REPLY TO COMMENTS OF NOLAN AND COOK

We appreciate and would like to respond to the comments made by Nolan and Cook. We make three points in our reply, which are directed at clarifying our position and responding to a few of Nolan and Cook\u27s assumptions. First, as a matter of clarification, the Palmer Drought Severity Index (PDSI) is not a measure of summer rainfall as Nolan and Cook (2010) imply. Instead it is a theoretical measure of soil-moisture, the value of which evolves over several months in response to fluxes of precipitation, evaporation, and runoff. In practice, PDSI is usually heavily weighted toward the precipitation side of the soil water balance. Second, with regard to archaeological facts, the known developmental trajectory of the greater Cahokia region is not quite as Nolan and Cook describe it. In their discussion of the timing of the precocious development of social complexity in the American Bottom, Nolan and Cook refer to the American Bottom during both the Edelhardt (A.D. 1000-1050) and Lohmann phases (A.D. 1050-1100) as being relatively wet but not the wettest areas in the Upper Mississippi and Ohio Valleys. However, Cahokia\u27s big bang dates to the end of the Edelhardt phase and continued through the Lohmann phase, so the overall climatic state of the Edelhardt phase is not at issue. Nolan and Cook\u27s Figure 2 supports the concept that the Cahokia area was extremely wet during the Lohmann phase as opposed to the Edelhardt phase

Cahokia\u27s Boom and Bust in the Context of Climate Change

During the early Mississippian Lohmann phase (A.D. 1050-1100), the American Bottom experienced a political and economic transformation. This transformation included the abrupt planned construction of central Cahokia, a large-scale influx of people to downtown Cahokia, the abandonment of pre-Mississippian village settlements, the reorganization of farming in the Mississippi River floodplain, and the founding of the Richland farming complex in the Illinois uplands. New tree ring-based records of climate change indicate that this rapid development occurred during one of the wettest 50-year periods during the last millennium. During the next 150 years, a series of persistent droughts occurred in the Cahokian area which may be related to the eventual abandonment of the American Bottom. By A.D. 1150, in the latter part of a severe 15-year drought, the Richland farming complex was mostly abandoned, eliminating an integral part of Cahokia\u27s agricultural base. At about the same time, a 20,000-log palisade was erected around Monks Mound and the Grand Plaza, indicating increased social unrest. During this time, people began exiting Cahokia and, by the end of the Stirling phase (A.D. 1200), Cahokia\u27s population had decreased by about 50 percent and by A.D. 1350, Cahokia and much of the central Mississippi valley had been abandoned