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Tree-ring data helps recreate causes of mega Asian droughts

Tree-ring data helps recreate causes of mega Asian droughts
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First Published: Sat, Apr 24 2010. 12 30 AM IST

Updated: Wed, Apr 28 2010. 10 02 PM IST
New Delhi: Even as India recovers from its worst drought in 37 years and policymakers await a crucial monsoon forecast from the nodal weather agency, a team of researchers at Columbia University’s Earth Institute have for the first time ever mapped major droughts that crippled Asia over the last 700 years.
While historical records have alluded to these major droughts, and blamed them for the decline of powerful kingdoms, this reconstruction recreates to the best detail yet the climate conditions—from rainfall patterns to temperatures that led to these droughts.
Experts associated with the study say this data could be useful for existing weather models which attempt monsoon forecasts as well as those trying to predict the ambiguous, highly contested impact of global warming and associated climate change.
Though scientists who study paleoclimate have relied on ice cores and coral deposits to estimate temperatures before the 16th century, this study, which involved intergovernmental cooperation and unprecedented access to tree-ring data, shows that it’s possible to accurately construct even paleo-monsoon variability charts.
Edward Cook, head of Lamont-Doherty Earth Observatory, a research organization of the Earth Institute, who led the study said, “Reliable instrumental data goes back only until 1950. This reconstruction gives climate modellers an enormous dataset that may produce some deep insight into the causes of Asian monsoon variability.”
Also See | Mapping monsoon failures in Asia (Graphic)
Click here to view a slideshow of graphics from the Monsoon Asia Drought Atlas
Called the Monsoon Asia Drought Atlas, the research paper published in Friday’s edition of the journal Science analyses tree rings from over 300 sites across Asia, including nine from India, and spanning Siberia, Indonesia, Pakistan and Japan.
The tree rings, which are a popular proxy for variables such as temperature, soil moisture and carbon dioxide content at a specific place and time, were used to date and map the regions that hosted four of the biggest droughts that plagued Asia since AD 1300: the Ming dynasty drought (1638-41), the Strange Parallels drought (1756-68), East India drought (1792-96) and the Great Drought (1876-78).
Among these, the most devastating was the Victorian-era Great Drought of 1876-78, felt most strongly over India; its effects were felt across the tropics; by some estimates, resulting famines killed up to 30 million people.
The tree-ring evidence suggests that though India bore much of the brunt, the drought’s effects were felt as far away as China and present-day Indonesia.
Another finding has been that the monsoons over China, India and regions in South-East Asia were closely linked even in the frequency of droughts.
In an interview, Cook added that the El Nino-Southern Oscillation, the warming and cooling of the tropical Pacific atmosphere-ocean system that is blamed for many of the recent droughts over India—and even last year’s monsoon failure over India—wasn’t strongly associated with the previous, so-called “mega droughts”.
“It’s only in the 20th century that we are seeing this increasingly powerful relationship between the El Nino warming phase and increased droughts over Asia. If you look back at these mega droughts, there isn’t always an El Nino at work. So probably regional, more local factors were driving those droughts.”
The June-September monsoon generates nearly 80% of the annual rainfall over India and is vital for the economy, being the main source of water for agriculture, which generates about 17% of the country’s gross domestic product. Other than the 60% of the country’s workforce that depends on agriculture, the rains are also important for traders dealing in food and cash crops as any shortfall can inject volatility in the markets.
India’s failed monsoon last year triggered unprecedented levels of food price inflation.
In India, monsoon records of rainfall and temperature were maintained only since the late 19th century, so this attempt at recreating monsoon variations and patterns using tree-ring data is novel, said M. Rajeevan, a weather expert with the Indian Space Research Organisation. “At the very least, current models that are trying to model the impact of global warming on monsoon, now have to show that their predictions agree with these maps too. So it’s a good and useful scientific attempt,” he added.
However, Rajeevan expressed doubts over the definition of drought employed. The analysis, he said, heavily relied on the Palmer Drought Severity Index, a complex scale used to define drought and popular in the US, and didn’t necessarily correspond with the India Meteorological Department’s (IMD) measures.
“We can’t be sure if a 10% deficiency of normal monsoon, as is the case with IMD, qualifies as a drought. Moreover, the tree-ring sites are still limited to capture the entire variability of the monsoon over Asia,” he added.
K. Krishnakumar, whose group at the Indian Institute of Tropical Meteorology (IITM), Pune contributed to the tree-ring samples referenced in the study, said that the work was exciting, but had its shortcomings. “Most of the tree samples in India are from the Himalayas and that region doesn’t reveal a lot on the Indian monsoon. More samples from central India were necessary to capture the Indian monsoon’s complexity,” he said.
The IITM group recently published data from 600-year-old tree-ring samples that threw light on patterns of droughts and floods over south India.
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First Published: Sat, Apr 24 2010. 12 30 AM IST
More Topics: Tree-ring data | Rainfall | Monsoon | Droughts | Asia |