While Europe was going through the Middle Ages, western North America experienced what is known as the Medieval megadroughts. A study explaining the causes of these events suggests there is a high danger the south-western United States in particular could be heading for an undesirable form of medieval re-enactment.

The megadroughts saw much of the American west experience extreme dryness for at least a decade at a time, starting around 800 CE and suddenly stopping 800 years later. Evidence for these great dry periods has been found from remnants of trees growing in what normally are lake beds, pollen (or its absence) in sediment cores, and rings from some of the region's ancient trees. However, climatologists have been uncertain of the causes.

The problem, according to Dr Nathan Steiger of the Lamont-Doherty Earth Observatory, is that people have been looking for a single explanation, but it took several factors working together to produce such lasting climatic events.

Steiger reconstructed conditions over the last 2,000 years and found three things coincided with the megadroughts. Two of these make intuitive sense: increases in sunlight and tightly bunched La Niña events, where cooling of the Eastern Pacific reduces the rain-bearing winds that otherwise water the western coasts of North and South America. The extra sunlight, in turn, could result from increased solar activity, a lull in volcanic ash production, or both. The third factor is more surprising, with warmer waters in the distant Atlantic having an effect on the other side of the American continent.

The most urgent question for those living in the region is whether higher global temperatures mean the megadrought from 2000-2018 will be the first of many. The booming populations of California, Arizona, and Nevada makes water increasingly scarce even under normal conditions.

The answers are not clear. Even global climate models that have proven successful in other ways struggle to predict the balance between El Niños (when the eastern Pacific warms) and La Niñas. That's a problem because Steiger estimates in Science Advances that La Niñas are at least twice as important as either of the other two factors.

However, we know the Atlantic will keep warming as long as the atmosphere's carbon dioxide levels keep rising. Higher temperatures could also replicate the effects of increased sunlight by upping the rate at which soil moisture evaporates.

"Because you increase the baseline aridity, in the future when you have a big La Niña, or several of them in a row, it could lead to megadroughts in the American West," Steiger said in a statement. Since the 1980s, the southwest has been getting dryer, pushing most of California into extreme drought by 2016.