How does global warming cause drought?

It does not because warming produces more moisture in the air and so more rain except in regions that have no lakes or oceans to get water from. Drought is most prevalent during long cold periods when the sun is not warm enough to evaporate the needed water for rain. We really needed at least 1 more degree of warming and another 2 to 300ppm of co2 to get into a good crop producing climate. We are still in an ice age and while it is more comfortable then a full ice age we still have a noticeable number of large glaciers present. During a true optimum the only major glaciers are on very tall mountains and Antartica.