In a small office in an old lab on the U of M campus, Hai Cheng sorts through a cardboard box full of stalagmites from all over the world.
"Those are from Peru, and those dark-colored samples are from Turkey. This yellow one is from Ukraine. I collected that last summer."
Hai Cheng grew up in China, but now he's a research scientist in the Department of Geology and Geophysics at the University of Minnesota.
His stalagmites are six-inches to a foot long and just two or three inches wide. They look a little like agates. They've been split in half length-wise to expose wavy lines -- they're growth rings, like in a tree. Some of them are almost 1,000,000 years old. These ancient rocks are nature's time capsule, showing the ebb and flow of climate on the earth.
Hai Cheng collaborates with colleagues in China, and with Lawrence Edwards, who is the George and Orpha Gibson Chair of Earth Systems Science and Distinguished McKnight University Professor at the University.
They've been refining techniques to wring secrets from these rocks. They measure uranium and thorium to count the years as the stalagmite built up from the floor of the cave. They can tell which years were wet and which were dry, based on different types of oxygen found in the rock.
They plotted the drought years and the wet years on a graph that parallels periods in Chinese history.
"If you start at around 700 A.D., basically you have above-average in terms of wetness," Larry Edwards explains. "And then you have gradual falling, so it's gradually getting drier, and this is all within the Tang Dynasty."
The Tang Dynasty was a period of great power, when big armies controlled trade along the Silk Road.
But in the 800's, it got very dry, and in 906, following years of disastrous harvests, the Tang Dynasty fell.
Around 1,000 A.D.the monsoons returned and there was enough water to grow rice in much of the country. This was the time of the next successful dynasty, the Song Dynasty.
"And it's the first time rice became the staple of the Chinese diet. Of course rice cultivation requires water," says Edwards. "It was kind of a golden age in China: the population doubled."
That was from about 1000 A.D. to 1300.
Then, droughts returned, and twice more they coincided with the fall of dynasties: the Yuan in the 1400's and the Ming around 1600.
"In China, the view is that the emperor, the dynasty, has a mandate from heaven to continue on, and the dynasties end when the perception is that they've lost that mandate, and part of the mandate is climatic conditions," says Edwards.
After all, if an emperor can't feed his people, what's he doing on the throne?
But it's not just China that experienced these cyclical monsoons and droughts. Hai Cheng and Larry Edwards came up with such an accurate weather history that they could link it to what was happening in places as far away as Mexico.
The Mayan civilization -- also suffering a drought -- fell at almost exactly the same time as the Tang Dynasty.
"So here the Mayans are, and the Tang Dynasty Chinese, and they're fighting against the same natural phenomenon," says Edwards. "They don't even know each other."
On opposite sides of the world, peoples' lives collapsed when the rains didn't come.
And later, at the same time as the generous monsoons during the Song Dynasty, Europe and the North Atlantic were experiencing what scientists call the Medieval Warm Period -- from about 800 to about 1300 A.D.
"And so virtually at the same time of this expansion of rice cultivation in China, the Vikings settled southern Greenland, and they were able to because it was relatively warm."
During the drought of the 1400's that contributed to the fall of the Yuan Dynasty, Europe was experiencing the Little Ice Age, that forced the Vikings out of Greenland.
Their research shows an ancient natural pattern in the earth's climate, Edwards says. Warm temperatures and generous rains in China coincided with warm temperatures in Europe. When it was cool and dry in China it was cold in Europe. It was like that for at least 100,000 years.
"And it falls apart in the last 50 years."
The pattern that had ruled the climate of the globe changed right around 1960. Europe's weather has been record-breaking hot, while it's getting drier in western China.
There's no proof, but scientists think the pollution and carbon dioxide we've put out since the industrial revolution are changing the way the monsoons behave. Weather is being driven by soot and greenhouse gases in the atmosphere rather than by the natural mechanisms of wind, temperature, and pressure.
And that magnifies the lessons that Larry Edwards and Hai Cheng brought back from the cave.
Geology and history teach us that when the climate shifts, human societies are affected, often dramatically.
The work is published in the November 7th issue of Science magazine.