This chapter discusses the climate anomaly called the 4.2 ka event. It starts with a description of this climatic event and how it affected civilizations that developed along river valleys. The stories of some of these societies include that of the first empire in history that thrived in Mesopotamia, called the Akkadian empire. The chapter tells the story of the rise of this empire, its monumental accomplishments, its weaknesses, and final collapse. The chapter alludes to some studies that show evidence for climate change at the time of the collapse. This, in addition to the story of the Old Kingdom in Egypt, its greatest achievements exemplified by the great pyramids, and the prolonged drought that brought about its demise. Then, the chapter covers some scholarly studies that provide evidence for climate change at the collapse time. The chapter also includes the story of the Liangzhu culture in ancient China on the Yangtze delta from its emergence to its demise. This is followed by the story of the Indus Valley Civilization, called the Harappan. The chapter overviews the development of its great cities, and the abandonment of these cities due to climate stress, and the evidence for this association.