All droughts are ultimately caused by a lack of precipitation. This reduced precipitation does not have to be local; areas that receive water from rivers, for example, can be affected by a lack of precipitation thousands of miles away.
While weather tends to follow patterns and cycles, the causes of these patterns can become disrupted over time. The Gulf Stream, for example, carries warm water near the equator north, where it warms up the western portion of the European continent. If the Gulf Stream stopped flowing, parts of England and other areas might receive less rain, which could cause droughts.
Droughts are more common than many people imagine, and there are always droughts occurring in the United States. Over the years, experts have learned which areas are most vulnerable to drought, and they have prepared infrastructure to help drought-stricken areas get the water they need until local conditions return to normal.
Parts of the developing world, however, might not have this infrastructure, and even relatively minor droughts can cause widespread crop failures and other potentially devastating problems. Experts concerned about global warming point to the possibility of more droughts in particularly vulnerable areas and the cascade effect they would have on civilization around the planet.