April 21, 2010
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| Tree ring scientists Ed Cook (left) and Paul Krusic trekked for nearly two weeks to reach this 1,000 year old hemlock in the Himalayas of Nepal. Credit: Brendan Buckley. (CLICK TO ENLARGE) |
The seasonal monsoon rains in Asia feed nearly half the world’s population, and when the rains fail to come, people can go hungry, or worse. A new study of tree rings provides the most detailed record yet of at least four epic droughts that have shaken Asia over the last thousand years, from one that may have helped bring down China’s Ming Dynasty in 1644, to another that caused tens of millions of people to starve to death in the late 1870s. The study, published this week in the journal Science, is expected not only to help historians understand how environment has affected the past, but to aid scientists trying to understand the potential for large-scale disruptions of weather in the face of changing climate.
By sampling the wood of thousands of ancient trees across Asia, scientists at Columbia University’s Lamont-Doherty Earth Observatory assembled an atlas of past droughts, gauging their relative severity across vast expanses of time and space. “Global climate models fail to accurately simulate the Asian monsoon, and these limitations have hampered our ability to plan for future, potentially rapid and heretofore unexpected shifts in a warming world,” said Edward Cook , head of Lamont’s Tree Ring Lab ,who led the study. “Reliable instrumental data goes back only until 1950. This reconstruction gives climate modelers an enormous dataset that may produce some deep insights into the causes of Asian monsoon variability.” There is some evidence that changes in the monsoon are driven at least in part by cyclical changes in sea-surface temperatures. Some scientists have speculated that warming global temperatures could alter these cycles and possibly make some of them more intense, but at this point there is no consensus on whether or how they might change.
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Collaborators helped Lamont’s Tree Ring Lab collect samples in more than 25 countries, including from this spruce tree in Japan. Credit: Brendan Buckley. (CLICK TO ENLARGE) |
Another severe monsoon failure came in 1756-1768, coinciding with the collapse of kingdoms in what are now Vietnam, Myanmar and Thailand. The drought roiled political structures all the way to Siberia, and the tree rings also indicate that western India was severely affected. This drought is not documented in historical records; scientists first identified it in teak rings from Thailand, and later in Vietnamese cypress trees. Some historians have speculated that climate must have played a role for such sweeping political changes to have happened simultaneously; fragmentary accounts suggest that dry periods may have been punctuated with devastating floods. The study appears to provide an explanation for the so-called “strange parallels” that Victor Lieberman, an historian at the University of Michigan, has spent his career studying. “It provides confirmation that there are very strong climate links between monsoon regimes in India, Southeast Asia and southern China,” said Lieberman in an interview.
Then, the so-called East India drought hit in 1790-1796. This one appears to have been felt worldwide, spreading civil unrest and socioeconomic turmoil. For instance, in Mexico, water levels at Lake Pátzcuaro fell so much they gave rise to ownership disputes over the land that emerged. In Europe, drought led to crop failures that preceded the French Revolution. Famines hit India.
Perhaps the worst drought, the scientists found, was the Victorian-era “Great Drought” of 1876-1878. The effects were felt across the tropics; by some estimates, resulting famines killed up to 30 million people. According to the tree-ring evidence, the effects were especially acute in India, but extended as far away as China and present-day Indonesia. Colonial-era policies left regional societies ill-equipped to deal with the drought’s consequences, as historian Mike Davis details in his book Late Victorian Holocausts . Famine and cholera outbreaks at this time in colonial Vietnam fueled a peasant revolt against the French.
The atlas is valuable to monsoon forecasters because the record is long enough and the spatial areas detailed enough that modelers can pick out short-term and long-term patterns, said Bin Wang, a meteorologist and monsoon modeler at the University of Hawaii who was not involved in the study. “It is extremely valuable for validating climate models’ simulation and understanding their origins in terms of model physics,” he said.
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