THEY trekked for a fortnight through Nepal to reach a hemlock tree that had survived for more than 1000 years. In the snows of Japan they cut wood cores from ancient spruce and in the mountains of Vietnam they sought out centuries-old po mu trees.The challenge in northern Australia was to find old cypress pines that had not been chopped down by farmers for fence posts.This 15-year project, sampling wood from thousands of trees at more than 300 sites from Siberia to Indonesia, and Pakistan to Japan, has allowed a history of the Asian monsoon rains to be reconstructed for the first time.The annual tree rings – a natural record of wet and dry years – have revealed unprecedented details of devastating droughts that have shaken civilisations during the past 1000 years. One dry spell may have played a powerful role in the 1644 fall of China’s Ming dynasty, according to the researchers, led by Edward Cook, of Columbia University in the United States.Their analysis has narrowed the drought down to a three-year period, from 1638 to 1641. It was particularly severe in north-eastern China, which suggests it could have influenced a rebellion of peasants that hastened the demise of the dynasty in Beijing more than 350 years ago.Nearly half the world’s population relies on the monsoon rains in Asia for water to drink and to cultivate their crops. Yet scientists cannot simulate this vital weather pattern well in their climate models, which makes it difficult to predict what will happen as the globe continues to warm.Reliable instrumental records of rainfall and temperature only go back to about 1950, which is why the researchers turned to the trees. ”This reconstruction gives climate modellers an enormous data set that may produce some deep insights into the causes of Asian monsoon variability,” says Dr Cook, whose team’s study is published in the journal Science.An Australian tree-ring specialist, Patrick Baker, of Monash University, says the research is ”hugely important”. With so many people dependent on the rains, it provides a picture of weather events that have had a big impact on societies in the past and which could occur in future.”It says, over the past 1000 years, these are the extremes. And it’s very reasonable to ask, ‘Are we prepared for this?’,” he says.Not many tree species in the tropics have clearly defined annual growth rings. To identify an Australian tree with rings that reflect rainfall, Dr Baker searched through the CSIRO’s vast collection of woods in Melbourne, where he hit upon the cypress pine, Callitris intratropica.While a lot of trees in the north of Australia are rotted by termites, cypress pines are resistant to their attack. This made finding large, long-lived ones a challenge. ”The problem is the older ranchers and cattle farmers used them for fence posts,” he says.With colleagues, however, he has been able to track down some pines up to 160 years old in the Northern Territory, and help reconstruct a record of monsoon activity over this period.In Asia, a severe failure of monsoon rains between 1756 to 1768 is not documented in historical records, but it is obvious in the ancient teak trees of Thailand and the cypress trees of Vietnam. The story they tell appears to confirm suspicions of historians that climate must have played a role in the simultaneous collapse at that time of kingdoms in Thailand, Vietnam and Burma.Another drought that hit in 1790 to 1796, causing famines in India, had an impact as far away as Mexico, where the drying up of a large lake led to disputes over the land that emerged.In a study published last month, members of the team, along with University of Sydney researchers Professor Roland Fletcher and Dr Daniel Penny, also used tree ring data to suggest climate was a factor in the demise of the Khmer empire in Cambodia nearly 600 years ago.