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.

各种文献学和考古学证据表明，连接美索不达米亚、波斯湾和印度河流域政体的庞大海上商业网络出现在公元前三千年后半期（共同时代之前）。在这里，我们提出公元前第三个千年期间西印度洋的气候对这一海上交换系统的发展具有重要但迄今为止未被认识到的影响。最近的重建显示，印度夏季风 (ISM) 在过去 10,000 年中逐渐减弱。然而，在第三个和第四个千年的部分时间里，数据显示趋势暂时逆转，ISM 加剧——至关重要的是，向东吹过阿拉伯海和西印度洋的信风的力量也是如此。这种强化在第三个千年后期达到顶峰，正是在上述海上接触急剧扩大的时候。随着 ISM 系统的加强，促进海上贸易的相关夏季西风也加强了。我们认为，这允许美索不达米亚南部的政体将其许多资源采购工作从北部和东北部重新集中到波斯湾，并通过使用远洋帆船更有效地进口大块金属矿石和出口而指向南部和东部历史上第一次大规模生产低价值、高散货的农牧产品。我们进一步提出，由ca。公元前2200年，