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Climate study puts Incas’ success down to 400 years of warm weather

Supreme military organisation and a flair for agricultural invention are traditionally credited for the rise of the Incas. However, their success may have owed more to a spell of good weather — a spell that lasted for more than 400 years.
According to new research, an increase in temperature of several degrees between AD1100 and 1533 allowed vast areas of mountain land to be used for agriculture for the first time. This fuelled the territorial expansion of the Incas, which at its peak stretched from the modern Colombian border to the middle of Chile.
“Yes, they were highly organised, and they had a sophisticated hierarchical system, but it wouldn’t have counted a jot without being underpinned by the warming of the climate,” says Dr Alex Chepstow-Lusty, a palaeo-ecologist from the French Institute for Andean Studies in Lima, Peru.
As the treeline moved higher up the mountains, the Incas re-sculpted their landscape to maximise agricultural productivity. They carved terraces into the mountainsides and developed a complex system of canals to irrigate the land.
Although the climate remained dry, the gradual melting of glacial ice meant that they had a constant supply of water to nourish their crops.
The resultant surplus of maize and potatoes freed a large part of the growing population for activities outside food production, such as constructing roads and buildings, and serving in an increasingly ambitious army.
“It was the perfect incubator for the expansion of a civilisation,” says Dr Chepstow-Lusty, who led the study.
The Incas’ Royal Road, which went through the highlands for a distance of 3,250 miles, and the Coastal Road, which stretched for 2,520 miles, were both constructed during the warm spell. So, too, was Machu Picchu, “the Lost City of the Incas”, where temples, sanctuaries and houses stand remarkably intact today, demonstrating the scale and the skill of Inca architecture.
By the time the Spanish colonials arrived in 1533, the Incas had built up food supplies to last the population more than ten years.
However, internal divisions, the Spanish invasion and the consequent introduction of new diseases led to the Inca population plummeting.
The team behind the study identified the change in climate by examining the sediment on the floor of a small lake called Marcacocha, in the Cuzco region of the Peruvian Andes, from where the Incas began their expansion.
Each layer of sediment represents a period of time, rather like the rings in the trunk of a tree. By analysing pollen, seeds and other environmental indicators buried in the layers of mud, the team were able to find clues to the climate at the time.
They noted the appearance for the first time of a range of trees and crops at the lake, which is 11,000ft above sea level, over the critical period, corresponding to a tree line edging upwards.
The lake sediments also revealed a major drought that took place around AD880, and that may have been responsible for the collapse of a previous empire, known as the Wari.
The study’s authors say that the findings have important implications for Peru and other countries facing the prospect of the most extreme shifts in climate because of global warming. For many countries, the prospect of warming is unwelcome. However, with the correct landmanagement techniques some of these countries might be able to turn a warmer climate to their advantage.
INCA FACTS
Dr Chepstow-Lusty recommends a national reforestation programme in Peru and the restoration of the country’s irrigation canals to increase water security in the region.
•The Inca empire was a theocracy in which the king, Sapa Inca, was said to descend from Inti, the god of the Sun
•At its peak the empire supported about eight million people
•The remaining Inca populations, such as the Quechuas and the Aymara people, retain a strong cultural heritage but are economically marginalised
•The Inca civilisation remains the largest empire to have existed in the Americas