Consider the wet-bulb temperature, the threshold at which a person resting in the shade with plenty of water would still suffer injury or death due to heat. While the threshold is lower in humid air and higher in dryer air, even desert air can get too hot for humans to survive.
Remember me talking about environmental foreclosure? Climate change means that the currently small areas of uninhabitable heat are growing. As these expand into more populated areas, people will flee for their lives, triggering waves of climate refugees. You thought Syria was bad? You have seen nothing yet.