Tree Rings Tell Of Killer Droughts In Indochina

-- by David Fogarty, Climate Change Correspondent, Asia

Along the mountainous spine of Vietnam grow ancient conifers whose tree rings tell of droughts lasting more than a generation that helped push civilisations towards collapse, a climate change conference heard on Tuesday.

Research by scientists from the United States and Japan has revealed a record of drought in Indochina that goes back more than 700 years by studying tree ring core samples from Fokienia hodginsii, a rare species that lives in Vietnam's cloud forests.

What the samples show are two lengthy droughts between the late 1300s early 1400s, around the time the vast and wealthy Angkor civilisation in modern-day Cambodia collapsed.

"There was a very significant multi-decadal drought in the early 1400s with the worst drought year being 1417," said Brendan Buckley of the Tree Ring Laboratory at the Lamont-Doherty Earth Observatory in the United States.

Another major drought lasting at least 30 years hit in the mid-18th century, said Buckley, speaking by telephone from the sidelines of the conference in Dalat, southern Vietnam, that is focusing on climate variability along the Mekong River basin.

"All of the kingdoms in Southeast Asia collapsed, in Thailand, Vietnam and Laos between 1750-80," he said.

Buckley teamed up with Masaki Sano and Tatsuo Sweda of Ehime University of Japan to study the tree rings of Fokienia.

The research is helping unravel the complexity of the annual monsoon that usually begins during March-May and on which millions rely to grow crops, particularly rice in the Mekong Delta. It could also help understand how climate change could affect the densely populated region and its economies.

Buckley said the chronology constructed from the tree rings showed a strong correlation between dry spells and the El Nino weather pattern that typically brings drought to Southeast Asia and eastern Australia.

CAUSE A MYSTERY

But what caused the much longer periods of drought was a mystery, although it could possibly be linked to a recently discovered multi-decadal switch of sea surface temperatures in the Pacific.

Buckley said the chronology was built from cores from 36 conifer trees found in a national park near Dalat.

"It's an amazing site, really, so it's a real breakthrough in tropical tree ring studies," he said, adding that a lot of the sites where the tree is found in Vietnam are being logged.

"Fokienia is an exceptional tree species because of the way it grows and responds to drought," he said, adding the data was corroborated with a lot of historical records, such as ancient lanna palm leaf texts from Thailand and accounts of foreign merchants, plus tree-ring data from teak trees in Thailand.

"What the Fokienia trees are most keenly tuned into is the length of the monsoon. Longer the monsoon, the trees grow more."

Dan Penny, part of a separate project investigating the collapse of Angkor, said drought was likely to be have been one of many reasons for the collapse of the city, which covered about 1,000 square kilometres and was the most extensive of its kind.

"There's no doubt that it was a contributory factor," he told Reuters from the conference.

Angkor was built around irrigation channels and was heavily focused on growing rice, he explained, but said evidence suggested the city was already in decline by the time the drought hit.

Buckley said the tree-ring data painted an awful scenario for the region, particularly the Mekong Delta, where sea levels are predicted to rise a metre or more in coming decades.

"When you measure the sea level rises that have already taken place across the Mekong Delta, it's really scary," he said.

"There's a real threat right there. Picture that with a major drought that lasts 30 years in the area and you can imagine the kind of chaos that could easily lead to some significant turmoil and societal collapse."