Historians have pointed to a prolonged dry spell as one of the contributing factors to the decline of China's Tang dynasty in the late 9th century. Now they may have the rock to prove it.

An ancient stalagmite found rising from a cave floor in northwest China has allowed scientists from Lanzhou University in China to chart the rise and fall in strength of Asian monsoons for over 1,800 years.

The stalagmite, which built up from minerals in dripping water over 1,810 years, had unusually high uranium concentrations, allowing researchers to chemically date the periods of high and low rainfall during its lifetime.

The monsoon cycles they discovered corresponded not only with known climate effects in other regions, such as glacial retreat in the Swiss Alps, but also with periods in China's long history, the researchers reported in Friday's edition of the journal Science.

The monsoon, they found, was generally weak during the final decades of the Tang (618-907 AD), Yuan (1271-1368) and Ming (1368-1644) dynasties, suggesting poor climate might have been a contributing factor in those upheavals. Likewise, they found periods when the monsoons were stronger corresponded with periods of growth in China.

"During strong monsoon periods, dynasties such as the Northern Song (960-1127) enjoyed increased rice cultivation and a booming population," the researchers said.

The 1.18 metre-long structure was found in Wanxiang cave, some 1,200 metres above sea level, in a region where the monsoon season stretches from May to September.

The researchers also found what they say is further evidence of man's role in climate change. While rising temperatures were linked to stronger monsoon periods, that relationship appears to have changed over the past half century, with temperatures rising while monsoon periods remain relatively weak.

"Variability in late 20th-century precipitation and temperature in the region is probably caused, in large part, by both anthropogenic greenhouse gases and aerosols," they wrote.

With files from Reuters