Hansen says that the climate used to be stable and droughts used to be rare before we loaded the dice.
A 1998 study by federal scientists, for instance, found that droughts as widespread and severe as that of the Dust Bowl of the 1930’s, when 65 percent of the country was affected at the drought’s peak in 1934, have occurred once or twice a century over the last 300 to 400 years. A decade-long drought, the study found, occurs about once every 500 years.
More recent analyses of tree-ring data have identified a 16th-century megadrought that affected much of the continent for years, far outstripping any drought of the 20th century in persistence and severity. It was during this drought that the first English colony in America, at Roanoke Island in North Carolina, disappeared, and experts now believe the drought is what killed it off.