One convincing explanation for how this cultivating population might have been assembled as state subjects is climate change. Nissen shows that the period from at least 3,500 to 2,500 BCE was marked by a steep decline in sea level and a decline in the water volume in the Euphrates. Increasing aridity meant that the rivers shrank back to their main channels and the population increasingly huddled around the remaining watercourses, while soil salinization of water-deprived areas sharply reduced the amount of arable land.
