Book excerpt: A boom in the production and export of cotton turned Iran into the richest region of the Islamic caliphate in the ninth and tenth centuries. Yet in the eleventh century, Iran's primacy ended as its agricultural economy entered a steep decline. Richard W. Bulliet advances several provocative explanations, for example that the boom in cotton production paralleled the spread of Islam and that Iran's agricultural decline stemmed from a significant cooling of the climate that lasted more than a century. Substantiating his argument with innovative quantitative research and scientific discoveries, Bulliet first establishes the relationship between Iran's cotton industry and Islam and then outlines the evidence for what he terms the "Big Chill." He then focuses on a lucrative but temperature-sensitive industry of cross-breeding one-humped and two-humped camels, concluding with an unusual concatenation of events that had a profound and long-lasting impact not just on the history of Iran but on the development of the world.