Alternative regimes of surfaceandclimate conditions in sandy arid regions: possible relevance to Mesopotamian drought 2200–1900 B.C.

Ten feedback factors acting in sandy arid regions tend to create two alternative regimes of surfaceandclimate conditions: an equilibrium with Vegetated, Crusted soil, and Convective Precipitation, commonly in the 200–400 mm year−1range (VC–CP), and an equilibrium with a Saltating-Sands surface and Extreme Aridity (SS–EA). The recently described 300-year long drought in Mesopotamia following a deposition of volcanic dust in 2200 B.C. over the region may be explained as a transition from a VC-CP to an SS-EA regime. The transition is attributed to creation of saltation-favoring conditions by the volcanic layer, brought about by the choking-off of the plants. After the destruction of the soil-crusts, the high albedo saltating-sands surface adversely affected probabilities for convective precipitation. The change again to the VC-CP conditions around 1900 B.C. may have been triggered by unusual patterns of cyclonic activity that brought far-above-average precipitation to the region.