As the evidence for ongoing climate change has grown incontrovertible, there is increasing urgency to the question of what these changes hold in store for us. Some wondered why Al Gore and the IPCC should be awarded the Nobel peace prize for promoting climate science? Is there really a connection? One source of insight involves querying the past: what consequences have past climate changes had for human society?
In a relatively new study (published in December 2007 — I’m a little behind the curve here), David Zhang and colleagues exploit new high-resolution paleo-temperature records to address this question. The paper assembles evidence from five to ten centuries of human history to show that climate variation drives changing food production, which among animals typically results in what we ecologists call “intraspecific competition”, that is, competition among members of the same species. Among humans we call it war.
The Figure at left shows the northern hemisphere temperature variation (as anomalies from the long-term average, panel A), war frequency, and rate of change in human population from 1400 to approximately the beginning of the industrial revolution in 1900. The number of wars is shown for the northern hemisphere as a whole (bright green line, and right axis scale), Asia (pink), Europe (turquoise), and arid areas of the northern hemisphere (orange). Panel C shows number of wars worldwide as recorded by three different authors using different thresholds for defining war — as can be seen, the trends are roughly similar. Finally, panel D shows the 20-year population growth rate in Europe (turquoise), Asia (pink), and the northern hemisphere as a whole (blue), as well as the 50-year fatality index in the region (bright green). Cold phases of history are shaded in gray. The bright green curves correspond to the right axis.
What to make of all this? First, we can see that climate varied during this time between cool and warmer periods that lasted decades to a few centuries. More importantly, these cool and warm periods coincided with times of unrest and relative tranquility, respectively. Considering the whole global data base, there is a strongly significant negative correlation between war and temperature, with temperature anomaly explaining 28% of the variation in war frequency. Even more telling, this “rhythm of history” was roughly synchronous across the northern hemisphere. Since, during these centuries, China and Europe were still largely isolated from one another, the synchrony of these trends over such an area, comprising much of the northern half of the planet, is difficult to explain by any factor other than the clear signal of large-scale global climate.
The relationships are even more pronounced in the finer-resolution record for China during the longer period from AD 1000 – 1900 (see figure below): here each of the cool periods (gray shading) saw a major spike in the number of wars.
What is the mechanism behind these patterns? The answer appears to be pretty simple, and readily predictable from basic principles of population ecology. Climate cooling reduces agricultural production, mainly by shortening the growing season and reducing available land for cultivation. Because the political boundaries of states in these agrarian societies were less porous than they are today, there was little opportunity for mass migration during the resultant shortages of food (and, since the problems were regional, nowhere to go in any case). So the four horsemen — death, famine, war, and pestilence — mounted up and rode in.
The figure at right summarizes in diagrammatic form the pathways by which long-term climate variation influenced frequency of war and human population dynamics in China and Europe during preindustrial times (i.e. up to 1900). Solid lines indicate direct effects, and dotted lines indirect feedbacks. Thicker arrows indicates stronger correlations. There are several inrerrelated impacts of climate change mediated by the interactions of humans with our food supply and with each other, but food supply is central.
So, OK, this data comes from back when people rode horses and peasants grubbed for potatos all winter long and so forth. Why should we care in the 21st century? We have refigerators and grocery stores! Besides, the climate now is warming, rather than cooling, so it should all be good, right? The most general message for us today is that climate variation has profound impacts on the global ecosystem’s ability to provide vital services, which in turn have profound implications for human society and well-being. Although a warming climate has been good for us in the past, and will surely be good for some people in some places in coming decades too, we are facing a much bigger and faster warming than the earth has seen in a very long time. And one of the consequences is change in rainfall, which is an even more powerful regulator of agricultural productivity than temperature. And when food runs low, conflict is inevitable, as we are seeing in Darfur. Too little (crop)land to go around was evidently a key match to the flame in Rwanda during the 1990s also.
Scholars have long sought, with only partial success, to explain the conflicts that repeatedly plague civilization. The results of this paper indicate that human ecology is –- or was — forced to a surprising degree by the same basic environmental drivers and by similar, if more destructive, mechanisms of competition that regulate populations of other animals.
There is hopeful news too. We have learned a thing or two in the last millennium. As the authors note:
“In the long run and at a global scale, technological and social development raised the population growth rate . . . reduced climate dependence of growth rate of population (after A.D. 1400), postponed the time of population decrease, and accelerated subsequent population recovery . . . The gradual increase in time delays for [northern hemisphere] population declines as we moved into the modern era may reflect that at least some social mechanismsmay becoming more effective over time at the macroscale.”
At the same time:
“these adaptive choices that are positive to humanity have not let the human race escape from social calamities such as population collapse caused by severe cooling at both the global and continental scales as shown in the history of the past millennium. For armed conflict, the positive social mechanisms could neither reduce the number of wars nor indefinitely postpone the times of war outbreak in any cooling periods . . . Although we have more robust social institutions at both international and national levels, and much more advanced social and technological developments at present, a much larger population size, higher standard of living, and more strictly controlled political boundaries will limit some adaptive choices to climate change. We hope that positive social mechanisms that are conducive to human adaptability will play an ever more effective role in meeting the challenges of the future.”
[Original source (open access): Zhang, D.D., P. Brecke, H.F. Lee, Y.-Q. He, and J. Zhang. 2007. Global climate change, war, and population decline in recent human history. Proceedings of the National Academy of Sciences of the USA 104:19214-19219.]
[The painting shows the English victory over the French at the battle of Agincourt in 1415.]