There’s a new paper by Jared Diamond out, always a welcome event. Alas, it is behind a paywall at Nature, but it is fairly easily summarized. Drawing on recent studies of the collapse of Classic Mayan civilization in Central America and Angkor in Cambodia, and the rise of the Inka empire in the Andes, it makes two main points. One, that civilizations may collapse for multifarious reasons, but “human overexploitation of natural resources never helps.” Two, that “climate can change in either direction.” Not particularly novel or surprising conclusions, but Diamond does his usual slick job of marshaling disparate, multidisciplinary evidence from tourism hotspots all over the world and from throughout history to advance arguments of great contemporary relevance, slight whiff of environmental determinism notwithstanding.
In this case, what struck me particularly was the success story. That is, how climatic change after AD 1100, during the Northern Hemisphere’s Medieval Warm Period, may have helped the Inka’s conquests. The argument is that higher temperatures allowed them to extend agriculture to higher altitudes, increase their arable-land area and make more use of irrigation, leading to greater production and the possibility of feeding large armies. Of course, “military and administrative organization was essential to their conquests, [but] climate amelioration played a part.”
The evidence for agricultural expansion comes from the work of Alex Chepstow-Lusty and colleagues, who analyzed the pollen and other plant parts (and, indeed, other organisms) in mud cores from a high-altitude Andean lake bed. Some of the plant parts came from an alder-like tree. I had no idea that the Inka planted the nitrogen-fixing Alnus acuminata. Its remains increase in lake sediments dated from about AD 1100, along with ambient temperatures and the bodies of llama dung-eating mites, showing that there was starting to be more agriculture and livestock-keeping around the lake at that time.
The success story came to an end, of course. The Spanish cut most of alders down for fuel. Chepstow-Lusty is calling for “massive reintroduction of native tree varieties, such as the alder, to trap moisture blowing over from the Peruvian Amazon to the east. He also recommends repair of the now derelict Incan canals and terraces so they can once again support agriculture.” Have there been similar suggestions for the restoration of the agricultural infrastructure — physical and biological — of the Maya and Khmer? Maybe all the tourists could be put to work on this — or at least tapped for cash.