamined with the improved data. We believe that the East/West synchrony finding will be confirmed.

            This phenomenon of East/West urban and empire synchrony in middle-term growth/decline phases has been subjected to several different methods of analysis, and it holds up across all of them. Both changes in the size of the largest cities and changes in the steepness of the city-size distributions have been used. And earlier studies have used two different kinds of spatial units of analysis: constant regions and expanding political-military networks (interaction networks of fighting and allying states). The East/West synchrony has been found with both.

 Detrending is important because the long-term trend for city and empires sizes to increase. Two different methods of detrending have been used: partial correlation controlling for year and decadal change scores in which the earlier year is subtracted from the later year. In the studies of empire sizes, empires that touch adjacent macro-regions such as the Mongol Empire of the thirteenth century CE have been removed from the analysis because they build in a degree of synchrony by appearing in both regions at the same time. The synchrony finding is strong even when this case has been removed from the calculations.

Frederick Teggart’s  (1939) path-breaking world historical study of temporal correlations between events on the edges of the Roman and Han Empires argued the thesis that incursions by Central Asian steppe nomads were the key to East/West synchrony. An early study of city-size distributions in Afroeurasia (Chase-Dunn and Willard 1993; see also Chase-Dunn and Hall 1997: 222-223) found an apparent synchrony between changes in city size distributions and the growth of largest cities in East Asia and West Asia-Mediterranean over a period of 2000 years, from 500 BCE to 1500 CE.  That led to an examination of data on the territorial sizes of empires for similar East/West synchrony, which was also found (Chase-Dunn, Manning and Hall 1999). The empire size data also allow the examination of rise and fall sequences of large empires in South Asia, but these were not synchronous with the growth/decline phases of empires in East Asia or West Asia (Chase-Dunn, Manning and Hall 1999). Chase-Dunn and Manning (2002) re-examined the city size data using constant regions rather than political-military networks to see if the East/West synchronous city growth hypothesis holds when the units that are compared are constant regions rather than expanding political/military networks (see Figure 1).

Figure 1: Sizes of Largest cities in East Asia and West Asia/Mediterranean

Comparable other instances of distant systems that came into weak contact with one another can be found.  Within the Old World, the Mesopotamian and Egyptian core regions were interacting with one another by means of prestige goods exchange from about 3000 BCE until their political-military networks (state systems) merged in 1500 BCE. Chase-Dunn, Pasciuti, Alvarez and Hall (2006) have already examined this case for synchrony and have not found it, though the data on Bronze Age city and empire sizes are very crude with regard to temporality and accuracy. It is also possible to study the temporality of rise and fall and oscillations among distant regions in the New World (e.g. Peregrine 2005).

Chase-Dunn, Alvarez and Pasciuti (2005) also report detrended correlations between constant regions for total population estimates taken from McEvedy and Jones (1975). These total population estimates at 100-year intervals show rather high growth/decline synchronies for several regions, also noted and discussed by McEvedy and Jones (1975: 343-48).

The East/West growth/decline synchrony seems to be rather robust, though better estimates and finer temporal resolution of empire and city sizes might challenge it. Interregional synchrony can be caused when two cyclical processes get simultaneously reset, either by the same cause or by different causes. This could be a one-shot occurrence. Or a process that is similarly cyclical can cause synchrony. Candidates for the East/West synchrony are: climate change, epidemic diseases, trade interruptions, or attacks by Central Asia steppe nomads. Sorting this out will require data on these phenomena for the relevant regions over the relevant time period.

 

Possible Explanations

            Climate change might affect regions by causing growth and decline of agricultural productivity that in turn affects cities and empires. Perhaps because South Asia is nearer the equator, its climate change history is different and this might explain why its growth/decline pattern is different. The simplest thing would be to find “little ice ages” or other large climate changes that correspond with the big changes in city and empire sizes.

            But climate change could also be involved in somewhat more complicated ways. Central Asian steppe nomads (discussed below) were very susceptible to climate change because their pastoral economy was greatly affected by changes in temperature and rainfall. It is possible that climate change in Central Asia affected the nomads, who then carried out incursions and military campaigns that affected the cities and agrarian empires of the East and West.

            The above hypotheses all conceive of climate change as an exogenous variable. But it is also possible that city and empire growth change the climate. We know that population growth and the development of complex civilizations changes the environment by means of deforestation, soil erosion and the construction of large irrigation systems (Diamond 2005). These changes may have affects on climate. Modern studies show that the construction of large cities creates an “urban heat island” that changes the environment in the immediate vicinity and downwind of cities. Cities ingest and egest water, air and energy, and while industrial cities do this on a much larger scale, earlier large cities also did it to some extent. So large-scale agriculture and city-building may be causes of climate change. Thus climate change may also be an endogenous variable.

            We know that Central Asian steppe nomads who raised horses and sheep periodically formed large confederacies and attacked the agrarian empires of the East and the West (Barfield 1989). Famous examples are the Huns and the Mongols. Perhaps there was a cycle of Central Asian incursions that impacted upon the agrarian civilizations of the East and the West and that accounts for the synchrony.

            We know that epidemic diseases spread across Eurasia killing large numbers of people in cities, for example the Black Death (Bubonic Plague) of the 13th century. Perhaps earlier pandemics (e.g. the plague of Justinian) caused the synchrony.

            We also know that the Roman and Han empires were linked by long distance trade routes across the Silk Roads and by sea. Perhaps interruptions to trade, or periods of greater and easier trade flows, affected the Eastern and Western civilizations simultaneously.

            It is also possible that two systems that are cycling independently can become synchronized if they are both reset by a simultaneous accidental shock. This is the so-called “Moran Effect” known in population ecology. We have discussed this possibility in Chase-Dunn, Alvarez and Pasciuti (2006).

            Figure 2 is a propositional inventory that includes most of the possible causes of East/West synchrony.

Figure 2: Possible Causes of East/West Synchrony

More research is required to find out which of these possible causes was responsible for the East/West synchrony. We have found some evidence that temperature changes in China are not associated with the growth/decline phases of cities or empires in East Asia. This is evidence against the climate change hypothesis.

East Asian Climate Change and Growth/Decline Phases

We have located time series data on two indicators of climate change in China, and they do not seem to be at all related to the East Asian rise and fall of empires or changes in the size of the largest cities. If this is true, then logically climate change can be ruled out as a cause of interregional synchrony.

The first indicator we have of climate change from China is an estimate of changes in average temperature that are inferred from measurements taken of stalagmites formed in the Shihua cave near Beijing (Tan et al 2003).[2] Ideally the indicator of climate change should be geographically near the area of city and empire growth. Recent research on El Nino and the Southern Ocean Oscillation shows that climate dynamics on a global scale are linked by huge inter-regional “teleconnections” in which changes in ocean temperature in one region affect rainfall and temperature in distant other regions, but the resulting patterns are very different from region to region (Davis 2001). This means that we need to have information on climate change that is spatially near to the areas where we are studying the possible impacts. Knowing what happened in Greenland or Europe is essentially irrelevant for our purposes. We need time series measures of climate change from East Asia, West Asia, Central Asia and South Asia.

The estimated temperature series as well as the plots for largest city and largest polity from 650 BCE to 1800 CE are shown in Figure 3.

Figure 3: Temperature Change (Shihua Cave), Largest City and Largest Polity in East Asia (n=26)

Figure 3 graphs the temperature changes inferred from Shihua Cave. Bivariate correlations among the selected variables (see Table 1 below) were performed in order to examine the hypothesis that city and/or empire growth/decline phases may be correlated over time with changes in average temperature.  Though there is no long-term trend for temperature in the time period studied, we also calculate partial correlation coefficients controlling for the variable DECADE in order to detrend the city and empire series. Both the bivariate and the partial correlation coefficients are shown in Table 1.

Table 1 reveals a significantly steady rate of empire growth throughout the period, when plotted against DECADE (correlation is 0.587, p < 0.01).  Similarly, the population of the largest city in East Asia has an even stronger positive correlation (0.800, p < 0.01) with DECADE, confirming the general trend of urban growth evident in Figure 2 above.

 

Table 1:  Bivariate and partial correlations: largest city, largest empire, temperature estimate from Shihua Cave and decade (n=26)

            Table 1 also confirms an earlier finding of a significant bivariate and partial temporal correlation between city and empire sizes in East Asia (Chase-Dunn, Alvarez and Pasciuti 2005). Cities and empires grow and decline in substantially the same periods within the East Asian region. But the most important result for the purposes of this paper is the finding of no significant relationship between temperature change and either city or empire growth in East Asia. This suggests that climate change may be ruled out as an important cause of the East/West synchrony. Logically, if climate change did not cause city or empire growth/decline phases in East Asia then it cannot be the cause of East/West synchrony.

            The results in Figure 1 and Table 1 are based on only 26 time points because of the paucity of data on city population sizes, which have been estimated at only very widely-spaced intervals, especially for the earlier time periods. When we examine the relationship between empire sizes and temperature change separately we have far better temporal resolution. Figure 4 graphs the empire sizes using ten-year interpolated values and the ten-year moving average of yearly temperature estimates. The bivariate correlation is .02.

Figure 4: Largest Empire and Temperature Change in East Asia (n= 216)

Figure 5 shows the same relationship, but it is easier to see what is going on because the scores have been standardized.

Figure 5: East Asian Largest Empires and Average Temperatures (standardized values)

Figure 6 depicts the relationship between East Asia largest city sizes and empire in a different way from Figure 3 above. Here a polynomial trend line has been fitted to the city values and the temperature estimates are from a ten-year moving average.

Figure 6: Largest Cities and Average Temperature with city polynomial

The results so far imply that there is no regular relationship between climate change and the growth/decline phases of cities and empires in East Asia. We are not completely satisfied with these results, however. It is plausible that temperature may affect city and empire growth in a complex way. It may be either too hot or too cold for productive agriculture. It is possible to compute deviations from average temperature, either hot or cold, and to examine their relationship with city and empire growth decline phases. We plan to do this in the next version of our paper.

            Also temperature is only one indicator of climate change. Yearly rainfall, the distribution of rainfall throughout a year, the frequency of large and destructive storms – all these are important aspects of climate change that may have large effects on agriculture and irrigation systems but are not well reflected in temperature changes. It is also possible that climate change near Beijing is not a good indicator of climate change in other areas within the East Asian region. The farther a large city or state is from Beijing, the more likely is it that a different pattern of historical climate changes occurred there. Thus we also examine an indicator of precipitation fluctuations estimated from Dongge Cave in Southern China (see below).

            We also want to examine the possibility of time lags between climate change and city and empire growth decline phases. The methods employed above presume a simultaneous causality, whereas it is likely that changes in climate take some period of time to affect the sizes of cities and empires. This can be systematically examined using the techniques employed by Turchin and Korotayev (nd) for studying lagged dynamical relations among time series variables. We plan to do this in the next version of our research.

Understanding Climate Oscillations in the Mediterranean and Asian Regions

            Much of the climate change within the Mediterranean region is a product of the North Atlantic Oscillation (NAO), a hemispheric meridional oscillation in atmospheric mass between the polar regions near Iceland and the subtropical regions of the Atlantic.  The alternating periods of the NAO are indicated by a positive/negative index based upon on the surface pressure difference between the subtropical high and the subpolar low.  During a positive NAO phase, there exists a stronger than usual subtropical high and a deeper than normal subpolar low resulting in colder and drier conditions in South-Eastern Europe and the Mediterranean, as well as water shortages throughout the Middle East.  During a negative NAO phase, there exists a weak subtropical high and a weak subpolar low, resulting in moist conditions in the Mediterranean.

Figure 7: Atmospheric regions responsible for North Atlantic Oscillation

http://sciencebulletins.amnh.org/earth/f/nao.20040910/essays/28_1.php

 

           

 

Figure 8:  Atmospheric effects and climate results of the NAO positive phase.

http://www.ldeo.columbia.edu/~visbeck/nao/presentation/html/NAO.htm

 

 

Figure 9: Atmospheric effects and climate results of the NAO negative phase

http://www.ldeo.columbia.edu/~visbeck/nao/presentation/html/NAO.htm

            Climate conditions within China are a product of the Asian Monsoon.  In the summer months the Asian continent heats up more that the surrounding ocean creating a large area of low pressure over north-central Asia and a smaller low pressure area over India. The disparity in pressure between the inland and ocean regions results in a moist onshore wind.  During the winter months the Asian continent cools rapidly, reversing the previously low pressure regions over north-central Asia and India.  This large high pressure area over north-central Asia is known as the Siberian High. The shift from a condition of low to high inland pressure blows wind offshore, creating the dry monsoon season.  The dynamics of the Asian Monsoon differ somewhat  in  East Asia and South Asia.   The East Asian summer monsoon covers both the subtropical and mid-latitude regions and its rainfall tends to be concentrated in rain belts that affect China, Japan, Korea, and the surrounding areas.  The South Asian Monsoon region includes parts of the Arabian Sea, the Indian Peninsula, and the Bay of Bengal.

Figure 10.  Regions affected by the Asian Monsoon

http://daac.gsfc.nasa.gov/atmosphere/dynamics/images/monsoon.qt

 

Connections between the NAO and the Asian Monsoon

          Recent research has supposedly established a synchronic connection between regions affected by the NAO and regions affected by the Asian Monsoon (Wang et al. 2005).  Generally speaking, many of the abrupt climate change events in the paleoclimatology record of the Asian Monsoon correlate with abrupt climate change in the Mediterranean and North Atlantic.  These include the collapse of both the Neolithic Culture of China and the Akkadian Empire around 4200 years before present, as well as the correspondence between observed weakening in the Asian Monsoon and ice-rafting events in the North Atlantic (Wang et al 2005:855).  More specifically, a comparison of precipitation fluctuations in Greenland (dictated by NAO) and Dongge Cave, China (dictated by the Asian Monsoon) demonstrated a Pearson-r coefficient of .57 when a 150-year phase lead was introduced.  Because of fine-scale uncertainties in the dating records of both the Greenland and China data sets, Wang et al (2005) were not able to conclude if decadal-scale variations were correlated (Wang et al 2005:856).

             

Figure 11: Comparison of the smoothed (5-point running average) detrended China climate proxy data (green) with the smoothed 20-year averaged Greenland climate proxy record (5-point running average, red) over the past 9000 years. The broad correlations between the China and Greenland records are apparent at the multicentennial scale.

From Wang et al. 2005, http://www.sciencemag.org/cgi/content/full/308/5723/854

 

Comparison of Mediterranean and China Proxy Climate Data

 

    

 

Figure 12: Geographic locations of proxy measures for climate conditions

            In order to further examine the hypothesis of East/West climate synchrony we compared proxy precipitation data from the Red Sea (Lamey et al. 2006) with Dongge Cave (Wang et al. 2005).  The Red Sea proxy data is based on a comparison of stable oxygen isotopes derived from the shells of planktic foraminifer and epibenthic foraminifer.  The resultant proxy data is presented as the difference between the stable oxygen isotopes of planktic and epibenthic foraminifera.  The Dongge Cave proxy data consists of oxygen isotope ratios derived from a 962.5 mm stalagmite sample.  The sample produced 2124 measurements with a temporal resolution of 4.5 years and an age uncertainty of 50 years.

            Contrary to the previous research cited above, a positive correlation between Mediterranean and Asian climate change was not observed (see Figure 13).  Our analysis instead observed a Pearson r of -.14.

 

Figure 13: East/West Estimated Precipitation Oscillations (Sources: Wang et al 2005 and Lamey et al 2006)

With regard to the correlation between the precipitation estimates from Dongge Cave and the temperature estimates from Shihua Cave (Tan et al. 2003), our analysis reports a Pearson r value of .16 (see Figure 14)

 

Figure 14: East Asian Precipitation and Temperature

The relationship between the Red Sea climate proxy data and the Shihua temperature data appears inconsequential, demonstrating a Pearson r value of -.006.

 

Figure 15: Red Sea Precipitation and East Asian Temperature

At this point we are lacking firm conclusions. There do not appear to be big climate events in either East or West Asia that might have reset the urban and empire cycles. And the correlations between climate indicators and city and empire growth/decline phases in East Asia are very small. It would be nice if we could completely rule out the climate change hypothesis at this point and focus on the other possible causes of East/West synchrony. But our results also suggest a lack of confidence in the reliability of the estimates of climate change. Not only do we fail to replicate the result reported by Wang et al (2005) of an East/West precipitation correlation (see Figure 13), but we find that temperature and rainfall estimates are not correlated between the Shihua and Dongge Caves in China. Is this because of small scale differences in weather patterns between north and south China, or is it that temperature and rainfall do not vary together, or are these measures flawed? The mystery of East/West synchrony remains. If it was not climate change, then the large scale human interaction networks discussed above did the deed. But which ones?

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[1] This was first noticed in 1992 by Chase-Dunn and Willard (1993) who were studying changes in the city-size distributions of several different regional world-systems. A striking similarity between the East Asian and West Asian city-size distribution trajectories was first confirmed by overlaying the graphs and holding them up to the bright light of a window.

[2] Several studies have demonstrated the usefulness of utilizing the mineral composition of stalagmites as high-resolution climatic indicators (Tan et al. 2003).  According to these studies, the composition and growth of stalagmite layers appear to be correlated with the temperature and rainfall of the environment in which they are located.