What evidence suggests that Arabia once had more rainfall and vegetation?

Evidence suggests that the Arabian Peninsula once experienced significantly higher levels of rainfall and supported richer vegetation than it does today. The primary physical evidence for this past climate is the presence of large, dried-up watercourses, known as wadis, that intersect the landscape.

These ancient river systems, which date back to the Pleistocene Epoch, were once capable of carrying vast loads of sediment from the interior toward the Persian Gulf. Remnants of the deltas formed by these rivers are comparable in size to the delta of the Nile River, indicating that the region was once shaped by much more substantial and frequent water flow than the current arid climate allows. While these features point to a wetter past, it is important to note that climatic conditions in the region have remained relatively stable over the last five millennia, with human life continuing to be a struggle against the peninsula's harsh, arid realities.12