Does a warming world beget more wars? A new study that investigates the relationship between climate change and clashes among the Classic Maya believes so, drawing an explicit link between temperature increases and growing conflicts.
The study, published in Quaternary Science Reviews, examined about 500 years of Maya history, from 363 to 888 A.D.
This is the so-called Classic period in which the Mesoamerican civilization boomed, with its people constructing extensive cities and massive pyramids, as well as developing one of the earliest writing systems in the Americas.
Indeed, the Maya began a tradition of recording historical events on stone monuments.
"The inscriptions that have been translated provide often remarkably detailed accounts of myths and political events, including conflicts between city-states," said the report, which was authored by Mark Collard, Canada research chair at Simon Fraser University in British Columbia and professor of archaeology at the University of Aberdeen in Scotland, along with Christopher Carleton and David Campbell, both of Simon Fraser University.
The researchers cataloged inscriptions on monuments related to violent struggles and compiled temperature and rainfall records for the regions inhabited during the Classic period: the lowlands of the Yucatán Peninsula, which includes parts of southern Mexico, Guatemala, and Belize.
A total of 144 unique conflicts emerged from inscriptions on monuments from more than 30 major Maya centers. The research team then compared conflict records to palaeoclimate data, and the correspondence was impressive.
"The change in conflict levels between 350 and 900 A.D. was considerable," they wrote. "The number of conflicts increased from 0 to 3 every 25 years in the first two centuries to 24 conflicts every 25 years near the end of the period."
They noted the exacerbation of conflicts could not be explained by change in the amount of rainfall. It was instead associated with an increase in summer temperature.
"There's been quite a bit of discussion about the impact of climate change on the Classic Maya, but this discussion has focused on drought," Collard told Seeker. "Our study suggests that we've been looking in the wrong place and that the impact of temperature needs to be looked at more closely."
Experts think that there are two potential mechanisms by which increases in temperature can lead to greater conflict.
One is psychological — when temperatures rise, tempers shorten. Several studies suggest it is possible that increased average summer temperatures made the Classic Maya more bellicose.
The other mechanism, which Collard and his colleagues find more likely and compelling, is economic, and involves the staple crop for the Classic Maya: maize.
Throughout the Classic period, average temperature fluctuated between 82.4 degrees Fahrenheit (28 degrees Celsius) and 84.2°F (29°C). During periods when the temperature was around 82.4°F (28°C) or less, maize yields were reasonably stable, with little or no food shortage and little conflict.
But as temperature continued to rise and the region experienced days at or above 86°F (30°C), crop shortfalls occurred frequently. Large-scale deforestation throughout the Classic period caused by urban expansion worsened the effect, increasing regional temperatures by reducing soil moisture availability. The result was food shortage, which led to spiking levels of conflict.
"Small year-to-year changes in climate can result in large, negative effects over the long term. This is a problem for us humans, because most of us are oriented towards the short term."
"We had originally thought that it all came down to starvation, but after talking with Maya specialists, we decided that wasn't convincing," Collard said.
He explained that maize would have been difficult to transport, in which case the idea of attacking neighbors for food did not seem very likely.
"Instead, it's probably better to consider the increase in warfare in a way that we often think about warfare today — namely as a tool for the elite to maintain support," Collard said.
With declining maize yields, a ruler could not have relied on opulent festivals or fed large labor forces needed to build impressive monuments. Consequently, going to war more often would have been an effective tactic to maintain status, prestige, and power.
"I think of it as being similar to the way that some modern political leaders seem to use conflict with neighbors to distract from problems within their country," Collard said.
Eventually, the growth in conflict became explosive.
The researchers believe the findings have implications for the debate about contemporary climate change. Concern is growing that climate change effects would increase violence within and between human societies.
The Intergovernmental Panel on Climate Change has cautioned that climate change will exacerbate conflict at a range of scales, from inter-personal violence to civil war, while the US Department of Defense has classified climate change as a threat multiplier, suggesting that it could lead to political and social unrest and increased terrorism.
"Our study shows that small year-to-year changes in climate can result in large, negative effects over the long term," Collard said. "This is a problem for us, humans, because most of us are oriented towards the short term."
"We run the risk of ignoring changes that will affect our children and grandchildren, because we can't perceive those changes," he added.
Some very important questions still need to be investigated.
"Most obviously, we need to know whether the effect is a regional one, specific to the Maya area, or one that holds for other parts of the world," Collard said.
But he warned that without government support it won’t be possible to answer this and other crucial questions.
"The data we used in the study were collected by researchers funded by US agencies that have been targeted for massive cuts by President Trump and his administration," he noted, pointing to the impact of politics on his research. "I think most people — most voters — want evidence-based government policies, and we can't have evidence-based policies without evidence."
Originally published on Seeker.