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Historical ifs and weathers or not

by Rowan Hooper

To suggest that history is shaped by chance weather events and climatic variation doesn’t lend it quite the same gravitas as if it were wrought by great leaders. It certainly isn’t as inspirational. But such processes can be just as important — and the weather can sometimes foil even the best-laid plans for world domination.

Take the storms that blew up off Kyushu in 1274 and 1281. Luckily for Japan, they happened just as Kublai Khan and his Mongol horde were trying to invade. Later named kamikaze (divine winds), the storms helped repel the Mongols — but what has not been appreciated is that climate change had helped them to expand their empire in the first place.

Mongolia is a country of extremes, with short, hot summers and long winters in which temperatures of minus 30 degrees are not unusual. It doesn’t rain much, and during most of the year — on average for 257 days — the sky is cloudless.

But it hasn’t always been so across those boundless tracts of central Asia.

Amy Hessl from the department of geology and geography at West Virginia University, and colleagues, studied tree-ring data from 107 living and dead Siberian pines in central Mongolia stretching back more than 1,100 years. The rings — made by new growth each year — can be used for dating, but also to make inferences, mainly from their widths, about the climate.

Hessl, working with Neil Pederson of the Tree Ring Lab at Columbia University, New York, found they could pick out the big climatic events of the past 1,000 years. From 950-1250 there was an unusual warming called the Medieval Climate Anomaly. Then, from around 1350-1850, there was a cold snap that’s now dubbed the Little Ice Age.

But the most unusual thing the team found was a persistent wet period in Mongolia from 1211-25 which, they suggest, boosted grassland productivity. For a culture based on horses and other livestock, this was incredibly beneficial. At the time of this sudden increase in fodder, the ruler of Mongolia was one Genghis Khan.

Under him, the empire grew to reach as far west as central Europe and as far east as the shores of Japan, becoming the largest — in terms of contiguous land area — that the world has ever seen.

Adverse climate events such as drought are often linked with the decline of complex societies, said Hessl. But what hasn’t been much studied is the opposite: The link between beneficial conditions and a rise in the complexity of a society. (The work is published in the journal PNAS, DOI reference: 10.1073/pnas.1318677111.)

By the time Genghis Khan’s grandson, Kublai, came to power in 1260, Korea was a tributary state of the Mongols known as Goryeo. But Kublai Khan wanted Japan, too, and he amassed huge fleets for his invasion attempts in both 1274 and ’81 — with an arsenal that included devastating gunpowder bombs.

Faced with this common foe, on both occasions Japan’s samurai fought together rather than battling among themselves as they had always done. For all their fabled prowess, though, they were stymied by their age-old rules of engagement which involved warriors calling out the name of an enemy with whom to engage in single combat. Similarly, archers would take aim at just one target, and fight in single combat by firing arrows at each other.

The Mongols paid no heed to such stuff, instead sweeping into the Japanese en masse, killing and maiming any which way they could — including by loosing fire arrows, poison arrows or ceramic shells filled with gunpowder into their ranks.

It was only a storm in 1274 and a typhoon in 1281 that led to the Mongols’ defeats, as their fleets were scattered and sunk. Yet the Japanese victories engendered a sense of invincibility that persisted until 1945.

But it’s not all ancient history. The tree-ring data also show the effect of a drought afflicting Mongolia in the present century. This is a mark of the Anthropocene, the era of history in which human activities themselves influence the climate and impact Earth’s ecosystems.

The warming we’ve seen over central Asia of late is related to human activity, said team member Kevin Anchukaitis of the Woods Hole Oceanographic Institution in Massachusetts. But that doesn’t mean it is possible to state scientifically that particular events — such as Mongolia’s drought — are caused by global warming.

“It is difficult to attribute the 21st-century drought in Mongolia to climate change,” he said. But he expects warming to “become even more of a factor in future droughts.”

Hessl agreed that droughts are going to be more likely. “We would expect to see similar events in the future based on past moisture variation in Mongolia and predictions of warming in the region,” she said.

The contention that history is shaped by climate — with the British Empire, for instance, said to have been built by a people escaping their homeland’s grim weather — may not be so wide of the mark after all.

Rowan Hooper (@rowhoop on Twitter) is the News Editor of New Scientist magazine.