Global warming played a role in Incas' rise, report says

Global warming is not necessarily always bad.

A 400-year warm spell in South America fueled the Incas' rise, British archaeologists reported Monday, helping them build the largest empire that ever ruled the continent.

A several-degree increase in temperature allowed the Incas to move higher into the Andes mountains, opening up new farmland and providing a water source through the gradual melting of glaciers at the top of those mountains, paleoecologist Alex Chepstow-Lusty of the French Institute of Andean Studies in Lima reported online Monday in the journal Climate of the Past.

"They were highly organized and they had a sophisticated [governance], but it wouldn't have counted a jot without being underpinned by the warming of the climate," he said in a telephone interview.

Other experts were cautious about his interpretation. "The premise that the Incan expansion was driven by climate change is quite revolutionary," said archaeologist Mark Bush of the Florida Institute of Technology in Melbourne.

Confirming it will require a great deal more work for paleoecologists and archaeologists alike.

The new research is important because "data on Andean climate during this time period are scant," said archaeologist Warren Church of Columbus State University in Georgia, who works in Peru. "However, it is important to remember that climates do not make empires. People do."

Chepstow-Lusty, geographer Mick Frogley of the University of Sussex and their colleagues have been studying a 26-foot-long core of mud drilled from the sacred Lake Marcacocha in the Patacancha Valley of Peru, near Cuzco. Seeds, pollen, charcoal bits and other debris from successive layers in the core paint a picture of climate and agriculture in the region for 4,000 years.

Analysis of the core showed that a major drought began in the region around the year 880 and lasted for at least 100 years.

That drought, Chepstow-Lusty speculated, may have been the cause of the demise of the Wari empire, which lasted from 550 to 1000. The region was also colder than normal for the 3,000 years before 1000.

Beginning about 1150, the climate began warming and eventually got "several degrees centigrade" warmer, Chepstow-Lusty said. That had the net effect of extending arable regions to an altitude about 300 yards higher in the mountains, he suggested, vastly extending the area that could be cultivated. It also might have caused the Peruvian glaciers to melt slowly, providing water that the Incas captured with large irrigation systems and agricultural terraces.