A changing climate with falling temperatures may have been the reason why the people living on the fringes of the Tibetan Plateau abandoned it, or dramatically changed their lifestyles, around 2,000 years ago. This is the conclusion from a study by an international team of scientists led by Jade D’Alpoim Guedes from Washington State University. According to Guedes, the reason for the collapse of the Plateau civilisations was that their primary food source, millet, became impossible to grow in the new climatic conditions. 
 
 Millet, although having a shorter growing season than wheat and barley, is more heat dependent and less frost resistant. It apparently used to be grown on the Tibetan Plateau during the Holocene Climatic Optimum, when temperatures in the northern hemisphere of the earth were similar to the ones we have today. The period, which lasted around 4,000 years, was caused by variations in the Earth’s orbit. When these variations expired, temperatures quickly dropped. 
 
 Archaeological research at ancient sites on the Plateau has yielded an abundance of wheat and barley seeds, evidence suggesting that these two crops very quickly replaced millet as the primary source of food for the local inhabitants. Guedes and her team believe that the two crops came to the right place at exactly the right time – when it was becoming impossible to grow millet. It remains unclear whether this impossibility first drove the indigenous people from the Plateau, to be replaced with a civilisation that grew wheat and barley, or whether the indigenous people simply changed their lifestyle. Whatever the actual event, Guedes says it may well be the case that this was the first place in Asia that wheat and barley were grown on a large scale, and it was from the Tibetan plateau that they spread across the eastern part of the continent. 
 
 This theory, based on agronomical models for growing millet, wheat, and barley, goes counter to what was considered a consensus on the subject: that the inhabitants of the Tibetan Plateau grew millet 2,000 years ago because of its shorter growing season. Guedes joined forces with agronomists and discovered that they do not judge the viability of crops in high-altitude, cold regions like the Plateau based on their growing season but on measurements of their total heat requirements, that is, the amount of heat the crop needs over its whole lifetime. This provided the basis for the models that Guedes used to prove that the Tibetan Plateau could not have provided enough heat for millet to grow there at the time under research, despite the crop’s short growing season. 
 
 According to the study author, there are implications from these finds for today’s inhabitants of the area that spans the Tibet Autonomous region and southwestern China. Since temperatures today are as high as they were during the Holocene Climatic Optimum, there is a good chance for millet to stage a return as a staple crop, after a long period in which it was more or less forgotten. 
 
 Image courtesy of Wikimedia Commons user: Jochen Westermann ||||| PULLMAN, Wash.--Climate change may be responsible for the abrupt collapse of civilization on the fringes of the Tibetan Plateau around 2000 B.C. 
 
 WSU archaeologist Jade D'Alpoim Guedes and an international team of researchers found that cooling global temperatures at the end of the Holocene Climatic Optimum, a 4,000 year period of warm weather, would have made it impossible for ancient people on the Tibetan Plateau to cultivate millet, their primary food source. 
 
 Guedes' team's research recently was published online in the Proceedings of the National Academy of Sciences. Her results provide the first convincing explanation for why the area's original inhabitants either left or so abruptly changed their lifestyles. 
 
 They also help explain the success of farmers who practiced wheat and barley agriculture in the region 300 years later. 
 
 Unlike millet, wheat and barley have high frost tolerance and a low heat requirement, making them ideally suited for the high altitudes and cold weather of eastern Tibet. Guedes argues this made the two crops an important facet of subsistence immediately after their introduction around 1700 B.C. 
 
 "Wheat and barley came in at the opportune moment, right when millets were losing their ability to be grown on the Tibetan Plateau," Guedes said. "It was a really exciting pattern to notice. The introduction of wheat and barley really enabled Tibetan culture to take the form it has today, and their unique growth patterns may have played a crucial rule in the spread of these crops as staples across the vast region of East Asia." 
 
 One offshoot of the research: The ancient millet seeds that fell out of cultivation on the Tibetan Plateau as the climate got colder might soon be useful again as the climate warms up. 
 
 "Right now, these millets have almost become forgotten crops," Guedes said. "But due to their heat tolerance and high nutritional value, they may be once again be useful resources for a warmer future." 
 
 An archaeological enigma 
 
 At Ashaonao, Haimenkou, and other archeological sites in the Tibetan highlands, researchers for years had noticed a growing trend. An abundance of ancient wheat and barley seeds found at the sites suggested the crops rapidly replaced millet as the staple food source of the region during the second millennium BCE. 
 
 The findings were puzzling considering that the scientific consensus of the time was the region's climate would have actually favored millet, due to its shorter growing season, over wheat or barley. 
 
 The conundrum intrigued Guedes so she dove into the agronomy literature to investigate. She found agronomists tended to use a different measurement than archaeologists to determine whether crops can grow in cold, high altitude environments like the Tibetan Plateau. They used total growing degree days or the accumulated amount of heat plants need over their lifetime rather than the length of a growing season. 
 
 "My colleagues and I created a new model based off what we found in the literature," Guedes said. "It revealed that global cooling would have made it impossible to grow millet in the Eastern Tibetan Highlands at this time but would have been amenable to growing wheat and barley. Our work turned over previous assumptions and explained why millet is no longer a staple crop in the area after 2000 BCE." 
 
 Guedes' work points to climate cooling as the culprit behind the collapse of early civilization on the Tibetan Plateau. Ironically, the region is today one of the areas experiencing the most rapid climate warming on the planet. There are some areas in the southeastern plateau where temperatures are 6 degrees Celsius higher than they were 200 years ago. 
 
 Rapid temperature increase is making it difficult for the region's inhabitants to raise and breed yaks, a staple form of subsistence in the central Asian highlands, and grow cold weather crops, once again endangering their survival. 
 
 "So now we have a complete reversal and climate warming is having a big impact on the livelihood of smaller farmers on the Tibetan Plateau," Guedes said. 
 
 ### ||||| Climate change may have been be responsible for the abrupt collapse of civilization on the fringes of the Tibetan Plateau around 2,000 B.C. - but it wasn't the modern political connotation of climate change, with man-made carbon dioxide causing warming, it was global cooling.At the end of the Holocene Climatic Optimum, a 4,000 year period of warm weather, it was no longer possible for ancient people on the Tibetan Plateau to cultivate millet, their primary food source, and after productivity declined most left. Not forever, a few hundred years later they were growing wheat and barley. That's because wheat and barley have higher frost tolerance and a lower heat requirement than millet, making them better suited for the high altitudes and suddenly colder weather of eastern Tibet. In a new paper in PNAS, a team argues that the change in climate led to the change in culture still evident today.The success of those two crops causes them to spread as staples. this made the two crops an important facet of subsistence immediately after their introduction around 1700 B.C.Intriguing speculation is that if the climate heats up the way leading computer models say it will, millet may be en vogue again.Barley cultivation in Jiuzhaigou National Park, located in the Min Shan mountain range, Northern Sichuan in South Western China. Not much has changed in 2,000 years but before that it was millet. Credit: Washington State UniversityAt Ashaonao, Haimenkou, and other archeological sites in the Tibetan highlands, researchers noted ancient wheat and barley seeds when the consensus was the region's climate would have actually favored millet, due to its shorter growing season, over wheat or barley. The difference in thinking was because agronomists tended to use a different measurement than archaeologists to determine whether crops can grow in cold, high altitude environments like the Tibetan Plateau. They used total growing degree days or the accumulated amount of heat plants need over their lifetime rather than the length of a growing season."My colleagues and I created a new model based off what we found in the literature," says archaeologist Jade D'Alpoim Guedes. "It revealed that global cooling would have made it impossible to grow millet in the Eastern Tibetan Highlands at this time but would have been amenable to growing wheat and barley. Our work turned over previous assumptions and explained why millet is no longer a staple crop in the area after 2000 BC."Guedes' work points to climate cooling as the culprit behind the collapse of early civilization on the Tibetan Plateau. Ironically, the region is today one of the areas experiencing the most rapid climate warming on the planet. There are some areas in the southeastern plateau where temperatures are 6 degrees Celsius higher than they were 200 years ago.
– Civilization at the edge of the Tibetan Plateau fell apart—or at least changed very suddenly—4,000 years ago, and now, researchers believe they can explain why. In fact, it has to do with climate change, though not the way we currently think of it, Science 2.0 notes. Rather than warming, it was global cooling that led to the sudden shift, researchers say in a press release. The changing temperatures prevented locals from growing their key source of food: millet. That either prompted their departure from the area or forced them to change the way they lived, New Historian reports. Most of those who lived there left, according to Science 2.0, but three centuries later, farmers in the area were growing wheat and barley, which can handle the temperatures. Jade D'Alpoim Guedes and her team made their findings by looking at data from a different perspective. Experts had been mystified by the apparent shift from millet to wheat in the area, given that previous research suggested millet would have grown well there at the time. But that conclusion was based on assessing the growing season of millet—rather than the total amount of heat it needs. Guedes' model "turned over previous assumptions," she says. Now, history seems to be repeating itself: Climate change's effects are being felt particularly acutely in the same region, though this time it's warming rather than cooling. That means millet could play a big role in the region's agricultural future, Guedes says. (Speaking of cooling, climate change doesn't explain recent harsh winters, researchers say.)