Warmer temperatures will lead to more evaporation and therefore more rain, so global warming will not lead to a drier planet.
Fact:
Warmer temperatures will indeed lead to more evaporation. In fact, for every 1°C of warming, there is a 7 percent increase in water vapour, meaning more of the planet’s finite supply of fresh water winds up being trapped in the atmosphere. Even if rainfall increases, the Earth will still be drier.
An analogy might be a bath which, instead of letting water drain away, pumps it straight back into the bath. The system is primed so that the amount of water leaving via the drain is exactly matched by the amount being put back in via the tap. This closed system is in equilibrium – like the Earth’s hydrological system at any point in time. Now, if we elongate the pipe, allowing it to hold more water, the amount in the bath goes down. If we also speed up the rate at which the pump pulls water from the bath and spits it back out of the tap, the system moves to a new equilibrium. This time, however, more water is being pumped into the bath (i.e. more rain) but less is actually in it (i.e. a drier Earth).