"Since the Arab Muslims were from desiccated Western Arabia, they may have been better at dealing with a dry climate; Muslim water-management techniques were superior to those of other civilizations in that era. They may also have had advantages in logistics and fighting technique. The Bedouin tribesmen of Arabia that were the core of the Arab Muslim army had been used to raiding across arid territory. Camels need less water than horses and can cover more territory per day, so in dry conditions a camel cavalry has advantages over a horse cavalry. Bedouin had been probing Byzantine defenses in Syria all along; why were they suddenly able to over-run Damascus in 634 CE? Many historians have focused on the esprit de corps and unifying ideology they derived from the new religion of Islam, but other explanations should continue to be considered." (Thanks Leila)
Juan Cole suggests a climate-related explanation as an explanation of the success of the Arab Muslim conquests. My take is that we should steer away from simple, uni dimensional explanations of history, especially when they appear to fit current scientific and popular interests such as climate change. The most likely course of event probably falls into the "perfect storm" category, as has happened many times later in history, such as right after the fall of the Ottoman Empire and during the early days of the British mandate. This is when market forces, droughts, "modernizing" mandatory policies and the creation of regional nation states (mainly Ibn Saud's Arabia) together acted to start a chain reaction that will eventually culminate in massive exit from Bedouinism. Do not also underestimate the impact that single individuals can have: Mohammad for the Islamic conquests, Glubb and Ibn Saud in the early 20th century.