History may be bound to repeat itself as Earth’s climate continues to warm, with changing temperatures causing food shortages that lead to wars and population declines, according to a new study that builds on earlier work.
The previous study, by David Zhang of the University of Hong Kong, found that swings in temperature were correlated with times of war in Eastern China between 1000 and 1911. Zhang's newer work, detailed in the Nov. 19 online edition of the journal Proceedings of the National Academy of Sciences, broadens its outlook to climate and war records worldwide and also found a correlation between the two.
"This current study covers a much larger spatial area and the conclusions from the current research could be considered general principles," Zhang said.
The research does not represent direct cause-and-effect, but rather suggests a link between climate and conflict.
Looking to the past
Because water supplies, growing seasons and land fertility can be affected by changes in climate, they might prompt food shortages that could in turn lead to conflicts, such as local uprisings, government destabilization and invasions from neighboring regions, the researchers speculate. These conflicts and the food shortages that cause them could both contribute to population declines, they add.
To see whether changes in climate affected the number of wars fought in the past, the researchers examined the time period between 1400 and 1900, when global average temperatures reached extreme lows around 1450, 1640 and 1820, with slightly warmer periods in between.
Using records reflected in tree rings and ice cores, the researchers compared temperature changes to a database of 4,500 wars worldwide that co-author Peter Brecke of Georgia Tech compiled with funding from the U.S. Institute of Peace.
The results of the comparison showed a cyclic pattern of turbulent periods when temperatures were low, followed by more tranquil times when temperatures were higher.
This correlation doesn't necessarily mean that all-out war is imminent, William Easterling of Pennsylvania State University, who was not affiliated with the work, had said in regards to Zhang's earlier work. However, the changing distribution of resources could certainly increase international tensions, he added.
The new study also showed population declines following each war peak. Specifically, during the frigid 17th century, Europe and Asia experienced more wars of great magnitude and population declines than in more temperate times.
Projecting into the future
To connect temperature changes of less than 1 degree Celsius (1.8 degrees Fahrenheit) to food shortages, the authors used price increases as a measure of decreases in agricultural production and found that when grain prices reached a certain level, wars erupted.
Though these historical periods of climate change featured cooler temperatures, current rising global temperatures could still cause ecological stress that damages agricultural production.
"Even though temperatures are increasing now, the same resulting conflicts may occur since we still greatly depend on the land as our food source," Brecke said.
"The warmer temperatures are probably good for a while, but beyond some level, plants will be stressed," Brecke explained. "With more droughts and a rapidly growing population, it is going to get harder and harder to provide food for everyone, and thus we should not be surprised to see more instances of starvation and probably more cases of hungry people clashing over scarce food and water."
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Andrea Thompson is an associate editor at Scientific American, where she covers sustainability, energy and the environment. Prior to that, she was a senior writer covering climate science at Climate Central and a reporter and editor at Live Science, where she primarily covered Earth science and the environment. She holds a graduate degree in science health and environmental reporting from New York University, as well as a bachelor of science and and masters of science in atmospheric chemistry from the Georgia Institute of Technology.