WASHINGTON, D.C.: Dry-spells known as flash droughts, with a surprisingly rapid onset and often devastating impact, are becoming more frequent as human activity warms the planet, according to a study published Thursday. Though droughts are generally thought of as long-term phenomena, some can occur quite suddenly, in a matter of weeks, when the conditions are right. Global warming is a recipe for increasing those special conditions around the world, creating in certain areas a decrease in precipitation and increased evaporation, which dries out the soil more quickly. For the study, published in the journal Science, researchers analyzed a combination of…