If you've lately found yourself wondering when the worst drought North America has suffered occurred, here's your answer: 1934. At least, as far as the last millennium goes. In a new NASA study, scientists say that a combination of atmospheric conditions (a high pressure ridge off the West Coast that blocked wet weather) and dust storms (the product of grass-eviscerating activities like overfarming and overgrazing that left topsoil susceptible to wind) culminated in the most severe drought in the past 1,000 years. In fact, the Dust Bowl drought of 1934 was so bad that it was 30% worse than the next most severe drought, which occurred in 1580, reports Nature. Scientists drew this conclusion using an analysis of tree rings between the years 1000 and 2005.
The 1934 drought extended across 71.6% of Western North America, while the extent of 2012's drought didn't quite reach 60%. As io9 reports, drought led to 14 dust storms in 1932 and 38 in 1933, but farmers kept working the land, expecting rain to return. "As it turns out, the successive dust storms were making the droughts even worse." NASA explains why: "Dust clouds reflect sunlight and block solar energy from reaching the surface. That prevents evaporation that would otherwise help form rain clouds, meaning that the presence of the dust clouds themselves leads to less rain." While the agricultural practices that led to those dust storms are no longer used today, scientists say the risk of droughts is only going up, and that agricultural producers will need to adapt accordingly. (The latest drought, meanwhile, is actually causing the West to rise.)