Climate link to fall of Tang dynasty questioned

PUBLISHED : Friday, 30 November, 2007, 12:00am
UPDATED : Friday, 30 November, 2007, 12:00am

Beijing ordered scientists to refute study blaming empire's demise on drought

It has been a millennium since the sun set on the golden age of the Tang dynasty, but the political fallout of its decline is still being felt today in a debate over exactly what brought the empire down.

In a paper published this year in Nature magazine, a German-led research group suggested an escalation of wintry monsoons caused by a global climate change was the catalyst for the empire's collapse.

However, linking global climate change with government collapse touched a nerve with some Chinese officials. So the government pushed mainland scientists to collect evidence and develop a theory that proved the German scientists were wrong. The results, regarded by anonymous academic reviewers as 'strong' and 'scientifically robust', were published this month in the same magazine.

The German team, led by geologist Gerald Haug of the University of Potsdam's Institute for Geosciences, concluded that between AD700 and AD900, winter monsoon winds in China were stronger and summer rainfall weaker than normal, leading to prolonged droughts and a decline in crop yields. Less food then magnified the impact of losing a war with the Arabs, which led to a rapid decline of the empire.

They based their conclusions on titanium levels in sediment in Lake Huguang Maar, on Guangdong's Leizhou Peninsula, which they believed represented the amount of dust carried by winds from the north.

'From the coincidence in timing, we suggest that [global climate change] could have contributed to the declines of both the Tang dynasty in China and the Classic Maya in Central America,' the authors wrote in their paper.

Zhang Deer , chief scientist at the China Meteorological Administration's (CMA) National Meteorological Centre, said she came under government pressure after the German findings were published.

In the beginning, she merely looked at the Nature paper out of personal interest because the conclusions were different from her knowledge, acquired over more than three decades of mining historical documents for weather records.

But the overseas study soon picked up steam on the internet, and in just a few days online copies of the paper attracted several hundred thousand clicks from across the mainland.

It generated so much interest that the party mouthpiece People's Daily sought her out for an interview.

'The next day, they published the interview with a title saying the German scientists were wrong, even though at the time we didn't have much academic evidence against them,' Professor Zhang said.

Qin Dahe , then CMA director and Beijing's chief representative on the Intergovernmental Panel on Climate Change, ordered Professor Zhang to respond.

'So I gave up other responsibilities in order to concentrate on this one,' she said. She led a team of mainland scientists who spent most of this year compiling evidence they say proves the climate-driven decline conclusions are wrong.

'Our result is based on science, not political opinion. That is why Nature agreed to publish it,' she said.

Professor Zhang said the German scientists had done a brilliant job, but their conclusion was 'total nonsense'.

'Over-simplification is a common mistake among people who know little about Chinese history and understand little about its culture.'

Professor Zhang said records showed that the later years of the Tang dynasty were particularly wet, rather than a period of drought. In fact, a comparison between documented weather and Professor Haug's titanium record reveals they are actually inversely related.

'I am not saying the scientific data is meaningless,' Professor Zhang said. 'But when you have such a strong anti-correlation between the data and historical record, you know there is a problem.'

The problem, according to Guangzhou Institute of Geochemistry researcher Zhou Houyun , was that the sediment samples the German scientists extracted from the lake had nothing to do with wind.

Dr Zhou, who also published his findings in Nature this month, said most of the sediment was the result of local erosion and runoff into the lake, rather than northern dust carried in by the wind.

'In the sediment we have found lots of strontium and neodymium, two isotopes common in the lake area but rarely found in migrated dust,' Dr Zhou said. 'The composition of other rare elements also supports our argument that rainfall, rather than wind, was the main carrier of the lake's sediment. Most [Nature] reviewers agreed with me.'

Nonetheless, the German researchers said the case against them 'is weak' and 'can be ruled out'. They said the mainland scientists only looked at their interpretation of titanium and overlooked other equally important evidence that the lake sediment was formed by airborne materials.

'We believe that their case against our interpretation of the titanium record is weak, and that their interpretation can be ruled out if the other measurements are taken into account,' Professor Haug said.

Professor Zhang said the Germans' conclusion was misleading, and Chinese culture was not as vulnerable as they made it out to be.

'The ultimate reason behind the fall of the Tang dynasty was corruption, a weakened central government and a split in the nation,' she said. 'It is wrong to simplify the rise and fall of Chinese civilisation based on a single factor like weather.'

When you have

such a strong anti-correlation between the data

and historical record,

you know there

is a problem

Zhang Deer, meteorologist