The American west’s drought is its second-worst for 12 centuries

THE SOUTH-WESTAD of the United States, together with adjacent parts of Mexico across the Rio Grande, is one of the driest parts of the North American continent. But, over the past two decades, even that expected dryness has been taken to the limit. According to Park Williams, who works at Columbia University’s Lamont-Doherty Earth Observatory, the current lack of rainfall in the area constitutes a megadrought of a magnitude seen on only four other occasions in the past 1,200 years.Dr Williams and his collaborators studied the annual growth rings of 1,586 ancient trees from Oregon and Montana all the way down to the northern reaches of Mexico, in order to reconstruct soil-moisture patterns going back to 800. During warm, wet years trees grow fast, producing wide rings. During cold, dry ones they grow more slowly, producing narrow rings. During a drought, a tree might not grow much at all.