Scientists say the heavy metals are still damaging the environment today

Smelting silver causes other heavy metals to leach into the environment

The Mongols oversaw large scale silver mining and smelting in the region

They were feared as barbarians who built one of the largest empires in the world from horseback, but it appears the Mongolian emperors were also some of the planet's biggest polluters.

A new study into the silver mines of Kublai Khan, the grandson of Genghis Khan, shows that the people produced four times as much pollution as modern industrial mining activities.

It is thought to be one of the few examples where pre-industrial period pollution has exceeded modern day levels.

Kublai Khan (above), the grandson of Genghis Khan, founded the Yuan Dynasty during which pollution from silver mining activities in the Yunnan region of south west China increased to four times modern levels

Geologists from the University of Pittsburgh and the Chinese Academy of Sciences have found pollution from heavy metals like lead, zinc and cadmium spiked between 1271AD and 1368AD in the Yunnan region of south west China.

HOW GENGHIS KHAN OWED HIS POWER TO THE WEATHER He was one of the most feared warriors in history, waging bloody war across Asia and Europe. But Genghis Khan’s 13th-century rampage may have never happened had it not been for a spell of good weather. The leader of the Mongol armies created a vast empire that eventually stretched across China, India, Russia and Eastern Europe. Historians used to think his armies of nomadic horsemen were fleeing the bleak, cold and dry Mongolian plains for warmer regions. But now scientists have discovered that the rise of the Mongol Empire coincided with a 14-year spell of weather that was the warmest and wettest for 1,000 years. The academics, from Columbia University in New York, discovered the weather anomaly by studying the rings of ancient trees. They think the conditions created those needed for a boost in lush growth of grass, which would have fuelled the soldiers' horses and fattened their livestock. The good weather lasted from 1211 to 1225 - the exact period when Genghis Khan and his armies rose to prominence. Advertisement

They say that this pollution was caused by large amounts of silver smelting driven by the Mongolian hunger for the precious metal, which they used to make coins, jewellery and art.

Writing in the journal Environmental Science and Technology, Aubrey Hillman, who led the work, said that the impact of this pollution was still being felt today by the wildlife and environment in the area.

She said: 'Culminating during the rule of the Mongols, known as the Yuan Dynasty, these metal concentrations approach levels three to four times higher than those from industrialised mining activity occurring within the catchment.

'Notably, the concentrations of lead approach levels at which harmful effects may be observed in aquatic organisms.

'The persistence of this lead pollution over time created an environmental legacy that likely contributes to known issues in modern-day sediment quality.'

Miss Hillman and her colleagues extracted sediment cores from Lake Erhai in Yunnan.

The mud within these sediments catalogued around 4,500 years of debris deposited on the floor of the lake.

By scanning the cores for heavy metals they were able to determine how pollution levels had changed through out the areas history.

In 1500BC they noticed a rapid increase in copper levels in the silt - marking the onset of the Bronze Age in southeast Asia and the birth of metallurgy in the area.

They say that from around 1100AD there was an increase in lead, silver, zinc and cadmium, all linked to the smelting of silver.

This map shows Lake Erhai in south west China where Mongol rulers caused heavy pollution by mining silver

The researchers took sediment core samples from Lake Erhai (pictured above) in Yunnan, south west China

The Mongol's Yuan dynasty prized silver for coins and jewelry similar to the belt shown in the picture above

However, the levels of pollution remained relatively low until 1271AD, around the time when Kublai Khan founded the Yuan dynasty and became Emperor of China.

The Mongols are thought to have established the first government operated silver mine in Yuann in 1290AD and taxes from silver production came to account for 47 per cent of the national revenue.

Silver smelting (shown above) can release heavy metal pollutants like lead into the environment

The Yuan Dynasty remained in power until 1368AD when they were defeated by the Ming Dynasty.

The researchers found that lead pollution in Lake Erhai peaked at 119 micrograms per gram of sediment in 1300AD before then declining to to around 30 micrograms per gram in 1420AD. By 1980 they had reached fallen to 8 micrograms per gram of sediment.

However, Yuann is known to be particularly rich in mineral resources and has large deposits of copper, tin, gold, silver and iron, and mining activity continues there today.

Miss Hillman said: 'Our findings are unique - while pre-industrial pollution has been detected in lake sendiments over many time periods and regions of the world, only a few studies have found pre-industrial pollution levels to be greater than the modern and none of these have been in China.

'Modern pollution issues rest on a long history of decline in sediment quality at Ehrhai.

'A study of Idaho wetlands documented that the persistence of lead in sediments impacted organisms several centuries after mining activity was reduced.