Review: Zhang DD, Brecke P, Lee HF, He Yuan-Qing, Zhang J (2007) Global climate change, war, and population decline in recent human history. Proceedings of the National Academy of Sciences, 104(49):19214-19219.

Feature Paper: Zhang DD, Brecke P, Lee HF, He Yuan-Qing, Zhang J (2007) Global climate change, war, and population decline in recent human history. Proceedings of the National Academy of Sciences, 104(49):19214-19219.  

Author Abstract: Although scientists have warned of possible social perils resulting from climate change, the impacts of long-term climate change on social unrest and population collapse have not been quantitatively investigated. In this study, high-resolution paleo-climatic data have been used to explore at a macroscale the effects of climate change on the outbreak of war and population decline in the preindustrial era. We show that long-term fluctuations of war frequency and population changes followed the cycles of temperature change. Further analyses show that cooling impeded agricultural production, which brought about a series of serious social problems, including price inflation, then successively war outbreak, famine, and population decline successively. The findings suggest that worldwide and synchronistic war–peace, population, and price cycles in recent centuries have been driven mainly by long-term climate change. The findings also imply that social mechanisms that might mitigate the impact of climate change were not significantly effective during the study period. Climate change may thus have played a more important role and imposed a wider ranging effect on human civilization than has so far been suggested. Findings of this research may lend an additional dimension to the classic concepts of Malthusianism and Darwinism.  

Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.  

Review: After a couple of weeks discussing modern science, we return this week to a review of a scientific paper. We'll discuss this paper out of the dozens on climate change because it also addresses how war versus peacetime can be affected by climate change. In essence, all conflict in the world is over environmental resources, even religious or ideological conflicts. People generally fight over who should have the right to certain resources and whether the excuse is ideological or not, the result is the same: some people are in power over certain lands and resources while others are not.  

The paper discusses how climate change can lead to social unrest and population collapse. Whereas many people assume that population will continue to increase, many scientists believe that the Earth's current population level is unsustainable for the resources that are being used. What is interesting about this paper is that the authors found that war frequency and population changes followed periods of climate change. The authors further found that "worldwide war-peace, population, and price cycles in recent centuries have been driven mainly by long-term climate change."  

The authors performed a review of past studies that addressed the concept of environmental conflict and how in middle-latitude regions long-term climate change acted as a control on population size. The authors discussed how past studies have lacked large-scale quantitative data to make significant conclusions towards clearly linking climate change with wars and conflicts. The authors were able to make their conclusions by seeing the effects of climate change on agricultural production. During periods of environmental cooling between AD 1400 - 1900, agricultural production in many areas of the world decreased. The authors then determined how as the "carrying capacity" of land goes down, food resources grow short and the incidences of "armed conflicts, famines, and epidemics" increased, leading to reductions in population size. As population decreased historically as a result of the above stressors, food resources increased and population increased. In essence, it is similar to a "boom and bust" in predator-prey models, only instead of predators and prey the metrics are agricultural production and climate change. The authors then linked agricultural production levels with human conflicts and population size.  

A good paragraph of the paper that sums up several of the authors' key tenets is: "Under ecological stress, adaptive choices for animal species are the reduction of population size, migration, and dietary change. Depopulation typically takes place through starvation and cannibalism. Humans have more pathways, social mechanisms, to adapt to climate change and mitigate ecological stress. Besides migration, they include warfare, economic change, innovation, trade, and peaceful resource redistribution. We believe that in late agrarian society established political boundaries in populated areas limited mass migration; the result of such mass migration, when it occurred, often was war. Economic change was a costly and slow process that involved changing cultures, technologies, and habits. When the speed of human innovation and its transfer were not fast enough to keep pace with rapid ecological change, famine and disease became difficult to avoid. Trade and redistribution under the condition of shrinking resources would not help much because the ecological stress was at a global or very large regional scale. Finally, human social development in the form of international and national institutions was not strong enough to buffer the tensions caused by food resource scarcity. Therefore, war and population decline became common consequences of climate-induced ecological stress in the late preindustrial era."  

The authors quantitatively showed how their conjectures were supported by data from several databases that had not previously been linked, summarized in their figure below. 

The only period of time within the last 600 years of the study where the patterns of temperature, agricultural production, warfare, and population decrease did not follow predicted patterns was during "the late 19th century when the temperature of the Southern Hemisphere was its coldest in the last millennium and a great number of wars occurred in the southern part of Africa." Based on the above figure, the authors were able to conclude that "synchronous periods of relative peace and turbulence during [the 500 years from 1400 - 1900 AD] were a global phenomenon seemingly linked to temperature change."  

And while the authors only had high-resolution temperature data for the last 600 years, the authors were able to link temperature and warfare in China back to 1000 AD as a result of climate change causing certain tribal groups under temperature stress "to enter central China" with warfare aimed at securing resources that were lost through climate change in the north of China.

The authors also found that after 1900 AD (as a result of changes in society to industrialized regimes) climate change only had a limited affect on agricultural production, the great conflicts of the 20th century, and changes in population densities. Therefore, technology may have some ability to stave off certain pressures on human population decline following environmental stress. What is unclear is how the current increasing temperature as a result of human-induced global climate change and increasing greenhouse gas production may complicate historical models of climate, population decline, and warfare. It is also unclear whether technology will enable societies to stave off population decline should the new "model" of increasing greenhouse gas production cause crop failures and lower levels of agricultural production. Also, there are now pesticides and fertilizers that have increased agricultural production levels higher than at any time in history.

In essence, we (as a society) are entering a grey area. On the one hand, technology has enabled us to better quantify effects that humans have made to the environment, but on the other hand as those effects become larger, the chances of humans having to deal with environmental conditions unlike anything else in recorded history for the Earth also increase. Furthermore, many scientists have stated that as global temperature increases with increasing carbon dioxide production (as a result of industrialization), droughts will also increase and potentially lead to water shortages and conflicts (e.g., wars) over control of those resources. It seems fairly certain that the Earth is not going to enter a period of lower temperature soon but it remains unclear whether the cycle of war and population pressure will ever end.

I firmly believe that all of the Earth's environmental problems can be solved using already-developed technologies but that such technologies are not being implemented through a lack of will. Individuals and corporations generally act in their own interests, whereas the stresses that future generations will face through global climate change must involve a global response, which only governments can  deliver.