The Maya civilisation, which dominated southern Mexico for hundreds of years, appears to have been brought to its knees at least in part by a series of severe, decades-long droughts, scientists say.
Conditions were so bad, says Nicholas Evans, a geochemist at the University of Cambridge, UK, that rainfall decreased by 50% on average. During the worst periods, he says, it decreased by up to 70%. The drought was further exacerbated by a 2-to-7% drop in relative humidity, his team found.
The climate shift coincided with an era called the Terminal Classic Period, between 800 and 1000 CE, when the Maya civilisation was in decline and permanently abandoned many of its cities.
The idea that drought may have contributed to this collapse isn’t new. “[It] has been debated for at least 100 years,” says Christopher Baisan, a dendrochronologist, or tree-ring scientist, at the US University of Arizona’s Laboratory of Tree-Ring Research, who was not involved in the new study.{%recommended 7544%}
But just how severely the climate had changed was not clear. All that was really known was that it was drier than at the height of Maya influence.
To find an answer, Evans’ team took core samples of sediments in a lake in the central Yucatan peninsula. “These sediments contain muds,” Evans says, “but importantly, they also contain a mineral known as gypsum.”
Gypsum is a crystal that precipitates out of water when the mineral content grows too large — something that can occur during a drought. It is predominately composed of calcium and sulfate, but it also includes trapped water molecules.
By examining hydrogen and oxygen isotopes in these molecules and comparing them to water in lake today, Evans says, scientists can chart changes in the lake. From these, he says, it’s possible to deduce variations in rainfall patterns.
The result isn’t perfect. To begin with, gypsum only forms during periods of drought, when minerals become concentrated enough to precipitate to the bottom. Also, the isotope levels of the trapped water reflect multi-year averages of climate conditions in and around the lake, not an instantaneous measure.
But it’s good enough, Evans’ team reports in the journal Science, to reveal a pattern of severe multi-decade droughts, interspersed with brief interludes of relative normality.
Other scientists are impressed. Baisan calls the research “interesting”, with a “nice new method” for evaluating the severity of the drought.
Richard Alley, a geophysicist at Pennsylvania State University who frequently uses isotopes to reconstruct paleoclimates in other settings, calls the analysis “careful and thorough, applying a wide range of modern techniques in clever ways to provide convincing results”.
The find, Alley says, follows a pattern in which archaeologists and palaeontologists first discover a history of change in some population, then uncover a climate change that in some way stressed it.
“These results may not be surprising to military leaders who have noted that climate change is a ‘threat multiplier’ or [to people] who have read [John Steinbeck’s novel] The Grapes of Wrath, about the Depression-era effects of societal stresses that arose in part from a drought in the Great Plains,” he says.
Baisan adds that intense climate cycles such as the ones that appear to have hit the Maya may be harder to cope with than more gradual changes.
“As a general rule, oscillation between extremes is more difficult for societies to adapt to,” he says.
The relevance to modern society is less clear. Evans notes that modern agriculture, with its drought-resistant plants and genetic modifications, is very different from the crops relied upon by the Maya, making it difficult to draw direct parallels between what happened to them and what could happen to us. But the fact remains that studies like his show that prior civilisations may have experienced droughts far worse than anything in written history.
Baisan agrees. “We know from the paleoclimate record that there have been changes and events in the past that have no modern analogue,” he says. “We should expect the same in the future.”
And, he notes, “the fact that we are currently ‘turning up the heat’ is not particularly comforting.”
