LAUSR

A variety of philological and archeological evidence indicates that a vast maritime commercial network linking polities in Mesopotamia, the Persian Gulf, and the Indus Valley emerged in the second half of the third millennium BCE (before the common era). Here, we propose that the climate of the western Indian Ocean during the third millennium BCE was an important but heretofore unrecognized influence on the development of this maritime exchange system. Recent reconstructions have revealed a gradual weakening of the Indian Summer Monsoon (ISM) during the past 10,000 years. However, during parts of the third and fourth millennia, the data show the trend temporarily reversed and the ISM intensified—as, crucially, did the power of eastward-blowing trade winds across the Arabian Sea and the western Indian Ocean. This intensification peaked late in the third millennium, precisely at the time when the aforementioned maritime contacts expanded dramatically. As the ISM system strengthened, so too did the associated summer westerly winds which facilitated maritime trade. We argue that this allowed polities in southern Mesopotamia, to refocus many of their resource procurement efforts from the north and northeast toward the Persian Gulf and points south and east by employing ocean-going sailboats to more efficiently import high-bulk metal ores and to export low-value, high-bulk agricultural and pastoral goods at scale for the first time in their history. We further propose that by ca. 2200 BCE, some coastal Arabian and Indus Valley participants in this trade may have used bulk imports of grain and textiles from Mesopotamia as a way to mitigate the effects of drought at that time.